Wednesday, March 18, 2009

Electroless Copper

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

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)

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.

Tuesday, March 17, 2009

NC Drill Files

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

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


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*


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.

Monday, March 16, 2009

PCB Glossary : S to Z

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.

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).

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.

A method by which successive exposures to a single image are made to produce a multiple image production master.

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

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.

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.

A common term for conductor.

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

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

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).

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

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

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

The top and bottom sides of any type of circuit board.

See Land.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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