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Capacitor reliability

Discussion in 'Electronic Components' started by mark, Oct 11, 2006.

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  1. mark

    mark Guest

    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
     
  2. Nermal

    Nermal Guest

    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.
     
  3. tlbs101

    tlbs101 Guest

    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
     
  4. mark

    mark Guest

    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
     
  5. tlbs101

    tlbs101 Guest

    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
     
  6. Ken Finney

    Ken Finney Guest

    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.
     
  7. mark

    mark Guest

    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
     
  8. Eeyore

    Eeyore Guest

    What's the failure mode though ?

    Graham
     
  9. mark

    mark Guest

    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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