Thanks, Doug.

1) The coating is LPI.  Not too many people using dry-film anymore, I think. (?)

2) We tested some PCBs with dyne pens, but only on the most recent lot.  The results weren't really conclusive.  60 did not wet, 58 wetted, 56 did not wet, 54 did not wet.  Unfortunately, 54 is the lowest pen I have.

3) Our IPA is 99.98% purity, at least per the test report with our latest receipt.

4) We are using UV cure material with a secondary heat cure for shadowed areas.  We remove masking after UV cure but before heat cure.  I can have them try to remove it while the coating is wet.  I agree, part of the problem is removing the masking.  Once the edges lift around the masked areas, the whole film sheet goes.  I've also wondered about the masking - we use a PVC film with acrylic adhesive.  I don't really know how PVC reacts with UV light, but we do use this masking material quite a bit across the board with no other apparent issues.

I considered the partially uncured soldermask.  It looks ok visually, but I guess that is no indication.  As a test I have asked the techs to run PCBs through the UV curing tunnel prior to coating, with theory being that the UVA 365nm light @ 2,500 mW/cm^2 might bump cure the soldermask.  I'm not sure which particular soldermask brand this is (right now), so I'm assuming it has a UV bump cure ability.


Kevin Glidden
Manufacturing Engineer
Astronics Luminescent Systems Inc.<http://www.astronics.com>

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, February 14, 2013 11:05 AM
To: TechNet E-Mail Forum; Glidden, Kevin
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]<mailto:[log in to unmask]>>
To:        <[log in to unmask]<mailto:[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]<mailto:[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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