Bob,

Yes, been there, done that.

We use surface energy as one parameter to match the conformal coat (c/c) to
the solder mask.  We started specifying a minimum surface energy of the
mask to insure our c/c would wet correctly.  We also had our c/c materials
characterized.

You also should check for process compatibility, i.e. - run samples through
the complete PWA manufacturing process and conformal coat.  These sample
then should be run through some sort of qual plan, including cycled
humidity and adhesion type tests.

Beware that what may appear to be benign process changes in the PWB
manufacturing process (like UV bump or no UV bump) may greatly affect your
c/c performance.  And thermal exposure of the mask in your PWA process may
affect surface energy and c/c performance.

Brian E. is correct about c/c on laminate rather than mask.  However we
have had issues with minute amounts of HASL flux entrapped in the laminate
causing c/c cure/adhesion issues so we actually prefer solder mask.

You always need to evaluate and qualify your solder mask AND c/c together
in YOUR process.  The solder mask and c/c manufacturers won't guarantee
compatibility.  And don't forget cleanliness...

Good luck,
-- Gregg

At 02:00 AM 12/14/2004, Bob Wilson wrote:
>Hi,
>
>I'm studying for the CID+ test and, among other
>things, I've discovered there is a bunch of things I
>don't know or didn't realize that were important in
>the fab/assy world, even after 20+ years in the biz.
>
>I had boards that needed to be conformal coated.  I
>used one fab house and a different assy house and a
>3rd conformal coat house.  Now that I'm studying, I
>see it is pretty important that the LPI and conformal
>coat "match" in compatibility.
>
>Has anyone run into this (potential) problem?  How do
>I specify a compatibility between disparate
>disciplines?
>
>Sigh...I swear my fab and assy notes take up more room
>on the drawings than the biggest boards I've ever
>done, by far.
>
>Regards,
>Bob
>http://www.cstone-design.com/home.html
>
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