Old tantalum capacitors (solid type) Would you use them? - Page 1
Forum section: electronics / projects

author: cdev
author_rank: Super Contributor
timestamp: October 23, 2018, 12:43:50 am
content: What would be a super conservative approach. What would you do?They are small to medium sized jelly bean shaped tantalums, similar to the ones that used to be used in telephones back in the day, and the higher values weigh a lot.The polarity on themisclearly marked. I got them from the estate sale of an electronics engineer and they are top quality, but likely date back to the 1980s or 90s - more likely 80s.When I have used them they have worked great. I always have derated them a lot.None of them have ever had any issues at all. When I have used them, I have used them at a quarter of their rated voltage or less.

author: helius
author_rank: Super Contributor
timestamp: October 23, 2018, 01:51:19 am
content: They aren't going to be any worse than when they were manufactured, until you solder them—that's what produces the defects. Use a Sn62 solder at ~600°F for as brief as you possibly can.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 23, 2018, 02:23:15 am
content: If they were from the '60s I might worry, but those should be fine. I have a bunch from the same era and have had no trouble with them. Remember to derate them properly and never use them in low impedance power circuits where current isn't limited. That's true for brand new ones too.

author: coppercone2
author_rank: Super Contributor
timestamp: October 23, 2018, 02:40:26 am
content: I have fairly giant military grade hermatically sealed tantalums. They are quite nice.Good idea to put a RC on the enable pin on modern LDO.

author: woodchips
author_rank: Frequent Contributor
timestamp: October 23, 2018, 12:06:16 pm
content: If they have a red epoxy positive end, possibly called Stantalum or similar, then they are silver tantalums worth a significant amount of money from a precious metal re-cycler.

author: coppercone2
author_rank: Super Contributor
timestamp: October 23, 2018, 04:28:38 pm
content: I like them too much. If I make lab equip powered from a lab PSU I always use them as decoupling. I just turn the I limit down when connecting them. I feel like my equipment is very safe because they are humungousI would use gold braze if I could afford it. I like nice shit

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2018, 06:27:55 pm
content: Not a problem. If you like, you can test a statistical sample, measuring leakage current or noise* under bias, and before and after soldering. Go up beyond rated voltage if you like, find typical failure points. Maybe with temp cycle, too. Would be good practice just to understand their behavior, really.*Noise is due to self-healing, where a partial short or discharge occurs, locally heating the electrolyte which decomposes it to an insulating compound. Catastrophic failure occurs if this happens too rapidly on a low-impedance supply, driving runaway self-heating, triggering a thermite-like reaction.Tim

author: jmelson
author_rank: Super Contributor
timestamp: October 23, 2018, 07:17:46 pm
content: Quote from: cdev on October 23, 2018, 12:43:50 amWell, I would NOT. I have been maintaining military and commercial gear for 40+ years, and had a LOT of Tantalum caps go bad. The worst case is gear that is used for some years, then put on a shelf for SEVERAL years, and then plugged in again. Suddenly charging the caps tends to blow the dielectric. I'm guessing that a gentle reforming of the dielectric might be OK, but the sudden application of the DC supply is too much. Sometimes these burn in a rather nasty way, too. In the same time, I have had VERY FEW Aluminum electrolytics blow, and almost NEVER in such a dramatic way. Sometimes I've had Aluminum electrolytics on one-shot timing circuits get leaky and have to be replaced.And, those dipped-epoxy tantalum caps seem to have the worst failure record. On the other hand, if you reform them with a series resistor, and then put them in equipment that is always left on, they may do fine.Jon

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 23, 2018, 07:30:19 pm
content: IMO, if they weren't reliable, they weren't used properly. Tantalums are known to fail due to inrush current. They're great for timing and filtering in moderate impedance circuits, but very risky for PS decoupling unless there's some series resistance. Here's a good place to start:https://www.avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: October 23, 2018, 07:39:29 pm
content: Operating voltage/voltage derating –FUfactorindeed it is a FU factorI found one bad tantalum in a wavetek pulse generator before. Fucker shorted out. But compared to the # of bad electrolytic I found, I think they are pretty reliable. It was not blown up or anything weird like that though. But it was one of those epoxy coated corn kernel looking ones.

author: cdev
author_rank: Super Contributor
timestamp: October 23, 2018, 07:48:09 pm
content: What I have done is used the current limiting on my bench supply to apply a low voltage with current limited to just a few ma for a while. (how long is appropriate, though?)Just because I've seen that suggested here. None of my capacitors have ever gone bad but I am still wary of them. Ive never seen one pop in real life, only on YouTube.I have thought about bringing my bench supply outdoors along with some of these tantalum caps to see under what conditions they fail, and documenting it.Until I feel totally confident they will literally never fail under the conditions I use them at, I think I'll still use electrolytics in everything for now.

author: nctnico
author_rank: Super Contributor
timestamp: October 23, 2018, 08:53:30 pm
content: Quote from: Conrad Hoffman on October 23, 2018, 07:30:19 pmI agree. Over the years I have replaced a lot of tantalums which failed as a short. That is one of the reasons I never use tantalums (the others: conflict minerals and environmental damage due to mining). Also if you connect them wrong they can explode violently. Fortunately I wear glasses.

author: cdev
author_rank: Super Contributor
timestamp: October 24, 2018, 12:09:25 am
content: This sounds like a useful test and guaranteed to be educational. How would you do it? I have the usual beginning/midlevel hobbyist tools. I could probably rustle up a chart recording tool. Is there any standard testing protocol?Quote from: T3sl4co1l on October 23, 2018, 06:27:55 pm

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2018, 12:51:30 am
content: The description suggests the protocol -- if I were doing it, I'd be interested in the short-term (self-healing) noise voltage, and the average leakage current, with respect to time, voltage, temperature, thermal history and thermal cycling. (That's a lot of variables, yes; it would be very involved to test all of them exhaustively, all the while taking a statistical sample of components! Just a few combinations would be fine.)The noise measurement would look very much like a Geiger counter: the expected mechanism is impulsive, so amplifying the AC voltage, and logging the magnitude of the impulse, and rate (presumably following Gaussian and Poissonian statistics, respectively?), and how they vary with various conditions, would be interesting.And, actually logging that data as such (say, magnitude and time stamp), would be a good random number generator, but not very useful given the amount of effort required to collect it (essentially a photomultiplier spectrometer -- an impulse size and rate counter, but slower), so simpler methods could be used, like taking the AC RMS voltage over a suitable duration (it might not be something you can measure with a TRMS DMM, but an RMS converter chip and a minutes-long filter would do).It would then be interesting to correlate noise with DC leakage, and DC leakage with time, and DC leakage or integrated noise against capacitance change (if any). Capacitance change is quite pronounced in self-healing film caps, but probably not that strong here? Would be an interesting hypothesis to test.Standard testing -- and expectation -- for capacitors in general, is applying a voltage via current-limiting resistor, and monitoring the leakage current over time. There will always be a long time constant due to dielectric absorption (which may not be a time constant at all, but a 1/sqrt(t) -- diffusion -- dependency), but there may be additional effects at work. They can be detected by teasing apart the different responses stacked on top of each other, assuming simple RC time constants, and/or a diffusion element.Presumably, there will be some reforming or self-healing effect, i.e., the leakage starts high then falls gradually, at a given voltage; subsequently, that voltage (or below) will retain a low leakage level, but raising the voltage higher causes more leakage, until that level is "formed", and so on.The test can be accelerated somewhat, by using a low impedance driver. The current should still be limited to low levels, but the dynamic impedance (change in voltage / change in current, for small changes) can be quite low. A good example is an op-amp follower, set for a constant input (and therefore output) voltage, and measuring its output current. That way, as soon as leakage is less than the current limit, output voltage will be forced to exactly the setpoint, eliminating the potentially long or compounded time constant an RC test has. Downside: output noise voltage isdifferentiatedinto noise current (Inoise = C * dVnoise/dt), making this a difficult method to combine with an AC noise test (depending on how much excess noise the DUT has, relative to the amp).But yeah, the basic test would be logging leakage current (hopefully microamperes) into a plot, for a bunch of individual parts, and raising and lowering the voltage stepwise to see the various effects. Then the temperature stepwise, and so on. Take as many steps as you like, each one as long as needed to verify the time constant(s).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 24, 2018, 01:16:24 am
content: How does this self healing stuff factor into real reliability?

author: Gyro
author_rank: Super Contributor
timestamp: October 24, 2018, 09:41:16 am
content: I'm really not sure that dry solid Tantalum capacitorshavea self healing mode. It's more a case of which weak spot fails first and whether there's enough thermal energy for it to propagate to device meltdown.Personally I have a theory that old non-hermetic bead tants suffer degradation due to moisture ingress in storage. It would certainly explain why so many have a tendency to fail in the hours and days after old equipment is powered up, when they presumably originally worked without problem for years.Hermetic axial dry (and wet) tants seem to last for ever.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2018, 01:58:08 pm
content: Yeah, that would support the idea of reforming.Appnotes, at least, describe the process as: a momentary short, causing the MnO2 electrolyte to heat up and decompose to Mn2O3, an insulator. The permeability of MnO2 for H+, OH- or H2O is probably not nonexistent, and that would have some relevance in storage. A precaution might then be to treat them like moisture-sensitive SMDs: bake at elevated temperature before applying power.This should be measurable as an increase in the reforming voltage... unless the thermal cycling itself causes more trouble.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 24, 2018, 06:54:14 pm
content: would you bake, cool down, then power, or would you bake, power, turn off heat, then slowly decrease current to cool it down?if something is reforming, I get the idea its like, squishy or plasticky or something, so it might work better if its warm to start with, if stuff has to migrate around with less stress.. i.e. gas diffusion might be higher in hot parts so there is less pressure build up

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 24, 2018, 11:39:56 pm
content: If I see any measurable noise caused by a tantalum cap, it goes straight to the bin. Ditto for DC leakage anywhere near the datasheet values.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 25, 2018, 02:43:04 am
content: The "sudden failure" mechanism of tantalum capacitors is referred to as Field Crystallisation in this paper:https://www.researchgate.net/publication/27357654_Failure_Mechanism_of_Solid_Tantalum_CapacitorsThis paper seems to indicate that field crystallisation requires a voltage to be applied in order to happen. - This sounds like good news for anyone with a stash of old tantalum caps!

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2018, 03:55:29 pm
content: AVX has apaper about field crystallizationas well. But the various papers I have read all agree that it only applies to higher voltage (25 volts and greater) parts and I am not convinced that proper derating would not allow self healing to handle these defects.I like T3sl4co1l's idea about monitoring noise; I will have to try that. So far I just burn NOS (new old stock) solid tantalum capacitors in for a couple days while monitoring leakage current as shown below.Epoxy packaged solid tantalum capacitors (but probably not hand soldered leaded parts) suffer from stress induce defects when heated during soldering and perhaps over the long term with humidity changes. These defects can be cleared after soldering during a separate burn in stage. Check out this video this video from Kemet at about 34 minutes:Also, is there any way to include a video link without embedding it???