Bob,
You are correct that the solder mask/resist and the conformal coating
should be chosen to be compatible with each other. I have generally found
that if the mask qualifies to IPC-SM-840 and the coating qualifies to
IPC-CC-830, and are processed correctly by the fab house and assembler,
respectively, then they will be compatible. Unfortunately, the part about
the materials being used correctly has a bigger impact on material
compatibility than the material itself.
You are also correct that defining "compatibility", testing for it, and
interpreting the results are an art form in themselves. If you get a lack
of adhesion and ask the solder mask vendor the problem, then the problem is
that darn conformal coating. If you ask the coating vendor, the problem is
that darn solder mask. The present IPC philosophy, as shown in J-STD-001,
is that it is the responsibility of the OEM to determine that the materials
they use are compatible with each other and to generate objective evidence
of that compatibility. There are many ways to do this, from lab tests to
life test on product.
The easy way is to put on the drawing that the solder mask must be
qualified to IPC-SM-840 and the coating qualified to IPC-CC-830, and do a
J-STD-001 qualification on the combination.
Doug Pauls
Bob Wilson
<[log in to unmask]
M> To
Sent by: TechNet [log in to unmask]
<[log in to unmask]> cc
Subject
12/14/2004 01:00 [TN] LPI and conformal coating
AM
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
Bob Wilson
<[log in to unmask]
M>
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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