How can I Determine the Breakdown Voltage of Recycled SMD Capacitors? - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 16, 2023, 06:45:19 pm
content: I recently purchased a heat gun for desoldering work and find that I can get a pile of components to fall off a salvaged PCB board using it. In particular,I am interested in reusing SMD epoxy encapusulated rectangular >.1uF valued capacitors.I plan to repurpose these salvaged caps in doing repair work or even in experimenting with circuits and making new PCB prototypes.This recycling idea is both convenient and economically interesting to me because these caps are now in short supply and fairly expensive and not easy to find at the local Wallmart at 2am.Some salvaged are no doubt tantalum, and although their capacitance in uF is clearly marked on most of them, the maximum operating voltage remains a mystery for most of these salvaged parts because it is just not printed or else printed in some unfathomable code on the cap.I can easily determine the actual capacitance of a cap at a low V by inserting them into a R-C single op-amp sawtooth-wave oscillator and my MCU code automatically displays on 7-seg LEDS the capacitance derived from the freq. of oscillation.I also seem to find that polarity of the caps can't always be precised by the line on one side.For instance, some (orange colored types), there is a distinct + sign, for others (i.e. black case with a white line) I have traced the installed mounting of the capacitor to find out that the line at one end is the negative connection, but in other cases I find that the line indicates the positive terminal!My Questions:1) Is their a way (non-distructive) to determine the polarity and breakdown voltage?2) How does a capacitor behave in terms of leakage current at a voltage just below breakdown voltage that would possibly cause it to short-circuit? Do caps short-circuit and then heal if the voltage is allowed to rise slowly to exceed their rated MaxV using a series resistor?3)How much greater than the specified published maximum operating voltage is the breakdown voltage?If I knew this I could determine the safe operating voltage of the salvaged cap.4)In short, how can I posit the correct positive polarity and safe op V of a questionably-marked SMD cap in my nearly filled recycled capacitor bin?

author: Someone
author_rank: Super Contributor
timestamp: February 16, 2023, 11:49:52 pm
content: Quote from: SuzyC on February 16, 2023, 06:45:19 pmNoQuote from: SuzyC on February 16, 2023, 06:45:19 pmUnknownhttps://www.eevblog.com/forum/projects/mlcc-discharge-curves/msg735041/#msg735041

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 17, 2023, 02:54:39 am
content: I have copied and posted the graphs attached to the the linked topic above. Many Thanks to Someone!It seems to me that the top graph, leakage plateaus at the highest voltages at a constant current charging, this can only mean the leakage current is increasing or the capacitance is increasing.In the case of the bottom graph, charging versus leakage current seems to decrease with applied voltage and in the top graph, at the plateau point the voltage plateaus with a constant current. Thus, the two graphs contradict each other about leakage current.I donno what to make of this??But taking the info shown on these graphs a little further into consideration, if I apply a bias voltage to a capacitor in a R-C single op-amp triangle wave oscillator I should see the frequency rise with applied bias voltage above the nominal working voltage if it is not a tantalum cap.Based on an observable doubling in frequency, could I then determine the type and working voltage of any >.1uF cap and find out if it is an XLR rather than a tantalum? This determination, of course based on comparison to known values of purchased capacitors.It would seem, with a >.1 uF capacitor, there exists and can be found a certain catastrophic breakdown voltage area on a charging curve, and at this point the sudden discharge of energy in the cap should cause the capacitance to decrease from damage/disconnection of the metalization, and even if the cap heals itself, this post-traumatized cap will reveal its new cap value at a low voltage in my test setup I am using (R-C triangle op-amp Osc)?I would, of course, use safety glasses for this type of breakdown voltage test because a tantalum cap could possibly explode since the energy in Joules stored in the cap prior to catastrophic failure = (CV squared)/2 might be enough to shatter the epoxy case?I am also surprised to discover that the leakage current observed with some >.1uF caps marked with a line on one end the SMD case seems to be the same regardless of voltage polarity!

author: Someone
author_rank: Super Contributor
timestamp: February 17, 2023, 04:25:19 am
content: Quote from: SuzyC on February 17, 2023, 02:54:39 amThe relationship between the capacitance vs DC bias, or leakage vs DC bias are not unique to specific capacitors. There is no 1:1 mapping from measured characteristics and the specifications:Quote from: Someone on February 16, 2023, 11:49:52 pmEven if you have a continuous curve of the capacitance vs DC bias characteristic, different voltage rated parts will appear identical.Quote from: Someone on December 02, 2016, 12:40:01 am

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2023, 04:53:56 am
content: the solution is to go onwww.digikey.com, search for capacitors, and buy new ones. If in doubt, throw it out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2023, 05:27:50 am
content: The voltage rating is almost certainly higher than the -30% (or whatever cutoff you would like) point on the C(V) curve, and more to the point, you most likely don't want to use it above such voltage anyway. So that's fine.More importantly is the tempco, is it X5R, X7R, Z5U, etc.? Many are available; these three are only the most common [type 2 dielectrics]. You will have to test at high temperature to see how it drops off, and by how much. Easy enough with a hot air machine, point at it during the test and there you go. Not so easy with a heat gun or soldering iron: the temperature is far too high so you can't know how hot you're getting the part; going from ~300°C down to somewhere specific in the 85-150°C range is nontrivial.Note that C0G has no tempco and only breakdown to limit the operating voltage. I would assume most picked parts would be up to 50V, in 0805 and below -- good enough for signal purposes. Maybe make note of where they come from; snubber capacitors in SMPS may be high voltage, even in 0805.You can always "hi-pot" them; if they don't fail at a given voltage, it'sprobablysafe to use them at or below, say, a 20-50% derating from that voltage. What you don't know is how many will fail at a given voltage. Have fun.Most important of all, if you have any kind of employment, it's literally not worth your time to pick and characterize such things. And that includes what you may learn from testing; I suppose unless you're a very hands-on type, there's little you can learn from testing that isn't covered by a handful of appnotes.(Conversely, if you aren't employed in those hours... go right ahead, have some fun picking boards apart!)Tim

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 17, 2023, 07:22:20 am
content: Economic practicality and patience are off-topic!Thanks T3sl4coLi for giving me some idea of what I could do to identify the characteristics of caps in my pile by simple tests.I don't (ever never) need caps that are guar. to work over -40 to +150 C, while a hair dryer will safely divine temp stability in seconds.Working on a project, I hate having to waits days to months to get (maybe one) specific part (in this case a SMD cap). This causes me to loose concentration and fill up my workbench with unfinished project setups.Definitely do not want always to wait days(DigiKey), weeks or mths(Ebay China..good prices) to get the SMD caps I need at once, while spending big bucks and filling a half-dozen parts bin cabinets with values or sizes that I may never find a use for.On the other hand, it is worthwhile while working on a project, that I could spend some minutes with experience using a simple test setup (incl. hair dryer. P/S, ESR meter, scope, my triangle-wave osc/MCU augmented cap val meter) to get something in SMD size and temp, ESR and other characteristics that I need from my big bin of free(except for time) parts.Don't forget that it takes real wage working time to earn the money lost searching online as well. It takes hours to search for and then do a lot of thinking/value judgements, to best buy a (optimal number and priced) good assortment of parts and then unpack maybe a zillion no-static plastic bags, only then to spend hours to organize these small piles into labeled storage bins, only to realize yrs later that most were not the most popular values/sizes/char needed and are just gathering dust.Can't get what I need when I want it with a visit to Wallmart at 2am!

author: Echo88
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:05:13 am
content: I would suggest a smd capacitor sortiment, bought once it will suffice for a few years and will be neglegible in cost if ordered from Alibaba/similar.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 17, 2023, 11:44:49 am
content: Quote from: Echo88 on February 17, 2023, 11:05:13 amPlease don't?

author: Haenk
author_rank: Super Contributor
timestamp: February 17, 2023, 01:16:49 pm
content: Depending on the age, it is possible that a lot of the salvaged caps are just trash and not up to spec. You really don't want to use 20 year old, used electrolytic caps.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 01:44:53 pm
content: Quote from: Ice-Tea on February 17, 2023, 11:44:49 amActually no reason not to, unless you're manufacturing a product for military, healthcare and the like.For hobby and prototyping they are perfectly fine and very likely much better than parts salvaged from dead boards. Order once a set of N values 100 pcs each, which will arrive in a couple weeks, and they'll be cheap and last forever. Yes the 10uF ones will likely end up measuring 7-8uF, but that's fine too: your design must tolerate for that. Precision parts is a whole different topic, and you dont normally use salvaged parts for this anyway.For a commercial product it's probably better to order parts from a reputable supplier.As for the original question, a capacitor of a particular type and value in a given form factor will very likely have a typical rated voltage. Just check a few datasheets for known components and then it should be more or less safe to assume that your salvaged parts will have a similar rating. Don't use in critical circuits, since who knows how they degraded from being used and desoldered.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 01:46:10 pm
content: Quote from: Haenk on February 17, 2023, 01:16:49 pmAnd yes, this ^Ceramics should be fine though. Usually.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 17, 2023, 01:59:45 pm
content: Quote from: shapirus on February 17, 2023, 01:44:53 pmAre you kidding me?? You would make a commercial product with Alibaba components?QuoteTo each their own, I guess. But if you've spend some hours or days chasing after ghosts that end up to be bad eBay or Alibaba components, there's a good chance you'll decide it's not even worth your hobby time, much less proffesional..QuoteProbably???QuoteNo offense but that is just terrible advise.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 02:24:03 pm
content: Quote from: Ice-Tea on February 17, 2023, 01:59:45 pmIf it gives more profit in the long run, even despite the possible warranty replacement/refund cases and the estimated reputational impact, then why not. Commercial products are always made to maximize profit, so you pick whatever option works best for this.Quote from: Ice-Tea on February 17, 2023, 01:59:45 pmRemember, the original problem was finding a particular capacitor for repair/experimenting/hobby which is not available at a local store in the middle of the night. This is what an assortment of components conveniently ordered a few months ago from AE solves very well. But then again, you can order the same set from a local supplier, if the price is okay.Now, from my personal experience, there's nothing terribly wrong with those unbranded Chinese capacitors or other parts (with some caveats of course). Yes the actual cap values can be somewhat off, but it's not a problem per se, and they're most frequently actually within the +/- 20% tolerance, better than which you'd not expect from such parts anyway. Precision parts, as I said earlier, is a different matter.If in doubt, test a few specimens from each batch for their breakdown voltage before and after heating, measure their capacitance to know what to expect.Quote from: Ice-Tea on February 17, 2023, 01:59:45 pmDepends on the calculated financial and reputational outcome, as I mentioned above. If, for example, you're making a disposable battery-powered toy, then you don't need top quality components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2023, 07:12:02 pm
content: Quote from: Ice-Tea on February 17, 2023, 01:59:45 pmGiven their flag, the reduced labor costs might justify more assembly or component testing, which might sort out wholly incorrect values, perhaps even incorrect types (e.g. Z5U where X7R is required).One must mind, this is not true in general; there are parts of the world where replacing whole boards is economical (even when one has the equipment, experience, and replacement components, to do otherwise). There are also parts of the world where OP's activity is indeed economical (well, maybe more just the harvesting and sorting, who knows about testing). Which is, honestly, a bit terrifying.. but, so it is.Tim

author: switchabl
author_rank: Frequent Contributor
timestamp: February 17, 2023, 09:27:20 pm
content: I would be wary of re-using MLCC capacitors in particular. They have a nasty habit of developing cracks when exposed to excessive mechanical or thermal stress, and eventually failing short-circuit as a result.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 11:05:28 pm
content: Quote from: switchabl on February 17, 2023, 09:27:20 pmWhich, when accounted for the concerns listed in the previous posts, makes it boil down to "reusing pretty much any SMD capacitor is inadvisable". Which is no surprise, after all :).

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2023, 11:10:14 pm
content: if you make some thing complicated even as a hobby if there are bad parts there it could waste TONS of your time. If you only make like... small projects that are simple then yes, but I am kind of thinking, I wonder how many wasted days are acceptable before you decide that buying specified capacitors is worth while. I already feel bad if I waste one weekend because of some stupid defect. Like I can see someone losing alot of their time dealing with shady parts. So many other things you can be doing.

author: switchabl
author_rank: Frequent Contributor
timestamp: February 18, 2023, 12:02:43 am
content: Quote from: shapirus on February 17, 2023, 11:05:28 pmWell, generally yes and personally I wouldn't do it systematically (but sometimes we all do what we have to in order to get the job done in time). But while electrolytics for example may degrade over time, it is fairly unlikely that a seemingly good one will just randomly short out your power rail. I've actually seen that happen with an MLCC capacitor, damaging the PCB in the process.

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 18, 2023, 09:00:26 am
content: No one has yet answered my question about MaxV of a cap.I have perused datasheets for various ceramic/tantalum/COG SMD epoxy encased caps and these sheets always specify a voltage rating for a capacitor,but how is that determinedsince they will tolerate, if not capacitate operation at much higher voltage?I think the MaxV answer for tantalum caps is much more accurate as I have found by experimentation that they will blow up or short out when subjected to operating voltages above specified.Unfortunately, most tants taint able to handle overvoltage gracefully, but the mfg's mostly had the wisdom to usually advertise their working voltage rating and always showing a + clearly on their case.I once tried using a 4.3V wet tantalum caps(As J. Cash said in his Folsum Blues song,"..Just to see him die!") as a typical bypass on a 5V power bus that was current limited(7805) to <1.5 Amp and saw that some would heat up, straight-up short out, while others weren't satisfied with that and would become more violent subjected to this insult and heat up and explode first with a short pleasant pop and a tiny wisp of smoke.I am still puzzled that I can't determine the polarity of some >1uF SMD one-end line-marked assumed to be pol. caps by measuring their leakage current which, while in the uA range, would nobly obey the sacred laws of symmetry.You would think that just writing a simple + sign on a SMD cap would disambiguate pol better than the idiot who chose to use a meaningless line as the standard!On the other hand, I would test the caps I would re-use for Cap value, high temp (hair-dryer test) and ESR and even HF ESR at a >V than would be encountered in my circuit desperately needing a cap at 2-am with my simple test setup and get a device or circuit to work well (usually for the long while).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2023, 09:28:08 am
content: Ceramic caps are probably determined statistically. There will be defects in the microscopic ceramic (and metal) layers, and the voltage rating is simply selected for some process yield.It could also very well be that the highest voltage part in a family is manufactured, and simply sold as lower voltage ratings with no modification at all -- perfectly identical parts. In which case, the rating might be adjusted to reflect the yield at that voltage, and consequently the higher ratings go for higher prices (compensating for the poorer yields).I haven't tested nearly enough parts to have any idea whether this is more than just a thought.Tim