Fellow technetters:

        I would appreciate any help in the following subject....

        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. The surface mount components
that we are using are attached to the board with a small dab of adhesive
in the middle of the component.  Then, after the components are wave
soldered to the board, two very small gaps exist under the components
between the adhesive and the solder connections.  The air bubbles that
we see in the conformal coat are next to these gaps, leading us to
believe that the air under the components escapes during the coating
cure thereby forming the air bubbles.  Some of these bubbles are large
enough to bridge between the two solder connections.  It is suspected
that these bubbles are accumulating moisture which causes leakage
currents between the conductive surfaces on the SM components.
        How do we get the air out from under the components without
causing bubbles in the conformal coat?  Is it acceptable to have
conformal coat flow under the SM components?

> 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]
>