[SI-LIST] : Some questions about capacitor reliability

Bob Perlman (perlman@acuson.com)
Thu, 11 Dec 1997 15:45:18 -0800

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