While in the US Navy, we had to do equipment inspections.
One quarterly was to examine the capacitors in the power supply of
one piece of equipment.
We were to look for leakage (sulfuric acid) from CL65 type wet slug
tantalum capacitors.
Shortly after that CL65 type capacitors were disapproved for military use.
I never saw one that leaked in that equipment, but, have seen a
number of boards damaged from seal leakage on CL65 capacitors.
Something to look out for.
The CL65 capacitors probably have a pure silver case a sulfuric acid
as an electrolyte.
The seal is Teflon.
We also had an interesting failure mode for ATC ceramic capacitors.
This failure mode will only occur in a sealed environment (submarine.
Just an observation.
73
Glenn
WB4UIV
At 09:46 AM 2/8/2010, you wrote:
The history of tantalum failures is wide and varied, but
there are some common characteristics:
1) The tantalum is in a power supply circuit and receives
a rapid ramp from 0V to operating voltage.
2) The tantalum is spec'd close to its operating voltage,
very close.... 5V on a 6.3V part, 12.5V on a 15V part...
3) The tantalum is dry slug, and is sealed with epoxy.
4) The instrument has been powered down for an extended
period.
HP equipment from the 1980's is pretty immune to the problem
because they typically use hermetically sealed mil spec
tantalum capacitors. Tektronix equipment from the 1980's
is infested with tantalum problems because they used the
cheap epoxy dipped parts.
Tantalum failures are pretty rare in equipment that is
run continuously. Tantalum has a self healing feature that
corrects any small problems while in operating... Large problems
result in detonation.
Dipped tantalum capacitors of any age are prone to failure.
The tendency can be mitigated largely by never allowing a
tantalum capacitor to see voltage above 50% of its rating.
And finally, powering a tantalum in reverse, will cause instant
and irreparable damage.
-Chuck Harris
Tom Van Baak wrote:
I powered up a 5071A to watch the end of Loran-C today
and was greeted by the special smell that only a mother
board could love.
Does anyone know the history of tantalum capacitor
failures in ten-year old [HP/Agilent] test equipment?
This is not my first. Last one was more like July 4th.
Thanks,
/tvb
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