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Tin/Lead (Solder)
Plating Tank

Introduction

Tin/Lead pattern plating is often considered a part of the imaging process since the deposited material acts as one of the most effective forms of etch-resist in systems based on acidic (and some alkaline) etchants. Solder plating, like acid copper plating, can be a very forgiving process, as long as the bath and equipment are well maintained.The equipment, if anything, is even easier to build since air agitation is not necessary (nor desired).

Bath filtration

The action of the ionic current in an electrolytic solder plating tank is such that,during plating, any and all suspended particulates will be forced toward the cathode (the panel that you are plating) and deposited. If the particles are large enough (5 to 20 microns), their presence can disrupt the local electrical field enough to reduce the effectiveness of the levelers present in the electrolyte. The net result of this "electrophoretic" migration is rough plating with the contaminating particles trapped in the electrolytic layer (right on top of a critical circuit element, no doubt). Roughness in a tin/lead film is often not a major problem since the material is usually reflowed a some point in the processing of the panel. Nonetheless, in a production setup, the plating electrolyte is usually filtered continuously using one or more 1 micron filters specially made for "polishing" electrochemicals. If you anticipate using your plater more than once a week and if the bath volume is greater than 2 gallons, you should plan to incorporate this continuous filtration into your system design. In the case of small and/or infrequently used plating setups, you can maintain an acceptable level of bath "cleanliness" by making use of the electrophoretic pumping described above. The trick is simply to manually stir up your bath with a wand of some sort, and plate a "dummy board" at full plating current (20 ASF) for 1 hour before plating you PCB. Dummy plating will remove virtually all particulate material from the electrolyte, but has the disadvantage of consuming relatively expensive anode material.

Basic construction

Tin/lead (solder) plating systems can be run without solution agitation, although mechanical agitation (20 ea. 7 cm strokes per min.) is recommended for effective electrolyte delivery to the interior of through-holes. Air agitation (sparging) must not be used!. The system should be based on a polyethylene, polypropylene, or PVC tank that is at least 10" wide, 3" longer than the longest PCB you anticipate making, and 4" deeper than the width of your widest board. Once you have selected your tank and installed the sparger, finish the plater as follows:
  1. Mount your anode segments (or anode baskets) to 1/2" thick PVC plates such that the bottom of each segment is 1" higher than the bottom of the board that you are plating. Adjacent segments should be mounted about 3" apart (surface to surface) for optimum plating uniformity on the workpiece. The total width of the array should be 4" less than the width of the workpiece. This geometry will minimize the tendency of the fringing field to excessively plate the bottom, left and right edges of the panel.
  2. Similarly, attach vanes (a.k.a. shadow vanes) to the anode mounting plates to move the effective tops of the anode segments 1" deeper than the top of the board. The whole point here is to force the plating field into a more planar configuration in the vicinity of the workpiece (cathode). This will improve the plating uniformity along the edges of your board, which is especially important if you at plating through-holes that are near one of the edges.
  3. Short all of the anode segments together with #10 PVC insulated, solid copper wire. Use stainless steel fasteners to make the connection between the anodes and the wire. Use liquid vinyl (e.g. Plastidip™) to coat the stainless steel fasteners as well as any exposed copper wire. Even though these connections will not be immersed in plating solution, aerosol from the sparging bubbles bursting at the surface will ultimately coat everything on the top of the mounting plates and will corrode any exposed copper.
  4. Mount the anode "arrays" into the tank with a minimum of 4" between the surface of each segment and the surface of the cathode. This separation will help the "planarize" the plating field in the vicinity of the board.
  5. Fabricate a stainless steel or titanium holder for the workpiece that will allow you to hang the board totally immersed midway between the anode arrays.
  6. Attach the board hanger to the negative (-) side of your plating power supply (low voltage, high current, capable of sourcing full current into 20 to 50 miliOhm loads), and both anode arrays to the positive (+) terminal.
  7. Your plating tank is now ready for use.


Established 1990

On the web since 1994

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Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 8:36:56 AM