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Measuring continuity

Discussion in 'Electronic Design' started by TTman, Jul 20, 2011.

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  1. TTman

    TTman Guest

    Between two/multiple nodes across connectors. The manual test spec says less
    than 1 Ohm. Easy, just use a milliOhm meter.
    The automatic test spec uses a fancy test box that costs mega bucks.(PC +
    endless cards +30 odd 25 way D sockets/plugs)
    From the test print out, it seems the test gear is measuring millivolts.
    This leads me to believe that they are injecting a constant current into one
    node, grounding another node and measuring the small voltage at the first
    node using an ADC(if that makes sense). I think the test gear uses generic
    Nat semi industrial measurement cards.
    Is this how any of you would expect a continuity test to be accomplised
    using ATE ?
    If not, anyone got any experience of doing this ? Typical values are in the
    low tens of milliohms.
    TIA
     
  2. tm

    tm Guest

    Google "four wire resistance measurement".

    Any good bench DVM from HP or Fluke can do this.



    tm
     
  3. Winston

    Winston Guest

    Yes. That would be a Kelvin connection:
    http://www.cirris.com/testing/resistance/fourwire.html

    I've used it in industry to test PCB trace resistance using
    non-automatic tools. It would be a simple setup using a
    data acquisition system.

    --Winston
     
  4. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    if you want to measure milliohms and even microohms, the four wire
    method is essential and you need a measuring current high enough to
    generate at least some ten to hundred microvolts over the measured resistor.

    Bye
     
  5. TTman

    TTman Guest

    ATE's great for doing lots of tests on a board, its an expensive way
    of doing just one.

    Qualification....
    I know about 4 wire/kelvin. There are a large number of boards with a large
    number of continuity checks. Hence the ATE route. This route precludes the
    use of 4 wire. The accuracy is unimportant, 10%,20% is no problem. The spec
    is simply that is must be <1ohm and we measure typically low tens of
    milliohms.
    The bottom line/original question is how does the ATE achieve this with 2
    wires ?
    My ultimate goal is to design a stand alone micro system (AVR) to do the
    tests.
     
  6. Winston

    Winston Guest

    TTman wrote:

    (...)
    Clarification, please?

    If you now "measure typically low tens of milliohms" with acceptable
    precision, accuracy and repeatability using your ATE, and you need to
    resolve only to ~ 780 mOhms +- 20 mOhms, what is the problem?

    If your ATE cannot now provide that level of precision, accuracy and
    repeatability, Kelvin is your only choice.
    Bed of nails or 'flying probe'?


    'Bed of nails' Kelvin is possible if you have a tame machinist
    on staff. 'Flying Probe' Kelvin is a bit of a challenge but
    quite possible if you can spin the PCB to accommodate testability.

    :)

    --Winston
     
  7. TTman

    TTman Guest

    I'd like to replicate what the ATE does, which is provide a reasonable
    measurement of the contact resistances.
    From a test board with various connectors, there are cables connecting to a
    PC using regular wires and connectors. I can only assume that they use a
    'group' resistance in their calculations that are representative of the
    overall resistance and subtract that from measurements..... Our company uses
    a mixture of technologies- Takaya flying probe machines, bed of nails (
    which does reasonable milliohm contact resistances) and this other archaic
    machine....
    I agree, resolving continuity to 1 ohm is easy, but I'd like a better( more
    accurate) solution. None of our current machies fit this particular project
    because of the large setup costs. We also have a Cirrus system running 4
    blocks which could be a solution.....All of these ( except Takaya maybe) are
    2 wire systems....
    It looks like one solution would be to use high side current sense circuits,
    fed by a constant current source and a load dump to ground. Seems feasable
    to me ?
     
  8. Winston

    Winston Guest

    TTman wrote:

    (...)
    Kelvin is your answer then. See the link at the bottom of this page!

    Tailor your maximum forcing voltage to be well below the saturation
    threshold of your lowest semiconductor and read your resistance as
    the voltage drop divided by the constant current.
    This meter is really nice, for example. (100 uV forcing voltage, max.)

    www.simpsonelectric.com/uploads/File/datasheets/444-2.pdf

    (...)

    When I last looked into this, I was checking out *coaxial* pogo pins.
    You get two contacts for the price of one, theoretically.

    And look at this! They actually exist now!

    http://www.teeys.co.jp/english/product/product_8.html

    Note that just about everybody else calls their spring loaded contacts
    'coaxial' but they are still only one contact per channel.

    --Winston
     
  9. josephkk

    josephkk Guest

    You may be in for some reverse engineering here. There is another variant
    that i have seen, to place many board runs in series and then measure the
    differential voltages at successive connection pairs. Kind of like this:

    Current Measurement Selector
    Source Points "Relays"

    ------------------+----------------------------------- X
    |
    ^ contacts / connectors
    ^
    | v1
    R trace
    |
    ^ contacts / connectors
    ^
    |__________________________________ X
    |
    ^ contacts / connectors
    ^
    | v2
    R trace
    |
    ^ contacts / connectors
    ^
    |___________________________________ X
    |
    ^ contacts / connectors
    ^
    | more voltages
    ~ ellipsis
    ~
    |
    ------------------+----------------------------------- X

    ?-)
     
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