Think & Tinker, Ltd.
P.O. Box 1606, Palmer Lake, CO 80133
Tel: (719) 488-9640, Fax: (866) 453-8473
Sales: Sales@ThinkTink.com, Support: Support@ThinkTink.com
Think
&
Tinker
Ltd.





SkypeMe at
"thinkntink"
Tin/Lead (Solder)
Plating

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.

Panel vs. pattern plating

Electrolytic tin/lead, as used in the printed circuit industry, is virtually always deposited via pattern plating. The only exception is when dummy plating is used to sweep the the electrolyte of particulate contamination.

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 material from the anode bank and usually requires that less copper be removed during the etching process. This decrease in load on the etchant results in a reduction in etchant bath analysis and maintenance requirements.

Pattern plating copperclad substrates proceeds as follows:

The following process assumes that you are pattern plating both the bulk copper and tin/lead layers.
  1. Calculate the total plating time.

    A tin/lead (solder) plating bath based on the SolderPlate 60/40 additive system deposits 40 microinches (40e-6 in., 1 micron) of matte solder in 1 minute at 20 ASF (Amps per Square Foot). For effective resistance to peroxy-sulfuric etchants, 0.0002" to 0.0003" ( approx. 6 to 8 microns) of tin/lead should be deposited.

    Example: To plate up 0.00025" (2.5 "tenths" or 6.4 microns) at 20 ASF, the total plating time (T) will be:

    2.5e-4 in./ (40e-6 in./min.) = 6.25 min. = TL
  2. 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 and plating current from the circuit pattern.

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

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

    (during pattern plating, the total current is the same for both
    acid copper and tin/lead deposition)
  3. Carefully inspect the substrate for deep scratches and nicks that might impair the quality of the finished circuit.
  4. Format the drilling stack to minimize burr formation during drilling.
  5. Drill the through-holes and mounting holes, and mill/router any slot or cavity that is to be plated.
  6. Activate the hole-walls.
  7. While the ink is curing, take a few minutes to analyze the electrolyte (both the tin/lead and acid copper baths). 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.
  8. After the ink is cured, thoroughly clean both sides of the panel.
  9. Through-hole plate the panel. To protect the conductive ink in the holes, it is a good idea to deposit at least 0.0004" (4 tenths, 10 microns) of copper onto the hole walls which will prevent damage during dry-film processing.
  10. Rinse the board thoroughly in deionized water before proceeding.
  11. Clean the board.
  12. Laminate both sides of the board with a plating grade photoresist.
  13. Image (and develop) both sides of the board with the desired positive circuit patterns.

    Make sure to leave 0.75" bands of bare copper along each side of the board (on both sides) to act as robber bars and to provide convenient connection points for the cathode clamps.
  14. 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.
  15. Attach the cathode clamp to the board, making certain that both copper surfaces have good electrical contact to the negative terminal of the plating power supply.
  16. Turn the power supply on.
  17. Lower the board into the copper plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
  18. Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
  19. 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.
  20. Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
  21. Plate the board for ½ the total copper plating time (TC).
  22. Turn the current down and flip the board in the clamp top to bottom and left to right. This will help minimize any plating non-uniformity that results from asymmetric, inconstant plating conditions.
  23. Lower the board back into the bath.
  24. Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
  25. 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.
  26. Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
  27. Plate the board for the remaining ½ TC.
  28. Remove the board from the bath and thoroughly rinse in the "acid copper" 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, and if the rinse tank is only used after acid copper plating, the water in the "acid copper" 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.
  29. Dip the board into a 10% solution of SolderPlate 60/40 acid to minimize the introduction of contaminants into the tin/lead cell.
  30. Attach the tin/lead cathode clamp to the board, making certain that both copper surfaces have good electrical contact to the negative terminal of the plating power supply.
  31. Turn the power supply on.
  32. Lower the board into the solder plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
  33. Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
  34. Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
  35. Plate the board for the total tin/lead plating time (TL) calculated above.
  36. Remove the board from the bath and thoroughly rinse in the "tin/lead" rinse tank to remove most of the electrolyte. Rinse the board under running tap water to remove the rest. Blow dry.

    Note: If no outside contamination is introduced, and if the rinse tank is only used after tin/lead plating, the water in the "tin/lead" 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.
  37. The plated board is now ready for further processing.


Established 1990

On the web since 1994

Payment Processing
Sales: 1-(719) 488-9640    Tech Support: 1-(719) 488-9640    Fax: 1-(866) 453-8473
Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 7:36:58 AM