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November 1997

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Subject:
From:
"Kane, Joseph" <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Mon, 3 Nov 1997 17:07:57 -0500
Content-Type:
text/plain
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text/plain (77 lines)
Bill:

Don't know about bubbles accumulating moisture - I'm not clear
whether your problem goes away in hot or after you bake out the
assembly.  One thought: we've had problems in the past with type
UR coats over thick film chip resistors - thermal cycling cracks
the glassivation on the top of the component, the crack propagates
into the resistive element, resistance changes.  Same problem
with adhesive if the parts are mounted glass-side down.  Obviously,
these failures are non-reversible.  They are easy to spot under
magnification, or measure the component by probing through the
coating.

Joe Kane
Lockheed Martin Control Systems

> ----------
> From:         BROMLEY, Bill[SMTP:[log in to unmask]]
> Reply To:     TechNet Mail Forum.;BROMLEY, Bill
> Sent:         Monday, November 03, 1997 3:13 PM
> To:   [log in to unmask]
> Subject:      ASSY: SMT/Temp. Cycling Test Failures revisited
> Importance:   High
>
> 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]
>
>

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