Jim Yanik wrote:

"ian field" wrote in
:


"Grant" wrote in message
. ..

On Sun, 21 Mar 2010 08:01:20 +1100, Franc Zabkar
wrote:


On Fri, 19 Mar 2010 02:24:18 -0000, "Arfa Daily"
put finger to keyboard and composed:


Anyone read anywhere what the failure mechanism is for solid
dielectric caps
of the tantalum variety ? In my experience, no matter what the
value, working voltage, or format (bead, bullet or box), they always
seem to fail
leaky. Not open or short (well, very occasionally short). Just
leaky.

I've had one particular board where the same tantalum cap (bead)
would go leaky.

What voltage across it? Maybe they're no good at near zero DC?


All other faults were dead shorts across a supply rail. I must have
replaced over a hundred of them (bead) in all manner of equipment. I
recall seeing one tantalum cap explode and catch fire like a match
head.

I've only seen them fail when reverse connected. Spectacularly in
one case with reverse connection on 24V battery -- high pitch bang
behind my back, when I turned around to look at the board and there
was the remains of the cap, standing on one leg, glowing red hot,
cooling off. No sign of the other leg or case...

So far I've never seen one blow - only go leaky in small signal
circuits. Wherever possible I try to find a non-polarized cap (film or
ceramic chip) small enough to fit the space - that strategy never
bounced a repair.





In TEK scopes,they were often shorted,and were used in PS decoupling
circuits ALL over the scope.
Sometimes,I could see a tiny pinhole in the epoxy coating.
I've seen them burned,too,even just the leads remaining,sticking out of the
PCB pads.

IMO,it's usually OV spikes or polarity reversals that take out tantalums.
Or manufacturing defects.

Oh I love tantalums. "Ashes to ashes, dust to dust"