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electrolytic caps in toaster oven

Discussion in 'Electronic Design' started by Jamie Morken, Nov 7, 2004.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi all,

    How well do SMT electrolytics (the silver can type) stand up to heat
    during soldering in a toaster oven? I have had to rework a part on a
    small board and it has been in the toaster oven 4 times now :) The
    board is looking a little charred but everything still works, but I am a
    bit concerned about the internals of the caps.. thanks,

    cheers,
    Jamie Morken
     
  2. Robert Baer

    Robert Baer Guest

    One could get the manufacturers spec sheet on the particular type and
    model of capacitor in question.
    That is to say, each manufacturer has their own manufacturing and
    quality standards, and ther are different models within a give type.
    For example, there are many models of aluminum capacitors, and each
    one has their own temperature rating, recommended SMT temperature
    profile, etc.
    BUT.
    The best way is to actually test some in the environment you use.
    Take ten or more and mount them on a PCB with test leads (say common
    ground (negative side) and seperate test points; maybe even guild the
    lilly and put a diode and resistor in series to common charging voltage.
    Heat them, cool them, charge them to rating, let them sit for 10
    hours, discharge each one with resistor and test capacitance and ESR.
    repeat 20 times or until 2-3 die.
    Make a record of all readings, heat times, etc.

    Ig you change brands, types or models, start again.
     
  3. Terry Given

    Terry Given Guest

    why only 20 times - repeat until dead. automated test rigs are great for
    this. Its funny to see light switches being tested in a standards lab -
    a test finger (literally) is used to turn the switch off and on again
    with a known load, until the switch dies.

    A have often obtained a production version of a psu, and set it up with
    some timed relays to power up into a load, shut down, discharge all
    caps, repeat in some suitably short time interval; then just left the
    darn thing in a corner

    The real problem with the test-and-see approach is of course the lack of
    repeatability in terms of temperature, gradients etc. I suspect that
    from job to job your toaster oven will have wildly different temperature
    profiles (for example, I betcha it varies strongly with AC line voltage).

    If you look at smt capacitor datasheets they ought to have reflow
    profiles, which would be a good start. Its been a few years since I did
    any significant work on capacitor lifetime, but as I recall there is an
    upper temperature limit, beyond which the dielectric (usually a modified
    carboxylic acid, but I'm no chemist) is buggered. I have seen a LOT of
    cap data from Hitachi AIC (including, for example, the distribution of
    cap lifetimes - they used -3 Sigma as their rated lifetime), and they
    specifically mentioned the maximum tolerable hot-spot temperature.

    rate-of-change of temperature (dT/dt) is usually quite important, to
    minimise mechanical stresses due to CTE mismatch in materials. Probably
    less of a concern with electro's than say ceramics.

    Cheers
    Terry
     
  4. Robert Baer

    Robert Baer Guest

    I mentioned 20 times, because i think the caps will die long before
    then.
     
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