In a message dated 11/7/06 16:30:23, [log in to unmask] writes:

> Does the length of time that the joint is held at solder flow/reflow
> temperature allow for more of the gold to go into solution with the solder?
> A: No, the only metal dissolving in solder faster than Au is Sn—or solder 
itself. Thus, all the Au contacted by the liquid solder is dissolved essentially 
before there is a metallurgical bond to the Cu.
> 
> Does the amount of solder that is added to the solder joint allow for more
> of the gold to go into solution with the solder?
> A: In special circumstances, i.e., thick Au and a very thin solder gap, you 
can run out of Sn to dissolve all of the Au; but that is not the case in normal 
circumstances.
> 
> What does gold embrittlement look like, or does it even happen, and if so to
> what extent, with the new solders that are being used as we go 'lead free'?
> A: Au-embrittlement looks like the last 3 pics of Ingemar's 6 pictures—but it 
is clearly the worst 
I have seen. I have seen similar devices from assemblies from China that also 
had a lot of Au [not quite as much] and that got hot enough for the solder on 
one side to actually melt. Now, I had always assumed that Inge etched the 
remaining solder in the last picture [all those platelettes are AuSn4 IMC, and I 
have seen this pic before], but it is actually an enlargement of the left 
side-fillet. No idea how it got to look like this, but it sure is ugly [from 
Inge's statement, it looks like there was so much Au that it consumed all the Sn in 
the Sn-rich phase, but I am guessing].
> 
> Are there any specific 'lead free' solders that are better in avoiding gold
> embrittlement that the others, and why?
> A: Actually, LF solders that are Ag-bearing are worse, because they 
potentially add Ag-embrittlement to Au-embrittlement. Ag3Sn-IMC looks essentially the 
same as AuSn4-IMC and produces the same weakening of the solder joint structure 
due to their geometry and brittleness.

By the way, when Ingemar makes a statement, I absolutely take his word for 
it, having known him long enough to KNOW he does not dish out BS [well 
Wodka-nights may be an exception].

Werner

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