Complete Circuit Board Lab & POV Business Card

Step 9 Tin Plate the PCB

If you so desire, it is a simple affair to chemically plate a thin layer of tin on the surface of the copper traces. Most commercial PCBs at least have tin plated solder pads in a addition to a soldermask to prevent short circuits over the traces (usually the distinctive green color, though sometimes other colors are used). In our case, however, to prevent corrosion and improve the solderability of joints a tin plating will do nicely.

The Chemical Plating Solution
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I have gotten very nice results using Tinnit, a powdered "just-add-water" plating solution. It is quite inexpensive at around $10 including shipping, has a 6+ month shelf life, and can plate many boards before needing to replaced (475 mL or 1 pint will plate up to 600 sq. inches of copper). It works by chemically depositing a very thin layer of tin on to the exposed copper of the circuit board. This produces the same result as an electroplating method, and is quite a bit simpler and cheaper.

The copper traces must be clean of oxidation or tarnish as well as etchant before you can plate it. If you just etched it, it should be clean from removing the toner. If not, first you will need to polish it up with a scotch brite pad and give it a quick rinse.

You will also need some way of heating the solution to 50º C and a container to do it in. I was able to come up with a stirring hotplate, which definitely gives me a head start. It's the kind that has both a heater and a magnet mounted to a motor underneath so the solution can be stirred by special glass coated-magnet simultaneously. If you can't find a good hotplate, a kitchen stove will work just as well.

The Plating Vessel
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The tricky part is finding a non-metallic container to do the plating in. A real lab would some kind of thermal-shock resistant borosilicate glass tray, also known as Pyrex. You'd think you could just use your casserole dish, but that is a very bad idea. What most people don't know is that kitchen "Pyrex" is not the same thing; the Pyrex name is actually licensed by Corning Glass to a company called World Kitchen. They use an entirely different kind of glass, called soda lime glass. It is known to shatter catastrophically when exposed to a temperature differential such as that on an uneven stovetop. So don't do that.

There is a cheap alternative, though. In my research I came across an early Corning product called pyroceram,  marketed as "Corningware." It is not glass, but actually a glass-ceramic, and incredibly reistant to thermal-shock. Real hi-tech stuff, and Corning actually made casserole dishes out of it! You can get them at Savers or Goodwill for under $5.00, complete with lid. Look for the distinctive white color and the "Corningware - Safe For Rangetop Use" label on the bottom.

I also needed a little wire harness to suspend the board over the spinner stir magnet at the bottom of my dish.

Plating the Board
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Safety Note: Tinnit contains Acidic tin salts, so wear gloves and goggles and don't reuse anything for eating. Follow the warnings on the package. Please!

Tinnit comes as a powder that first needs to be dissolved in hot water. Following the instructions on the back, heat 475mL (1 pint) of distilled water to 55º C (120º F). Once it's fully dissolved, it will be a milky white-yellow color with a few solids floating around. Immerse the board face-up in the solution and give it 10-30 minutes, stirring at 5 minute intervals. If everything has gone to plan, you should see the copper almost immediately start to turn a dull gray. The longer you leave it in, the thicker the plate. Just don't heat it over 150º C (302º F) or it will start to put off weird ammonia and sulfur dioxide fumes.

Once you're satisfied, pull the board out and rinse it off thoroughly. The instructions say to clean it off with ammonia to remove corrosive residues that would dull the board. Window cleaner with ammonia will do in a pinch. Then just shine it up with a scratch free Scotch Brite pad and it's ready to solder!

Tools & Supplies

• Tinnit plating solution
• Laboratory thermometer
• Plastic tweezers
• Rubber gloves, goggles
• Borosilicate tray, or Rangetop-safe casserole dish