We recently had a lot of problems with a socket which was copper, immersion
nickel, immersion gold.  We found lots of staining which we presumed to be
copper diffusion through a very thin and very porous nickel barrier.  The
stains were redish orange and these areas had very poor solderability.

To test your theory I tried to contaminate some gold plated components by
dipping them in a copper loaded nitric/sulfuric bath and was unable to
visually see any copper or staining on the gold.  (perhaps I didn't give it
enough time)  When I dropped a nickel/Iron component into the bath it
immediately plated with the copper.  My suspicion is that the copper won't
plate out of solution onto the gold but if you can get copper in contact
with the gold in the presence of heat then you can cause the same copper
gold intermetalic which renders the gold unsolderable.  Perhaps one of the
plating gurus can explain the likelihood of electroless copper plating onto
gold.

We have had so much trouble with aluminum contamination on gold surfaces
that we ban the use of aluminum trays, fixtures, foil, etc from our
production areas.

-Russ Winslow

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of doug smith
Sent: Wednesday, February 17, 1999 12:18 PM
To: [log in to unmask]
Subject: [TN] Immersion Gold contaminated with Copper


I am observing a variation in the color of gold immersion finished boards.
My theory is the gold finish is being contaminated by subsequent cleaning
processes prior to assembly.

Does anyone have any experience with copper contamination of immersion gold?
I know that nickel is used as a barrier between the copper SMT pad and the
gold finish. Without the Nickel barrier the copper would migrate into the
gold creating an unsolderable finish. So there is a chemical reaction
possible between copper and gold.

So what if the gold immersion finish is subjected to subsequent processes in
the fabrication facility that have copper in solution. Is the copper
attracted to the gold surface out of solution? Can the concentration of
copper in solution be so high as to make the gold finish unsolderable? Is
there an acceptable limit of copper contamination in an immersion gold
finish from an assemblers solderability viewpoint?

Doug Smith
Technical Support Engineer
Yamamoto Mfg. (USA), Inc.

(408)944-8308
[log in to unmask]

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