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Silver Sulphide Contamination of Resistors

Dave E

#5302

Silver Sulphide Contamination of Resistors | 21 February, 2001

We have had a problem with some resistors on some conformally-coated boards which, after a few months in the field, started going bad -- a real worst-case situation. The resistors would increase in value dramatically and sometimes develop diode-like behavior. It was suggested that this was probably due to Silver-Sulphide contamination. I would like to know where I can find information and/or discussions on this topic. Thanks.

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#5312

Silver Sulphide Contamination of Resistors | 21 February, 2001

References on corrosion are: * "ASM Corrosion Handbook No. 13" [0871700077]. * "Electronic Packaging and Corrosion" ME Nicholson [0871702916].

Please explain how you determined that you have silver-sulfide contamination.

Where is the silver coming from? Silver has not been used in component terminations for a long time, I want to say since the early �70s.

Where is the sulfur coming from? Ooops, that�s your question.

Sulfur is a horror show contaminant, regardless of your solder type or termination material.

You weren�t specific, but since your board is conformally coated, it�s probably reasonable to assume the sulfur was on the board before it was coated. [I guess that also assumes the conformal coat was effective.] You need to find the source of the sulfur that is down-stream of the coating process.

Talking out loud about sources of sulfur, consider: smokers, some types of paper, auto batteries, card board, elastomer (rubber) products, plastics that degrade slightly and release sulfuric acid, poorly vulcanized rubber sometimes has unbound sulfur, adhesives that release sulfur over time

As a final note, if you�re correct that you have silver in your terminations, silver tin intermetallics are very brittle and may present you with an alternate path to chase in determining the source of your problem. [If you consider gold-tin IMs bad, silver-tin IMs are like having your mother-in-law coming to visit for a month!!!] And this may be the excuse you need to get some metallurgical analysis done on the terminations of these problem components.

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Dave E

#5317

Silver Sulphide Contamination of Resistors | 21 February, 2001

Realistically it seems like a component problem that the vendor is trying to blame on our processes. The problem is internal to the 0805 resistors so I suppose they are perhaps porous to some degree. I have only heard 3rd hand about the analysis that was done but hope to find others who have experienced this type of resistor problem.

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#5319

Silver Sulphide Contamination of Resistors | 21 February, 2001

I don't know where sulfur would get into your resistor supplier's process. Your processes are more likely candidates.

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Dave E

#5320

Silver Sulphide Contamination of Resistors | 21 February, 2001

Well, the vendor claims the contaminant caused the component failures. So? The only feasible possibilities would be contaminated solder paste, wash water or coating -- or maybe the problem is due to defective resistors?

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#5323

Silver Sulphide Contamination of Resistors | 22 February, 2001

I guess the answer to that would have something to do with the distribution of the problems across the various components on the board. [I hear you saying that it's isolated to a specific component, from a specific supplier; rather than being more loosely distributed.]

Years ago, we had problem similar to yours. It was a wide spread across the components on the board. It was caused by the paper our board fabricator used to seperate each board.

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#5345

Silver Sulphide Contamination of Resistors | 23 February, 2001

We still build a legacy product with a primitive backplane using silver plated fork terminals for outside world connections. After some 20 years of no problems the fork terminals on raw stock PCBs were black and almost unsolderable. Because of the terminal protrusion our vendor had always packaged these boards with rubber bands over a poly bag. Turns out he ran out, went to Office Depot and bought a box. The sulphur outgassing from these rubber bands was enough to contaminate every board in a stack of 10 through the tiny holes the fork terminals punched in one side of the stack. As Dave says, there are lots of sources of sulphur contamination.

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Dave E

#5352

Silver Sulphide Contamination of Resistors | 23 February, 2001

>The sulphur outgassing from these rubber bands was >enough to contaminate every board...

Well, if every rubber band is a threat then sulphur contamination must be a common problem. Are sulphur test kits available?

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#5354

Silver Sulphide Contamination of Resistors | 24 February, 2001

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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#5361

Silver Sulphide Contamination of Resistors | 26 February, 2001

ASTM E 443 is a test for sulfur by Oxygen Flask Combustion. Other instruments will detect sulfur (I assume a SEM with EDS capability will pick up the sulfur). We had an addition cure silicone potting compound inhibition problem last year that was traced to excess sulphur in the compression pad we were using. Most rubbers have sulphur added for vulcanization.

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