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March 1999

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Wed, 24 Mar 1999 09:39:07 -0500
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I recently heard of a similar problem, and the F/A  found that there were
defects in the polymer coating, as described by John Maxwell in the other
reply to your query.  What was unusual about the failure mode, was that the
resistance measurements across the part, changed when the meter leads were
reversed, which changes the polarity of the measurement.  Apparently, the
flaws in the coating, let in trace amounts of trapped flux & washwater,
which formed a small galvanic cell with the dissimilar metals. The
description reminded me of the old lemon battery experiment. Anyway, this
resulted in polarity sensitive resistance readings.

Bruce Tostevin
Benchmark Electronics
Hudson, NH
> ----------
> From:         Bob Willis[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Bob Willis
> Sent:         Monday, March 22, 1999 2:47 AM
> To:   [log in to unmask]
> Subject:      [TN] Chip Resistor Corrosion
>
> Has any one seen failures of chip resistors after 1-2 years where the
> junction between the silver termination and the restive
> element on the top of the ceramic has corroded through. This results in a
> high resistance failure after this period of time.
>
> I understand that some people in the automotive industry have seen this
> even on conformally coated boards, the example I have been
> examining is not in the same type of environment.
>
> Look forward to hearing from a few people, many thanks.
>
>
> Bob Willis
> Process Engineering Consultant
> Electronic Presentation Services
> 2 Fourth Avenue Chelmsford Essex CM1 4HA
> England
> Tel: (44) 01245 351502
> Fax: (44) 01245 496123
> Email: [log in to unmask]
> Web Page: www.bobwillis.co.uk
>
> Europe's leading supplier of training products and training workshops
> on electronic assembly and your guarantee of quality training.
>
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