I recently ran a few test assemblies (with immersion Ag finish, of course)
through thermal cycling (for 2nd level reliability).  At the end of the
test, some of the non-soldered metal area (EMI contact) got the yellow
tarnish.  When probed with a multimeter I measured significant increase in
contact resistance.

I read a lot of comments on the presence of sulfur and chlorine (or their
ionic counterparts) as being the possible root cause.  My question is: what
is the chemical reaction(s) here?  Are those agents attacking the Ag or the
organic co-deposited with the Ag?  Does anybody know what the yellow stuff
is?

Mason Hu

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