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A problem that used to occur when printed circuit boards were Tin/Lead plated happened when the board was left too long in a soak tank of acid immediately prior to Tin/Lead plating.
In that tank, the Copper substrate would react with dissolved Copper to form BAD tarnish. The board would still plate, but the plating would not be adherent.
(Problem was made worse, if acid tank was not changed out regularly, and dissolved Copper content was allowed to build.)
Is it possible you are getting this phenomenon? If a drop of fairly conc. Hydrochloric Acid dissolves black, it points to this being the problem.
Rudy Sedlak
RD Chemical Company
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