Hi Ed: Take a close look and see if the discoloration is only on the mask or
is it also on the laminate in the exposed area  around the pads. If it is on
both it could possibly be the flux being over baked during preheat and the
wetter being left behind causing a hazy appearance. If it is just on the
mask, it's probably that the very top resin portion on the surface of the
mask has broken down and exposed some of the filler in the mask. The
DSR-3241 is a semi-matte to matte style of mask that uses quite a bit of
filler to create its appearance.
Have you checked the cure to verify that the mask is fully
cured/crosslinked. Insufficient cure both thermal and UV can contribute to
this problem.
I'm also assuming you are seeing this only on the board bottom side i.e.
flux/solder side and before any cleaning operation.





Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076
-----Original Message-----
From: Ed Hare [mailto:[log in to unmask]]
Sent: Wednesday, May 23, 2001 10:12 AM
To: [log in to unmask]
Subject: [TN] Discoloration of LPI solder mask


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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