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August 2000

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Subject:
From:
Gabriela Bogdan <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 1 Aug 2000 19:25:49 +0300
Content-Type:
text/plain
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text/plain (68 lines)
Hi, Jason,
I could not see the pictures, there is an error.
Please, could you send them again?
Gaby

"Jason Larson, AScT" wrote:

> 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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