Another method of reducing the tarnish is to use one of the stripper additives.  We add
a just a gallon a day of an additive that keeps the choline level up and this keeps our
panels looking better.  After strip we place the panels in citric acid for a few
seconds and this works like an anti tarnish too.  Makes double treat look a whole lot
better.

Chuck Brummer

Rudy Sedlak wrote:

> I do love it when someone asks a question that we are real knowledgeable on.
> Thank you.
>
> Tarnish on Copper after stripping is endemic to resist stripping.  It is made
> much worse by buildup of Copper in the stripping chemistry.  In other words,
> most strippers will not tarnish when new, but begin to tarnish as they are
> used, and Copper builds in the sump.
>
> The problem is cureable by using a  resist stripping chemistry with the
> proper anti-tarnish in it.  Most resist stripping chemistry has 250 ppm of
> Copper when the stripper is dumped.  It is possible to get resist strippers
> that will not build up more than 30 ppm of Copper for the same number of
> panels stripped, and average less than 2 false defects per 18" X 24" panel.
> This has a nice added benefit of keeping the Copper in the rinse waters so
> low that they usually do not have to be treated, and can legally be put right
> down the drain.
>
> Many stripper chemistry vendors have resolved this issue by simply going to
> high volume feed and bleed, thus turning the sump over often, and keeping the
> dissolved Copper levels low.  This is great for the chemistry vendor, but
> expensive for the PCB fabricator, and not required with the proper
> anti-tarnish chemistry.
>
> As you probably have surmised, we are one of the vendors who can supply
> resist strippers with the proper anti-tarnish properties.
>
> Rudy Sedlak
> RD Chemical Company
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