Hi, Daan,
From the little I know about moisture and board assemblies, covering the
components with Humiseal, or not, the components will still absorb moisture
and lose it again. The boards themselves, plastic components and 1B31 all
transpire moisture as a sort of natural "breathing" cycle. I asked this
forum a while back about coatings that exclude moisture, and got the
resounding response that there was no such thing. Even if there was, a
coating that keeps moisture out, also keeps absorbed moisture in, and it
would delaminate from the board as trapped moisture tries to escape.
Apart from aesthetics, the only case against not coating component bodies
is the lack of complete integrity of the coating film. It might start to
lift off at the edges.
Peter
"d. terstegge" <[log in to unmask]> 14/08/2002 08:28 PM
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Please respond to "TechNet E-Mail Forum."; Please respond to "d. terstegge"
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Conformal coating on top of plastic IC's
Dear Technetters,
We've just finished our first few boards with Humiseal 1B31 conformal
coating, applied with a selective spraycoating machine, and we asked the
customer to inspect the products.
He commented that some areas on top of the IC's were not covered
completely, which is required (so he says) because these are plastic
components which may absorb moisture if left uncovered.
I'm not gonne argue about this with the customer, but still I'm
interested to hear from you wether the fact that the IC's are plastic is
indeed a reason to require them to be completely covered with coating.
As far as my knowledge goes the problem with plastic IC's and moisture
is limited to the soldering process only, but I may be wrong.....
Anyone can comment ?
Daan Terstegge
Unclassified mail
Personal Website: http://www.smtinfo.net
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