Edward,
As with any material, the end user must check for compatibility between the
material and the processes the material will be exposed to and for
compatibility between different materials.  In the fine print at the bottom
of virtually every materials data sheet the manufacturer will not guarantee
that their material will work in any or all applications.  They are just
guaranteeing the material will meet the parameters they have specified on
the data sheet.

Just because vendor "A" says their material may work with vendor's "B"
material or that both materials "meet the IPC spec" doesn't mean that will
work together in all instances.  The same solder mask material, processed
differently (or exposed to different processes) at the PWB fab may interact
differently with your conformal coating material.  You have to be careful
or you could have a catastrophic problem on your hands.

It's not a dirty little secret that some conformal coats won't adhere to
some substrates, it's surface science.   Surface energy and spreading
coefficients come into play.  Cleanliness is important for adhesion but
more critical for long term reliability since the conformal coat is there
to keep the surfaces of your assembly clean (not dry since moisture will
permeate most coatings).

As for the tape and knife "X" test, I would only use it as a qualitative
measure of adhesion.

A few years back I consulted with Dr Chris Rulison at
http://www.augustinescientific.com/ for some surface science work related
to conformal coatings.  He has some decent white papers on surface science
which may help your understanding of the physics involved.

Best regards,
-- Gregg


At 01:33 PM 12/16/2004, Edward Mines wrote:
>To the best of my knowledge there is no written standard for solder
>mask/conformal coating compatability, but I have heard instances where
>poor adhesion of conformal coating to the solder mask was preceived as a
>problem. It's a dirty little secret that not all conformal coatings adhere
>to all solder masks; I've seen some that peel right off, no matter how
>clean the underling solder mask is.
>
>At one time Enthione had a flier listing the "compatability" of popular
>conformal coatings to their solder masks. THer criteria that they used to
>judge this was vague. I don't know if it's still available.
>
>HumiSeal tried to develop a correlation between the surface tensions of
>conformal coatings and solder masks to predict adhesion but was unsuccessful.
>
>1B31S - a goosed up version of 1B31-  has improved adhesion to some
>problem solder masks, but there are still some solder masks where 1B31S
>will  peel right off.
>
>Doug Pauls is correct that cleaning problems are sometimes mistaken for
>incompatabilty of solder masks to conformal coating. Frequently this was
>solder mask batch related, sometimes process, sometimes the chemistry of
>the batch itself.
>
>Somewhere IPC (IPC-CC-830?) calls out an adhesion test for conformal
>coating to solder masks. The test cuts a grid larger than 1" X 1" into the
>coating with a $100 plus apparatus and then trys to lift off coating
>squares with tape. That test is  operator dependent (subjective) and it's
>hard to find a 1" square area free of components on most boards.
>
>
>
>Edward Mines BEChE, MEChE
>238 Willard Dr.
>Hewlett, NY  11557
>516 205-6523
>__________________________________________________
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