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Subject:	Re: Solder mask for touch up
From:	Dennis Fritz <[log in to unmask]>
Reply To:	TechNet E-Mail Forum.
Date:	Wed, 4 Apr 2001 00:51:55 EDT
Content-Type:	text/plain
Parts/Attachments:	text/plain (38 lines)

Good idea to use more of the original solder mask for touch up - but a couple
of further pointers to help make your life easier.

1. If the original mask is LPI - it is cured with both a photo exposure and a
bake. Doubt an assembly shop has UV light of that intensity - so really
watch the bake - don't short cut that. The bake at touch-up has to cure as
much as combined expose and bake at original fab. Also, be sure this extra
bake is not damaging any surface finish conditions - OSP, immersion tin or
silver, or forming extra copper/tin intermetallic from the tin/lead solder of
a HASL board. If original mask was dry film - you obviously have to use a
different liquid mask for touch up.

2. Lots of mask formulations come as two part - resin and catalyst. You have
to get both parts from your board fab, and then you have a mixed mask pot
life of a few hours. Mix no more than you will use quickly in the touch-up
operation. Likewise, most masks are carried in solvent. Some solvent may
evaporate from your mixed resin/catalyst, so get some extra solvent from your
fab shop. Or, you may want to make a more dilute mask formulation to be able
to brush-on touch up areas more easily.

3. If you don't like these complications, you might consider touching up
with a one part thermal cure mask from the old days. That stuff is pretty
simple.

Denny Fritz
MacDermid, Inc.

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