Don't know why Ceramic Chip Capacitors fails after few weeks ... - Page 1
Forum section: electronics / projects

author: luk
author_rank: Contributor
timestamp: November 02, 2020, 12:35:58 pm
content: Hello,Some of our products are failing after few weeks of usage due to ceramic capacitor failure.There are seven 22uF 0805 capacitor rated @ 6.3v on the PCB. They have few Ohms resistance after failure.Information:1- the board is a probe that senses the electro-conductivity of water. It is powered by an isolated power source (push-pull transformer) that delivers 4.7v to an LDO that transform it to 3.3v. The power source is equipped with a Zener diode that prevents voltage to exceed 4.7v. The power source is switched off for 5 minutes and switched on to measure the conductivity of the water then switched off again.2. the failing capacitors can be the ones that are connected to Vin and Vout of the LDO but also others that should never experience more than 2v voltage. Only the 22uF fails, not the 100nF 50v rated also present. It's random and there can be one capacitor or many failing at the same time.3. the probe is molded in epoxy resin and are used few weeks to few month successfully before failing. about 80% of them fails.My questions are :a. the supplied 4.7v is under the capacitor rating of 6.3v, and I learned in engineer school that ceramic capacitor rating do not need to be overestimated (no need to add huge margin when choosing components). Is it still possible that some higher brief impulse voltage make those capacitor to fail ?b. is it possible that the epoxy resin enclosure breaks the internal structure of the capacitor after some time ?c. is it possible that an ESD breaks only some capacitors and not the any other chip ? there is an instrumental amp on the board and none has failed with the capacitors.Anyway, I would be happy to hear your opinion about this because I'm a bit lost here.Thank you very muchPS: last question, do you think a 22uF rated at 25v or 16v would solve the problem ?

author: dmills
author_rank: Super Contributor
timestamp: November 02, 2020, 12:45:08 pm
content: B is likely.Ceramic caps have a fairly well known failure mode involving cracking if the substrate is placed under strain.Epoxy resins shrink slightly as they cure, I would try an acid free curing silicone rubber encapsulant instead of epoxy.Are the caps by chance aligned parallel to a board edge and are there mouse bites, a mounting hole or or a V score line close to the caps?The cap datasheets (from decent manufacturers) go into some detail about this failure mechanism.

author: 2N3055
author_rank: Super Contributor
timestamp: November 02, 2020, 12:47:04 pm
content: Quote from: luk on November 02, 2020, 12:35:58 pmEpoxy resin, if hard enough will crush ceramic capacitors. You need to use soft conformal coating on these. They can also crack if board flexes...

author: tunk
author_rank: Super Contributor
timestamp: November 02, 2020, 03:51:02 pm
content: A question out of curiosity: Would it be possible tofirst "use soft conformal coating" and then epoxy?

author: 2N3055
author_rank: Super Contributor
timestamp: November 02, 2020, 05:29:27 pm
content: Quote from: tunk on November 02, 2020, 03:51:02 pmSoft layer will still be able to create hydraulic pressure if hard compressed inside closed vessel. Best would be to unload mechanical pressure on the board...

author: helius
author_rank: Super Contributor
timestamp: November 02, 2020, 05:51:07 pm
content: Urethane-acrylic encapsulant might be water resistant enough to use as a substitute for epoxy.

author: luk
author_rank: Contributor
timestamp: November 02, 2020, 07:07:24 pm
content: Thank you for you answers !Quote from: dmills on November 02, 2020, 12:45:08 pmI thought the same, that's why I covered one of the PCB with hot glue (the sticks used with hot glue guns) then molded in epoxy resin. It failed this morning, it has 2 shorted capacitors next to each other, completely separated from the LDO and where voltage can't exceed Vdd/2 (1.65v).After visual inspection under a microscope, there are no cracks. I know that the damages can be internal but both failed at the same time after 3 weeks. The resin has finished curing long time ago above the soft plastic glue layer.I will still try to add a 3d printed plastic shield above the electronics to keep a thin layer of air above the capacitor and see if it fixes the issue.I will also try to use 22uF/25v and 10uF/25v.Quote from: helius on November 02, 2020, 05:51:07 pmI will also check this, thanks !We will see if it worked only in few weeks

author: chrisl
author_rank: Regular Contributor
timestamp: November 02, 2020, 07:26:21 pm
content: Mechanically stressed failure will result caps that are opened. Since the failed caps are shorted I would put a scope and check for any possible spikes. Zeners are sometimes not fast enough to suppress spikes.

author: Yansi
author_rank: Super Contributor
timestamp: November 02, 2020, 07:30:15 pm
content: Isn't it the classic overvoltage problem? Only single ceramic input cap, to which is brutally connected to a stiff power supply through a long cable with significant inductance, that creates an overvoltage spike, that can even kill the LDO at times.

author: georges80
author_rank: Frequent Contributor
timestamp: November 02, 2020, 07:38:31 pm
content: ^ quite likely given the small voltage headroom of the caps.OP: What cable length are you running?I'd suggest if it is a cable length/ceramic issue that you need to consider a lossy electrolytic at the front end. As stated, Zeners can be quite slow to react. You need to 'harden' your front end power input circuitry to suppress spikes.Long cables/inductance and low ESR Ceramic form a classic LC circuit that can create 100's of volts on power connect/disconnect.cheers,george.

author: jbb
author_rank: Super Contributor
timestamp: November 03, 2020, 07:03:29 pm
content: My sympathies. This kind of problem can get very annoying.How about some differential testing? Here are a few options:- deploying a couple of ‘original design’ boardsunpoweredright next to a life test board? Give them a few weeks and see if they die? (Not definite.)- swapping caps for 0603 size? (Maybe less sensitive to mechanical forces)- swapping caps for 0805 tantalum polymer? (- running a couple of test boards ‘always on’?- running an un-potted board through lots of power cycles?We’ve had issues with our products that were exposed by long power cycling tests.Oh yes: how are you soldering the 0805s? Hand soldering might be introducing some stresses.

author: coppercone2
author_rank: Super Contributor
timestamp: November 03, 2020, 07:25:39 pm
content: use through hole parts

author: ehughes
author_rank: Frequent Contributor
timestamp: November 03, 2020, 07:59:18 pm
content: QuoteNo! MLCC will absolutely short circuit. This is a huge issue in PCB assembly (one of the reasons I almost never v-score) and there are IPC guidelines as to placement near the edge of a PCB.I have experience this 1st hand in production. As a general rule I only spec AEC-Q200 parts as the soft termination helps the issue.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 03, 2020, 08:02:38 pm
content: It seems to mejbbquestions are close to the problem.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 03, 2020, 08:21:47 pm
content: QuoteThe fact that other caps fail suggests strongly that it's a mechanical, not electrical issue, unless spikes can get to the "others".As mentioned above. if the cap is on a cable, and the other end is connected/disconnected from power by plugging/unplugging or via a mechanical switch, the low ESR of the cap combined with the inductance of the cable can cause voltage spikes - I've seen this kill DC/DC converter chips.My guess is stresses built up during the assembly and potting process.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 04, 2020, 12:14:36 am
content: maybe exposure to outdoor weather, hot-cold (pot expand-contract) cycle?

author: jmelson
author_rank: Super Contributor
timestamp: November 04, 2020, 12:30:38 am
content: Quote from: luk on November 02, 2020, 12:35:58 pmYes, this is a WELL documented issue with hard potting. You should NEVER pot an electronic assembly directly in hard epoxy, the delayed failure is commonly seen. You need to coat the entire assembly in a soft material first (thickness needs to be determined by experiment, but larger assembles require thicker soft coating) before embedding in the hard-setting epoxy.Jon

author: wraper
author_rank: Unknown
timestamp: November 04, 2020, 12:31:17 am
content: Quote from: chrisl on November 02, 2020, 07:26:21 pmNope it often ends with shorted caps, especially with high capacitance for the size since layers are very thin.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 04, 2020, 10:22:10 am
content: Yes more likely mechanical than overvoltage. LC hotplug transients seldom get much over 2-3*Vin (they theoretically can get over 2*Vin if damping R is very close to zero AND the DC bias C drop is significant), and MLCCs can generally withstand voltage spikes quite well. You should still pay attention to damping of the input. I suggest you add 2*C + series R, either using the same MLCCs + a discrete resistor, or use a standard-ESR electrolytic capacitor if your temperature ranges allows it.MLCCs they often fail in a very awkward "nearly short" state where they have low enough resistance to cause significant heating and local damage, yet high enough R not to blow fuses or trig active protection.Are you 100% sure you really want potting? Why are standard waterproof cases out of question?Potting seems more demanding than people usually think. If you want to dampen vibration, it's going to require understanding on mechanical resonant modes, how different potting materials absorbs it instead of directing it to the components, and how PCB flexing and stress to the components and solder joints can be avoided. At the same time, bare open PCBs perform as expected and you can protect them from the elements with a water/gastight enclosure with silicone gaskets.

author: jonpaul
author_rank: Super Contributor
timestamp: November 04, 2020, 10:57:30 am
content: Hello what potting compound? Precoating PCB? Cure cycle? Anneal cycle?maximum thickness of finished part?Potting is an art and takes time to learn and debug.Our 12 kV DO-160 aviation HVPS modules took on YEAR to design and debug THE POTTING PROCESS!Typically we say diode and cap failures after potting or temp cycling.Just a thought!Jon

author: serg-el
author_rank: Regular Contributor
timestamp: November 04, 2020, 01:52:12 pm
content: You may find this book useful.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: November 04, 2020, 03:14:18 pm
content: Without size and cost information to restrict options we can explore some alternatives.Ceramic capacitors like that are attractive and specified because they are small and cheap. They often are in the reference design.1. A key issue with MLCC capacitors is the loss of capacitance as voltage goes up. There are threads around here on that topic, such as:https://www.eevblog.com/forum/beginners/capacitor-voltage-rating-safety-margin/Therefore, your part may actually be operating at only around 5 uF to 7 uF anyway.2. If your regulator is a linear LDO, and the "push-pull" transformer supply is below perhaps 100 kHz, then you could use Al-Poly electrolytic capacitors:https://www.digikey.com/en/products/detail/kemet/A700V106M016ATE045/1829934This suggests that it would be worthwhile to measure the actual frequency spectrum of the power going into the LDO, along with the actual capacitance at the DC bias voltage.Don't look at just the price of the capacitors. Factor in engineering time, warranty costs, reliability estimates, etc.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 04, 2020, 06:03:51 pm
content: Quote from: serg-el on November 04, 2020, 01:52:12 pmSeems a very useful book.OCR then DeepL gave very useful English version. Unfortunately trial version of DeepL didn't do the whole thing in one shot, and I used up my tries before successfully splitting it into two parts.Here is the translated first part, and the OCR second part in Russian. Perhaps someone else can get the second part translated.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 05, 2020, 06:54:21 am
content: Apparently the DeepL trial version limit on translation is some sort of words per hour limit. Here is the second half of the book in English.

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2020, 10:51:54 pm
content: Can they be replaced with tantalum capacitors? Obviously you'll need to go with a higher voltage rating, like 10V, but they should be much less susceptible to cracking, than ceramic.

author: rstofer
author_rank: Super Contributor
timestamp: November 06, 2020, 02:34:16 am
content: Is that 4.7V in RMS? If so, the peak value is 6.58V, somewhat higher than 6.3V capacitor rating.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 06, 2020, 07:40:13 am
content: Quote from: Zero999 on November 05, 2020, 10:51:54 pmDEFINITELY no 10V tantalum on 5V hot-pluggable input. It's almost guaranteed to go up in flames one day. At standard 40-50% voltage derating, peak current is very important.At even higher voltage derating, higher peak current can be accepted, so a 16V tantalum could work but I'd definitely skip tantalums anywhere else exceptactively current-limitedregulator outputs.It's another can of worms.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 06, 2020, 08:31:31 am
content: Quote from: Siwastaja on November 06, 2020, 07:40:13 amStronglyagreed, tantalums are a tremendously bad idea here.In general I never use them on anything directly accessible to the outside world, because it is an invitation for future disaster. Inside the circuit where the designer has (nearly) full control they are fine, when otherwise suitable of course. This isn't the '90s anymore, there's no need to fear them... as long as you play by their rules. One of which is to keep them inside!(Tantalum polymer is rather different but I still don't use them on externally accessible lines; their benefits are usually not adding much, so there's no reason to bother.)If something low profile is needed, one of the aluminum polymer caps in 7.3x4.3mm package (SP-Cap, AO-Cap, etc) may be suitable.Oh, and +1 on the potting being the source of this problem. MLCC cracking is a very common issue... even seen it stop production lines before. Nothing here implies to me that the applied voltage is a particular issue. MLCCs are quite tolerant of small overvoltages (or large but fast overvoltages, like ESD). And even if they weren't, the parts seem to often be binned: manufacturer characterization data for two capacitors that differ only in voltage rating is often suspiciously similar... so similar that the most convenient explanation is that they're the same part, differently marked. (It may also be possible that lot testing shows some lots are underperformers. But at the volumes the major manufacturers operate, that's unlikely. And they do produce a tremendous quantity of distinct part numbers, so this argument certainly doesn't apply for every part number.)So your 6.3V MLCC is probably not going to die suddenly if you feed it 7V peak. It might (might!) not even care if you hit it with 100V. But it absolutelywillfail, possibly open, possibly short, possibly in-between, if you wrench on the thing.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2020, 08:59:21 am
content: I knew I'd get flamed for suggesting tantalum, but they're much less prone to the failure mode described by the original poster, than ceramic. I've found tantalum capacitors to be very reliable and have only encountered a few fire incidents. In this application, they're not exposed to ESD or high currents. The impedance of the transformer, driver and LDO should limit surges nicely.People get all hot about tantalums, because they can fail violently and it's happened to them in the past, but in reality, when used properly, they're reliable and very robust. They don't dry up like aluminium electrolytics, or crack like ceramic capacitors do, when flexed and shaken. Don't zap them with high currents or voltages and suitable derate them and they'll be fine.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 06, 2020, 09:41:20 am
content: Quote from: Zero999 on November 06, 2020, 08:59:21 amYou are wrong.One of the core principles that sets good engineering apart from poor engineering is how risk is managed. In this case, it is necessary to consider what you are getting in exchange for taking on the fire risk of a component in a potted assembly. Because there are components of substantially equal performance available (flex-term MLCCs, polymer capacitors, ordinary aluminum electrolytics), and the capacitance is small, the benefit from tantalum is approximately zero.So it's foolish to take on the risk. There is no point to high-risk options when they gain nothing over low-risk options!Look, I use loads of tantalum capacitors, I know how to apply them in production designs. I know how to argue against design reviewers stuck with the backwards mindset that every tantalum capacitor is evil. But this is not an appropriate place for a tantalum.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2020, 10:21:15 am
content: Quote from: exmadscientist on November 06, 2020, 09:41:20 amI don't see the fire risk. It's potted in resin, which will exclude oxygen and should be fire retardant, thus even if the tantalum does burn out, it will be confined to a small area.I agree about the other options, but wouldn't they require the boards to be changed? It's possible a tantalum could be used, which has the same footprint as the ceramic capacitor, so no changes to the board are required.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 06, 2020, 10:39:12 am
content: Tantalum is fine if surge current is controlled.LC ringing overvoltage being discussed earlier suggested a part is used as an input cap in hot-pluggable circuit, in which case surge current is not controlled.It has significant ESR, through (one reason to use it).Is the low ESR of the original MLCCs required, or is it not?

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 06, 2020, 12:05:44 pm
content: There is a possibility to use a parallel-series connection of MLCC cases (caps). And we may use large valued caps in series with a small value resistors to achieve some damping and thus lower the possible peak oscillation voltage at the input of a device.

author: 2N3055
author_rank: Super Contributor
timestamp: November 06, 2020, 12:30:15 pm
content: It would be simple enough to get few boards that are not potted, connect them to power and see what happens..MLCC can be damaged from potting stress, they can be damaged with soldering process. Or maybe there is electric overstress..Divide and conquer...Take the potted one and try to power it with finely controlled PSU DC to see if it happens then.Take one unpotted board and try to ramp up the voltage to find out what happens..Take a simple 5mm styrofoam, put it as a plug over board and pot over it, to relieve stress on components.Then test that one..You need to experiment a bit.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2020, 01:29:30 pm
content: Quote from: Siwastaja on November 06, 2020, 10:39:12 amI think it's unlikely to be overvoltage because the zener diode will get rid of any high voltage transients. A higher ESR would also damp any transients, making high voltages less likely.Oh and I don't mean that the original poster should definitely use tantalum, just that it's a possible solution, which could potentially avoid modifying/respinning the PCBs. The one thing to bear in mind is the polarity, which would not be indicated on a silkscreen designed for ceramic capacitors.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 06, 2020, 01:45:50 pm
content: Tantalums are very sensitive to surgecurrents. I only discussed the ringing overvoltage as ahintthat surge current is also uncontrolled, and the zener that clamps the voltage does not help with the inrush current.Quote from: 2N3055 on November 06, 2020, 12:30:15 pmPossible but very very unlikely with 0805 parts, I'm assuming it's p&p & reflow'n, not hand soldered.Unless there is unlimited time to run all the tests, I think it's safe to assume the potting is the reason (unless there is an additional board flex due to mounting screws or similar; if there is, it needs to be rectified as well).Checklist in order of importance:* Remove potting* Eliminate board flex, especially due to mounting holes* If board flex cannot be completely removed, or high reliability is required, usesoft terminatedMLCCs, preferably with automotive qualifications as well.* For the input cap, add damping for LC parasitics. An elcap or MLCC + series resistor will do.* Alternatively, or in addition, consider using 10V rated MLCC although I suspect this isn't the problemIf potting is really required, hire an expert on potted PCB design, reserve a year to debug the process, extensive field testing, simulated harsh conditions etc.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2020, 08:16:47 pm
content: Quote from: Siwastaja on November 06, 2020, 01:45:50 pmThe impedance of the transformer would limit current surges.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 06, 2020, 09:24:45 pm
content: As mentioned by several others earlier, the potting and mechanical stress are among the most likely root causes.- Is it possible to get a picture of the actual board where we can see the location and orientation of the failing capacitors?- Depanelization method (mouse bites, v-cut, routing)?- Operating temperature range? Constant or changing rapidly? Dipping the probe into hot liquid?

author: thm_w
author_rank: Super Contributor
timestamp: November 06, 2020, 10:20:01 pm
content: Quote from: Zero999 on November 06, 2020, 08:59:21 amYeah, any sort of modern tantalum is quite reliable. I avoid them in designs for moral reasons, but, out of ~10k units out there with a 16V rated tantalum on a 13.5V rail, haven't seen more than a few fail (more ceramics failed). Although.. they do specify it as "surge-robust" so maybe its really a 35V tant in disguiseDatasheet here:T495D336K016ATE150Kemet

author: Siwastaja
author_rank: Super Contributor
timestamp: November 07, 2020, 07:46:23 am
content: Understanding tantalum ratings and datasheets is black magic. The tradition of rating the voltage in such way that the expected lifetime is just hours, then give another "recommended" actual ratings table - often inanother"application note" sheet, is just nuts. And it's confusing if some other part does not require such derating but is actually rated properly from the start.