Hi Glenn,
You sure got a smorgasboard of different metals in your solder joints.
Both AgxSny and AuxSny intermetallic compounds (IMCs) are brittle and weak.
In fact, AgSN IMCs were the reasons for the formation of the IEEE Compliant
Lead Task Force in 1983 when TI PLCCs with silver-plated leads started to
fall of the PCBs at IBM-Austin. Your Ag is not 'leaching' (a word describing
a slow process) into the Sn63/Pb37 solder, it is virtually rushing into it.
You are likely hard pressed to find a layer if the silver metallization left
on your resistor pot leads unless nearly all Sn has been consumed by the IMC
formation.
You also are likely having a large thermal expansion mismatch between the
ceramic pots and your FR-4 (?) PCBs which is likely unmititgated by
stiff/short pot leads.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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