SIR Douglas O. Pauls is correct that masking tape should not be left on,
but there are instances where removal of the tape causes problems with
the soft coatings peeling away, and as yet I have not found a solution
to that problem, so it was left on. I did not consider the fact that the
adhesives could pose a problem, but I am sure that is possible. I would
recommend removing the tape whenever possible. Never, never attempt to
cut along the tape with a knife and then pull it away. I know many
companies do this, but it is too dangerous, as traces can be cut
inadvertently.

As to the moisture condensation issue, both are correct. It is very
difficult to obtain a perfect hermetical seal with the coatings, but as
Doug pointed out, it can be done with the proper methods and materials. 

However, a word of caution. There have been many instances where
moisture was not completely removed prior to coating. If this happens,
or if a board is immersed or has liquids spilled on it after the coating
was applied, the liquids can be absorbed under the coating and trapped,
and this presents a much worse problem if there are any active flux
residues left behind.
This was the subject of Terry Munson's article regarding the dendritic
growth posted here last week. I urge all of you to read that and the
article by Doug. They reveal a lot.
  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls
Sent: Friday, December 01, 2006 2:10 PM
To: [log in to unmask]
Subject: Re: [TN] conformal coating as under fill

Mr. Stadem's comment about conformal coating wicking is true. I have
seen instances where it got inside connectors making them impossible to
connect or gluing them shut.

**Yup, we agree there.

  The IPC should make recommendations regarding conformal coating
masking in their hand book. Should it be left on after coating? Subject
to qualification?

**If there was any recommendation, it should be to never leave tape on.
Too many tapes have too much crap in the adhesive to make it a long term
reliability material.  Especially after you throw varying amounts of
solvent at it from the coating.  What kind of tape you use for masking
depends on what materials you are using, whether you clean afterwards,
the end use environment, etc.  Talk with your coating supplier to get
the right kind of masking tape for the coatings used and their
recommendations on how it should be used.

  Mr. Stadem is mistaken regarding conformal coating and "moisture
condensation". Moisture yes, condensation no. A lot of other engineers
make this incorrect assumption. Conformal coating applied as liquid
doesn't provide water proofing on 3 dimensional objects. If it's
expected to then the qualifying tests should be changed.

**OK, now we have an argument, and it depends on how far you want to go
with the term "water proofing".  In general, conformal coatings are
present to resist the flow of moisture and to prevent shorts from liquid
water contacting the energized assembly.  No coating makes an assembly
truly water proof as all coatings have varying levels of water
permeability.  We have applications here where assemblies are quite
likely to have liquid water running across them due to condensation in
the aircraft avionics bays.  In that case, we might pick a silicone
coating and make sure that we have good coverage.  In these
applications, conformal coat does provide protection on 3 dimensional
objects from liquid water.  It all requires selecting the right material
and process.

Doug Pauls

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