RE: Scott Westheimer's tarnish with OSP problem

Scott, this problem intrigues me because I see something almost 
similar from time to time.  I suspect you have a galvanic effect as 
a previous reply suggested. 

I sometines see bga's have one pad or several in the middle that
plate electroless nickel differently  than the rest of the pads.  It's
not
contamination because the same pad on both sides of the panel will
have the exact same appearance that looks like the pad has a thinner 
deposit than it's kin. 

My theory  is it is caused by one or some pads being connected internally

to areas near the top of the circuit and others being connected to areas 
deeper in the tank during  plating.  Then stratification or localized
depletion 
in the chemistry causes a  potential difference and hence different
plating 
rates. 

Your problem may be similar, in that gold and copper are quite different
in the hierarchy of metals.  When you soak them in a conductive media 
you create a battery.  I would offer a guess that bringing the pH of the
OSP bath closer to neutral would help, and keeping the rinses as clean 
as possible would help, especially the rinse after the microetch since it

is most likely quite conductive.  (ie. use DI water if possible).  


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