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Tin stains from electrodeposited process

I am an end customer that is experiencing a brownish to black staining on my copper basis material cans from an electroplating house. The finish is 100% tin. The staining varies from part to part and is concentrated on one side of the part. Our end customer has performed material analysis on the staining and found evidence of tin and carbon.
A baking soda [in bulk on eBay or Amazon [affil links] bath cleans the parts to reveal a clean, non-stained surface and does not tend to return. Using Tarn-X tarnish remover ⇦ this on eBay or Amazon [affil links] cleaning solution cleans the stain but returns after sitting for about 20 minutes.
I am looking for a solution to the problem and wondering if this is a result of the rinsing process at the plater.
Please help.

Richard Holcomb
Electronics component manufacturer - Riverside, California, USA
2006

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2006

Hello Richard,
You stated the staining is present on the same side of the part. One or two possible causes come to mind. Rinsing may be inadequate, leaving electrolyte residues behind, or the tin plating thickness is low in that area allowing for migration of the copper through the tin. If this is migration you would surely have copper present in the spotted area. I'm guessing this is a acid tin sulphate bath. If so, there are a few things the plater can do.
1) Make sure a 10% sulfuric dip is being utilized before going into the tin.
2) Go into the tin with live current.
3) Check tin plating thickness at spotted area, could be a low current density area with minimal thickness.
4) After tin plate go back into the 10% sulfuric acid dip, rinse well.
I've read that going back into a mild alkaline cleaner after tin plate is effective to get rid of staining, but I've never tried it. It probably wouldn't hurt to dummy plate the bath on a regular basis to keep copper contamination down. Good Luck!

Mark Baker
process engineer - Malone, New York