>>> Tantalum make excellent decoupling capacitors ****PROVIDED*** that you 
>>> can
>>> absolutely guarantee that their voltage rating is not exceeded by even 
>>> the
>>> shortest duration voltage spike. Almost always you can't.

> ?  Almost always we do, as other parts like semiconductors also react to 
> very short spikes.  What is different?

> I have never had any tantalum blow, and on things I repair the problem of 
> dried electrolytics is more frequent than shorted tantalums.  Also, i have 
> replaced a handful of cracked multilayer chip caps, but unly a couple 
> tantalums.  I dont have the same kind of experineces like you seem to 
> have...

It's not just my experiences, but I have personally seen the failure modes 
that others tell about.

The key problems are these:

- When a tantalum capacitor is connected to a "high energy" source (such as 
a power supply rail) and is subject to an over-voltage spike of relatively 
low magnitude, the capacitor can (and often does) fail short circuit and 
allow destructive high energy dissipation in the capacitor. It's not the 
spike energy that causes the destruction - all the spike needs to do is 
start the initial breakdown and the power supply does the rest. Other types 
of capacitors are nowhere as near as sensitive to fault initiation by over 
voltage and do not have the catastrophic failure mode. eg Aluminium wet 
electrolytics (the standard electrolytic capacitor) can take significant 
overvoltage or limited reverse polarity without catastrophic breakdown.

- The failure mode is often (usually?) a very hard short circuit. A bead of 
metallic tantalum forms between the leads. This clamps the power supply to 
ground and can be quite hard to find. On a board with many tantalums used 
for power supply decoupling with one only shorted to ground,troubleshooting 
without suitable tools can be difficult. (A current tracer or very low ohm 
meter maybe required - or unsolder each one in turn and test it :-(  ).

- As well as short circuiting on failure the capacitor may do some or all of 
smell REALLY bad, smoke, shriek, emit a jet of fire or explode. I have seen 
a single tantalum capacitor do all 6! :-)

A tantalum capacitor connected to a high energy supply can be thought of as 
a randomly operated power supply crowbar.
If you want a product that generates service calls out of warranty, use 
tantalum decoupling caps and set the warranty period as short as possible 
:-).



        RM


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