We are PCB manufacturers and seem to be having a enigmatic problem on one of
our boards we manufacture for one of our customers.

The board is a double sided 0.062" SMOBC board with flash gold goldfingers(
1-3 microinches). The assembly process consists of screen printing of solder
paste, pick and place SMT machines , Reflow, manual insertion of through hole
components and finally wave solder. The problem manifests itself after IR
reflow. There are tiny silverish/grey spots on the goldfingers. Suspecting
that the spots were caused by solder paste particles which were either
deposited because of contaminated equipment or degassing from the PTH causing
splattering of solder paste we ran the next batch of boards using Kapton tape
on the gold fingers. Unfortunately we saw the same spots again. The straw
which broke the camels back was when a recent datecode boards fresh out of the
cartons were run without Kapton tape and exhibited the same symptoms.

Our dilemma:

We manufacture 30 P/Ns for this customer. The problem is only on this P/N. The
same lines are used to run all the boards.

We have manufactured thousands of these boards without any problem and now
suddenly we have this issue.

The same assembly line used to assemble this board is used to run other boards
we manufacture and no other P/N exhibits this problem.

Hope somebody has seen such a manifestation and can advise.

Thank you.

Richi

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