Hi Christopher,
Even with 40-50 micro-inches of Au, I doubt very much that you would be 
soldering directly to the Au. The dissolution rate of Au in Sn is so fast that the 
dissolution of the Au in normal soldering will only stop if essentially all 
the Au is dissolved or there is no more Sn to go into solution with. Magarete 
Nylen of the Swedish Metal Research Institute showed that very nicely quite some 
years ago now.
Also, it is not so much that AuSn is brittle—in fact all the IMCs are 
brittle, but it is brittle and weak, and like AgSn IMCs forms platelet-shaped 
crystalline forms that act as stress risers in the solder matrix.

Werner



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