We recently started using a white tin finish on a select few assemblies.
Results have been good up until the last batch or so.  After a few boards
went through reflow, there were some small "crispy" sections, where the
finish became very oxided and turned black.

After inspecting raw pcb's directly out of vendor packaging, I noticed a
yellowish sticky substance that seemed to be oozing out of the through holes
and vias.  Supplier has the opinion that we added some kind of flux. Not the
case however, we are using noclean 63/37 process and peak pcb temp is only
220C.  Sent our returned pcb's for analysis, results still pending.

Any ideas?  My opinion is there's something slightly wrong with final
cleaning after white tin is applied, prior to ship.  Thickness is higher
than usual at 0.120, probably not helping the situation.

Thanks!  Bill C

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