Hi technet, I need help to investigate a failed component issue.

Some history first;
*       Received modules back from warranty return for analysis,
*       performed electrical testing on modules and found current
measurement on one of the pins to be higher than typical,
*       dissected the potting compound away from the component ( 1N4751 30 v
zener ) associated with that part of the circuit,
*       the diode measurement was incorrect for this component ( i.e.
measured .23 volts in both directions ), there is a cap in parallel with the
diode but compared the reading to a known good module and the cap doesn't
affect the reading,
*       then viewed the component under stereoscope and noticed the glass
package had cracked and there was discoloration with the component
- sent away a couple of the units to the diode manufacture for failure
analysis
- requested mtbf data from manufacturer on diode
- reviewed insertion process for any indication if the ai process was the
root cause,
- 100 % of finished goods rechecked to verify the current draw was typical,
found no nc's

The modules are enclosed in a plastic case and then potted with polyurethane
( Conathene En-2521 ) for weather resistance
sample size of 12 put through validation phase that included thermal shock
and vibration, this failure mode was not evident in these modules ( this was
done before the modules were mass produced )

The 30 v zener is used in conjunction with other components as a transient
suppresser,

What I am wondering is,
*       is this application suitable for a zener?
*       will the diode crack from electrical stress?
*       is there a chemical available that will breakdown the potting
compound without harming components (currently I place the module in the
thermal chamber and heat it up then carefully pick away the compound for the
circuit area that I am interested in)
*       are there any ideas on how to get this fault to replicate this
failure in-house

Thanks for any suggestions, I am new to the manufacturing arena and
unfortunately there are no mentors in  failure analysis in the company to
learn from.

Jason

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