If you get bubbles that 'look like foam', there is something wrong in
the process. We use Humiseal acrylic coatings, and have tried Urethane
before. Single microscopic bubbles are acceptable, which means in the
range 1 mil or less. We use to say, that the bubbles must not exceed one
third of the total thickness of the coating.

A bubble is entrapped atmosphere, and the oxygen content can
theoretically cause oxidation. The oxygen content in such little a
bubble is not enough to cause any oxidation. When you use the word
corrosion, one use to think of chemical reactions serious enough to
damage the whole solder joint, or make the lead fall off the package. I
would like to see the microscopic bubble that could cause such damage!
Would be no less than a miracle! Single air bubbles is nothing to be
worried about, foam of bubbles is something you need overlook.

Inge 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joel Alexander
Sent: den 23 april 2007 21:47
To: [log in to unmask]
Subject: [TN] Conformal Coating Voids

We are using Urethane coating on an assembly that uses  63/37 solder. We
are using organic water soluble flux. We are Ionic testing after wash
and we are well within IPC cleanliness specifications. We installed a
selective coating machine and in our initial run of product we had some
small bubbles that look like foam. Our customer does not like it but our
stand is that it meets IPC-A-610 and J-STD-001 requirements since there
in no obvious voids between conductors. They were worried about
corrosion if a bubble was against a lead. The bubbles are fully
encapsulated. Does anyone think this is a valid concern? 



Joel Alexander
Quality Assurance Manager
TT APSCO, INC. 

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