Re: Overetch of exposed copper that has exposed gold plating electrically
attached to it

The problem that you allude to is real common, and one of the more maddening
aspects of applying OSP's.

You are correct that the problem is called "Galvanic Etch".  Remember that
the etching reaction is an electrical one,  where electrons are passed from
the metal to the etchant.  This passing of electrons changes the Copper from
that insoluble orange metal to the blue chrystals.

When you etch a surface that is more or less uniformly attacked by the
etchant, the etching chemicals, in this case the persulfate, which is the
electron acceptor, actually are blocked from the surface by the Copper
escaping from the surface as it is etched.  However, if you have an area that
is relatively passive, like a gold surface, which is electrically connected
to the Copper being etched, the Copper dissolving passes its electrons down
the trace to the gold, and the Persulfate is all lined up at the Gold surface
to accept those electrons, with no Copper coming off the Gold to interfere
with the orderly (and quick!) exchange of spent and new Persulfate.

Hope this view of the reaction process is helpful.

Rudy Sedlak
RD Chemical Company