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October 1997

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Subject:
ASSY: Temperature Cycling Test Failures
From:
"BROMLEY, Bill" <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Fri, 17 Oct 1997 11:19:37 -0400
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        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]

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