Glenn,

We have intentionally used liquid epoxy over chip resistors in the past.
The thermal cycling results indicate significant improvement in thermal
cycle life. These were chip resisotrs on insulated metal substrates and due
to the CTE mismatch between the substrate (23ppm/C) and the chip resistor
(6.5ppm/C), the thermal cycle life for unencapsulated parts was lower than
on traditional FR-4 laminate. The epoxy coated boards surived 3500 thermal
cycles from -40C to +125C with no failures. I may be able to locate a
publication on this if you are interested.

Wayne

At 09:18 AM 3/19/99 PST, you wrote:
>Does anyone have information on the effect of applying epoxy around SMT
>components after they've been soldered?
>
>The situation arises due to space limitations on the board.  A die
>encapsulant is used to protect wire bonds but overflows onto adjacent
>chip caps and resistors.  Two conditions occur; fully encapsulated
>components, and partially encapsulated components.
>
>I have some concern with the mechanical stress that it may see at
>different use temperatures.  But tell me if my thinking is correct here:
>The epoxy is cured at an elevated temperature (150 to 160 C).  So, given
>that it is stress free at the cure temperature, at room temperature, the
>epoxy should be in tension.  Right?
>
>Thanks for your input.  This will probably result in an experiment to
>prove out the design.  But, maybe there's some technical paper already
>published or personal experience that would help design the experiment.
>
>Glenn
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