SV: [sdiy] Solder won't stick to old PCB

[David G. Dixon]

A little Google research has revealed a currently available product --
Weiman Instant Tarnish Remover -- good for both copper and silver.  Looking
up the MSDS, I see that it is composed of a dilute solution of thiourea and
phosphoric acid.  Thiourea is a well known complexing agent for coinage
metals (copper, silver, gold), and phosphoric acid is, well, an acid.
Interestingly enough, thiourea is also one of the key ingredients in Liquid
Tin PCB tinning solution.

I have both thiourea and phosphoric acid in my lab.  I'll try detarnishing
one of my boards this week, and I'll report back on whether it worked or
not.  I have lots of tarnished boards, because I only recently took to
spraying them with TremClad clearcoat when I finish soldering them.