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Capacitor reliability

M

mark

Jan 1, 1970
0
I've been seeing some disturbing occurances re Tantalum capacitor (both
thru-hole and SMT). It seems that the reliability is not what I was hoping
for. (well, I was hoping for infinte life but ...)

Some have suggested replacing the Tant device with MLCC capacitors. I am
concerned that the reliability may be worse w ceramics.

If one goes into Mil-HDBK-217F, tantalum cap have a higher base reliability
then do ceramics. As long as the derating is below say 50%, then tant caps
((according to 217F)are more reliable then ceramics.

Any comments? Does someone have information that says ceramics are more
reliable? Tks

M Walter
 
N

Nermal

Jan 1, 1970
0
Hermetic tantalums do not like surge currents, high ripple, and reverse
bias.
Non-hermetic tantalums do not like moisture (their useful life is
shortened). The voltage rating becomes slowly degraded over time.
Surface mount capacitors are destroyed and/or damaged if the board is
flexed. Otherwise they are relatively immune to: surge currents, high
ripple, reverse bias, and moisture.
 
T

tlbs101

Jan 1, 1970
0
mark said:
I've been seeing some disturbing occurances re Tantalum capacitor (both
thru-hole and SMT). It seems that the reliability is not what I was hoping
for. (well, I was hoping for infinte life but ...)

Some have suggested replacing the Tant device with MLCC capacitors. I am
concerned that the reliability may be worse w ceramics.

If one goes into Mil-HDBK-217F, tantalum cap have a higher base reliability
then do ceramics. As long as the derating is below say 50%, then tant caps
((according to 217F)are more reliable then ceramics.

Any comments? Does someone have information that says ceramics are more
reliable? Tks

M Walter

Are you using military grade caps, or, have you compared at the
reliability numbers for commercial grade caps in 217F? That's one
point.

The other point -- when I do reliability analyses, I go to the
manufacturer for the latest measured failure rates. It is only as a
last resort that I use 217F's empirical failure rates (although they
are still very useful). Check with the manufacturer and see if they
have published failure rates for their caps.


Tom
 
M

mark

Jan 1, 1970
0
tlbs101 said:
Are you using military grade caps, or, have you compared at the
reliability numbers for commercial grade caps in 217F? That's one
point.

The other point -- when I do reliability analyses, I go to the
manufacturer for the latest measured failure rates. It is only as a
last resort that I use 217F's empirical failure rates (although they
are still very useful). Check with the manufacturer and see if they
have published failure rates for their caps.


Tom
Tom
thanks for the thoughtfull reply. Yes, I am at the last resort stage.
My problem is: should my designs go wholesale to MLCC (if I can) instead of
tantalum chip? Is there a failure mechanism in tantalum which tells me I
should avoid them? At this time I am not aware of any. What is the
Reliability Engineering community seeing?

M Walter
 
T

tlbs101

Jan 1, 1970
0
mark said:
Tom
thanks for the thoughtfull reply. Yes, I am at the last resort stage.
My problem is: should my designs go wholesale to MLCC (if I can) instead of
tantalum chip? Is there a failure mechanism in tantalum which tells me I
should avoid them? At this time I am not aware of any. What is the
Reliability Engineering community seeing?

M Walter

In my experience, which is limited to low volume production (1000s of
units), I have seen 2 tantalum chip caps (CWR type) fail
catastophically (explode on-board), while I have never seen a
multi-layer ceramic fail. Our designs are very conservative as to
applied voltage (stress). for one of the failed caps operator error
involved (i.e. hooking up a power supply backward, etc.). I didn't
read the final failure report as to the cause of failure (wasn't my
project), but I heard the parts lab "ruled" it an isolated event, not
indicative of a systematic type parts flaw. For the other failed cap,
it was on a prototype and the power supply was accidentally connected
backwards (this was my project).

Our company doesn't use "dipped" radial tantalums, so I can't speak to
that.

I still design with CWR tantalums, and have not been told by the
parts-engineering group to quit using them

If you can stand the difference in ESR, size, ESL, and other parameters
of the MLCCs compared to the tantalums, to me, that would be the
deciding factor -- especitally if you have any doubts about
reliability.

Tom
 
K

Ken Finney

Jan 1, 1970
0
mark said:
Tom
thanks for the thoughtfull reply. Yes, I am at the last resort stage.
My problem is: should my designs go wholesale to MLCC (if I can) instead
of
tantalum chip? Is there a failure mechanism in tantalum which tells me I
should avoid them? At this time I am not aware of any. What is the
Reliability Engineering community seeing?

There isn't a simple answer. Manufacturing operations and the hardware's
environment induce failure modes. And often, there are "failures" and there
are "FAILURES". If your hardware isn't intended to be repairable or live in
an explosive atmosphere environment some of the types of failures might be
non-issues. If you can stand the low voltages available, the tantalum
polymer types are a very good all around choice.
 
M

mark

Jan 1, 1970
0
tlbs101 wrote:
Tom,
Not sure I agree that all things being equal mlcc are the answer, hence why
I am asking
I am aware of some issues w tantalum caps bursting into flames. Not my major
concern (due to other design factors): I am more worried about inherent
reliability.

I base some of my answers/concerns from Mil-Hdbk-217F data. The '217F base
failure rate for tantalum caps is 10 times better then the base failure
rate for ceramics. The penalty for operating at elevated temperatures is
higher for ceramics, and if you were derate both to around 50% of rated
voltage the stress factors are about the same. Given that Mil-Hdblk-217
says use a tantalum cap where temperature will be high. Also, I get
concerned about cracking ceramics in the large case sizes required for say
a 4.7 uf device

If I jump onto the mlcc bandwagon to replace tantalums will I be going from
the frying pan into the fire?

M Walter
 
E

Eeyore

Jan 1, 1970
0
mark said:
tlbs101 wrote:
Tom,
Not sure I agree that all things being equal mlcc are the answer, hence why
I am asking
I am aware of some issues w tantalum caps bursting into flames. Not my major
concern (due to other design factors): I am more worried about inherent
reliability.

I base some of my answers/concerns from Mil-Hdbk-217F data. The '217F base
failure rate for tantalum caps is 10 times better then the base failure
rate for ceramics.

What's the failure mode though ?

Graham
 
M

mark

Jan 1, 1970
0
Eeyore said:
What's the failure mode though ?

Graham
I'm seeing some increased leakage and increased ESR after hi temp exposure.
Root cause was identified as cracks in the dielectric film. Also the amnuf
at least has said that it is due to reflow soldering process: ind test lab
say that cracks are from manf process faults.
M Walter
 
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