*gasp* Useful EDN article on tantalum caps

Tantalums fail from excess current, namely dV/dt. MnO2 is an oxidizer and tantalum is a fuel, and only a tiny ignition source sets it off. They are fine for current-limited applications but firebombs if used to bypass power rails. Derate them 3:1 on voltage if you must use them as power bypasses. Better yet use aluminums, polymer aluminums for wide-temperature appls

John

The interesting thing in the article is that soldering makes large tants vulnerable to overvoltage failure well below their rating, and that they can be reconditioned (and made much more reliable) by putting a current limit on the power supply. That's more useful for repairs or fixing problems with a shipping product than for new designs, admittedly. Still, I thought it was kind of cool, and remarkable for being in EDN!

Cheers

Phil Hobbs

Serious non-commercial technical content is a good thing for EDN to do now and then!

But tantalums are incredibly erratic, which is why different people form different theories about their failure mechanisms. My worst experience was with thru-hole gumdrops, hand soldered, which hadn't been exposed to much heat. I think I still have a few thousand around in a box somewhere.

I sometimes use tantalums as switching or linear reg output filters, because the ESRs are often right and aluminums don't work cold. But I try to limit dV/dT and derate about 3:1 on voltage.

The manufacturers mostly try to hide the failure mechanisms. One tant maker recommends that they *always* be voltage derated 2:1. So why do they call it a 25 volt cap is it's only safe to use at 12.5? Grrrrr.

ftp://jjlarkin.lmi.net/P14_reg.gif

C1 is a tantalum, and C40 limits its dV/dT.

I *have* exploded tantalums on the outputs of LM1117s.

John

Phil Hobbs wrote in news: snipped-for-privacy@electrooptical.net:

TEKTRONIX used tantalums for many years in their scopes,and the failures were not objectionably high. Usage was mostly on power rails,BTW.

yeh luckly I were glasses, I have a scratch one glass from an exploding tant on the output of a switching regulator

-Lasse

Yeah, except that they also used mostly mil spec components and production methodologies back then as well, and one (an engineer) could rely on the fact that they had been thoroughly conditioned before use on the production line.

A hermetically sealed mil EL cap has a shelf life of decades. A COTS EL cap does not. There are several mechanisms that are/were in place during their manufacture to ensure that reliability level on those specialized caps, even though they differ little at the physical/electrical level... ie they are both paste laminated foil roll-ups.

Gaze at circuits that you just 'fixed' as you re-energize them often, do ya? (you likely cause their failure with poor solder technique)

I tend to gaze at the input current numbers. I do not have such catastrophic failures often, as with proper bench technique, the failure modes can almost always be spotted prior to any such actual event occurs.

Wet-slugs don't detonate like MnO2 dry tantalums. The MIL CLR-types are very reliable. They have silver cases, so have gotten expensive.

I don't think these are roll-ups. They are a sintered tantalum slug floating in a conductive paste, in a silver can. The semi-liquid paste must put the fires out.

John

That might make a good argument for a properly thought out burn in procedure. There's too much pressure from management to just build it and ship it (airplanes included).

. snipped-for-privacy@electrooptical.net:

In the late '70s Tek decreed that its design engineers should only use tantalums with explicit series resistance (3ohms/volt IIRC), and enforced it with the design review process. Direct power supply bypassing was a no-no. This was after a lawsuit stemming from a fire started by a Tek product took out a large mobile lab. I think it was around that time that Tek also got much more serious about ensuring that connector bodies were flame retardant, and started worrying about their switch cams with all its flammable delrin.

Maybe the rate of failure was not "objectionably high", but when the consequences can be serious (as with the tantalum self-immolation, or with a certain current oil leak), you have to consider implications beyond your own immediate product.

-former Tek design engineer

They could have reduced the text body to eight letters:

:-)

We've had tantalums fail months or years after a product was shipped. On one of our VME modules, which we do burn in, well over half the field failures were tantalum caps. Field MTBF increased by about 3:1 when we fixed that problem.

This is more an engineering issue than a test/burnin thing. Dry tantalums have to be designed in very carefully, and the manufacturers mostly don't want to tell you what's safe or not safe.

John

Sounds just like LDOs :-)

Lots of LDOs need tantalum output caps to be stable. It's a conspiracy.

John

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How do polymer caps stack up?

G=B2

Assuming that's a serious question, and not a terrible pun, they're great, but ESR is so low that some LDOs won't like them.

They don't flame like tantalums, don't freeze like aluminums, and I'm not sure about long-term reliability.

John

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Z is even better. I'd like one more hour of Z but I have to service the pellet stove today ...