RE>>FAB:OSP:copper gold galvanic during*     9/20/96

I pulled out this TechNet submission from Thad McMillan from 4/25/96 which described the general problem.  Hope this helps!

Regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA  
[log in to unmask]
-------
     We have had a problem in the past with OSP/bare copper boards with 
     gold fingers.  It might be related.  It occurs along the gold/copper 
     interface.  When the board goes through the microetch prior to the OSP 
     coating the bare copper traces entering the gold fingers were over 
     etched at the gold/copper interface.  
     
     For some reason, the term the "experts" used was "galvonic etch", the 
     microetch at the interface becomes too agressive resulting in etch 
     outs at the interface.  Very difficult to see.  Since electrical test 
     is before the OSP treatment we have had opens in final product as a 
     result of this..
     
     Our design solution was to make sure the soldermask overlaps the gold 
     fingers and also that there was enough room to insure the taping prior 
     to gold plating completely covered any bare copper. 
     
     Never saw this in HASL boards with gold fingers.  The exposed 
     copper/nickel/gold interface seemed to be the cause.  
     
     Someone else will have to explain the chemistry.
     
     [log in to unmask]

--------------------------------------
Date: 9/20/96 12:59 PM
To: Greg Bartlett
From: Marc R. Sinclair


To: [log in to unmask]
	
	Dear Donald: 
					
	"Copper gold galvanic during OSP application. " If you mean that there is
copper sticking to your gold tabs after OSP treatment then this has been a
problem we've had before too. It could be two things. One is that there
might be an electrical leak in your microetch that deposits the copper back
on the gold. I've seen it happen when recleaning panels. Our heater was
shorting into the bath. The other is, that if your running Lea Ronal OSP,
and see only copper depositing on the same fingers repeatedly like we did,
then the copper in the Conditioner is to high. Like above 60 ppm copper.
Dilution is the solution. High copper in the Concentrate just gives you
spotting in which case dilution is the only solution again. Unless the
copper can be electrowinned out of the bath. I will be experimenting with
both bath soon. Other than that I couldn't tell you without any more
information.
Marc

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