To technetters:

Several interesting questions and answers about immersion gold on electroless
nickel.  
 
1.Solderability:  
 About phosphorus content and solderability----- It has been written many
times that the higher the phos the poorer the solderability.  However,  we
did not mention that many electroless nickel bath for electronic use are a
nickel-boron type and do not use any phosphorus in make up or as a reduciing
agent.  The boron electroless nickels are reasonably solderable even with out
the immersion gold overplate, but the gold does aid solderability.  The
palladium overplate also works well with the boron electroless nickels and is
perhaps will retain solderability better.

2.Storage life: 
  The question continues to come up as to storage life of immersion gold over
nickel.  In my experience, I tried to determine if one was better than
another, and therefore, used steam aging on the M-coupon as a test vehicle.
 The result was that the immersion gold is a poor barrier to retain
soldrability of the underling nickel in histile environments and that some
product even exuded nickel salts after the test.  Some product was better
than other, but it was not related to thickness.  In general the test
indicated that the gold is porous and with the most probable presence of
residual ionic salts will cause the nickel to react and become
very-very-poorly solderable.  For best storage results, it is best to
bake-dry the boards after processing and keep them dry up until soldering
time.  Note: a true non-porus immersion gold would only have to be about 4
angstroms thick or about a 1000 time thinner than the 3-8 millionths that we
apply

Phil Hinton
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