Silver Sulphide Contamination of Resistors

Well, here I am working Saturday. Seems my production Supervisor had the nerve to get sick on an overtime weekend and, for some peculiar reason, the execs expect the numbers to be met anyway. Go figure! Let me backtrack and clarify that it was the terminals that were contaminated and , for reasons I will state later, not the board itself. I am not aware of any simple (read cheap)test for the presense of sulpher. If there is it would likely indicate presense rather than concentration. Concentration level would be in the realm of rather sophisticated (read expensive) instruments. Then you would have to establish a reliable sampling procedure and limit criteria. I think a bit overkill. My tests to establish contamination were quite simple and crude. I lined up 3 jars, put a new (suspect) rubber band in one, an old rubber band in another and a piece of the affected PCB in another. Then I tossed in a few clean terminals in each jar and sealed them. In a few days the terminals in the jar with the suspect rubberband were visibly affected, especially those touching the rubberband. Both other jars were un-affected. We felt the boards were safe to use at this point and simply soldered a little more aggressively and took the precaution of soldering the terminal to the board rather that relying on the swage. We have not experienced a single failure of these boards in the years since. Although we no longer allow rubberbands for PCB packaging, this has been my only experience with this sort of contamination. I don't think it's a real problem for most. If you indeed have sulphide contamination of one or more reels of resistors it could be because of an isolated downstream incident like a rubberband on a reel at any time or something like some fool charging the forklift by the RI bench. Good luck in your investigation.

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