>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

Bruce & Bob
I'v been away from my e-mail so there has been a delay in my reply.
1) No resistance value shift was observed.
2) Most chip resistors and capacitors are nickel barrier plated over a
silver glass frit termination so there should be little exposed silver.
3) The chips that I investigated had exposed PdAg metallization between the
nickel barrier and resistive element.
4) The exposed areas where there was thin or missing polymer coating.
5) If I remember correctly from my capacitor manufacturing days nickel
sulphamate baths were used in barrel plating nickel barriers.

Hope that this adds something to solving the problem.

Regards,


John Maxwell

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################