Doug
You are correct, the schmutz factor is important. We've seen that issue here
before. The number one culprit in these cases is usually the surface energy,
lack of wetting. It's amazing how different coatings react to different
substrates as well. Conathane has lousy wetting characteristics and dewets
from KimWipe residue.
Lloyd E Duso
Diamond-MT
Plant Manager
(814) 535-3505
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Thursday, February 14, 2013 11:05 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat Adhesion to Gloss Solder Mask
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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