In a message dated 2/26/01 7:48:59 PM Eastern Standard Time,
[log in to unmask] writes:

<< is there an underlying Ni diffusion barrier
 between the copper and silver for surface mount applications as there would
 be for Cu/Ni/Au flash?  If yes, do you follow the same Ni thickness
 guidelines as for Cu/Ni/Au?  If no, why not?

 Is anyone gold wire bonding to an immersion Ag surface for Chip On Board
 applications.  I am assuming a Ni barrier layer is used.  Correct? >>

Steven -

No, there is no electroless nickel diffusion barrier when using immersion
silver.

Immersion plating is based on the electromotive potential between elements -
that is, the capability to form a "battery".   Gold is the most noble element
and could immersion plate on any other metal.  Electroless nickel is used as
a barrier because gold immersion plated on copper would form imtermetallic
very rapidly.  However, gold can replace nickel from a surface to give
immersion plated gold.

Tin and silver are not as noble as gold, and so far as I know, cannot deposit
over nickel (I am not at my desk as I write).  However silver forms
intermetallic very very slowly in contact with copper, and tin forms
intermetallic slowly enough with copper, that you get at least months of
shelf life - depending on immersion plated tin thickness.

Without a nickel barrier, I know of no one wire bonding to silver or tin on
copper.  I have no experience with electroplated silver or tin over nickel,
and then wirebonding.  However, I be surprised if that is ever done.

Dennis Fritz
MacDermid, Inc

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