Consider non porous white immersion tin. Without the propensity for
dendritic growth (like gray tin or silver) and performs well with all
tin/"something" alloys (silver works great with tin/silver; works good with
tin lead, works less-than-good with tin bismuth).

Non porous white tin's singular drawback is wirebonding  -  which may not be
an issue in your application.

As for gold, you'd do well to search TechNet for imbrittlement horror
stories (before the gold Rep's respond to this, re-read the ANSI J-STD-001B
specification concerning the removal of gold requirement for PCBs, and note
that GOLD stands alone from all other surface finishes in the
strip-it-before-you-use-it department).

Robert Lazzara
Georgia Tech Support Bureau
Circuit Connect, Inc.

-----Original Message-----
From: Rob Schetty <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, January 28, 1999 10:48 AM
Subject: Re: [TN] Thermosonic ball bonding to .... silver?


>Silver is prone to a phenomenon called "silver migration" which occurs
under
>certain conditions of humidity and voltage.  This can lead to bridging and
>short circuits.  Silver is also prone to tarnishing.  For these reasons,
>silver is mostly not allowed as a metallization on exposed areas of
>circuitry or on external terminations.
>
>Rob Schetty
>Shipley-Ronal
>Freeport, NY  USA
>
>
>-----Original Message-----
>From: Phil Hersey <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Wednesday, January 27, 1999 8:18 PM
>Subject: [TN] Thermosonic ball bonding to .... silver?
>
>
>>Has anyone experience with ball bonding to pure silver?  A thickfilm
expert
>>told me he tried it one time, thermal cycled the heck out of it, and the
>>bond got stronger!  He had no idea why silver was not used for ball
>bonding.
>>Aluminum and gold are the only metals I know of commonly used for
>>thermosonic ball bonding.
>>
>>If silver works, maybe I can convince my microvia PCB fabricator to
install
>>a silver bath- cheap'r 'n gold!
>>
>>Phil Hersey, Hytek Microsystems
>>
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