Ryan: "Butter Coat" is an old term and probably a lousy one. It is not
butter and is not a true coating.
It is not applied by the board fabricator. It is actually part of the
laminate manufacturer's standard process. It typically is an epoxy resin
that is incorporated into the lay up process that among other things helps
seal the outer layer of glass weave in the laminate to prevent moisture
penetration.
The board fabricator can disturb this coating by over aggressive chemical
and mechanical processing, exposing the glass weave. Thus creating pathways
for moisture and chemical intrusion into the laminate.


Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ryan Grant
Sent: Thursday, January 13, 2005 8:52 AM
To: [log in to unmask]
Subject: [TN] Butter coat


Hi Technetters,

I just learned a new PCB fabrication term today..."butter coat".  It's
supposed to be thin layer of resin between glass cloth bundles and
conductors to assure reliability.  In the few shop audits I've been
through, I've never seen or heard of a "butter coat".  What would the
process look like?  Is the resin pressed and frozen into a sheet that
can be handled or would it be squeegy'd on?  If it is squeegy'd, can
that be performed by hand or must a machine do it?  If a machine must do
it, how do you lay up the brick for the press?  Finally, is this a
standard process that I've just been unaware of?

Thanks,
Ryan

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