Gold-tin interconnections will degrade at Class I environmental levels
(Class I as described by the Battelle Environmental Study Group (BESG)) as a
result of corrosion (i.e. fretting corrosion).  Gold-tin interconnections
are more susceptible to corrosion than tin-tin interfaces.

Gold plating, as a noble finish, is free of surface films.  (This is true as
long as the application (i.e. plating) of the noble metal is continuous and
thick enough to prevent diffusion of the base metal to the surface (this
includes diffusion through an underplate, such as nickel, as well).)  Tin
and tin-lead plating, non noble finishes, will always have some amount of
surface film (oxide) present.  (This is true if no lubricants are present.)
Tin, being softer than gold, will transfer to the gold surface during the
mating/unmating cycle and the ever present surface oxides on the tin will
transfer as well.  The mating/unmating cycles will continue this build up of
oxides until it becomes an insulator.

In the case where the "spring" half of the interconnection is gold plated,
the contact normal force is normally specified (or designed) for mating to
another gold plated surface.  If this gold plated "spring" member is now
mating to a tin plated surface, the normal force (and wiping action) may not
be enough to "break through" this oxide film.

Martha Rupert
AMP Incorporated

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