Hi Technet, well for once I can post some information instead of asking!!!

Awhile back I posted some questions regarding the failure of a glass zener
diode, wanting to know what the failure was. Well after some research and
money, I came up with the root cause, thought I would share my findings.

Some pictures of the diodes can be viewed at
http://www.driveway.com/web/catalogView.jsp?dwSession=vapp03-36a84655aeb859b
c
(thanks Steve)

The module is used in an harsh environment, exposed to various thermal
cycles, and moisture. A polyurethane compound encapsulates the components
and a nylon-glass case houses the package.
The zener is used in the circuit for clamping transient voltage levels, the
failure mode of the zener is a short. Modules were being returned exhibiting
high current draw, following some circuit troubleshooting I determined that
the zener diode was the fault, after removing the potting compound the
observed diode conditions were noted, (see photos)
Removal of the nylon case revealed evidence of moisture ingress, white
residue, the diode being located on the edge of the board was visible
through the compound. The presence of moisture and voltage, as well as
contamination (board level, salt water from road) caused cathodic reduction.
which in turn created a leakage path through the diode.



        This leads me to one question, what is the minimum distance
components and signal tracks should be placed from the edge of the board,?
(through hole components)

Hope the information is interesting

Jason Larson

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