>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 ################################################################