PCB Fabrication or Manufacturing Process

Electroless Copper

noreply@blogger.com (Unknown) — Thu, 19 Mar 2009 01:32:00 +0000

Although electroless copper has been successfully used for more than three decades, limits on operator exposure to formaldehyde and difficulties in removing the electroless copper from the waste stream caused manufacturers to seek alternatives. Among the deficiencies are (ref. 30):

Use of formaldehyde as reducing agent.
The process is inherently unstable, requiring stabilizing additives to avoid copper precipitation.
Environmentally undesirable complexing agents, such as EDTA, are used.
The large number of process and rinse tanks causes high water consumption.

The electroless copper process consists of four basic operations: cleaning, activation, acceleration, and deposition (Exhibit 3-17). An anti-tarnish bath is common after deposition. Virtually all shops purchase a series of proprietary chemistries from a single vendor that are used as the ingredients for the several process baths in the electroless copper process line. Only the micro-etch, its associated sulfuric dip, and the anti-tarnish baths are likely to be non-proprietary chemistries.
Cleaning. The cleaning segment begins with a cleaner-conditioner designed to remove organics and condition (in this case swell) the hole barrels for the subsequent uptake of catalyst, followed by a microetch step. The cleaner-conditioners are typically proprietary formulations, and mostly consist of common alkaline solutions.

A microetch step can be found on the electroless line, oxide line, pattern plate line and with chemical cleaning if that is the cleaning method used. Three chemistry alternatives are available. Sulfuric acid-hydrogen peroxide (consisting of 5% sulfuric acid and 1% to 3% peroxide) is most common, followed by sulfuric acid-potassium (or sodium) persulfate (5% sulfuric, 8 to 16 ounces/ gallon persulfate) and ammonium persulfate. In each case, the microetch bath is followed by a sulfuric acid dip, which serves to remove any remaining oxidizer. About 40 microinches of copper are etched for the making holes conductive process. Based on a 3-4 ounce copper carrying capacity, approximately 0.0183 gallons of microetch are used per square foot of product run. This figure does not include any solution that may be dragged out when the panels are moved to the next tank. The sulfuric-peroxide alternative has some attractive waste treatment and performance features (ref. 31):

No spent etchant disposal. The etchant is replenished as it is used, and copper is removed with a recovery unit in the form of copper sulfate crystals. These crystals form when the solution is cooled to room temperature or lower. Smaller shops may use a batch treatment where the solution is pumped to another tank to cool and crystallize. After removing the copper crystals, the solution can be transferred back to the process line and reused.
Constant etching rate. The etching rate is dependent on temperature and hydrogen peroxide concentration, not the copper concentration.
Simple waste treatment. No chelators are present in sulfuric-peroxide microetchants.
A high copper capacity of 3 to 4 ounces/gallon.
Efficient copper recovery. Copper sulfate recovery is usually 90-95% efficient.

Persulfate microetchants must be treated in-house or shipped to a licensed disposal facility. The etching rate is difficult to control since it declines as panels are processed and copper builds in the solution. Ammonium persulfate is uncommon due to high waste treatment costs.

Activation and Acceleration. Activation, through use of a catalyst, consists of two process tanks. A pre-dip, for the drag-in protection of the expensive activation (also called catalyst) bath, usually contains hydrochloric acid and possibly tin or sodium chloride. The activation bath itself consists of hydrochloric acid, tin chloride, a palladium chloride. The Sn+2 ion reduces the Pd+2 to Pd, which is deposited on the panel. The remaining Sn+2 and Sn+4 are selectively removed from the hole barrels by the accelerator (also called the post-activator). Fluoboric acid is a common accelerator, as is sulfuric acid with hydrazine.

Copper Deposition. Electroless copper baths can be divided into two types: heavy deposition baths (designed to produce 75 to 125 micro-inches of copper) and light deposition baths (20 to 40 micro-inches). Light deposition must be followed immediately by electrolytic copper plating. The more common heavy deposition can survive the outer layer imaging process, and copper electroplating occurs thereafter. The main constituents of the electroless copper chemistry are sodium hydroxide, formaldehyde, EDTA (or other chelater), and a copper salt. In the complex reaction, catalyzed by palladium, formaldehyde reduces the copper ion to metallic copper. Formaldehyde (which is oxidized), sodium hydroxide (which is broken down), and copper (which is deposited) must be replenished frequently.

Most heavy deposition baths have automatic replenishment schemes based on in-tank colorimeters. Light deposition formulations may be controlled by analysis. Formaldehyde is present in light deposition baths in a concentration of 3 to 5 grams/liter and as high as 10 grams/liter in heavy deposition baths.

When light deposition is applied, the next process step must be electrolytic copper plate. This is either a full panel plate (the typical 1 mil is plated in the holes and on the surface) or a "flash" panel plate, designed only to add enough copper to the hole barrels to survive the imaging process. Flash-plated panels return to copper electroplating after imaging to be plated up to the required thickness. This double plating step has made heavy deposition the more common electroless copper process.

Process Waste Streams. The electroless copper line typically contributes a significant percentage of a PWB shop's overall waste volume. Water use is high due to the critical rinsing required between nearly all of the process steps. Copper is introduced into the wastewater stream due to drag-out from the cleaner-conditioner, micro-etch, sulfuric, accelerator, and deposition baths. Much of this copper is complexed with EDTA and requires special waste treatment considerations. Furthermore, waste process fluid generation is high. Micro-etch baths are exhausted when 2 to 4 ounces/gallon of copper is dissolved, and this bath life is usually measured in days. While the electroless copper bath is relatively long-lived (usually several weeks or months), a considerable bailout stream (including formaldehyde) is generated (several gallons of site concentrated bath chemistry per day in production shops). This waste must either be treated in-house or shipped off-site, which adds another cost to using electroless copper.

Electroless Nickel/Immersion Gold

noreply@blogger.com (Unknown) — Thu, 19 Mar 2009 01:08:00 +0000

Nickel-gold finishes may cover an entire circuit or be selectively plated onto certain areas of a circuit. Nickel-gold formulations can produce hard gold, with the addition of cobalt or another metal being co-deposited in small amounts, or soft gold by utilizing pure gold.

Hard Gold. Hard gold is electrolytically plated. The most common application of hard gold is edge connectors, but hard gold may also be plated over circuit areas as well. Automated edge plating machines are common since manual plating is quite labor-intensive. Typically a plater's tape is applied to the board masking off all of the circuit above the edge connector. The panel is then processed through a nickel-gold plating line, with just the edge connectors immersed in the plating fluid. Nickel is plated first and Watts or sulfamate nickel is common. Cyanide gold is the most common gold electroplating chemistry.

Soft Electrolytic Gold. Soft gold is a pure gold coating over a nickel deposit. It may be electroplated over the entire circuit or selectively over certain portions of a circuit (excluding edge connectors, which require hard gold). Selective electroplating requires a combination of masking and bussing (providing current to the portion of the circuit being electroplated). Selective gold applications include contact points (which may require hard gold), press pads, wire bond sites, or portions of a board that may reside in a corrosive environment. Selective gold plating can be labor-intensive and is not frequently specified for production lots (all gold plating is often substituted; the labor savings offset the extra gold required).

Electroless Nickel/Immersion Gold. The electroless nickel/immersion gold process is another method of applying soft gold. Electroless plating can be conveniently performed after etching because no bussing is required. Therefore, these all-gold boards can be processed with a standard tin etch-resist and processed identically as SMOBC, except the gold plating step replaces the HASL step. This process has advantages over SMOBC/HASL and electrolytic gold plating. When compared to SMOBC/HASL, electroless all-gold circuits have a much longer shelf life. The flat surface profile of the electrolessly plated surface-mount pad and overall excellent solderability make electroless nickel/gold ideal for surface-mount technology. Cost, however, is an obvious disadvantage when compared to HASL. When compared to electrolytic gold, electroless has the advantage of full copper encapsulation because plating is performed after etching, not before, as with electrolytic gold plating. Selective gold plating is made somewhat easier by the electroless plating method since no electrical bussing is required. Cost is the main disadvantage. Immersion gold and electroless nickel process baths are short-lived compared to electrolytic formulations and maintenance and control of these baths is more difficult. Immersion gold plating is a self-limiting process which, for common baths, cannot produce thicknesses of much more than 10 micro-inches. The main application of electroless nickel-gold coatings is chip-on-board technology, where component leads are ultrasonically or thermosonically bonded to gold pads rather than soldered.

Hot Air Solder Level (HASL)

noreply@blogger.com (Unknown) — Thu, 19 Mar 2009 01:06:00 +0000

The HASL process consists of a pre-clean, fluxing, hot air leveling, and a post-clean. Pre-cleaning is usually done with a micro-etch. However, the usual persulfate or peroxide micro-etch is not common in the process. Dilute ferric chloride or a hydrochloric-based chemistry is favored for compatibility with the fluxes that are applied in the next step. Fluxes perform the following functions:

Provide oxidation protection to the precleaned surface.
Affect heat transfer during solder immersion.
Provide oxidation protection during HASL.

Higher viscosity fluxes provide better oxidation protection and more uniform solder leveling, but reduce overall heat transfer and require a longer dwell time or higher temperature. A balance in flux use must be struck between better protection with high viscosity fluxes and superior heat transfer with lower viscosity fluxes (ref. 38).

Hot air level machines consist of a transport mechanism that carries the panel into a reservoir of molten solder (460°F, 237°C), then rapidly past jets of hot air. All areas of exposed copper are coated with solder and masked areas remain solder-free. Boards are then cleaned in hot water, the only step in the SMOBC process where lead may enter the wastewater stream, albeit in very small quantities. Once cleaned, the panels may again enter the screening area for optional nomenclature screening, or proceed directly to the routing process.

Copper, flux and other impurities build in concentration in the solder pot as panels are processed through the hot air leveler. These impurities can be removed to some degree by performing a procedure known as drossing. From the hot operating temperature, the temperature is reduced to 385°F (196°C) and the machine sits idle for 8 to 12 hours. The impurities will float to the surface of the solder where they are scooped out and placed in a dross bucket. This material can be returned to the vendor for reclamation of the metals. Some manufacturers go for years without changing the solder, they dross and make additions. When the time comes to change over the solder, vendors will issue credit on the purchase of new solder as long as the old solder is returned to them for processing.

The acid pre-clean will have some copper in solution and can be treated conventionally. The waste flux is collected and is sent off-site for treatment.

NC Drill Files

noreply@blogger.com (Unknown) — Wed, 18 Mar 2009 05:09:00 +0000

Excellon is a manufacturer of CNC control systems for both drillers and routers. This company has been in the business of CNC control for many years. The Excellon Company can be best described as one of the pioneers Printed Circuit board drillers and routers.

The term "Excellon Drill File" is a bit of a misnomer, as the company's name has been attached to what should be simply described as an NC (numeric control) drill file. The most popular format of this data is Excellon's and is described in the following section. Although the term "Excellon" has been coined into the CAD environment don't let it confuse the issue of NC drill data.

There are many manufactures of drilling and routing equipment today. Most are fully compatible with Excellon's control codes. Some PCB vendors still require an older format known as EIA-Binary because their drill systems use an older paper tape reader system. Modern drilling equipment now uses the preferred ASCII drill formats and a PC or similar control computer interpret the ASCII files.

Maximum drill block length is 100 characters, English (inch) or Metric (mm), Leading or Trailing Zeros, Incremental or Absolute modes are supported. Maximum m.n format supported is 2.4
NC Drilling File and Gerber file Similarities

An NC drilling file is very similar in nature to that of the Gerber file. The main differences are the absence of control codes in the NC drill file. The drill assumes that each X/Y pair is a hole location and the drill will plunge at each X/Y coordinate that is listed in the file. The NC drill file contains delineators that identify groups of X/Y coordinates to be drilled with a specific Tool Size.

The delineator is "T"+#, the ‘#’ is cross referenced to a customer supplied list. There is no sorting necessary or specific sequence required when identifying the T code.

T1 for instance could be a .2550" while T2 could be a .0200" etc. There is provision for header comments in the drill file as well.

Many CAD systems such as Tango, Orcad, and Protel for Windows place the T code sizes in the header of the NC drill file just before the Start of Data marker "%".

Gerber Data Format

noreply@blogger.com (Unknown) — Wed, 18 Mar 2009 05:00:00 +0000

The Gerber data format is an industry standard used for printed circuit board layouts. This type of data is used by photoplotter equipment which uses a light to "draw" a line using an aperture, or shape. The Gerber data file is an ASCII format that instructs the photoplotter with four basic pieces of information:

X/Y Coordinate Information
Aperture Selection - tool shape to chose
Shutter Commands - one of open, close, or flash
End of Line Character - typically an asterisk (*)

X/Y Coordinate Information

The X/Y coordinate information, like any numerical data in a Gerber data file, has an integer and decimal portion. However, the decimal point in not a valid character, and the decimal values are written as integers. The position of the decimal place in the integer is determined by three parameters:

Number of integer (whole number) digits
Number of decimal (precision) digits
Zero suppression

These parameters give rise to the "2,3" and "2,4" file format designations. The number in front of the comma (2) designates the whole number digits, and the number trailing (3 & 4 respectively) represent the number of precision digits. When written out, the decimal point of the value is removed, and the values are appended to one another.

For example, in "2,3" format, the value 12.345 becomes "12345". In the "2,4" format, the same value becomes "123450" in order to fulfill the precision requirement.

Zero suppression can be one of three types: leading, trailing, and none. Leading and trailing zero suppression remove "0" characters that are unnecessary to reduce the data file size.

For example, in "2,4" format, the value 0.0100 can benefit from zero suppression. Since 2,4 format requires 2 whole number digits (in this case 00), and four precision digits (in this case 0100), then the resulting value is converted to 00+0100, written as "000100" if there is no zero suppression.

With leading zero suppression, "000" at the front is not required, and so it is written as "100".

With trailing zero suppression, "00" at the end is not required, so it is written as "0001".

Zero suppression is only one method of reducing the size of Gerber data files. Another method is to eliminate the use of redundant data; this is referred to as Modality.
Gerber File Examples

There are two types of Gerber file formats. Refer to the Plotter Codes information for explanations on the information contained in the following data file examples:

Gerber RS-274D File Example

RS-274D uses two files to represent each layer of data. The first file is an aperture file which tells the plotter which shapes it can draw with. The second file is a list of coordinates and commands which tell the plotter how to draw the layer.

Aperture file

D10 10H 10V Round -Draw-
D11 20H 20V Round -Draw-
D12 15H 15V Round -Draw-
D13 5H 5V Round -Draw-
D14 40H 40V Round *Flash*
D15 34H 46V RCT *Flash*
D16 10H 10V Square -Draw-
D17 28H 100V RCT *Flash*
D18 60H 14V RCT *Flash*
D19 40H 40V Round -Draw-
D20 14H 60V RCT *Flash*

Coordinate Data File

%LPD*%
G75*D10*G1X98750Y71250D3*
%LPC*%
G75*D11*G1X19791Y-5996D2*X19878Y-6035D1*X19957Y-6114*X19996Y-6232*
Y-6272*X19957Y-6390*X19878Y-6468*X19791Y-6500*X19720D2*X19642Y-6468D1*
X19563Y-6390*X19524Y-6272*Y-6035*X19563Y-5870*X19602Y-5799*X19681Y-5720*
X19760Y-5681*X19524Y-6232D2*X19563Y-6114D1*X114510*Y130765*X119760*
X127890D2*Y140765D1*X122640*Y130765*X127890*X136020D2*Y140765D1*X130770*
Y130765*X136020*X144150D2*Y140765D1*X138900*Y130765*X144150*X111630D2*
X106380D1*Y140765*X111630*Y130765*X6800Y102445D2*X34800D1*Y140445*
Y140695*X28800*X6800*Y140445*Y102445*X129310Y70565D2*Y98565D1*X91310*
X91060*Y92565*Y70565*X91310*X129310*X45050D2*Y98565D1*X7050*X6800*Y92565*
Y70565*X7050*X45050*X87180Y38685D2*Y66685D1*X49180*X48930*Y60685*Y38685*
X49180*X87180*X129310Y6805D2*Y34805D1*X91310*X91060*Y28805*Y6805*X91310*
X129310*X45050D2*Y34805D1*X7050*X6800*Y28805*Y6805*X7050*X45050*X0Y0D2*
M02*

Gerber RS-274X File Example

Below is a sample of a RS-274X file, notice that aperture definitions are contained in the header of the file, there is no separate aperture file.

*G04 Output by CAMMaster V9.0.51 PentaLogix LLC*
*G04 Fri Feb 16 07:54:01 2007*
*%FSLAX44Y44*%
%MOIN*%
%IPPOS*%
%ADD10R,20.0X16.0*%
%ADD11C,0.008*%
%ADD12R,0.06X0.015*%
%ADD13O,0.06X0.015*%
%ADD14R,0.05X0.06*%
%ADD15R,0.03X0.04*%
%ADD16R,0.04X0.03*%
%ADD17R,0.12X0.12*%
%ADD18R,0.06X0.044*%
%ADD19R,0.06X0.05*%
%ADD20R,0.09X0.075*%

%LPD*%
G75*D10*G1X98750Y71250D3*
%LPC*%
G75*D11*G1X19791Y-5996D2*X19878Y-6035D1*X19957Y-6114*X19996Y-6232*
Y-6272*X19957Y-6390*X19878Y-6468*X19791Y-6500*X19720D2*X19642Y-6468D1*
X19563Y-6390*X19524Y-6272*Y-6035*X19563Y-5870*X19602Y-5799*X19681Y-5720*
X19760Y-5681*X19524Y-6232D2*X19563Y-6114D1*X114510*Y130765*X119760*
X127890D2*Y140765D1*X122640*Y130765*X127890*X136020D2*Y140765D1*X130770*
Y130765*X136020*X144150D2*Y140765D1*X138900*Y130765*X144150*X111630D2*
X106380D1*Y140765*X111630*Y130765*X6800Y102445D2*X34800D1*Y140445*
Y140695*X28800*X6800*Y140445*Y102445*X129310Y70565D2*Y98565D1*X91310*
X91060*Y92565*Y70565*X91310*X129310*X45050D2*Y98565D1*X7050*X6800*Y92565*
Y70565*X7050*X45050*X87180Y38685D2*Y66685D1*X49180*X48930*Y60685*Y38685*
X49180*X87180*X129310Y6805D2*Y34805D1*X91310*X91060*Y28805*Y6805*X91310*
X129310*X45050D2*Y34805D1*X7050*X6800*Y28805*Y6805*X7050*X45050*X0Y0D2*
M02*

RS-274D data files do not define apertures. Therefore, an application that lays out printed circuit boards and a photoplotter can interpret the aperture differently. To overcome this problem, an aperature file must also be included with the RS-274D file to ensure compatability - this is what the RS-274X Gerber format can solve. The RS-274X format has an aperture definition embedded in the file so that the separate aperture file is not required.
Additional Information

For additional information on Gerber files an their applications in printed circuit board layout, see the Gerber Files and Uses table.

PCB Glossary : S to Z

noreply@blogger.com (Unknown) — Tue, 17 Mar 2009 03:36:00 +0000

Scoring:
A technique in which grooves are machined on opposite sides of a panel to a depth that permits individual boards to be separated from the panel after component assembly.

Screen Printing:
A process for transferring an image to a surface by forcing suitable media through a stencil screen with a squeegee.

Single-Sided Board:
A printed board with conductive pattern on one side only.

Soldermask Over Bare Copper (SMOBC):
A method of fabricating a printed circuit board that results in final metallization being copper with no protective metal. The non-coated areas are coated by solder resist, exposing only the component terminal areas. This eliminates tin lead under the pads.

Surface Mount Technology (SMT):
Defines the entire body of processes and components that create printed circuit board assemblies with leadless components.

Solder:
An alloy that melts at relatively low temperatures and is used to join or seal metals with higher melting points. A metal alloy with a melting temperature below 427°C (800°F).

Soldermask:
Coating material used to mask or to protect selected areas of a pattern from the action of an etchant, solder, or plating. Also called resist or mask.

Step-and-Repeat:
A method by which successive exposures to a single image are made to produce a multiple image production master.

Stripping:
The process by which imaging material (resist) is chemically removed from a panel during fabrication.

Substrate:
A material on whose surface adhesive substance is spread for bonding or coating. Also, any material that provides a supporting surface for other materials used to support printed circuit patterns.

Test Coupon:
A portion of a printed board or of a panel containing printed coupons used to determine the acceptability of such a board.

TG:
Glass transition temperature. The point at which rising temperatures cause the solid base laminate to start to exhibit soft, plastic-like symptoms. This is expressed in degrees Celsius (°C).

Tooling Holes:
The general term for holes placed on a PCB or a panel of PCBs for registration and hold-down purposes during the manufacturing process.

Top Side:
See Component Side.

Trace:
A common term for conductor.

Traveler:
The list of instructions describing the board, including any specific processing requirements. Also called a shop traveler, routing sheet, job order, or production order.

Twist:
A laminate defect in which deviation from planarity results in a twisted arc.

UL:
Underwriters Laboratories, Inc., an independent product safety testing and certification organization.

Underwriters Symbol:
A logotype denoting that a product has been recognized (accepted) by Underwriters Laboratories Inc. (UL).

Via:
A plated through-hole that is used as an interlayer connection but does not have component lead or other reinforcing material inserted in it.

Void:
The absence of any substances in a localized area.

Wave Soldering:
A process wherein assembled printed boards are brought in contact with a continuously flowing and circulating mass of solder, typically in a bath.

X Axis:
The horizontal or left-to-right direction in a two dimensional system or coordinates.

Y Axis:
The verticle or bottom-to-top direction in a two dimensional system of coordinates.

Z Axis:
Perpendicular to the plane formed by the X and the Y datum reference.

PCB Glossary : N to R

noreply@blogger.com (Unknown) — Tue, 17 Mar 2009 03:32:00 +0000

Nomenclature:
Identification symbols applied to the board by means of screen printing, inkjetting, or laser processes. See Legend.

Outer-layer:
The top and bottom sides of any type of circuit board.

Pad:
See Land.

Pattern:
The configuration of conductive and nonconductive materials on a panel or printed board. Also, the circuit configuration on related tools, drawing, and masters.

Pattern Plating:
The selective plating of a conductive pattern.

Photographic Image:
An image in a photo mask or in an emulsion that is on a film or plate.

Photoplotting:
A photographic process whereby an image is generated by a controlled light beam that directly exposes a light-sensitive material.

Photo Print:
The process of forming a circuit pattern image by hardening a photosensitive polymeric material by passing light through a photographic film.

Phototool:
A transparent film that contains the circuit pattern, which is represented by a series of lines of dots at a high resolution.

Plated Through-Hole:
A hole with plating on its walls that makes an electrical connection between conductive layers, external layers, or both of a printed board.

Platen:
A flat plate of metal within the lamination press in between which stacks are placed during pressing.

Plating Void:
The area of absence of a specific metal from a specific cross-sectional area.

Plotting:
The mechanical converting of X-Y positional information into a visual pattern such as artwork.

Prepreg:
Sheet material (e.g. glass fabric) impregnated with a resin cured to an intermediate stage (B-stage resin).

Pressing:
The process by which a combination of heat and pressure are applied to a book, thereby producing fully cured laminated sheets.

Printed Board:
The general term for completely processed printed circuit or printed wiring configurations. It includes single, double-sided, and multi-layer boards, both rigid and flexible.

Printed Circuit:
A conductive pattern that comprises printed components, printed wiring, or a combination thereof, all formed in a predetermined design and intended to be attached to a common base. (In addition, this is a generic term used to describe a printed board produced by any of a number of techniques.)

Printed Wiring Board:
A part manufactured from rigid base material upon which completely processed printed wiring has been formed.

Registration:
The degree of conformity to the position of a pattern, or a portion thereof, a hole or other feature to its intended position on a product.

Resin (Epoxy) Smear:
Resin transferred from the base material onto the surface of the conductive pattern in the wall of a drilled hole.

Resist:
Coating material used to mask or to protect selected areas of a pattern from the action of an etchant, solder, or plating. Also called soldermask or mask.

Rigid-flex:
A PCB construction combining flexible circuits and rigid multi-layers usually to provide a built-in connection or to make a three-dimension form that includes components.

Router:
A machine that cuts away portions of the laminate to form the desired shape and size of the printed board.

PCB Glossary : H to M

noreply@blogger.com (Unknown) — Tue, 17 Mar 2009 03:27:00 +0000

Hole Breakout:
A condition in which a hole is partially surrounded by the land.

Hole Pattern:
The arrangement of all holes in a printed board with respect to a reference point.

Hot Air Solder Leveling (HASL):
A method of coating exposed copper with solder by inserting a panel into a bath of molten solder then passing the panel rapidly past jets of hot air.

Imaging:
The process by which panelization data are transferred to the photoplotter, which in turn uses light to transfer a negative image circuitry pattern onto the panel.

Impedance:
The total passive opposition offered to the flow of electric current. This term is generally used to describe high-frequency circuit boards.

Inner-layers:
The internal layers of laminate and metal foil within a multi-layer board.

Insulation Resistance:
The electrical resistance of an insulating material that is determined under specific conditions between any pair of contacts, conductors, or grounding devices in various combinations.

Known Good Board (KGB):
A board or assembly that is verified to be free of defects. Also known as a Golden Board.

Laminate:
The plastic material usually reinforced by glass or paper that supports the copper cladding from which circuit traces are created.

Laminate Thickness:
Thickness of the metal-clad base material, single- or double-sided, prior to any subsequent processing.

Laminate Void:
An absence of epoxy resin in any cross-sectional area that should normally contain epoxy resin.

Land:
The portion of the conductive pattern on printed circuits designated for the mounting or attachment of components. Also called a pad.

Layup:
The process in which treated prepregs and copper foils are assembled for pressing.

Legend:
A format of lettering or symbols on the printed circuit board: e.g. part number, serial number, component locations, and patterns.

Liquid Photoimageable Soldermask (LPI):
A mask using photographic imaging techniques to control deposition.

Line:
See Conductor.

Lot:
A quantity of circuit boards that share a common design.

Major Defect:
A defect that is likely to result in failure of a unit or product by materially reducing its usability for its intended purpose.

Mask:
A material applied to enable selective etching, plating, or the application of solder to a PCB. Also called soldermask or resist.

Metal Foil:
The plane of conductive material of a printed board from which circuits are formed. Metal foil is generally copper and is provided in sheets or rolls.

Microsectioning:
The preparation of a specimen of a material, or materials, that is to be used in metallographic examination. This usually consists of cutting out a cross-section followed by encapsulation, polishing, etching, and staining.

Minor Defect:
A defect that is not likely to result in the failure of a unit of product or that does not reduce the usability for its intended purpose.

Multi-Layer Board:
Printed boards consisting of a number (four or more) of separate conducting circuit planes separated by insulating materials and bonded together into relatively thin homogeneous constructions with internal and external connections to each level of the circuitry as needed.

PCB Glossary : D to G

noreply@blogger.com (Unknown) — Tue, 17 Mar 2009 03:20:00 +0000

Defect:
Any nonconformance to specified requirements by a unit or product.

Definition:
The fidelity of reproduction of pattern edges, especially in a printed circuit relative to the original master pattern.

Delamination:
A separation between any of the layers of the base of laminate or between the laminate and the metal cladding originating from or extending to the edges of a hole or edge of board.

Design Rule Checking:
The use of a computer program to perform continuity verification of all conductor routing in accordance with appropriate design rules.

Desmear:
The removal of friction-melted resin and drilling debris from a hole wall.

Dewetting:
A condition that results when molten solder has coated a surface and then receded, leaving irregularly shaped mounds separated by areas covered with a thin solder film and with the base material not exposed.

Dielectric:
An insulating medium that occupies the region between two conductors.

Dimensional Stability:
A measure of the dimensional change of a material that is caused by factors such as temperature changes, humidity changes, chemical treatment, and stress exposure.

Double-Sided Board:
A printed board with a conductive pattern on both sides.

Drilling:
The act of forming holes (vias) in a substrate by mechanical or laser means.

Dry-Film Resists:
Coating material specifically designed for use in the manufacture of printed circuit boards and chemically machined parts. They are suitable for all photomechanical operations and are resistant to various electroplating and etching processes.

Dry-Film Soldermask:
Coating material (dry-film resist) applied to the printed circuit board via a lamination process to protect the board from solder or plating.

Electroless Copper:
A thin layer of copper deposited on the plastic or metallic surface of a PCB from an autocatalytic plating solution (without the application of electrical current).

Electroplating:
The electrodeposition of an adherent metal coating on a conductive object. The object to be plated is placed in an electrolyte and connected to one terminal of a direct current (DC) voltage source. The metal to be deposited is similarly immersed and connected to the other terminal.

Epoxy:
A family of thermosetting resins. Epoxies form a chemical bond to many metal surfaces.

Epoxy Smear:
Epoxy resin that has been deposited on edges of copper in holes during drilling either as uniform coating or in scattered patches. It is undesirable because it can electrically isolate the conductive layers from the plated-through-hole interconnections.

Etchback:
The controlled removal of all components of the base material by a chemical process acting on the sidewalls of plated-through holes to expose additional internal conductor areas.

Etching:
The chemical, or chemical and electrolytic, removal of unwanted portions of conductive materials.

Flying Probe:
A testing device that uses multiple moving pins to make contact with two spots on the electrical circuit and send a signal between them, a procedure that determines whether faults exist.

FR-4:
The UL-designated rating for a laminate composed of glass and epoxy that meets a specific standard for fire-retardance. FR-4 is the most common dielectric material used in the construction of PCBs.

Gerber:
A software format used by the photoplotter to describe the printed circuit board design.

Golden Board:
See Known Good Board.

Ground Plane:
A conductor layer, or portion of a conductor layer, used as a common reference point for circuit returns, shielding, or heat sinking.

PCB Glossary : A to C

noreply@blogger.com (Unknown) — Tue, 17 Mar 2009 03:09:00 +0000

Check out The printed circuit board industry is full of specific terminology and acronyms. To assist you in designing and ordering your circuit board, we’ve put together the following list of common terms:

Annular Ring:
That portion of conductive material completely surrounding a hole.

Aperture Information:
This is a text file describing the size and shape of each element on the board. These are also known as D-code lists. These lists are not necessary if your files are saved as Extended Gerber with embedded Apertures (RS274X).

Array:
A group of elements or circuits (or circuit boards) arranged in rows and columns on a base material.

Artwork:
An accurately scaled configuration used to produce the artwork master or production master.

Artwork Master:
The photographic film that embodies the image of the PCB pattern, usually on a 1:1 scale.

Aspect Ratio:
A ratio of the PCB thickness to the diameter of the smallest hole.

Automated Optical Inspection (AOI):
Visual inspection of the circuit boards using a machine scanner to assess workmanship quality.

Automatic Test Equipment (ATE):
Equipment that automatically analyzes functional or static parameters in order to evaluate performance.

Barrel:
The cylinder formed by plating through a drilled hole.

Base Copper:
The thin copper foil portion of a copper-clad laminate for PCBs. It can be present on one or both sides of the board, and on inner layers.

Base Material:
The insulating material upon which a conductive pattern may be formed. It may be rigid or flexible or both. It may be a dielectric or insulated metal sheet.

Base Material Thickness:
The thickness of the base material excluding metal foil or material deposited on the surface.

Bed-Of-Nails Fixture:
A test fixture consisting of a frame and a holder containing a field of spring-loaded pins that make electrical contact with a planar test object (i.e., a PCB).

Bevel:
An angled edge of a printed board.

Blind Via:
A conductive surface hole that connects an outer layer with an inner layer of a multi-layer board.

Blister:
A localized swelling and separation between any of the layers of a laminated base material, or between base material or conductive foil. It is a form of Delamination.

Bond Strength:
The force per unit area required to separate two adjacent layers of a board by a force perpendicular to the board surface.

Bow:
The deviation from flatness of a board, characterized by a roughly cylindrical or spherical curvature such that if the board is rectangular. Its four corners are in the same plane.

B-Stage Material:
Sheet material impregnated with a resin cured to an intermediate stage (B-stage resin). Prepreg is the popular term.

B-Stage Resin:
A thermosetting resin that is in an intermediate state of cure.

Buried Via:
A via hole that does not extend to the surface of a printed board.

CAD:
See Computer-Aided Design.

CAM:
See Computer-Aided Manufacturing.

Chamfer:
A broken corner to eliminate an otherwise sharp edge.

Circuit:
The interconnection of a number of devices in one or more closed paths to perform a desired electrical or electronic function.

Circuitry Layer:
A layer of a printed board containing conductors, including ground and voltage planes.

Cleanroom:
A room in which the concentration or airborne particles is controlled to specified limits.

Component:
An electronic device, typically a resistor, capacitor, inductor, or integrated circuit (IC), that is mounted to the circuit board and performs a specific electrical function.

Component Hole:
A hole used for the attachment and electrical connection of a component termination, such as a pin or wire to the circuit board.

Component Side:
The side of the circuit board on which most of the components will be located. Also called the “top side.”

Computer-Aided Design (CAD):
A software program with algorithms for drafting and modeling, providing a graphical representation of a printed board’s conductor layout and signal routes.

Computer-Aided Manufacturing (CAM):
The use of computers to analyze and transfer an electronic design (CAD) to the manufacturing floor.

Computer-Integrated Manufacturing (CIM):
Software that takes assembly data from a CAD or CAM package and, using a pre-defined factory modeling system, outputs routing of components to machine programming points and assembly and inspection documentation.

Conductor:
A thin conductive area on a PCB surface or internal layer usually composed of lands (to which component leads are connected) and paths (traces).

Conductor Spacing:
The distance between adjacent edges (not centerline to centerline) of isolated conductive patterns in a conductor layer.

Conductor Thickness:
The thickness of the conductor including all metallic coatings.

Conformal Coating:
An insulating protective coating that conforms to the configuration of the object coated and is applied on the completed board assembly.

Connector Area:
The portion of the circuit board that is used for providing electrical connections.

Controlled Impedance:
The matching of substrate material properties with trace dimensions and locations to create specific electric impedance as seen by a signal on the trace.

Core Thickness:
The thickness of the laminate base without copper.

Printed Circuit Board Material Selection

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 08:27:00 +0000

Material selection for the construction of a printed circuit board is a critical step towards the final product.

Product Application Recommendations

Application recommendations for selection of a specific type of material is based upon consideration of the following:

Material Suitability
Best in Class Performance
Cost / Performance Ratio

Key Questions:

Will the (PCB / PWB) printed circuit board successfully survive the reflow process?
Will the (PCB / PWB) be fit for use and have maximum reliability in use after the reflow process?

Specific Material Selection Suitability

The application suitability of a specific type of material is based on consideration of the following parameters:

Tg - Glass transition temperature
Td - Decomposition temperature
Z axis expansion
PWB Processability
I.S/T/ performance
T260 and T288 performance
Solder float survivability
Thermal cycling performance
And additional data

Lead-Free Printed Circuit Board Surface Finishes

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 08:12:00 +0000

Printed Circuit Board finishes used to be simple consisting of either HASL (hot air solder leveling) or electrolytic nickel / gold. For years lead based finishes dominated the industry but technology and environmental concerns have altered the landscape for manufacturing printed circuit boards and more importantly the use of lead-free finishes used on circuit boards.

RoHS compliance , which took effect July 1, 2006, has established a new set of regulations requiring circuit board manufacturers to use lead-free finishes. There are a variety of lead-free finishes available today that meet RoHS compliance but it is important to understand the pros and cons of each finishing option and which to choose for your application.

Please select from the list below to learn more about lead free circuit board finishes:

Organic Solderability Preservative – OSP
Immersion Silver
Electroless Nickel Immersion Gold – ENIG
Hard Gold
White Tin
Lead-Free Hot Air Solder Leveling – Lead-Free HASL
Lead Free Finishes - Comparison Chart

Organic Solderability Preservative (OSP's) lead-free finishes are available in different thicknesses, are inexpensive, provide good surface oxidation, coplanarity and are easy to apply. OSPs are limited in the number of heat cycles and some require a nitrogen environment at time of assembly. OSPs have a poor shelf life, degrade with high temperatures, have poor wettability and are not suited for wire bonding or as a contacting surface.

Immersion Silver lead-free finishes offer good wettability, low costs, and are re-workable. Tarnishing is handled with an anti-tarnishing ingredient applied during the silver bath step. Immersion Silver finishes have a good shelf life and must be packed using sulphur-free paper.

Electroless Nickel Immersion Gold - ENIG This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Because this finish has special requirements Triangle Circuits has created a separate page for more details on ENIG.

Hard Gold This lead-free finish offers a variety of good properties including wettability, coplanarity, shelf-life as well as offering a good process window and process control. Unfortunately, Gold is a very expensive metal and is often too high priced to be used for some applications, in addition it can crack or break and cannot be reworked.

White Tin This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Beeches this finish has special requirements Triangle Circuits has created a separate page for more details

Lead-Free Hot Air Solder Leveling This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Because this finish has special requirements Triangle Circuits has created a separate page for more details

The Printed Circuit Board Manufacturing Process:

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 06:17:00 +0000

The process starts once your files and specifications have been sent to Triangle Circuits. Our engineers review each order to ensure design for manufacturability and proper selection of materials and technologies before manufacturing begins.

Patterning | Etching

The majority of printed circuit boards are manufactured by applying a layer of copper over the entire surface of the circuit board substrate either on one side or both sides. This creates what is referred to as a blank printed circuit board, meaning the copper is everywhere on the surface. From here the unwanted areas are removed, this is called a subtractive method, the most common subtractive method is known as photoengraving.

Photoengraving

The photoengraving process uses a mask or photomask combined with chemical etching to subtract the copper areas from the circuit board substrate The photomask is created with a photoplotter which takes the design from a CAD PCB software program. Lower resolution photomasks are sometimes created with the use of a laser printer using a transparency.

Lamination

Many printed circuit boards are made up of multiple layers, these are referred to as multi-layer printed circuit boards. They consist of several thin etched boards or trace layers and are bonded together through the process of lamination.

Drilling

Each layer of the printed circuit board requires the ability of one layer to connect to another, this is achieved through drilling small holes called "VIAS". These drilled holes require precision placement and are most commonly done with the use of an automated drilling machine. Theses machines are driven by computer programs and files called numerically controlled drill or (NCD) files also referred to as excellon files. These files determine the position and size of each file in the design.

Some files require very small vias to be drilled which results in heavy wear and tear of the drill bit itself. Drilling through different substrates may require the drill bit to be made of tungsten carbide and are costlier than other materials but required to provide a proper hole.

Controlled depth drilling can be used to drill just one layer of the circuit board rather than drilling through all the layers. This can be accomplished by drilling the individual sheets or layers of the PCB prior to lamination.

Blind Vias : Is when the holes connect a layer to the outside surface
Buried Vias : Is when the holes only connect interior layers and not to the outside surface.

The walls of each hole (for multi-layer boards) are copper plated to form plated-through holes that connect the conductive layers of the printed circuit board.

Solder Plating | Solder Resist

Pads and lands which will require components to be mounted on are plated to allow solderability of the components. Bare copper is not readily solderable and requires the surface to be plated with a material that facilitates soldering. In the past a lead based tin was used to plate the surfaces, but with RoHS compliance enacted newer materials are being used such as nickel and gold to both offer solderability and comply with RoHS standards.

Areas that should not be solderable are covered with a material to resist soldering. Solder resist refers the a polymer coating that acts as a mask and prevents solder from bridging traces and possibly creating short circuits to nearby component leads.

Silk Screen

When visible information needs to be applied to the board such as company logos, part numbers or instructions, silk screening is used to apply the text to the outer surface of the circuit board. Where spacing allows, screened text can indicate component designators, switch setting requirements and additional features to assist in the assembly process.

NOTE: "Red Print" refers to the silk screening of a one sided printed circuit board.

Testing

Unpopulated circuit boards are subjected to a bare board test where each circuit connection (as defined in a netlist) is verified as correct on the finished circuit board. In high volume circuit board production, a bed of nails tester or fixture is used to make contact with the copper lands or holes on one or both sides of the board to facilitate testing. Computers are used to control the electrical testing unit to send a small current through each contact point on the bed of nails and verify that such current can be detected on the appropriate contact points.

For small to medium volume production runs, a flying probe tester is used to check electrical contacts. These flying probes employ moving heads to make contact with the copper lands and holes to validate the electrical connectivity of the board being tested.

Printed Circuit Board Substrates

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 06:14:00 +0000

The majority of today’s printed circuit boards are made up of layers, also referred to as multi-layer printed circuit boards. Most circuit boards have between 1-16 layers and are separated by a substrate which is a laminated insulator that separates the circuitry design. Each layer of the circuitry is connected to each other through a series of drill holes.

Theses holes know as VIAS are plated holes that connect copper tracks from one layer of the printed circuit board to other layers they can be plated or have a rivet . There 2 types of vias used in multi-layer printed circuit boards:

Blind Vias: This is a vias that is visible on one side only.
Buried Vias: This is a vias that is not visible on either side

Printed Circuit Board Substrates

Substrates can be classified into five types, each having a unique set of characteristics for specific applications.

FR-2 Paper Printed Circuit Board Substrate

This lower grade of substrate is a made of impregnated paper, also called Phenolic, that is easy to machine over a fiberglass material substrate. The “FR” refers to the term Flame Resistant. This type of substrate is usually found on more inexpensive consumer electronics.

FR-4 Fiberglass Printed Circuit Board Substrates

Fiberglass substrates are made up of a woven fiberglass material and impregnated with a flame resistant material. The material is rigid and can also be drilled cut or machined but due to the abrasive nature of the fiberglass, tungsten carbide tools are needed. An FR-4 is a stronger substrate compared to an FR-2 and is more resistant to cracking or breaking and are usually found in higher end electronics.

RF Radio Frequency Printed Circuit Board Substrates

RF substrates are comprised of low dielectric plastics and used in printed circuit boards for applications in high power radio frequencies. Although the substrate has poor mechanical properties it has exceptional electrical performance properties.

Flex – Flexible Printed Circuit Board Substrates

Not all circuit boards use rigid core materials. Some are designed to be very flexible or slightly flexible called flex circuits. Thin and flexible plastics and or films are employed as substrates. The manufacturing process is more difficult than using rigid substrates, but offers benefits that cannot be achieved with rigid substrates such as saving space by bending the circuit board to fit a particular space or where repetitive movement requires a flexible layer.

Ceramic/Metal Core Substrates

Power electronics demand a low-thermal resistivity substrate. A ceramic core or metal core substrate provides the necessary characteristics to handle larger copper tracks and the high electrical currents used with these type of circuit boards.

Bright Acid Tin Plating

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:36:00 +0000

Overview

Electroplating bright tin is always considered a part of the imaging process where positive artwork is used, since the resulting tin plate is probably the best etch-resist available when using acidic or ammoniacal etchants. Acid tin plating tanks are identical to the tanks used in tin/lead plating with the exception that the anode bars are metallic tin instead of tin/lead.. The items that are not easily fabricated in a home shop are available from a variety of sources. A long lasting, very reliable plating solution can be mixed using readily available materials.

The Downside of Tin Plating

As good as it is, tin plating suffers from a tendency to form an intermetallic alloy with copper which changes the conductive properties of the bulk metal and can cause an increase in brittleness. Tin plating can also cause the growth of metallic whiskers between circuit elements leading to very hard to find shorts. As a result, the tin layer must be stripped after your circuit has been etched. Luckily, the thin layer of tin that is necessary for effective resist action is easily removed using a replenishable, environmentally friendly stripping solution.

Basic principles

An acid tin electroplating solution is a mixture of water, organic acid, and stannous tin. To this is added a number of organic constituents that serve to regulate and distribute the delivery of tin ions to the surface being plated. The two basic organic brighteners are commonly referred to as the "brightener" and the "starter".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of tin bars (or baskets of nuggets) arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical tin/lead electroplating bath has an effective full load operating "impedance" that ranges between 0.015 Ohms and 0.035 Ohms). Situated halfway between these anode "banks" is the copperclad substrate that is to be plated. It is variously referred to as the cathode or the workpiece.

In the simplest terms, metal deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of tin ions to the cathode where the ionic charge is neutralized as they plate out of solution. At the anode (in a properly maintained bath), sufficient tin erodes into the electrolyte, to exactly make up for the deposited material, maintaining a constant concentration of dissolved tin metal. As in all electrolytic solutions, there is a tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more metal ions than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of tin instead of the smooth, bright surface needed for reliable resist action inside the etching tank. Inhibiting and controlling this nonlinear behavior is where the organic brighteners come in to play.

Organic brighteners

In a well controlled plating bath, the starter supports the formation of a skin on the anode material which serves to regulate the diffusion of tin ions into the electrolyte. The material is also attracted to, but not co-deposited on the cathode (work piece) forming a layer (film layer) in close proximity to the surface that controls the rate of tin metal deposition.

The brightener works within the film layer to control alloy deposition on a microscopic level. It tends to be attracted to points of high electro-potential, temporarily packing the area and forcing metal ions to deposit elsewhere. As soon as the deposit levels, the local point of high potential disappears and the brightener drifts away. (i.e. brighteners inhibit the normal tendency of the plating bath to preferentially plate areas of high potential which would inevitably result in rough, dull plating) By continuously moving with the highest potential, the brightener prevent the formation of large clumps of tin whiskers, giving the smooth, bright deposition that results from a properly maintained and operated acid tin plating bath..

Mark Brelsford of QMS in Toronto, ON likens the action of the starter to the function of a doorman at a theater who regulates the flow of people into a theater, but doesn't really care where they go once inside. The brightener would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.

Tin/Lead (Solder) Plating

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:34:00 +0000

Overview

Electroplating tin/lead alloy is often considered a part of the imaging process, since the resulting solder plate is a very effective etch resist when used in conjunction with both acidic and ammoniacal ethcants. Most of the equipment, like tin/lead plating tanks, is easy to make and will last for many years if properly maintained. The items that are not easily fabricated in a home shop are available from a variety of sources. A high-performance plating solution can be mixed using readily available materials.

The Downside of Tin/Lead Plating

Laminating soldermask over solder plate is not recommended for boards that are going to be wave soldered or passed through an infra-red reflow oven. As the solder melts it can cause the mask to wrinkle and crack, creating pockets where solder flux and moisture can be trapped against circuit elements. Generally speaking, solderplating is only used on boards that are going to be hand assembled and soldered, or boards that are not going to be coated with soldermask. It is possible the strip away the tin/lead coating after etching your circuit, but the resulting bath will contain dissolved lead and will be difficult to dispose of responsibly. If you intend to make boards at production levels, and want to use a metallic etch resist, you might consider bright acid tin instead of tin/lead. Acid tin is plated with exactly the same equipment as tin/lead and has the advantage that it can be stripped off using a replenishable bath that is very environmentally friendly.

Basic principles

A tin/lead (solder) electroplating solution is a mixture of water, organic acid, stannous tin, and lead sulfate. To this is added a number of organic constituents that serve to regulate and distribute the delivery of tin and lead ions to the surface being plated. The two basic organic additives are commonly referred to as the "additive" and the "carrier".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of tin/lead bars (or baskets of nuggets) arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical tin/lead electroplating bath has an effective full load operating "impedance" that ranges between 0.015 Ohms and 0.035 Ohms). Situated halfway between these anode "banks" is the copperclad substrate that is to be plated. It is variously referred to as the cathode or the workpiece.

In the simplest terms, metal deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of both tin and lead ions to the cathode where the ionic charge is neutralized as they plate out of solution. At the anode (in a properly maintained bath), sufficient tin and lead erodes into the electrolyte (at the proper ratio), to exactly make up for the deposited material, maintaining a constant concentration of dissolved tin and lead. This all sounds quite nice, except for the annoying tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more metal ions than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of tin and lead spears instead of the smooth, matte surface needed for reliable resist action inside the etching tank. Inhibiting and controlling this nonlinear behavior is where the organic additives come in to play.

Organic additives

In a well controlled plating bath, the carrier supports the formation of a skin on the anode material which serves to regulate the diffusion of tin and lead ions into the electrolyte in the proper ratio. The material is also attracted to, but not co-deposited on the cathode (work piece) forming a layer (film layer) in close proximity to the surface that controls the rate of tin/lead alloy deposition.

The additive works within the film layer to control alloy deposition on a microscopic level. It tends to be attracted to points of high electro-potential, temporarily packing the area and forcing metal ions to deposit elsewhere. As soon as the deposit levels, the local point of high potential disappears and the additive drifts away. (i.e. additives inhibit the normal tendency of the plating bath to preferentially plate areas of high potential which would inevitably result in rough, dull plating) By continuously moving with the highest potential, the additive prevents the formation of large clumps of poorly mixed alloy, giving the smooth, matte deposition that is the hallmark of a properly functioning solder plating bath..

Mark Brelsford of QMS in Toronto, ON likens the action of the carrier to the function of a doorman at a theater who regulates the flow of people into a theater but doesn't really care where they go once inside. The additive would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.

Pattern plating copperclad substrates proceeds as follows:

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:28:00 +0000

Calculate the total plating time.

An acid copper plating bath based on the Lea Ronal PCM+ additive system deposits 0.0011" (1.10 mils, 28 microns, 0.81 oz) of high ductility copper in 1 hour at 20 ASF(Amps per Square Foot). Plating up "one ounce" of copper (i.e. plating 1 oz. of copper onto 1 square foot of board) is equivalent to plating a thickness of 0.0013" (1.3 mils or 34 microns).

Example: If you are starting with "half ounce" copperclad and want to plate up to a finished thickness of "one ounce", you will need to add .65 mils. The total plating time at 20 ASF will be:

[0.65 mils / (1.1 mils/hr.) x 60 min./hr. = 35.5 minutes = T

Calculate the required plating current.

Convert the total area of the pattern being plated into square feet (remember both sides!) and multiply the result by 20. Some CAM packages output the area of the pattern as a percentage of the total board area (area enclosed by the board outline defined in the ECAD or CAM software), while others can calculate the total pattern area in any unit specified by the user. To normalize the plating field, it is often beneficial to add an exposed ¾" boundary around the board to increase to total plating area and suppress the formation of high potential areas at the edges of the pattern. These are referred to as "robber bars" or "thieving bars" since they "steal" some of the electric field from the circuit pattern.

Example: If you are plating a double-sided board with a total circuit area equal to 25 sqin. (robber bars included) you will need:

[25/144] x 20 = 3.5 Amps = C

Carefully inspect the substrate for deep scratches and nicks that might impair the quality of the finished circuit.
Format the drilling stack to minimize burr formation during drilling.
Drill the through-holes and mounting holes, and mill/router any slot or cavity that is to be plated.
Activate the hole-walls.
While the ink is curing, take a few minutes to analyze the electrolyte. If you have a hull cell, this is a good time to run a test to insure that the organic components of the bath (which are very difficult to test directly) are in balance and present in the proper concentrations.
After activation and curing, both sides of the substrate should be thoroughly cleaned to remove any trace of conductive ink from both surfaces. Any ink that is not removed will almost certainly show up in the worst possible place so take your time cleaning the board and make a good job of it!
An abrasive pad (e.g. Scotchbrite® pad) can be used to remove ink that proves to be too stubborn for conventional cleaning, but be careful. You must be certain that you do not break the electrical contact between the conductive ink on the inside of the holes and the copper foil on the surface of the board or the holes in question will not plate properly.
Rinse the board thoroughly in deionized water before proceeding.
Dip the board into a 10% solution of sulfuric acid to make sure that no residual developing solution remains in the traces or through-holes and to minimize the introduction of contaminants into the copper plating tank.
Attach the cathode clip to the board, making certain that both copper surfaces have good electrical contact to the negative terminal of the plating power supply.
Turn the power supply on.

Note: The power supply should be adjusted so that, at its lowest setting, it establishes an electrical potential of about 0.25 Vdc when the board is first lowered into the bath. This will help prevent the formation of a low adhesion "electroless" copper layer that might lead to trace peeling and cracking during soldering.

Lower the board into the plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
Turn on the air compressor and adjust the air flow until a uniform blanket of agitation roils the top of the bath on both sides of the board. You only need about 2 CFM (Cubic Feet per Minute) of air flow per square foot of bath surface.
Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
Plate the board for 5 minutes to seal the conductive ink with a layer of electrolytic copper.
Remove the board from the bath and rinse thoroughly to remove any electrolyte.
Clean the board and laminate both sides with plating resist (photoresist used for etching will also works pretty well).
Image both sides of the board, being careful that the correct pattern in aligned on each side (component side on the top and solder side on the bottom).

Note: Be sure to leave a bare copper area on both sides of the board so that you can insure good electrical contact with the cathode clip.

Develop the circuit pattern.
Reconnect the cathode clip to the board.
Turn the power supply on.
Lower the board into the plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
Turn on the air compressor and adjust the air flow until a uniform blanket of agitation roils the top of the bath on both sides of the board.
Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
Plate the board for ½ the total time (T).
Turn the current down and flip the board top to bottom and left to right. This will help minimize any plating non-uniformity that results from asymmetric, inconstant plating conditions.
Reconnect the cathode clip and lower the board back into the bath.
Plate the board for ½T.
Remove the board from the bath and thoroughly rinse in the rinse tank to remove most of the electrolyte. Rinse the board under running tap water to remove the rest.

Note: If no outside contamination is introduced, the water in the primary rinse tank can be added back into the plating bath to make up for drag out and evaporative losses. This is crucial to reducing the effluent from this process to near zero.

Blow dry.
The plated board is now ready for further processing.

Panel plating copperclad substrates proceeds as follows:

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:25:00 +0000

Calculate the total plating time.

An acid copper plating bath based on the Lea Ronal PCM+ additive system deposits 0.0011" (1.10 mils, 28 microns, 0.81 oz) of high ductility copper in 1 hour at 20 ASF(Amps per Square Foot). Plating up "one ounce" of copper (i.e. plating 1 oz. of copper onto 1 square foot of board) is equivalent to plating a thickness of 0.0013" (1.3 mils or 34 microns).

Example: If you are starting with "half ounce" copperclad and want to plate up to a finished thickness of "one ounce", you will need to add .65 mils (0.00065"). The total plating time at 20 ASF will be:

[0.65 mils / (1.1 mils/hr.)] x 60 min./hr. = 35.5 minutes = TC

Calculate the required plating current.

Convert the total area of the substrate being plated into square feet (remember both sides!) and multiply the result by 20.

Example: If you are plating a 12" by 9", double-sided board, you will need:

[(12" x 9" x 2 sides)/144] x 20 = 30 Amps = C

Carefully inspect the substrate for deep scratches and nicks that might impair the quality of the finished circuit.
Format the drilling stack to minimize burr formation during drilling.
Drill the through-holes and mounting holes, and mill/router any slot or cavity that is to be plated.
Activate the hole-walls.
While the ink is curing, take a few minutes to analyze the electrolyte. If you have a hull cell, this is a good time to run a test to insure that the organic components of the bath (which are very difficult to test directly) are in balance and present in the proper concentrations.
After activation and curing, both sides of the substrate should be thoroughly cleaned to remove any trace of conductive ink from both surfaces. Any ink that is not removed will almost certainly show up in the worst possible place so take your time cleaning the board and make a good job of it!
An abrasive pad (e.g. Scotchbrite® pad) can be used to remove ink that proves to be too stubborn for conventional cleaning, but be careful. You must be certain that you do not break the electrical contact between the conductive ink on the inside of the holes and the copper foil on the surface of the board or the holes in question will not plate properly.
Rinse the board thoroughly in deionized water before proceeding.
Dip the board into a 10% solution of sulfuric acid to minimize the introduction of contaminants into the copper plating tank.
Attach the cathode clip to the board, making certain that both copper surfaces have good electrical contact to the negative terminal of the plating power supply.
Turn the power supply on.

Lower the board into the plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
Turn on the air compressor and adjust the air flow until a uniform blanket of agitation roils the top of the bath on both sides of the board. You only need about 2 CFM (Cubic Feet per Minute) of air flow per square foot of bath surface.
Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
Plate the board for ½ the total time (TC) calculated above.
Turn the current down and flip the board top to bottom and left to right. This will help minimize any plating non-uniformity that results from asymmetric, inconstant plating conditions.
Reconnect the cathode clip and lower the board back into the bath.
Plate the board for ½TC.
Remove the board from the bath and thoroughly rinse in the rinse tank to remove most of the electrolyte. Rinse the board under running tap water to remove the rest.

Blow dry.
The plated board is now ready for further processing.

Panel vs. pattern plating

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:23:00 +0000

There are two main approaches commonly used when electroplating PCB substrates, panel plating and pattern plating.

During panel plating, the entire copper surfaces on both sides of the substrate, as well as the hole walls are plated up to a desired final thickness. While this requires a fairly hefty current source for even a modest size PCB, the end result is a smooth, bright copper surface that is easy to clean and prepare for later processing. A major problem for folks without access to a photoplotter, is the need to use negative artwork to expose the circuit pattern into the more common contrasting reversing dry-film photoresists (contrast preserving films have been introduced from time to time but never seem to stay around for very long). When you etch a panel plated board, you end up removing most of the material that you plated, so the burden of extra erosion of the anode banks is exacerbated by an increased copper loading in your etchant.

Pattern plating, as the name implies, involves masking off most of the copper surface and plating only the traces and pads of the circuit pattern. Due to the reduced surface area, a much smaller capacity current source is generally needed. Further, when using contrast reversing photopolymer dry-film plating masks (the most common type), a positive image of the circuit is all that is needed. For many prototype PCBs, this artwork can be reliably produced on a relatively inexpensive laser printer or pen plotter. Pattern plating consumes less copper from the anode bank and requires that less copper be removed during etching reducing bath analysis and maintenance. The downside of the technique is that it requires that the circuit pattern be plated with either tin/lead or an electrophoretic resist material prior to etching and then stripped prior to soldermask application. This increases the complexity and adds another set of wet chemical baths to the process.

Acid Copper Through-hole Plating

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:21:00 +0000

Overview

Once the through-holes have been activated, your board is ready for acid copper electroplating (short for "electrolytic plating"). Considering the amount of trepidation that seems to surround the entire topic of through-hole connectivity, and the lengths that some people will go to avoid wet chemistry, putting a uniform, reliable sheath of copper on the insides of every hole turns out to be quite straight forward if not downright easy. Thanks to decades of work by the major electrochemical suppliers, the various chemical systems are well understood and readily available in most industrialized nations. Most of the equipment, like acid copper plating tanks, is easy to make and will last for many years if properly maintained. The items that are not easily fabricated in a home shop are available from a variety of sources. A high-performance plating solution can be mixed using readily available materials.

Basic principles

An acid copper electroplating solution is a mixture of water, sulfuric acid, copper sulfate, and a trace of hydrochloric acid. To this is added a number of organic constituents that serve to regulate and distribute the delivery of copper to the surface being plated. The two basic organic additives are commonly referred to as the "brightener/leveler" and the "carrier".

A basic electroplating cell consists of a tank full of the above electrolyte with arrays of copper anode bars arranged along two opposite sides. These bars are referred to as the anodes, and, as you might expect, are connected to the positive terminal of a current source. This supply must be capable of continuous sourcing into a near short circuit load (a typical copper electroplating bath has an effective full load operating "impedance" that ranges between 0.025 Ohms and 0.015 Ohms). Situated halfway between these anode "banks" is the copperclad substrate that is to be plated. It is variously referred to as the cathode (duh!) or the workpiece.

In the simplest terms, copper deposition occurs when an electrical potential is established between the anodes and the cathode. The resulting electrical field initiates electrophoretic migration of copper ions from the anodes to the electrically conductive surface of the cathode where the ionic charge is neutralized as the metal ions plate out of solution. In the Think & Tinker process, a thin layer of conductive ink extends the conductivity of the surface foil layers into the through holes. This ink forms a highly reliable surface for efficient electrolytic copper depositon. The figure at right shows a photomicrograph of the mechanically active surface that results when the ink is cured and the uniform layer of smooth, bright copper that has been deposited inside the through-hole.

At the anode (in a properly maintained bath), sufficient copper erodes into the electrolyte, to exactly make up for the deposited material, maintaining a constant concentration of dissolved copper. This all sounds quite nice, except for the annoying tendency of electrical charges to build up on the nearest high spot, thereby creating a higher electrical potential. This area of increased potential attracts more copper than the surrounding areas which in turn makes the high spot even higher. If this process were allowed to continue unchecked, the resulting plated surface would resemble a random jumble of copper spears instead of the smooth, bright surface needed for reliable electrical circuit formation. Inhibiting and controlling this nonlinear behavior is where the organic additives come in to play. This situation is especially critical at the rims of the through-holes. Here the field concentration is sufficiently high, that, in the absence of some mediating mechanism, electrodeposition would completely close off many of the smaller diameter holes.

Organic additives

In a well controlled plating bath, the carrier supports the formation of a black skin on the anode material which serves to regulate the diffusion of copper ions into the electrolyte. The material is also attracted to, but not co-deposited on the cathode (work piece) forming a layer (film layer) in close proximity to the surface that controls the rate of copper grain growth.

The brightener works within the film layer to control copper deposition on a microscopic level. It tends to be attracted to points of high electro-potential, temporarily packing the area and forcing copper to deposit elsewhere. As soon as the deposit levels, the local point of high potential disappears and the brightener drifts away. (i.e. brighteners inhibit the normal tendency of the plating bath to preferentially plate areas of high potential which would inevitably result in rough, dull plating) By continuously moving with the highest potential, brightener/levelers prevent the formation of large copper crystals, giving the highest possible packing density of small equiaxed crystals which resulting in smooth, glossy, high ductility copper deposition.

Mark Brelsford of QMS in Toronto, ON likens the action of the carrier to the function of a doorman at a theater who regulates the flow of people into a theater but doesn't really care where they go once inside. The brightener would then be the ushers who politely lead each person to a vacant seat until the theater is uniformly filled.

Bubble-assisted Developing of Aqueous Process Photopolymers

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:17:00 +0000

Wear a lab smock, gloves and eye protection when handling and/or using
developing and stripping solutions!

After you have imaged the laminated photopolymer (resist or soldermask) and have let the board sit for 15 minutes:

Preheat the developing solution [43°C±2°C for DF 8030 soldermask and 31°C±6°C for DF 4615 photoresist] and pour into the bubble tank.
Peel the Mylar cover sheet(s) off of the photopolymer.
Hang the board vertically in solution. The board must be totally covered by developing solution.
Turn on the compressed air just enough to insure vigorous, uniform agitation across the surface of the bath. About 2 CFM (Cubic Feet per Minute) per square foot of bath surface seems to be about the right amount, if you have the ability to measure such low flow rates.
Develop the board for 90 seconds.
Flip the board top-to-bottom, and left-to-right and develop for another 90 seconds.
Lift the board from the solution and allow most of the developer to drain back into the tank.
Rinse both sides of the board with warm tap water, using a spray wand if available.
While rinsing the board, vigorously rub the photopolymer with a wet kitchen sponge to remove any partially dissolved material from the nooks and crannies of the developed pattern.

This probably sounds like the last thing that you should do the the tiny little pads and traces that make up a resist pattern. However, if the dry-film will not hold up to the abrasion of a soft kitchen sponge, it will probably fail during etching. Even if the film does survive the etchant without lifting off and exposing the underlying copper, failure to remove partially dissolved resist (or soldermask) will almost certainly result in shorts between circuit elements and incomplete etching at points across your board. This is the reason that the surprising toughness of properly processed dry-films is so important.

Using an eye loupe, if available, carefully inspect the developed pattern to insure that all edges are sharp and steep, that any partially dissolved material has been removed, and that all tented holes are still securely covered.

Properly exposed and developed photopolymers, besides being fairly tough, display very distinct characteristics which are visible even under a very modest power (10X) magnifier. All trace walls should be nearly vertical and should meet the surface of the copperclad in sharp, well defined intersections. The photopolymer surface should be smooth and glossy (after drying) and should meet the trace walls along sharp (or very slightly rounded) edges.

If the board needs more developing, return it to the tank and agitate for another minute (flipping at 30 seconds).
Rinse (and scrub) the board and re-inspect.
Continue in this manner until the image is developed to your satisfaction.
Thoroughly rinse the board with warm tap water, blow dry, and place in a 212ºF (100ºC) oven for 5 minutes. Do not leave the board in the oven for more than 10 minutes or it may become VERY difficult to strip.
Remove the board from the oven and allow it to cool to room temperature.
The developed board is now ready for further processing.

Stripping Aqueous Process Photopolymers

When it is time to remove the photopolymer from the board:

Preheat the stripping solution to 130ºF (54ºC) ± 10ºF and pour into a Pyrex tray large enough to accommodate the board.
Put a "spacer ring" on each corner of the board and set into the tray.
Lift up one end of the the tray and position a 1/4" (6 mm) wooden dowel rod crossways under the approximate center.
Gently rock the tray back and forth across the dowel rod for 2 minutes.
Flip the board over and rock for another 2 minutes.
Lift the board from the solution and allow most of the stripper to drain back into the tray. Most of the photopolymer should also slip off of the surface of the board and drop into the tray. If it does not, return to the stripper for 2 more minutes (flipping at 1 minute).
When the entire surface of the copperclad is free of polymer, rinse the board thoroughly in warm tap water.
Using an abrasive copper cleaner designed for PCB use, clean and rinse the copper surface.
After drying, the board is ready for further processing.

Tray Developing of Aqueous Process Photopolymers

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:14:00 +0000

Wear a lab smock, gloves and eye protection when handling and/or using
developing and stripping solutions!

After you have imaged the laminated photopolymer (resist or soldermask) and have let the board sit for 5 minutes:

Peel off the Mylar cover sheet(s).
Put a "spacer ring" on each corner of the board and set into the developing tray.
Pour enough heated [48°C±2°C for DF 8030 soldermask and 38°C±6°C for DF 4615 photoresist] developing solution into the tray to insure that the top of the board is 1/4" (6 mm) below the liquid surface.
Lift up one end of the the tray and position a 1/4" (6 mm) wooden dowel rod crossways under the approximate center.
Gently rock the tray back and forth across the dowel rod for 2 minutes.
Flip the board over and rock for another 2 minutes.
Lift the board from the solution and allow most of the developer to drain back into the tray.
Rinse both sides of the board with warm tap water, using a spray wand if available.
While rinsing the board, vigorously rub the photopolymer with a wet kitchen sponge to remove any partially dissolved material from the nooks and crannies of the developed pattern.

Using an eye loupe, if available, carefully inspect the developed pattern to insure that all edges are sharp and steep, that any partially dissolved material has been removed, and that all tented holes are still securely covered.

If the board needs more developing, return it to the tank and agitate for another minute (flipping at 30 seconds).
Rinse (and scrub) the board and re-inspect.
Continue in this manner until the image is developed to your satisfaction.
Thoroughly rinse the board with warm tap water, blow dry, and place in a 212ºF (100ºC) oven for 5 minutes. Do not leave the board in the oven for more than 10 minutes or it may become VERY difficult to strip.
Remove the board from the oven and allow it to cool to room temperature.
The developed board is now ready for further processing.

Developing Dry-film Images

noreply@blogger.com (Unknown) — Mon, 16 Mar 2009 05:13:00 +0000

Overview

Unlike copper etching, developing properly laminated and imaged dry-film photopolymers is easily accomplished by swishing your board about in a conventional photographic processing tray. This derives from the fact that the organic and radiation chemists who developed (and continue to develop) these films have done an excellent job of creating an entire class of easy-to-use and environmentally safe materials that are almost as tough as nails if processed according to published guidelines. The benefit of this will become apparent below.

Processing Chemicals

Regardless of the developing equipment you use or the film vendor you buy from, most of the aqueous processable dry-films use the same basic developing and stripping solutions. The films are developed with a 1% (wt.) solution of sodium carbonate (common soda ash) operating at 100ºF (38ºC) ± 5ºF and stripped with a 3 to 5% solution of sodium hydroxide (caustic soda or lye) operating at 130ºF (54ºC) ± 5ºF. You can usually buy premixed concentrates of both from the film manufacturer (or distributor) but you will end up paying as much as 100 times what it will cost you to mix your own. In any case, it is always a good idea to start with fresh solutions until you have a good feel for the life expectancy of the developing and stripping chemistries based on your throughput.

The two easiest dry-film developing systems to implement in a small shop setting are tray developing and bubble-assisted developing. Tray developing is the easiest to set up but may severely limit the density of the circuit patterns that you are able to develop, as well as restrict the maximum hole size that you can reliably "tent" with photoresist. At Think & Tinker, we have tray developed 6" (15.2 cm) by 13" (33 cm) circuit patterns with typical features including 0.010" (0.25 mm) wide traces separated by 0.015" (38 mm) spaces. With care, we were able to tent 0.125" (3.18 mm) dia. through-holes (in 0.155" pads) well enough to survive low-pressure spray etching. Stripping is a straightforward matter of immersing the board in a heated solution of caustic soda and agitating the stripper until the photopolymer lifts off of the copper.

One of the prime advantages of aqueous process films is that the developing and stripping solutions are easy to neutralize (with hydrochloric acid), filter (to remove suspended particulates) and dispose of. In most cases, disposal does not become an issue until 10 to 15 square feet of 1 mil (0.001", 0.025 mm) dry-film has been processed for every gallon of solution.

Chemical Etching

noreply@blogger.com (Unknown) — Sat, 14 Mar 2009 07:40:00 +0000

The bad news first

Before you even think about setting up to chemically etch printed circuit boards at home, or in a small shop, it is a good idea to get a single ethic firmly planted in your mind.

It is fundamentally wrong to pour toxic chemicals down the drain, out the back door, or on your neighbor's property. Besides being unethical, it is downright wasteful since you end up throwing away materials that you have paid good money for.

The net result of this is that etching boards on a small scale with ferric chloride, the erstwhile standard of the hobbyist world, is out of the question. Generally speaking, it really does not make sense to use an etchant that cannot be recycled or replenished locally, without additional cost to you.

The good news!

Fortunately, recent improvements in an infinitely replenishable copper etchant commonly referred to as " peroxy-sulfuric" with its environmental compatibility and ease-of-use has come to the rescue. Peroxy-sulfuric is very aggressive oxidizer/corrosive that can be mixed on site from inexpensive ingredients, and, with proper use and maintenance, literally never wears out. The real beauty of this mixture of hydrogen peroxide, sulfuric acid, copper sulfate and organic stabilizers is that excess copper can be removed by simple precipitation, after which, the bath is ready to consume more copper. In addition, during operation, the etchant is is "self agitating". The bubbles and heat that evolve during etching, so thoroughly stir up the bath that the etchant works almost as well in a simple dip (immersion) tank as it does in a far more expensive spray etcher. Unfortunately, peroxy-sulfuric can be tricky to use in shops requiring high throughput. The two primary reactions that effect the erosion of copper (copper -> copper oxide, copper oxide -> oxygen + copper sulfate) are very exothermic. A lot of heat can build up in a bath with no provision for cooling the etchant. Generally speaking, if the bath "loading" exceeds 2 oz. of copper (1 ft² of double sided one ounce copperclad) per gallon of bath per hour, enough heat can accumulate that the stabilizers begin to fail. Once the effectiveness of the stabilizers is impaired, the peroxide reacts with the dissolved copper to spontaneously decay into oxygen and water, releasing even more heat. If left unchecked, this runaway reaction (often called "going exothermic") can melt plastic tanks and severely compromise the integrity of any plumbing attached to the system. Needless to say, it also eats up every bit of the hydrogen peroxide in the bath and, may leach enough material out of the tank walls and plumbing to thoroughly pollute the solution and render it quite useless.

As you might imagine, throughput can often be increased by pumping the etchant through a heat exchanger during etching to remove excess heat before it poses a threat. In a limited sense, you can also increase throughput (as well as the etch rate) by agitating the bath with compressed air (a.k.a. sparging). The benefits of "air sparging" are three fold.

The turbulence in the wake of the bubbles breaks up the depletion layer immediately adjacent to the board's surface, delivering fresh etchant to the unprotected copper. As a result, the etch rate can be significantly enhanced.
This same turbulence has the added benefit of similarly "freshening" the etchant down in the nooks and crannies of the resist pattern, effectively increasing the etching resolution of the bath. Resist geometries with 0.008" (0.2mm) traces and 0.008" spaces can be routinely etched with such a "bubble etcher".
Although the total air volume is fairly low, air bubbles tend to carry away some of the heat generated during etching. This cooling effect is further enhanced by the evaporative phase change that occurs at the bubble walls as they rise through the heated solution.

The primary downside of bubble etching is that it generates a significant quantity of corrosive aerosol. Effective fume collecting with active scrubbing must be implemented if a bubbler is used.

Please note that using an air agitation system with an aggressive etchant chemistry does not remove the danger of the bath going exothermic. It will however, significantly increase the size of the safe operating window and improve the overall performance of your etcher.

Peroxy-sulfuric etchants are most effective, and safe when used in spray etching equipment. Spraying:

increases the etch-rate by increasing the delivery of fresh etchant and removal of depleted etchant,
enhances the effective resolution by improving the delivery of fresh etchant into finer resist geometries,
cools the etchant before it impacts the copper, rendering a runaway exothermic reaction virtually impossible
can produce far greater uniformity of copper removal from large area panels

As with the bubbler above, spray etching with peroxy-sulfuric generates a significant quantity of corrosive aerosol. Effective fume collecting with active scrubbing must be implemented if spraying is used.

Preparing etching test coupons

Regardless of whether you use immersion, bubble-assisted, or spray etching, always etch a test sample to see how long it takes to totally etch copperclad with the same weight foil as you will be using. If possible, it is a good idea to image, and develop a set of copperclad test-coupons whose resist geometry is representative of the minimum sized feature in your circuit design. In most cases, mixed blocks (1" x 1") of horizontal, vertical and crossed (gridded) 0.010" (0.25mm) traces on 0.020" (0.51mm) centers act as very effective probes for measuring many facets of etchant performance.

Mechanical Etching

noreply@blogger.com (Unknown) — Sat, 14 Mar 2009 07:36:00 +0000

In response to the inexorable tightening of environmental regulations and the difficulty of obtaining strong acids and oxidizers in many locales, "mechanical etching" has seen unprecedented growth over the last 10 years. In spite of high up-front costs and lack of plated through-holes, its complete freedom from toxic chemicals has made this technique very attractive to many PCB prototyping shops.

As the density and complexity of circuit designs have increased, forming reliable electrical connectivity between the top (component) and bottom (solder) circuit patterns has evolved from a mere convenience to an absolute necessity. While some mechanical etch vendors offer manual and semi-automatic machines for inserting eyelets into all of the designated through-holes, the cost of the equipment, the penalty in PCB real-estate (holes must be drilled oversized to accommodate the eyelets), and the relative sluggishness of the process render it undesirable for many applications. As a result, many shops have adopted simplified electrolytic plating processes (like Green CirKit) to accomplish this critical task.

As mentioned above, mechanical milling involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace. There are a number of configurations currently available for these special mechanical etch bits, but most users report that bits with spiral flutes (vs. a flat "spade" geometry) are the most effective at removing copper debris and tend to stay sharp longer at higher cutting rates. Tip angles of 60° and 90° are the most common, with 90° seeming to offer the best combination of minimal substrate penetration and longer cutter life. If the circuit design also requires that some (or all) of the non-circuit copper be removed (clear milling), conventional carbide end-mills can be used to accelerate the copper milling process. Typical diameters range from 0.010" (.25mm) to 0.050" (1.27mm).

Mechanically etching a PCB proceeds as follows:

Lay out the circuit using a compatible PCB layout package (often proprietary to the milling machine vendor).
Run the layout through a post processor to generate the boundary paths that the cutter will need to follow to define the circuit elements and any cutter paths needed for clear milling (always vendor specific).
Mount the substrate (or flex circuit) in the machine as instructed in the user's manual.
Insert the drill size indicated by the operating software into the chuck and adjust the height (if no set ring is present) to insure that the bit drills all the way through the substrate and a short distance into the backing material.
Drill the first hole size.
Repeat the previous 2 steps for each drill size. If you will be plating the through-holes, remove the board from the machine for hole wall activation and electroplating. After the through-holes are plated, return the board to the milling machine for further processing.
Insert a mechanical etch bit and adjust the depth setting foot to the approximate cutting depth desired for your design. The cutting depth sets the width of the channel milled through the copper foil, so you must know the tip angle of your cutter prior to setting the depth.
Off to one side of the substrate, cut a couple of test channels and carefully measure the width of each. Adjust the height of the foot until the desired width is consistently achieved.
Mechanically etch (and clear mill) the first side of the board.
Carefully inspect the cutter. If the cutting edge shows signs of wear or of excess copper buildup, replace it with a new bit before proceeding.
If your design requires a double sided PCB, flip the substrate and repeat steps 8-9 for the second side (making sure, of course, to load the second side artwork into the mechanical etch software).
Mechanically etch (and clear mill) the second side of the board.
Remove the "etched" board from the machine.
Carefully examine both sides for copper debris that may have become wedged (or smeared) in the milled channels. You cant bet that any such material will short out the most expensive and unobtainable component on the entire board so you need to be very diligent during this inspection.
If any copper strands are found shorting out circuit elements, use the tip of an X-Acto knife to remove them. Be very careful not to cut thin traces during this operation.
After any shorts have been removed and the through holes cleared of plugs of copper and milled substrate detritus, the board is ready for further processing.