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June 1998

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"SEM Lab, Inc." <[log in to unmask]>
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Date:
Thu, 25 Jun 1998 13:46:59 -0500
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Tim,

Soldering to gold can be precarious due to the formation of Au-Sn and Au-Pb intermetallic
compounds (IMC).  Since some of these compounds consume a lot of Sn (e.g. AuSn4) a large volume
fraction of IMC can form with a relatively small amount of gold.  I'd recommend a careful
analysis of just how much gold is disolved into the solder on these boards.

Regarding SN62 vs SN63 for gold.  The silver is believed to use up some of the solvent capacity
of the molten solder and therefore slow down the gold dissolution process.

I've never heard that SN63 vs SN62 performed any differently with respect to disturbed solder
joints.  Could you please elaborate on this?

Thanks,
Ed
--
               SEM Lab, Inc.
Scanning Electron Microscopy and Failure Analysis
               Snohomish, WA
               (425)335-4400
           http://www.sem-lab.com

Tim Frigon wrote:

> I have recently gone to work with a company who is doing SMA assembly to
> PWB's which have gold plated traces and pads.  This is a historical concept
> as we have both ceramic and PWB substrates.  In some products, there is
> direct wirebonding to the pads, and hence some thick gold plating is
> required.  However, if there is no wirebonding to the board, is there any
> other reason this would be preferable to conventional lead/tin plating?  I
> would like to change the solder process from using a 2% silver solder used
> as a result of the gold plating to a conventional Sn63 eutectic solder as a
> guard against disturbed connections.  Secondarily, what is the chemical
> action which makes the 2% silver solder more effective on gold plating?
>
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