Ed,
We have seen exactly what you show in the pictures. After 9+ years of
wave soldering, we finally had this happen to us once. It was due to
the flux attacking the surface of the solder masking. We use Kester
water soluble flux and after contacting Kester, they said that it does
attack certain masking. Our board vendor switched to another brand of
masking, and all the problems went away. Prior to that, we had tried all
kinds of things - pre-baking boards, temp variations on preheats, speed
through pre heaters, etc. and could not eliminate it. Our customer sent
some of the boards out for analysis and it came back saying the
integrity of the masking had not been compromised, and the foggy
appearance was a fine etch in just the surface of the masking. As
further proof, we ran a couple of boards through the process after
sticking pieces of heat resistant tape on the boards, after wave solder,
the surface below the tape was not effected (no fog) since the flux
could not reach the board to attack the masking. We continue to use the
same flux today, switching to another type or brand would be too much as
far as re-qualification for all our customers.
Tom Parkinson
WinTronics, Inc.
Ed Hare wrote:
>
> Dear Technetters,
>
> Thanks in advance for any insight you can offer on this issue ...
>
> The issue is discoloration of LPI solder mask (ENPLATE DSR-3241 (MD)) that
> corresponds to areas that were selectively wave soldered I believe using OA
> flux. There are pictures at the link below ...
>
> http:\\www.eskimo.com\~ehare\EH_516
>
> The discoloration does not map with ground planes or other internal
> circuitry in the PWB.
>
> What might cause this type of discoloration?
>
> Thanks,
> Ed Hare
> SEM Lab, Inc.
>
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