Gee Bill, sounds mighty strange to me!  When we've had moisture related
failures in temp cycling it was attributable to corrosion/conductive paths
under the conformal coating caused by incomplete flux residue removal prior
to coating boards.  Is there a possiblity that your urethane coating is not
completely cured or that the boards weren't baked to drive off residual
moisture prior to coating?

Sheila Smith, Tracor AES











At 11:19 AM 10/17/97 -0400, you wrote:
>
>        We are experiencing temperature cycle (-37C to 55C) test
>failures on mixed technology (thru-hole one side, SMT other side) PWB
>assemblies coated with Polyurethane per Mil-I-46058. The same assemblies
>pass static temperature testing at the low, ambient and high points of
>the temperature range. Uncoated assemblies will pass the temperature
>cycle test. It is suspected that small bubbles in the conformal coating
>(within allowable size per specification criteria) are accumulating
>condensation during temperature transitions, causing bridging between
>conductive surfaces on the SMT components. Has anyone experienced
>similar problems?
>
>
>Bill
>
>Bill Bromley
>Lead Designer
>Lucas Aerospace PSA
>MailStop D960
>777 Lena Dr.
>Aurora OH 44202-8025 USA
>voice: 330-995-1000 ex 3092
>email: [log in to unmask]
>
>
>Attachment Converted: C:\EUDMAIL\TECHNETA
>

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