I have done some work looking at gold embrittlement of solder joints and we found a weight percent on the order of 4-5% in eutectic Sn/Pb solder joints can cause embrittlement, in cross sections we saw joints that started with Au-Sn rich globules, turn into needles and collect at the solder joint interface over time leading to embrittlement.  There was a paper from Paul Vianco showing a 4 weight percent of Au in Sn/Pb solder leading to embrittlement.

Brian J. Toleno, Ph.D.
Sr. Applications Chemist
Loctite Electronics
15051 East Don Julian Rd
Industry, CA 91746
Ph: 626-968-6511 x304
Cell: 626-390-9562
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