Capacitors packages for PCB  - Page 1
Forum section: electronics / general

author: joniengr081
author_rank: Regular Contributor
timestamp: March 23, 2023, 02:19:44 pm
content: Hi,I am wondering which package we normally use and what is the appropriate combination of the de-coupling capacitors for FPGA based PCB.I guess we can not get 100 uF or 220 uF in small packages like 0603, 0805 etc with voltage rating of 35 V for example. Then which package are suitable for these two 100 uF or 220 uF ?

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 23, 2023, 09:43:07 pm
content: That is a pretty general question. It all depends on what characteristics you need for a given capacitor.The capacitance is only part of the requirements usually.At the moment, the maximum reasonable and reasonably available capacitance you'll find with SMD ceramic capacitors is 47µF - obviously at relatively low operating voltages.10µF can be easily found down to 0603 at low voltages. 22µF will usually be found in 0805 packages and larger.For anything above 47µF, you can either resort to paralleling several ceramic caps, or go to electrolytic or tantalum.Now there can be reasons for using electrolytic caps or tantalum caps beyond the available capacitance. For instance, sometimes their relatively higher ESR is actually desirable (while the typ. ESR for ceramic capacitors is very low.)Typically, for a 100µF+/35V capacitor, if paralleling ceramic caps is not an option for your design constraints, a tantalum cap or electrolytic cap will do. Tantalum SMDs are usually smaller for a given capacitance and operating voltage. A typical 100µF/35V tantalum cap will come in "D" to "F" packages. Electrolytics are almost always much taller, so if height is a problem, they are not an option.Another factor, beyond characteristics (ESR, parastic inductance) is the cost. Tantalum caps tend to be the most expensive.Yet another factor is MTBF. Electrolytic caps (at least in general) will be the ones with the lowest MTBF compared to tantalum and ceramic, and will eventually end up leaking or drying up at some point. Can be a few years to a few decades depending on construction and operating conditions.Finally, another factor is failure modes. Each of these 3 types of caps have different failure modes. Electrolytics tend to dry up or leak over time. Tantalum don't wear out as easily, but if they ever fail, they'll usually fail as shorts - which can blow up themselves and the circuits around them. Not pretty. And, ceramic capacitors can suffer from cracking (which can either lead to open or short circuits), most often caused by mechanical constraints (shocks, vibration.)That's all for now.

author: thm_w
author_rank: Super Contributor
timestamp: March 23, 2023, 09:55:44 pm
content: The FPGA vendor will often give you recommendations, eg:https://www.latticesemi.com/-/media/LatticeSemi/Documents/ApplicationNotes/PT/FPGA-TN-02115-1-1-Power-Decoupling-Bypass-Filtering.ashx?document_id=8374

author: wraper
author_rank: Unknown
timestamp: March 23, 2023, 10:06:44 pm
content: Quote from: SiliconWizard on March 23, 2023, 09:43:07 pmMTBF is a quite meaningless figure for electrolytic capacitors and I haven't seen such rating in their datasheets.https://forum.digikey.com/t/mtbf-of-aluminum-electrolytic-capacitors/25074They are very reliable until they start failing in droves years later. MTBF does not show how long something lasts, just how often it fails in general. FWIW MTBF of tantalum caps is much worse, especially if voltage is not heavily derated.

author: joniengr081
author_rank: Regular Contributor
timestamp: March 24, 2023, 09:28:03 am
content: Thank you very much for reply.

author: joniengr081
author_rank: Regular Contributor
timestamp: March 24, 2023, 09:57:26 am
content: Some related questions.Aluminum capacitors and Tantalum capacitors are both under the category of Electrolytic Capacitors ?Ceramic capacitors have high tolerance compared to electrolytic capacitors, which tolerance is referring here, I guess capacitance, right ? Does high tolerance means they are more accurate in their capacitance ?Temperature stability of ceramic capacitors is good compared to poor temperature stability of electrolytic capacitors, it applies to both Aluminum capacitors and Tantalum capacitors ?

author: wraper
author_rank: Unknown
timestamp: March 24, 2023, 10:17:30 am
content: Quote from: joniengr081 on March 24, 2023, 09:57:26 amWrong. It's bad unless It's C0G type or similar which come only with a very small capacitance for it's size. Also it drastically drops with voltage applied. It's not uncommon that like only 20% of rated capacitance is left when used in actual circuit.

author: n4u
author_rank: Contributor
timestamp: March 24, 2023, 10:19:39 am
content: You need a whole set of capacitors for decoupling. To make good coupling for design caps need to extend the characteristics after resonanse frequency of previous used caps, to gain low impedance over range of frequency that u have on the design and enviroment. Lower impedance let you remove noise better.Usually bigger max voltage / bigger size or even bigger capasitance is equal to earlier resonance frequency. When you have low voltage fpga you should have low voltage capacitors with small size under the chip. Then u can move away and decouple the power supply layers / fields with bigger capacitor etc. 35V and above voltage may be used for decoupling power supply or power line in your design - not the fpga chip itself.

author: joniengr081
author_rank: Regular Contributor
timestamp: March 24, 2023, 11:39:43 am
content: I mean the operating thermal range for ceramic capacitors is between −55 °C to +125 °C compared to electrolytic capacitors where the maximum temperature is up to 105 °C. Meaning that ceramic capacitors have wide operating temperature range compare to electrolytic capacitors.The common rule that capacitor lifetime increases by a factor of 2 for every +10°C reduction in temperature. Is that only true for electrolytic capacitors ?I am still confused between electrolytic, aluminum and tantalum. Electrolytic capacitors are aluminum capacitors, tantalum capacitors or both are under the category of electrolytic capacitors ?

author: wraper
author_rank: Unknown
timestamp: March 24, 2023, 11:42:57 am
content: Quote from: joniengr081 on March 24, 2023, 11:39:43 amTake X5R for example, it's only up to +85oC. Also there are +125oC and even higher temp rated electrolytic caps. Wider temperature range does not mean its parameters are more stable. Or that part is necessarily more reliable. For example I avoid using MLCC in sizes larger than 0805 because they are prone to cracking due to mechanical stress. Also I avoid very large capacitance for the size. Too expensive, less reliable, loses most of capacitance under voltage bias and likely not that much more capacitance will be left than with capacitor of 2x smaller rating. Also MLCC shouldn't be used anywhere near mounting holes and board edges (especially v-cut) due to susceptibility to mechanical stress I mentioned.

author: joniengr081
author_rank: Regular Contributor
timestamp: March 24, 2023, 12:08:54 pm
content: It says that "The capacitance of electrolytic capacitors drifts from the nominal value as time passes, and they have large tolerances, typically 20%." What about the capacitance tolerance of ceramic capacitors ? Is that large or small ?

author: wraper
author_rank: Unknown
timestamp: March 24, 2023, 12:26:27 pm
content: Quote from: joniengr081 on March 24, 2023, 12:08:54 pmYou can buy X5R or X7R capacitor with 10% tolerance. The problem is that with ageing like 20-30% of initial capacitance will be lost over the years (re-heating returns initial capacitance and restarts the process). Within operating temperature capacitance can change within +/-15% range. Then the worst, under voltage bias up to about 85-90% of capacitance can be lost in worst cases when capacitance is extremely large for the package size. These factors don't exactly multiply, so it's somewhat better than losing another 90% under voltage bias after 40% already lost due to other factors. Datasheets usually don't show capacitance dependence under voltage bias so you need to dig further to find that. Murata for example has a simulator on their website which will show it for each particular part number.https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us&jis=false&md5=a60f38f6f45f84e55ead64588273760b

author: wraper
author_rank: Unknown
timestamp: March 24, 2023, 12:34:28 pm
content: For example this 47uF 6.3V 0805 cap under it's rated voltage has only 11.7uF left out of rated 47uF. And it's not even considering ageing and temperature.

author: joniengr081
author_rank: Regular Contributor
timestamp: March 27, 2023, 08:34:59 am
content: Yes, now I also have checked at the link provided in reply above that capacitance changes a lot as the DC bias increase but I guess they are class 2 capacitors, right ? How do I know if it is class 1 or class 2 ceramic capacitor. I was checking the Digikey's website to search ceramic capacitors, I don't find any option to select class 1 or class 2 in the filter.How about Tantalum, do they also have large variation in their capacitance as DC bias increase ?

author: mariush
author_rank: Super Contributor
timestamp: March 27, 2023, 09:34:48 am
content: Quote from: joniengr081 on March 24, 2023, 11:39:43 amYou can buy electrolytic capacitors rated for higher than 105c, like for example 130c ... Rubycon RX30 series comes to mind :https://eu.mouser.com/c/passive-components/capacitors/aluminum-electrolytic-capacitors/aluminum-electrolytic-capacitors-radial-leaded/?m=Rubycon&series=RX30In fact you can look at other manufacturers at catalogues or series charts and you'll find higher temperature series.Rubycon for example has them in the catalogue at page 3-4 :https://www.rubycon.co.jp/wp-content/uploads/catalog/aluminum-catalog.pdfI see there TSV series rated for 150c , T*V series rated for 125c, HGX rated for 135c (automotive series) , RX50 rated for 150c etc etcThe double lifetime for every 10c is valid for electrolytics... it's a more complex formula but it can be simplified to double life for every 10cpolymer capacitors have a different formula : L = Linitial x 10zwhere z is (Tm-Ta)/20 and Tm is rated temp, Ta is ambient tempSo for example a 4000h @105c polymer running at 65c would have an estimated life of L = 4000 x 10(105-65)/20= 4000x 102= 400k hours.well ... uploading the "complete" formulas here :

author: wraper
author_rank: Unknown
timestamp: March 27, 2023, 10:27:07 am
content: Quote from: joniengr081 on March 27, 2023, 08:34:59 amClass I capacitors like C0G or NP0 only come in very small capacitances for the size. SMD MLCC usually are white or grey-ish. Larger capacitance will be type 2, like X7R, X5R which are brown color, from light to very dark. There are even worse like Z5U and Y5V which fall into class III, although I've also seen them classified as class II. Not that colors mentioned are only for SMD types as they are not coated and you see bare ceramic.

author: alm
author_rank: Super Contributor
timestamp: March 27, 2023, 11:48:24 am
content: Quote from: joniengr081 on March 27, 2023, 08:34:59 amThey will let you filter by dielectric. Just look up which dielectric corresponds to which class, e. g. C0G and NP0 are class I, X5R and X7R class II, Z5U class III.Quote from: joniengr081 on March 27, 2023, 08:34:59 amNo, this severe capacitance decrease under DC bias is limited to ceramic caps. Tantalum caps just don't like being operated too close to their maximum rating, and need de-rating to be reliable.

author: asmi
author_rank: Super Contributor
timestamp: March 27, 2023, 02:52:59 pm
content: Ideally you'd want to pick the smallest package for a given capacitance/voltage rating, because inductance (ESL) is determined by the package, and for decoupling you want it to be as small as possible. Realistically, packages smaller than 0402 are harder to assemble - not all PnP machines will place them accurately enough, on the other side, depending on a ball pitch, larger packages will simply not fit within a via field, which places an upper limit on the package size, and then there is the price - smaller than 0402 caps typically cost more than 0402. I typically use 0201 for 1 mm and 0.8 mm packages, if you can use via-in-a-pad technology, you can get away with using 0402s for 1 mm pitch FPGAs.

author: wraper
author_rank: Unknown
timestamp: March 27, 2023, 03:15:01 pm
content: Quote from: asmi on March 27, 2023, 02:52:59 pmNo. For decoupling fast stuff generally you just want smallest package, not maximum capacitance available within smallest package. Then for really fast stuff you really need to use PCB layer stackup for decoupling because capacitors no mater how small cannot do all of the decoupling needed.

author: asmi
author_rank: Super Contributor
timestamp: March 27, 2023, 03:55:35 pm
content: Quote from: wraper on March 27, 2023, 03:15:01 pmYes, you need it all to cover as wide of a frequency range as possible. On many fast devices there are on-package decoupling caps to cover some of it, but they typically cover the highest frequencies, so you still need to cover lower frequencies.