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Bright Acid Tin
Plating

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.

Panel vs. pattern plating

Electrolytic tin, 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. An acid tin plating bath based on the TinPlate starter system deposits 100 microinches (100e-6 in., 2.5 microns) of metallic tin 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 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./ (100e-6 in./min.) = 2.5 min. = TT

  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 acid tin 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 acid tin 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 top to bottom and left to right. This will help minimize any plating non-uniformity that results from asymmetric, inconstant plating conditions.
  23. Reconnect the cathode clip and lower the board back into the bath.
  24. Plate the board for remaining ½TC.
  25. 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.

  26. Dip the board into a 10% solution of sulfuric acid to minimize the introduction of contaminants into the acid tin cell. Do not use the same acid dip that you used prior to copper plating. The intent here is to insure that each electrolyte remain as pure as possible and the minimum "carry over" occur between baths.
  27. 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.
  28. Turn the power supply on.
  29. Lower the board into the tin plating tank halfway between the two anode banks until the top edge is at least 1" below the surface of the electrolyte.
  30. Swish the board gently back and forth to drive any trapped air bubbles out of the through holes.
  31. Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
  32. Plate the board for the total tin/lead plating time (TT) calculated above.
  33. 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.

    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.

  34. The plated board is now ready for further processing.
Remember that the tin layer should be stripped after the board is etched. This is easily accomplished by setting the etched panel in a ***** for approximately ****, or until all of the tin has been visibly removed.


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On the web since 1994

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Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 8:36:58 AM