I'm on my first large board design, and it's only begun to dawn on me how infuriating capacitors can be to design with.
Exhibit A: I need 100uF for a simple circuit to keep my FPGA on for 1ms after power is cut. This is hard to find in a cost-effective ceramic, but because it is just being used for energy storage I can deal with tantalum. So I start looking for a 100uF tantalum that's good to 25V, should be easy right?
It probably is, but I've noticed that all tantalums quote endurances between 2000 and 10000 hours. This seems terrible, that's barely a year of continuous operation. These numbers are quoted at Tmax, so how do I derate for a lower temperature? For example, at 55 °C I imagine the lifetime is more reasonable, but I can't find any resources on this. Is there any way to know? Is it a crapshoot? If so, isn't designing with tantalum caps incredibly risky in something that expects to be used on the order of years? How do you deal with this conundrum?
Edit
I've since come across two resources that helped my comfort level with tantalum lifetimes, which I'll add here to help anyone else who comes across this question. AVX has a useful primer on tantalum derating that goes over the MTBF calculations:
Voltage Derating Rules for Solid Tantalum and Niobium Capacitors
Better still, Vishay has a very simple "FIT calculator" that does easy MTBF calculations for all of their tantamlum families. Playing around with this was super useful for framing typical MTBF values in my mind:
Bizarrely, Kemet only has such a calculator for aluminums, and while I can find references online to an AVX calculator I can't find it on their site as of Sept 2017.
Best Answer
If the manufacturer doesn't provide the information then you could test it yourself or email the manufacturer and see if they could provide more info. There are two reasons why they didn't:
One is the lifetime for a cap at room temperature is significantly longer than a high temperature capacitor and the testing time becomes prohibitively long.
The second reason is the degradation happens faster at high temperatures so they provide the spec at the high temperatures to provide and upper bound and to show that the lifetime will be degraded if you run them at +50C.
Another thing to note is a manufacturer can never provide an exact time of failure only a mean time between failure (MBTF), so if your application is critical, testing and buying high reliability parts will be necessary
I've never had a problem with tantalum capacitors running in products at 40C for years. And the specifications probably degrade slightly from the original specs, but in my application I use them as power filter capacitors and my design is tolerant to slight degradation of a few percents.
1) Don't use the capacitors for applications that are dependent on the capacitors value.
2) Don't run them at high temperature.
3) Use the right kind of tanalum capacitor, the electrolyte makes the difference.
4) Know what the faiure modes are and mitigate them: