Despite my interest in the precision high-end of electronic design (and thus being a subscriber to this list), I have very little experience with such high-fallutin' designs. Still, I've done and seen some "interesting" things in my career. It's amazing how the minutiae of even jelly bean components can cause product failures.
A few decades ago (many of my jobs were engineering positions at companies that made equipment for the POTS phone line), the company I was with had some rather large surface-mount ceramic capacitors that were failing short in a new product that used them across the phone line (they were RATED for this application right there on the data sheet). The concern was our product would some day short out the phone line and someone with another phone on the line would have an emergency and wouldn't be able to call 911 to get help. The possible legal liability to the company was obvious. In a discussion with other engineers and managers, someone wondered if it was the way the board was manufactured, maybe the solder process caused some caps to crack, or whether the failures were strictly the fault of the capacitor manufacturer. I volunteered to test the caps in an environmental chamber that wasn't being used at the time, putting them near the edge of, but still within, specs. I made a jig that put a bias voltage on many in parallel (using a mechanical spring connection, not soldered) and left them for a few days/weeks. Out of 100 to 150 devices, about 5 or 10 became unacceptably leaky. I wrote up my report and emailed it to the other engineers and managers involved, and didn't hear back anything more about it. The good news (!) is the product wasn't in production for very long for several reasons, more recently of course that the POTS phone line is no longer the most common form of telecommunications. The bad news is it's no longer helpful to have POTS product design on my resume. More recently, I saw this Kemet presentation on Digikey about tantalum capacitors. Certainly for aluminum electrolytic capacitors, the rated voltage is "the rated voltage" and as long as the capacitor never goes ABOVE that voltage (and has no overcurrent that would heat it up, etc.), the cap is good for its combination of temperature and lifetime rating. I (and as far as I know, everone I've known) assumed this was the same for tantalums, but it appears that's not the case (this presentation mentions several failure causes and shows how they are multiplicative). As you go from 1/2 rated voltage to full rated voltage, the chances of a tantalum failing goes up substantially. The implied rule seems to be for maximum reliability, don't operate a tantalum above HALF the rated voltage. I'd heard a lot of anecdotal things about tantalums suddenly shorting out for this or that reason, but hadn't heard of this, and here it is straight from the manufacturer. https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.