Jason -

You mention that the traces are tin immersion plated - have they been fused or
reflowed in any way?  Straight tin plate often loses its solderability very
quickly after plating resulting in conditions similar to what you describe.

I'm not a real proponent of immersion gold over nickel, but this may very well
be one of those cases where it should be considered.  Or, as an alternative,
one of the white tins: FST from Dexter; or Omikron from Florida Cir Tech.

Regards - Kelly

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