Rudy,
     
        Got a similar but different experience that puzzled me. If you
     plate selective solder (pads) over Nickel or Tin Nickel and you pass
     this combination thru your ammoniacal etcher, sometimes you will see
     certain solder pads completely disappear. Trying to investigate the
     source produces a commonality. The disappearing pads are the ones
     that are connected to large exposed plane areas. The actual issue
     takes place in the solder conditioner. (Thiorea) No stray current
     detected in any solution.
        The only theory I could come up with was the Thiorea in the solder
     conditioner generates some sort of electrical energy by being sprayed
     against the plane. This energy deplates the selective pads which try 
     to replate to the large planes. (??????) (BTW-The fully isolated pads
     are no problem.<?>) I believe the secret is how the Thiorea works.
     (Displacement or replacement or something like that) Unfortunately,
     I fell baffled for too long and decided to just turn the conditioner
     off and do that task off line if required. That worked. Still never
     found the answer though.
     
     Groovy
______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: FAB gold plating problem
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    4/26/96 1:17 AM


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