Ok, I'm probably showing my ignorance here,
but with a stable non-corrodable finish like
Sn-Ni, why do you need the gold?

Jack (a PCB layout guy)





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----- Message from Happy Holden <[log in to unmask]> on Thu, 22 May 2003
08:44:12 -0400 -----

 Subject: Re: Sn-Ni PCB
          plating



Tin-Nickel alloy plating is a unique metal that is just about uncorrodable.
We used it at Hewlett-Packard for about 23 years (1965-1988).  First as a
final finish (but in those days we had acid fluxes in the wave solder) than
as a barrier metal under bright tin and then gold.  You can read about it
in Coomb's PC Handbook 4th Ed. (pp. 19.42-19.44) or earlier.   It is not in
the new 5th Ed.  I don't know if it is being used anymore in the San
Francisco Bay Area like in the earlier days of pcb growth.  It is a
non-proprietary plating bath that has no organic additives.  It can be
difficult to control and its plating can be very stressed.  But it has a
number of unique metallurgical properties--It is non-magnetic--Gold WILL
NOT DIFFUSE INTO IT--It will not corrode unless you use concentrated
aqua-regia.  Stanford Univ once did a project to understand how the alloy
worked because the properties were SO MUCH different from either nickel or
tin.  The report was fascinating, especially for a metallurgist because the
alloy was truly unique.

Morton Adler of Bell Labs did a lot of research on it because 5 microinches
of gold on tin-nickel out performed their 100 microinches of gold on nickel
for switch contacts.

I think Electrochemicals developed a proprietary tin-nickel that had an
organic stress reducer in it and did not require hydrofluoric acid to
control the pH.  The Metal Finishing Handbook had the bath formulation in
it.  I know personally that it made a great finish for sail boat
parts--never a sign of salt water corrosion in 20 years and we helped some
artists in Estes Park, COL plate a bronze statue so it would look good
forever.

Happy Holden
Westwood Associates
375 Morgan Lane, #206
West Haven, CT 06516

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