just some infor..good luck Alan.
The vast majority of solid tantalums are the "dry-slug" or "solid
tantalum" capacitors. A layer of manganese
dioxide (MnO2) is layered over the pentoxide followed by a
layer of colloidal graphite and a layer of silver paint. There are
minor variations on this. With no liquid involved, they can
be sealed with just an epoxy dip, although the better ones
may have a molded body. The pentoxide layer is prone to
defects, and the key to the solid tantalum capacitor's reliability
is that the MnO2 provides self healing. If a flaw in the
pentoxide layer develops, the leakage will cause localized heating
in the MnO2 and convert it to Mn2O3, a much less conductive
oxide, sealing off the flaw. This mechanism is not perfect,
however, and failures due to dielectric flaws have been a
traditional problem for solid tantalums. Solid tantalums aren't
usually made with high working voltages because the particle
size limits the dielectric thickness that can be grown; 50
volts is usually the upper limit. There are however, a very
few solid tantalums with working voltages as high as 125
volts. Most dry tantalums are rated to no more than 85C to
125C, but a few are rated for use to 150C with voltage
derating. Solid tantalum capacitors are commonly available in
surface-mount packages, in molded bodies.
At 03:08 PM 8/8/00 -0700, you wrote:
>Alan
>
>I do not have an answer for your tantalum capacitor
>problem. However, if you are experiencing unwanted
>damage to your electronic assembly as a result, you
>should consider repairing the damage, rather than
>scrapping the boards. There are a variety of
>acceptable methods and procedures found in IPC-7721.
>For more information you can visit circuittechctr.com.
> The web site is full of useful information.
>
>Mike Connolly
>Circuit Technology Center
>--- Alan Peters <[log in to unmask]> wrote:
>> I have been asked to investigate two failures of 10u
>> 25v solid tantalum capacitors.
>>
>> They are rectangular plastic/resin encapsulated,
>> sintered tantalum/ manganese
>> dioxide/ graphite-silver types.
>> These items are "cremated" beyond investigation!
>>
>> I am lead to believe (hmm!) all correct operating
>> conditions have been met/not
>> exceeded i.e. excessive voltage, ripple and reverse
>> voltage.
>> The capacitors have seen service for at least 5
>> years in a very humid environment.
>> I am not aware if they are from the same batch. One
>> was fitted across a +15v rail
>> in one unit and the other was fitted across a -15
>> rail in another unit.
>>
>> Question? Does anyone know of an ageing condition,
>> or what the effects of humidity
>> are that can lead to failure of these capacitors?
>>
>> Alan Peters
>> Failure Analysis Engineer
>> Materials & Components Labs
>>
>>
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