When you place the kapton tape on the gold fingers and see the same spots,
were the spots on top of the tape or below it and showed only on the fingers?

If indeed on top of the tape, then it is caused by solder balling.  If below the
tape, then it is probably an oxidation of some material on the gold fingers.

If indeed it is solder balling, it is probably caused by the IR heating.  Solder
balling occurs when the solder is heated up too rapidly and the solder spatters.
Solder balling in this case can be eliminated by reducing the heat up rate,
which is more uniform in convection systems.

Humidity could also be causing the spattering.  Solder paste will absorb
moisture from the air.  Excess water in the paste will lead to solder
balling.  More convection heating will be required.

For more information on Convection Systems contact ETS at 509-483-0900 or go
to our homepage www.eznet.com/~ets or via e-mail [log in to unmask]

Brian Stumm
ETS





>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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