Tantalum cap failures

I don't understand how voltage ratings for tantalum caps are derived. Do they just ramp them up to 25V over 5 seconds, observe an absence of smoke for 5 more seconds, and stamp them "25V"?

Why isn't this considered a big problem by the manufacturers, if not the users, of these parts? It's getting to be the most-F of all FAQs: "why are my tantalum caps dying?"

-- jm

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This particular range of caps only goes up to 63V. Also space is at a premium, so increasing rated voltage would probably be at the cost of capacitance.

There is a 20 year shelf-life requirement on the product, so we can't use electrolytics. One way or another, our choices seem pretty limited.

Ted

I take your point - the extent of Arcotronic's response has pretty much been simply to confirm that returned caps have indeed gone s/c.

Ted

Tantalums hate AC ripple current. It causes local heating of tiny bits of the sintered pellet. If the contact media is MnO2, which standard solid tants are, the tantalum becomes fuel and the MnO2 becomes oxidizer and it explodes. It's an instant-ignition effect, not slow heating. It's very erratic from mfr to mfr, or even between lots.

Replace them with aluminums, or niobiums, or with the new polymer tantalums, none of which has the same detonation mechanism. The polymer tants have very low esr, too.

I'm selling VME modules to BAE, for a critical radar application, and we used all aluminum caps! I only use tants in slow RC-type circuits, never as power bypasses; I've learned that lesson!

John

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Tantalum failure modes and mechanisms are well-known and well-documented. Among the root causes are mechanical stresses during manufacturing, handling, soldering, and handling after installation. You can reduce the probability of failures by specifying 'surge tested' parts. Some datasheets and some customers specify a minimum series resistance between low impedance power and Ta caps. Most designers who have experienced the failure problems design them out. Unfortunately, high value ceramic caps are also subject to spectacular failures, especially if working voltages are high enough to sustain arcs. The surface mount types are especially susceptible to cracking due to mechanical stresses in soldering and board handling. As others have suggested, other cap technologies are available, and it's possible that you can get away with less capacitance, particularly if you minimize inductance in the connection of the parts. Paul Mathews

In similar situations, some makers recommend using capacitors derated to 1/3 or so of rated working voltage, so in your case about 100V. That may not be possible for you. Perhaps you have to look at using a different type of cap.

Best regards, Spehro Pefhany

Ceramics are getting close. You can get 22uF/50V X5R from TDK, though they are expensive. 10uF/50V is more reasonably priced, but you'd need a pack of them to get the same capacitance.

Best regards, Spehro Pefhany

Hello John,

The recommended derating alone can make tantalums unattractive. See #5 about low impedance apps which is in essence a high ripple situation:

Same here. Another hard lesson many of us had to stomach was similar to the energy crisis. Suddenly many of the tants were "on allocation". Distributor-speak for "can't touch this", maybe unless you were the big shot executive of a fortune-10 company.

Regards, Joerg

Hello Edward,

Thing is, if you don't properly derate tantalums that are exposed to lots of ripple they'll keep failing. Sometimes they do that with pyrotechnic gusto.

Check out whether the Vishay TVA ATOM series (formerly Sprague?) suffices. You will need to contact them as the data sheets are kind of skimpy. There may also be suitable mil spec versions as this stuff has to have long shelf life. Possibly John could suggest some.

From my lab stock of quality electrolytics I have yet to see one that has aged out. When I restored an old Astor Radio from 1959 I checked the electrolytics for leakage and capacitance. As good as new, plays like a champ. I did see electrolytics that passed away after 30-40 years but mostly because some engineer had placed them right next to the rectifier tube.

Regards, Joerg

Hello John,

Unless this is confidential or they cost an arm and a leg: Which aluminum cap series did you use?

Regards, Joerg

We're using a 63V part at a nominal supply voltage of 28V, but would have used a 100V part, as suggested by Spehro, had they been available.

The ripple current rating of these caps appears marginal, (2.7A rms per cap @ 20C), when assessed in terms of the peak ripple requirement of

So far as dV/dt and dI/dt ratings are concerned, I can find no figures for either on the data sheet, other than reference to charge/discharge at rated voltage through 0.5 ohms for 1E6 cycles. We have however modified our test regime, so that changes in supply voltage occur more slowly, (~1ms time constant), than was originally the case, It's worth noting however that some failures have occured when winding the supply voltage up to the new value on a bench power supply, so dV/dt was not an issue in these instances and all evidence points to the problem being voltage related, albeit possibly brought about by some other mechanism.

We have traditionally been prevented from using electrolytics in the military environment, due partly to our 10 year shelf-life shelf-life requirement, (not 20 years as I stated earlier in the thread). I will have to check with the powers that be regarding the possibility of using electrolytics. Clearly, the current level of failures is unacceptable and we are going to have to do something to address it.

Ted

The issue of tantalum caps regularly appears here. Even used for supply decoupling they have an alarming tendency to expire ( usually with some combustion ).

Derating the voltage seems to help.

Have you checked the ripple current rating ? Remember that ripple current is higher than the DC load current.

Have you checked the dV/dt - dI/dt capability of the caps ?

I wouldn't touch tantalums with a bargepole personally. Is there any reason you don't want to use low ESR aluminium electros ?

Graham

Have you looked at solid polymer? They seem to have much lower ESR than tantalum and better life than ordinary electrolytics. They also fail open circuit instead of short. Here's a Cornell Dubilier article from 2001 that describes the construction and failure mechanisms:

All the major cap manufacturers supply them now, so there should be plenty of data on reliability and product lifetime.

Mike Monett

Solid Aluminium?

SIC Safco or Rifa (wet) aluminium have some parts that are rated for a very long life.

Graham Holloway WPS/Accuphon Electronics

Panasonic ECE series, mostly. Since aluminums tend to fail by drying out and slowly losing capacitance, it's easy in most cases to just grossly overkill on capacitance and voltage rating and buy reliability that way. VME modules allow about 0.6" of component height, so a surface-mount aluminum can use the height and provide a lot more CV product than a flat tantalum.

We've had a number of products fail from tantalum shorts - they are the least reliable part we've ever used - but none from surface-mount aluminum elec failures.

John

can you store the energy at a higher voltage using alum cap then use a regulator to create your desired output voltage.

the engery is 1/2 C V^2 so you get better space efficeincy at higher voltages.

In other words at a higher voltage you need less CV and CV is prop to space

Mark

snipped-for-privacy@baesystems.com wrote: (snip)

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Have you considered a prescreen voltage test that blows the bad units up before they are attached to the boards? Perhaps you can design this test in cooperation with the maker, and get replacements for all the units that fail, since you are not exceeding their rated limits and would just be doing their quality control for them.

Hello John,

Thanks for the info. We have used Panasonic caps a lot as well. No problems. I also like their ECH and ECHU series when film caps are called for. Oversizing in capacitance is indeed the thing to do.

The only significant failures I ever saw from aluminum electrolytics was when people placed them too close to hot stuff, like processors, regulators or heat sinks. Even then they usually go gracefully, not 4th of July style like some tantalums.

Regards, Joerg

Hello Mike

I've had a look at solid polimers and, at first glance, they look very promising. However, their expected life is only 10 years or so, and we have a contractual requirement for not less than 25 years. (How realistic that really is is another matter, but that is what we are obliged to aim for).

Ted