Has anyone else seen a "knock-kneed" zener failure?

I worked for a defense contractor many years ago, doing troubleshooting thru final test on avionics switching power supplies. Part of the testing was hot/cold cycling under changing load with measurements taken at every cycle.

The last unit of the run kept intermittently failing the hot cycle with lousy regulation on load changes. Management wanted it "out the door" ASAP.

Other technicians had pretty much "shotgunned" every part on the board before it was handed to me.

I spent days trying to duplicate the failure on the bench by standard troubleshooting.

Finally I went to lifting "possible" parts from the board and spot heating every thing. I finally stumbled on a 22v zener that showed minute leakage (about 10 ua) at about 100F. I hooked up a lab supply on it and curved it out and heated it some more . At 160F it showed a partial "knee" at 17 volts with about 1ma leakage, then dropped off until it hit the rated 22v knee. Since it was on one input of a differential amp used as a reference, this explained the failure. The

1 ma draw was enough to give an erroneous output on the diffamp.

I drew out 40 of the same zener out of stores and found that 6 of them had the same type of failure. There had to be more of them installed in supplies that had already shipped, and there was no way of tracking which units got one of these knock-kneed zeners.

The only saving grace was that the customer's rep said that the units would never see that kind of condition onboard the aircraft. The temp cycling conditions were such that they figured that there was plenty of margin in real world operating conditions.

What's *YOUR* tale about weird component failures?

Reply to
nobody >
Loading thread data ...

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.