Hi Neil,
Are you sure you have de-wetting? You may never have had any wetting in the
first place or you may have too much brittle and weak Au-Sn and/or Ag-Sn
intermetallic compounds (IMCs) near the interface, which with  upwards of 2
microns of Ag or Au, and particularly with 2 microns each of Ag AND Au, is
likely. You can tell the difference by analyzing the lead surface. If there is
little or no Sn present, you never had wetting, if you have Sn and Ag/Au
present it is the IMCs.
The Ag does not reduce solderability, it is similar to Au in this respect.
With this much Au/Ag underplating with nickel will NOT help. The Ni-underplate
only helps if the brass is not solderable by itself.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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