Re: [SI-LIST] : Some questions about capacitor reliability

Fred Balistreri (fred@apsimtech.com)
Thu, 11 Dec 1997 17:54:40 -0800

Bob Perlman wrote:
>
> Hi -
>
> One recurring issue among designers and signal integrity engineers is
> how to properly bypass component supply voltages. I bring this up not
> because I want to reopen the issue of just how to do bypassing; I'm
> firmly convinced that bypassing in the form of discrete capacitors (as
> opposed to just relying on plane-to-plane capacitance, or power supply
> capacitance, or faith) is a requirement for high-speed designs.
> However, this does bring up a related, troubling issue: capacitor
> reliability.
>
> It's been my experience that capacitor reliability is a serious
> problem. Tantalum capacitors have a distressing tendency to fail short
> and burn up, and even multi-layer ceramic capacitors sometimes short,
> albeit with less spectacular results. Adding to the severity of the
> problem is the fact that these parts are used in great numbers,
> increasing the probability of failure. I'm familiar with some rules of
> thumb for selecting and using capacitors: don't use the largest value in
> a given package size; derate tantalum voltages by 2 to 1; follow the
> manufacturer's temperature profile recommendations when assembling the
> devices onto boards, etc. However, while following these rules lessens
> the chance of mishap, failures still occur at rates that dwarf those of
> other components (your mileage may vary, of course).
>
> So, my questions are these:
>
> 1) What rules do you follow when selecting and assembling bypass
> capacitors?
>
> 2) Do you know of any "capacitors-are-my-life" consultants who can give
> me better insight into capacitor failure mechanisms and how to avoid
> them?
>
> Thanks,
> Bob Perlman
> Acuson Corporation
I may be wrong but I'm sure that I have seen MTBF data on components
such as capacitors. Generally speaking the more expensive a capacitor
the better the MTBF. However that's not the case with tantalum. Needless
to say that capacitor voltage ratings should be taken seriously. Bypass
capacitors are expecially tricky becuase of voltage spikes than can
occur during turn on/off if the supply is not designed properly. Again
generally speaking the cheaper the supply......well you get the point.
Years ago I had the same problem. Except for the case of tantalum the
problems stopped once we stopped the practice in indiscriminately buying
by the bulk without getting engineers involved.

If the manufacturer does not publish the MTBF data ask for it! If they
can't come up with it find a manufacturer that will. At least one would
know what to expect.

Best regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com