Bill,
 Generally when small air bubbles appear in the conformal coat it is
because of trapped moisture in the PWB.  Board preparation prior to
conformal coat is essential.  The assembly must be clean and dry.  The
key word in your case is being dry.  The assembly may appear dry to
sight and touch but may have moisture in the inner layers.  The best way
to ensure the assembly is dry is to bake the assembly.  Generally they
can be baked at 170 F for four hours or vacuum baked at 170 F for one
hour.  These are estimates and your application may differ.  Also,
after baking the assemblies they should not sit longer than 1 or 2 days
before being coated, the sooner the better.  I hope this helps.

Jon Hemstad
[log in to unmask]

>----------
>From:  BROMLEY, Bill[SMTP:[log in to unmask]]
>Sent:  Friday, October 17, 1997 10:19 AM
>To:    [log in to unmask]
>Subject:       ASSY: Temperature Cycling Test Failures
>Importance:    High
>
>
>       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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