Hi all,
We are experiencing a cosmetic (and perhaps worse ?) defect that is
unfamiliar to us. The boards (mixed technology) are coming off the wave
soldering line with subtle but clearly visible black streaks on the
underside. These streaks appear to be within the board.

The effect is present only when flux is applied. Boards, both populated and
not, do not develop these streaks if no flux is applied. They will however
show these streaks even when minimal flux is applied and their appearance
(intensity, number) appear to be independent of both the amount of flux
applied and the thermal profile used (both were varied significantly). The
results are identical for boards (of this batch) that have not previously
been through SMT.

The markings cannot be removed by washing with either water or IPA. It looks
as though excessive flux was being absorbed by the solder mask and was
degrading in there during the thermal cycle. I can provide pictures.

We have built this product many times before without this occurring. The PCB
supplier is one of our regulars.

For these last reasons we have been assuming all along that this is a
problem with the batch of PCBs. However, the supplier claims to have never
seen this and that nothing different was done. We are suggesting an error of
some sorts occurred.

Can anyone suggest a likely cause for this failure ? Also, if someone knows
of a likely cause, is there a test we could do to confirm this and push back
on our supplier ? What is the likelihood that this problem would effect the
long-term reliability of the board (they behave normally in full testing,
both before and after burn-in) ?

thanks,
yves

Yves Dupuis
Process Engineering
Leitch Technology
Phone: 416-445-9929 X3389 / Fax: 416-445-7927
25 Dyas Road, North York, Ontario M3B 1V7

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