I manage a captive PWB shop that produces 4,000 square feet per week of
double sided plated thru hole product from .062 FR-4 copper laminate.  Our
product is a bit uncommon in that it has about .0004 inch hard nickel plated
over the copper traces.  Thru testing, we find a VERY thin layer (maybe 150
angstroms) of some contaminate that affects the surface conductivity when it
is used as a switch half with a stainless steel snap dome.  Using a
Scotchbrite pad to burnish the surface provides a solution, but I would like
to know the source of the substance and how to prevent it in the original
process.  Any ideas on the subject?