fact or myth? "higher mlcc cap voltage to increase reliability and lifetime" - Page 1
Forum section: electronics / projects

author: elwali
author_rank: Newbie
timestamp: January 23, 2017, 01:51:46 am
content: 1- For a 3.3V microcontroller board:a) does using 50V mlcc bypass caps increase reliability and lifetime more than 16V caps?b) NP0 as bypass caps, will it make a difference? my design already uses 0.1u NP0 do I need to use a different 0.1u X7R for bypass?2- Correct me if I'm wrong, class 2 caps are unstable but yet used as bypass caps in high reliability products like top brand computers.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 23, 2017, 03:44:08 am
content: 1 (a) There would be negligible improvement in using 50v caps for a 3.3v supply compared to 16v,However in a manufacturing situation, you may find it simpler to use 50v caps everywhere, rather than a mix of 16v and 50v1(b) NP0 or X7R will work similarly as bypass caps, but if your design needs to operate at -50c you need to allow extra X7R caps, most of the time, with 100nF per chip you have plenty of excess capacity, so this design approach is OK at -50c.Note however that if a LDO regulator is used, then it may be unstable at low temperatures, so if your LDO regulator requires 4uF for stability, and you have a 2u2 MLCC at the regulator and ten 100nF scattered around the PCB, then you might have problems at low temperatures.NPO capacitors are physically too large to use as decoupling caps. I use 47nF NPO's as timing capacitors, but above that value they get expensive.You can buy 10 class2 caps for the price of 1 NPO in the larger values (100nF and up), so better to use more caps and just sprinkle them around.2 The caps aren't "unstable" that implies they can oscillate. They are low stability (i.e. value changes with temperature and voltage), however you might only need ~ 50nF per chip to meet the noise spec while the designer allocates 100nF per chip or more, so you have twice as much as needed (to handle low temperatures), and you use 50v caps (to avoid the voltage sensitivity).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2017, 07:16:55 am
content: Wow. 100nF C0G are expensive!Type 2 dielectric (X7R, Z5U, etc.) only needs to be rated for enough voltage so that the capacitance is not too low under bias. (Capacitance varies as a function of voltage.)50V caps are cheap enough that you should have little reason to use anything less, except in larger values (1uF+), smaller packages (0402 and smaller), and as a final cost reduction step for high production items (100k+ qty/yr).Tim

author: hamster_nz
author_rank: Super Contributor
timestamp: January 23, 2017, 07:29:01 am
content: NASA had a few things to say about heavy derating of polarised caps (not that they apply here with MLCC). If installed with reverse polarity they don't go bang in testing, but can fail during flight with age...

author: elwali
author_rank: Newbie
timestamp: January 23, 2017, 08:07:04 am
content: Quote from: hamster_nz on January 23, 2017, 07:29:01 amI had a very interesting experience with a revered installed polarized 10u 16V cap as single bypass cap for a 3v3 micro, where the board works fine until reaches temperature rises to ~40C then a resistor in the 5v side blows up. it was a design error, so it took a while to solve.we had to recall a full batch for repair, really was a nightmare.

author: EEVblog
author_rank: Unknown
timestamp: January 23, 2017, 10:34:39 am
content: Quote from: T3sl4co1l on January 23, 2017, 07:16:55 amBack when I was designing Seismic recording gear we needed high value COG caps, best available at the time was like 2.2nF or something and they cost a fortune. We would have killed for a 100nF COG! Money was no object.

author: dmills
author_rank: Super Contributor
timestamp: January 23, 2017, 12:03:31 pm
content: Yep, when you need them then often nothing else will really do, but they are the wrong tool for most decoupling.High temperature, non microphonic, and stable with time and temperature, a dollar a piece you say? I will take 100.X7R should probably be the default for decoupling unless you are playing minimal cost games and can justify the time to read the datasheets of the other class II dielectric parts most carefully.I would also note that the very high value MLCCs (100uF and such) can be something of a poisoned chalice if you are not careful, the Q can be high enough to cause nasty overshoot if the supply wiring has some inductance, adding a low value series resistor helps make these better behaved.Regards, Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2017, 02:31:54 pm
content: Quote from: EEVblog on January 23, 2017, 10:34:39 amWow, couldn't swing PP films? (Temperature?)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2017, 02:33:54 pm
content: Quote from: dmills on January 23, 2017, 12:03:31 pmYep, the same is true of aluminum polymers (which, in many ways, are the low voltage equivalent of film caps -- speaking of them).Tim

author: BravoV
author_rank: Super Contributor
timestamp: January 23, 2017, 03:38:25 pm
content: Quote from: EEVblog on January 23, 2017, 10:34:39 amHere you go, my hobbyist grade stable/reference caps for verifying meter at low capacitance, Kemet COG 100nF.Although now that COG sized is norm, but still quite expensive, at Mouser cost about $23 a pop.

author: thm_w
author_rank: Super Contributor
timestamp: January 23, 2017, 11:48:02 pm
content: "top brand computers" are usually not exposed to extreme environmental conditions. Can you explain your situation, what specifically are you concerned about, any environmental conditions? Or do you just want the product to last longer.I don't see a reason to use NP0 unless you provide one. 0.1uF looks to only be available in 1206 or larger for ~11c, not very convenient. If you do use X7R/X8R, go with a larger package as well though. There was also a thread on series-ing ceramic caps in automotive designs for increased reliability. This might be relevant reason to go with the larger package size:https://www.maximintegrated.com/en/app-notes/index.mvp/id/5527For reference I have repaired over 10,000 boards and <0.1% of those failures were due to MLCC failure. The product is used outdoors, but not high heat or vibration.

author: elwali
author_rank: Newbie
timestamp: January 24, 2017, 05:07:14 am
content: Quote from: thm_w on January 23, 2017, 11:48:02 pmproduct will work outdoors, metal casing and waterproof, expected operating temperature 0C~55C, direct sunlight is possible. under these conditions, I'd like it to be both reliable and last longer.

author: metacollin
author_rank: Contributor
timestamp: January 24, 2017, 10:48:36 am
content: It is definitely a myth. The rated voltage of ceramic capacitors is a guarantee, not some conditioned and tightly controlled property. The rated voltage is the voltage that a vendor guarantees that a given ceramic capacitor can withstand indefinitely and at either extreme of the temperature range. Non-ceramic capacitors have physically-conditioned breakdown voltages that can be tightly controlled and measured, and will almost certainly undergo dielectric breakdown if they see a voltage just a little bit above their rated voltage and for just a little too long. This is why we derate those capacitors - because the rated voltage is the 'absolute maximum' voltage, not the typical operating voltage, so to speak. Tantalum capacitors are especially unforgiving in this regard, and as a bonus, undergo a reaction similar to thermite. Instead of iron oxide and aluminum as with thermite, it is between manganese dioxide and tantalum. It is nowhere near as hot as thermite of course, and it has a MUCH lower ignition temperature. So when 600mV over the rated voltage punches right through the layer of tantalum pentoxide and forms a 1-3 ohm short inside a tantalum capacitor (which is almost always directly across a power rail), tantalums will happily generate a lot of heat and release a lot of chemical energy. In the form of ejected flaming capacitor entrails, of course.I digress. Ceramic capacitors are not like this. The true voltage rating of a ceramic capacitor is highly dependent on subtle variations in sintering conditions, metallization, if that day is a thursday, etc. etc. And by highly dependent, I mean the same part number but two different batches of that part can vary as much as 10 times in what their true voltage rating is.So the voltage rating given is one that a vendor is sure will never lay within that huge variability. Furthermore, look at any ceramic capacitor datasheet for something called the 'dielectric withstand voltage'. This is a second voltage rating that is usually either 250% of the rated voltage, or just a fixed voltage (100V is one I see often). There will usually be another qualifier, saying it will withstand that voltage for up to 1 second or something like that.What does this mean, at the end of the day? That any ceramic capacitor effectively has a very heavy amount of derating already built into it due to the high variation suffered by ceramic capacitor manufacture methods, and the simple fact that the rated voltage for ceramic capacitors is a WORKING voltage, rather than a maximum voltage as with other capacitors. Usually. Of course, always double check the datasheet. But regardless, you will not see any benefit to longevity or reliability by choosing higher than needed voltage ratings.All of this information can be found on the lengthy but very good wikipedia article on ceramic capacitors, by the way. So don't just take my 'random dude on the internet' word for it.Now, this part IS just my random internet dude word:I have my suspicions that most if not all of the vendors actually sell one capacitor under several part numbers spanning several 'rated voltages'. This is likely because they just make that capacitor over and over, and test each batch, and sometimes they will be safe to label as 50V, other times just 35V, etc.I suspect this because there are several capacitors that are utterly identical in every specification, including having all identical temperature and capacitance change under DC bias curves. The only difference is the lower voltage ones have those curves truncated at the rated voltage, but I have actually overlayed the graphs and they are actually identical. *cough* I'm looking at you, Murata *cough*.Anyway, to actually answer elwali's question, no, there is no ceramic capacitor that is waterproof. Electronics are not waterproof. If a component were to be waterproof, it could not have metal leads to solder to. Obviously, all ceramic capacitors have metallic connections, otherwise there would be no way to use them. The rest of your conditions are not a problem for pretty much any ceramic capacitor, and will not be impacted by voltage derating.Based on your requirements, you will need a conformal coating. That should be your primary concern here. Just pick an automotive grade ceramic capacitor that meets your needs and leave the rest to the conformal coating. This is non-optional if you want the electronics to be outdoors and waterproof. Electronics must be protected from the ingress of water either by an enclosure, or chemically by a conformal coating. Or potting of course. That will also effectively remove the burden of environmental resistance (except for temperature and mechanical stuff) from the capacitors. There is no ceramic capacitor in existence that is waterproof. As far as I know, all of them are UV proof. MLCCs are not available in metal cases, at least not anymore (there are some older ceramic capacitors that were, though I don't think they are MLCCs, just normal ceramic), and you wouldn't want them to be. The epoxy coating used offers superior protection to the extremely brittle and hard ceramic inside compared to an easily deformable metal casing.You can find some non-MLCC ceramics in metal casing, but these are large high voltage types, and very expensive.

author: thm_w
author_rank: Super Contributor
timestamp: January 24, 2017, 11:03:49 pm
content: Quote from: elwali on January 24, 2017, 05:07:14 amI agree with metacollin. Conformal coating will be useful, ensure it is applied correctly (use UV inspection). If you arecertainthe enclosure is waterproof, can still throw in a desiccant pouch.Direct sunlight could push the temperature up, but it still won't be close to X7R/X8R rating.For reliability concerns, get some other people with experience to perform a design review. So many reliability problems are due to oversight or poor design (ignoring tolerance of components), and not the components themselves.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 25, 2017, 04:24:56 am
content: Yep conformal, coating is a must, this unit operates at ~0-50C and is next to a swimming pool:http://daisypoolcovers.com.au/roller/dp-daisy-power-series/It has two coats of spray on coating on the PCB (and uses X7R capacitors throughout), we have had zero moisture related failures so far of ~ 800 units.Note painting outdoor boxes white or silver is a good idea, as black boxes will radiate to the night sky and become cooler than their non-black surroundings, and dew will condense on the black boxes.

author: elwali
author_rank: Newbie
timestamp: January 25, 2017, 10:46:28 am
content: Thanks guys, conformal coating is a great idea.As for metacollin's theory here:Quote from: metacollin on January 24, 2017, 10:48:36 amI'd like to add that I've noticed there is no price difference between some mlcc of same brand, size, capacitance and tolerance but different rated voltage as shown in this query on digikey:http://www.digikey.com/products/en/capacitors/ceramic-capacitors/60?k=&pkeyword=&pv7=1&FV=fffc01ea%2C34003e%2C400004%2C440005%2C11401c5%2C1f140000%2Cffe0003c%2Cc0003&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25someone might say that components pricing is a different story. but I thick if a 50V cap has the same price of a 16V, so what is all this headache for? why does manufacturers still product lower voltage mlcc? any special application for them?

author: Fungus
author_rank: Super Contributor
timestamp: January 25, 2017, 11:30:32 am
content: Quote from: elwali on January 25, 2017, 10:46:28 amPresumably they want their parts to show up when people type "16V cap" into search boxes.It might even be the exact same part. 50V rating is still 16V rating, so... just label it differently and sell it as 16V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2017, 11:54:01 am
content: Marketing is certainly a thing. There are a vast number of engineers who don't understand the voltage rules of the many types of capacitors.Likely electrical difference is number and spacing of plates inside the chip. A lower voltage part needn't use the whole volume of the chip -- therefore it will store less energy at saturation.There may be a cost difference in extremely large quantities.A run of parts might not meet qualification for the higher voltage specs, but could be acceptable for the lower voltage spec. (I don't know if they would be able to do this, given that C(V) curves should at least be typical of the part, which would be determined by construction.)Tim

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 26, 2017, 01:23:58 am
content: Extending on Tims response,a 16v capacitor, with thinner dielectric, would require less plates, therefore you could make the metallisation thicker, this would lower the ESR and increase the Q.So from a user's perspective you could trade off ESR vs voltage coefficient by using a higher voltage part.Personally I like to minimise the number of inventoried items and stock1206 X7R : 1u100v, 10u35v ,0805 X7R: 10u10 , 1u50, (and 100n50, 47n50,22n50,10n50, then in 100v 4n7 2n2 1n)0805 NP0 5%: 47n50v , then in 100v 4n7 to 1p in a E3 sequence.There's no point having 22u or 2.2uf in stock, as two parallel capacitors have much better ESR and ESL , using two caps is cheaper if you are buying them by the reel vs having bags of a couple of hundred odd values.