Hi Bev! There is a fairly large set of industry data on the solder joint
reliability problems induced by Gold/Tin intermetallic  (Au4Sn) formation
and a smaller data set on the solder joint reliability problems induced by
Silver/Tin intermetallic  (Ag3Sn) formation.  See Klein Wassink's book ISBN
0-901150-24-X. I would expect that if you have both intermetallic species
in a solder joint it just would not be a "good" thing. The use environment
of the assembly would also play a role in just "how much" intermetallic
degradation would be allowable. Tough question - good luck.

Dave Hillman
Rockwell Collins
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Bev Christian <[log in to unmask]>@IPC.ORG> on 04/16/2001 08:33:58 AM

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Subject:  [TN] Embrittlement


Good morning!
Here is a met. question to start off the day.

We know that if we have over 4% gold in a 63/37 tin lead solder joint we
risk gold induced embrittlement because of the production of a  significant
amount of the tin/gold intermetallic.  It is also common practise to use a
slight variation of the eutectic solder with a 62/36/2 Sn/Pb/Ag
composition.
We also know that gold and silver are 100% soluble in each other.  My
question: if the final tin/lead based solder joint composition has a
COMBINED noble metal percentage in the 4% range how bad is that for solder
joint reliability?

regards,
Bev Christian
Research in Motion

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