[sdiy] Failure modes on bypass caps...

[Seb Francis]

Seb Francis wrote:

> Aaron Lanterman wrote:
>
>>
>>> But in general, ceramics "do not fail", so there isn't a whole lot 
>>> to worry about with them.
>>
>>
>>
>> So it sounds like the take home message is to always use ceramics for 
>> your bypassing pleasure?
>>
>
> Bad idea .. ceramic caps have very low ESR (a good thing for 
> decoupling!) but if you have a load of them all over a PCB, the 
> combined effect of their capacitance and inductance of the PCB tracks 
> in between will create a resonant network that will tend to 'ring' at 
> a particular frequency.
>
> This is overcome by putting a cap with a higher ESR ('equivalent 
> series resistance) in parrallel somewhere on the board.  A tantalum or 
> electrolytic is suitable.
>
> A general rule of thumb is:
>
> Lots of 100nF ceramic placed as near as possible to where they are 
> needed (e.g. by ICs)
> A single 20uF electrolytic or 10uF tantalum per power bus where the 
> power comes into the module.  For very big boards, larger electrolytic 
> or tantalum should be used (perhaps 20uF electrolytic per 10 100nF 
> ceramic caps, or 10uF tantalum per 20 100nF ceramics)
>
should have said "For very big boards, larger VALUE electrolytic or 
tantalum should be used ..."
it's still only necessary to use 1 (unless the power tracks are 
particularly thin, long, or carrying high current)