The surface energy of an incompletely cross linked polymer will be higher
than when completely linked as the "free ends" of the molecules can have
charges. Whacking the surface with a plasma can embed charges, another older
and cruder way to get some energy into a surface is to flame (part char) the
surface so breaking some molecules apart. [Not recommended for electronic
assemblies :) ]
Regards
Mike Fenner
Bonding Services & Products
M: +44 [0] 7810 526 317
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: Monday, February 18, 2013 4:44 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat Adhesion to Gloss Solder Mask
Lamar,
Interesting. So you are saying a post UV bump of the soldermask, while
increasing cure (cross link), can be detrimental to conformal coating
wetting and adhesion? What is the principle behind this? Is the UV bumped
and theoretically(?) more cross-linked material simply more glossy? It is
my understanding a post UV bump typically also increases hardness and
reduces VOCs.
We did run the sample test I wrote about in my last email, which was to run
the PCBs through a UV curing conveyor prior to any conformal coating
applied. On this one sample, it DID seem to make a difference. This test
piece shows good wetting and adhesion. In fact, we can't flake it off at
all.
Thanks,
Kevin Glidden
Mfg Engineer
Astronics Luminescent Systems Inc.
-----Original Message-----
From: Lamar Young [mailto:[log in to unmask]]
Sent: Thursday, February 14, 2013 9:59 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat Adhesion to Gloss Solder Mask
If the boards are already built, then I agree with Wayne that a plasma clean
may be your best bet.
If the boards are not yet built, then check with the supplier to see if they
are using a post-UV bump to further cross link the mask to further reduce
ionic contamination test results. In some cases, the UV bump could be
eliminated which may improve the wetting and adhesion, while still passing
ionic testing.
Lamar Young
Specialty Coating Systems, Inc.
7645 Woodland Dr.
Indianapolis, IN 46278
(317) 244-1200 Ext. 276
www.scscoatings.com
From: Wayne Thayer <[log in to unmask]>
To: <[log in to unmask]>
Date: 02/14/2013 09:48 AM
Subject: Re: [TN] Conformal Coat Adhesion to Gloss Solder Mask
Sent by: TechNet <[log in to unmask]>
A light plasma clean will make this problem go away. Test wetting with
water drop test before and after. You should see a dramatic difference even
if the surface looks unaffected.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: Thursday, February 14, 2013 9:30 AM
To: [log in to unmask]
Subject: [TN] Conformal Coat Adhesion to Gloss Solder Mask
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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