Tin has a great affinity for gold and if you can't use a tin free solder
(a similar MP indium/lead for example) then you are somewhat.... "stuck"
is the polite word. There are no alloying additions which will slow down
Au take up or "pre-saturate" the solder.
If you have to use a tin based solder then the only way to reduce the
amount of gold leaching is to reduce the amount of gold. (thinner
coating , mask areas, etc). You can slow down the rate of dissolution by
reducing the temperature, but this probably wouldn't be significant in
practice.
Sometimes if Au embrittlement is a concern you can wash the gold off in
(tin based) solder and then ideally re-coat (with tin based solder), but
that's rather expensive in time even if possible, and gives a two time
heat hit before assembly even begins.
Finally and perhaps counter intuitively you could consider a higher tin
content solder. If that were possible it would reduce the effective
concentration of the gold with respect to the tin.
Regards
Mike Fenner
Indium Corporation
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hugues Colicis
Sent: Friday, November 21, 2003 11:02 AM
To: [log in to unmask]
Subject: [TN] Gold diffusion into a tin based solder.
Hi all,
I'm looking for a way to reduce the gold diffusion into a tin based
solder (except the use of the expensive Au/Sn solder). Is there alloys
that have this effect? Or can it be reached by the contamination of gold
by some metals (like Pt, Pd, Cd,...)?
I know the gold embrittlement phenomenon and this is not an issue here
but well the "consumption of gold by the solder".
Thank you for your help,
Hughes Colicis
Process Engineer
Vishay - BCcomponent
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