Good morning Kevin,
A couple of questions to help refine the response.
1. Is the solder mask a dry film or a liquid photoimageable? I suspect
the latter since not many dry film masks are glossy.
2. Have you tested the surface energy of the mask on lot to lot to look
for variations? It is a simple way to look for lot to lot variations in
mask cure. Lots of asian suppliers are unaware that as their UV bulbs for
mask imaging age, they drop in output and so they run them far longer than
they should. The result is a mask with inconsistent properties from lot
to lot. If your boards are being produced in the same facility across
multiple fab lines, you might also get line to line variation.
3. What quality IPA are you using? The cheaper brands of IPA often have
lots of other schmutz in it. Could be leaving a residue.
4. Are you removing masking after cure, or after tack-free but prior to
cure? Can affect how much coating delam you get. I recommend prior to
cure so that you can do minor touch up before cure.
I partially agree with Wayne that plasma may be a good remedial method for
stuff you have in house. Even the briefest of exposures will take surface
energies above 60 dynes/cm. But, if you have solder mask that is not
completely or adequately cured, then it might have sucked in other process
chemicals along the way which then can leach out, especially with solvents
that normally go with urethanes, and can interfere with adhesion. Plasma
would not really address that absorption problem.
Doug Pauls
From: "Glidden, Kevin" <[log in to unmask]>
To: <[log in to unmask]>
Date: 02/14/2013 08:32 AM
Subject: [TN] Conformal Coat Adhesion to Gloss Solder Mask
Sent by: TechNet <[log in to unmask]>
Hi everyone,
We have a PCB that is using a gloss soldermask, and we are having extreme
difficulty getting urethane conformal coating to adhere. We use this
same soldermask on many PCBs, but only this one particular PCB gives us
issues. It has occurred for some time, across multiple lots of PCBs. We
do pre-clean all PCBs with IPA and DI water just prior to coating. This
is just a light surface clean just to remove any fingerprints or whatever
contaminants might be introduced between post solder cleaning, through
e-test, and then masking. I think much of the issue for this particular
PCB is due to geometry. The PCB is small and has large masked areas- a
few dots and all around the perimeter. When the masking is removed, all
the coating flakes off. Ultimately, converting this PCB to matte finish
is my goal, but I have many PCBs in stock to use up. HDBK-830 states no
primer is known to be needed for urethanes. Is anyone out there using
primers for urethanes, and would you care to recommend one? What about
alternate methods to roughen the gloss surface? I am thinking perhaps if
the PCB supplier could somehow dull the finish with an acid etch, or
possibly light grit-blast internally?
Just looking for ideas.
Thanks,
Kevin Glidden
Mfg Engineer
Astronics Luminescent Systems Inc.
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