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Burn In Change in Characteristics

Discussion in 'Electronic Design' started by rickman, May 14, 2013.

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  1. rickman

    rickman Guest

    I have an audio I/O board that I test by looping back the analog I/Os
    externally, stimulating the digital input and examining the digital
    output. The functional test shows a -0.9 dB drop in gain at 20 Hz and a
    -2.5 dB drop in gain at 20 kHz. After a 24 Hr burn in the 20 kHz gain
    is the same within measurement error, but the 20 Hz gain is always down
    to -1.0 dB.

    I don't know what could be causing this. Do capacitors age have an
    initial rapid aging of some sort? The low end response is controlled by
    a couple of X5R caps on each channel. I can't think of anything else
    that would affect the frequency response.

    I suppose there could be a small effect due to a drift in the DC bias
    point which is set by zener diodes. This would modify the capacitance
    of the coupling caps a bit. Do zener diodes shift characteristics in
    the initial 24 Hrs of operation? I guess I could measure the DC set
    point on some before and after boards.
  2. rickman

    rickman Guest

    So you are saying this is purely a function of time from exposure to the
    Curie temperature? What I am seeing is happening at a normal operating
    voltage not far above room temp. 24 hrs under power and *every* one of
    the boards shifts the 20 Hz attenuation from -0.95 dB (give or take a
    couple of 0.01 dB) to -1.05 dB (again about 0.02 magnitude variation).
    I didn't even notice this for the first few hundred boards tested for
    frequency response. But the current order is a lot larger than usual
    and I'm staring at these numbers all day long. So I finally realized
    how consistent the connection is to before and after burn in. It is
    virtually a perfect correlation.

    I don't think this is a matter of just time as the boards have been
    sitting for varying amounts of time since they were soldered. But the
    burn in time is pretty consistent since that is the bottle neck in
    testing and after 23.5 hours they are outta there to make way for the
    next batch.
  3. Guest

    I say that something that means -1dB at 20Hz for audio isn't terrible
    critical, maybe more it down to 10 to be sure

  4. How big a change in C is that? Could it just be a temperature thing.
    Are the boards warmer after 24 hrs of burn-in? Or is the sign in the
    wrong direction for that?

    George H.
  5. Guest

    I read it as the caps aging in use, and reset if you bring them up to
    curie temperature

    try resoldering the caps on a board that has been burned in and
    see if it goes back to start

  6. Robert Baer

    Robert Baer Guest

    I think that you will see different patterns of burn-in "drift" with
    different manufacturers, and most certainly with different ceramics
    (X7R, Z5U, X5R, Y5V, U5V, X7T, and the "gold standard" C0G).
    Since your correlation is so good, perhaps a sample as small as 5-10
    would be sufficient.
  7. Is it just a thermal effect, is the board still warm from the burn-in?
  8. rickman

    rickman Guest

    This is not a problem at all. I just noticed that is was very
    consistent. I thought this was just variation in the measurement until
    I realize the correlation. No, the goal is just to be better than -3 dB
    at 20 Hz and 20 kHz so the units are all fine. I'm just curious about
    the effect.

    BTW, the frequency limiting parts mostly are pushing the
    size/capacitance tradeoff and there is no more room on the board for an
    extra via much less another huge cap. I think the ones on the output
    are 10 uF. Inboard there are 2.2 uF IIRC.
  9. Robert Baer

    Robert Baer Guest

    One can double or triple the capacitance,using the SAME capacitors
    and the SAME board space very easily; stack them.
    With a small change in board area, one can put the SMT caps on the
    pads sideways and maybe get three in almost the same area.
    Then stack another 3 above...
    Any of the above tricks require NO extra via.

    BTW, there is nothing wrong in putting a via in a pad - takes ZERO
    extra board space.
  10. Yeah, more data! does the voltage change? What about T?
    Say is there enough room an X7R cap?

    George H.
  11. rickman

    rickman Guest

    What does that mean, 2% per decade? Decade of what?

    There is no reason to expect a temperature difference. The boards are
    pulled from a chassis where they aren't really warm anyway, and sit as
    they are tested one at a time. This takes about an hour and there is
    virtually no difference from the first to the last. I expect this is
    definitely related to a part being under voltage. Obviously the caps
    are the most likely, but I don't know much about such a subtle effect,
    so I'm asking here.

    Changed relative to what? The only factor I see is the time under
    voltage. Most of the caps on this board are as small as I can get them.
  12. rickman

    rickman Guest

    What about T? There is only minimal T fluctuation. The boards are
    always at room temp, even in the burn in chassis, they are fan cooled
    and not very warm to begin with, so the ICs just don't get warm enough
    to even feel.

    What voltage are you talking about? I don't see any reason for a
    voltage to change other than possibly the zeners that set the bias point
    of the amps. I think I mentioned them in my first post. I don't have
    time right now to mess with it. I'm trying to get product out the door.
    I was just curious if anyone knew of such an effect in caps or the
  13. rickman

    rickman Guest

    I have no reason to think there is any difference in temperature in the
    two measurements. It takes me an hour to test a batch of board after
    being under voltage. So any temp difference would show as the effect
    wearing off with time out of the burn in chassis. This is not elevated
    temperature burn in and the chassis is fan cooled being designed for
    much higher power boards.

    I'm starting to wonder if the zeners are drifting while powered, 6.2
    volt units.
  14. rickman

    rickman Guest

    The "board" is 51 daughter boards of fairly low power consumption. The
    burn in chassis is a production card rack with fans. So I doubt you
    could measure 1 degree difference from off to powered on. Then it takes
    half an hour to swap out the daughter cards and the testing takes an
    hour as each one is tested separately, one at a time. So there would be
    lots of time to cool down and a drift in the effect would be seen.
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