Using tantalum capacitors with zero voltage margin - Page 1
Forum section: electronics / testgear

author: octillion
author_rank: Contributor
timestamp: May 09, 2020, 11:38:49 pm
content: I just repaired two lock-in amplifiers by EG&G PARC (models 5207 single channel and 5208 two phase of slightly different vintage), and both had a few shorted tantalum capacitors on the +/-15 V rails. The capacitor voltage ratings on these rails are only 15 V in one unit and 16 V in the other. After replacing a couple shorts in each unit, each unit had one more fail after about 30 seconds. It's almost comical.I chose replace only the failed tantalums, as there are quite a number, and there have been no more fails after about an hour of burn-in. The +5 V rails have 6 V capacitors, which I'm less concerned about.What were they thinking? Was it/is it acceptable to rely on the component manufacturer to provide enough safety margin to allow usage right up to the rated value? This isn't 2nd tier equipment designed by morons, but there clearly is an issue here.

author: maginnovision
author_rank: Super Contributor
timestamp: May 09, 2020, 11:49:16 pm
content: Tantalums didn't use to be derated, there was "no need". Now the recommendation is to derate. As things fall out of tolerance and age those tantalums that weren't chosen with proper voltage derating are more likely to fail.

author: David Hess
author_rank: Super Contributor
timestamp: May 10, 2020, 12:14:10 am
content: Solid tantalum capacitors were originally advertised as not requiring voltage derating.

author: CaptDon
author_rank: Super Contributor
timestamp: May 10, 2020, 07:01:36 pm
content: Sadly it seems the aged tantalums failing is an almost daily occurrencein older test gear even when they are running in a sensible fashionlike 12vdc on a 15 or 16vdc rated tantalum polarized capacitor. Thosethings were failing on a daily basis since new, they seemed to be plaguedwith infant mortality issues from day one. I worked in the video gameindustry for many years and the 8080A based Midway games like Gunfight,SeaWolf and so forth often had shorted tantalums in the memory area.They used tri-voltage D-Rams. We would apply about two volts at upto around 10 amps on each rail one at a time and the shorted tantalumwould explode revealing which one was the bad guy!!!

author: Gyro
author_rank: Super Contributor
timestamp: May 10, 2020, 07:18:16 pm
content: My pet theory about Tantalums failing on old equipment after a period of storage is moisture ingress. The failure rate definitely seems to drop off after boards have been run and warmed up nicely for a while. Unfortunately, after another long period of storage, the cycle tends to repeat, until you've replaced them all.They definitely didn't live up to the zero derating claim though. Even back in the '80s I remember a company edict coming round for our military radio gear - "Thou shalt include a series resistor of (I think, 1 or 10R per volt) on the supply before the Tant".

author: helius
author_rank: Super Contributor
timestamp: May 10, 2020, 07:37:38 pm
content: Today tantalum caps are available with polymer electrolyte (Sanyo née Panasonic calls them POSCAP, by association with the polymer aluminum OSCON). The polymer doesn't runaway thermally and ignite like the older manganiese dioxide electrolyte. So it is possible to use those with "zero derating"; the issue is that if your circuit actually does have voltage excursions above the intended voltage, that does still cause damage.

author: maginnovision
author_rank: Super Contributor
timestamp: May 10, 2020, 07:48:51 pm
content: As far as I know derating is still recommended.https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2020, 09:10:30 am
content: Quote from: Gyro on May 10, 2020, 07:18:16 pmThat is consistent with the problem only occurring with epoxy packaged parts. When thermally cycled during soldering, stress induced in the tantalum pellet causes new defects. I suspect the same happens over time from changes in humidity which affect the epoxy used for packaging.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: May 11, 2020, 10:33:58 am
content: And just yesterday I powered up a WaveTek 148 that I bought some years ago.You guessed it - shorted tantalums.After I replaced 2 of them (all rail shorts gone), I repowered the unit and kablam, more shorted out.I did check the rails with a scope and no over-volt, spikes or ripple was noted.Checked the supply rail voltages and they were slightly below 15V.The tantalums were rated at 15V but still blew.I replaced them with 16V ones, 7 in all.They have not spoken up since.

author: garrettm
author_rank: Frequent Contributor
timestamp: May 11, 2020, 10:56:20 pm
content: I just had a solid tantalum in my Fluke 81 function generator go out the other day. Very similar situation, 16V solid tant on a 15V rail. Replaced it with a 25V solid tant for good measure. Still better than an electrolytic, but the fail-shorted nature of tantalums can cause extraneous damage to traces and regulators unless current limited. Thankfully, when derated to 80% or 50% they seem to last 30+ years. Especially the wet hermetically sealed tantalums (axial metal case with glass or epoxy seal). I just pulled one of these off a scrap HP board circa 1974 and the Q is around 80 at 120Hz and still within 5% of the stated value. Pretty amazing for an electrolytic.Quote from: helius on May 10, 2020, 07:37:38 pmI don't know if commercial parts do this, but the mil spec variants have a "surge rating" so you can run them at rail voltage and still be relatively safe against voltage surges/ringing. But I'd still derate to 80% just for the increased MTBF.

author: garrettm
author_rank: Frequent Contributor
timestamp: May 11, 2020, 11:12:31 pm
content: Found a pretty good site talking about reliability and wearout mechanisms:http://my.execpc.com/~endlr/reliability.html

author: maginnovision
author_rank: Super Contributor
timestamp: May 11, 2020, 11:19:47 pm
content: AVX and Kemet have plenty of information on tantalums if people actually care. Just a google search away.

author: cdev
author_rank: Super Contributor
timestamp: May 11, 2020, 11:34:28 pm
content: I keep thinking to myself that in order so that I have a good idea of what can happen if they fail that I should 'sacrifice' some tantalum caps outdoors, deliberately test them, make them fail so I can see what the relative risk is. Suppose I wanted to make them fail, what would I need to do, simply give them a voltage way above their rated voltage, and make a note of what it is and what the failure current let through is?Is there any standard testing protocol?

author: maginnovision
author_rank: Super Contributor
timestamp: May 12, 2020, 12:38:38 am
content: You could over voltage, uncontrolled surge currents, reverse polarity... All you'll get is a small mess but have a blast.