Grant,
Unfortunately, without seeing the board or pictures of the problem I
can only give some general comments:
a) Differences in colour may be down to thickness variations in
the soldermask coating. Areas around vias or edges of high plated tracks may
appear lighter than the bulk.
b) Lightening of soldermask may also be due to moisture
absorption, sometimes seen after HASL or any other hot, wet post-processing
(ENIG for example). You may find that baking the board at 140°C - 150°C
(284°F - 302°F) for ~20 min will remove this effect.
As you also comment on a different finish (matte for light areas, glossy for
dark) I suspect your problem might be more related to (a) above, because you
are also seeing 'artwork marking' from the UV exposure process. You might
see this if you have a thicker ink deposit that has not been fully dried
prior to exposure.
I can't comment on possible effects on conformal coat, although I would
suspect that if the colour differences are due to moisture absorption you'll
want to get that out before considering overcoating.
Cheers,
David Albin
Coates Circuit Products
-----Original Message-----
From: Grant Emandien [mailto:[log in to unmask]]
Sent: 19 March 2002 09:36
To: [log in to unmask]
Subject: [TN] Soldermask color
Hi,
We have received PCBs with solder mask exhibiting color variarion i.e. light
and darker hues on the same PCB. The color differences are more prevalent
around vias but also evident on other areas. The light hue has a more matt
finish while the darker hue exhibits a sheen, almost an oily appearance but
is not easily removed.
Any comments on what it may be and possible effects (for conformal coating)?
TIA
Grant
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