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Capacitors packages for PCB

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joniengr081:
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 ?

SiliconWizard:
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. ::)

thm_w:
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

wraper:

--- Quote from: SiliconWizard on March 23, 2023, 09:43:07 pm ---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. ::)

--- End quote ---
MTBF 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/25074 They 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.

joniengr081:
Thank you very much for reply.

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