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Thread: PCB true Position Failures

  1. #1
    Associate Engineer
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    PCB true Position Failures

    I am interested in hearing comments regarding major contributors to true position failures; Design? Material? Method? Drill; Complexity of PCB such as datum structures and # of holes........

  2. #2
    Lead Engineer RWOLFEJR's Avatar
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    Hi and Welcome...


    (To be said in the voice of Mr. Mackey from South Park...) True position failures are bad... mmm-kay...

    Sorry, couldn't resist.

    Well first of all you question design and as a designer of things... and I think I can speak for all engineers out there on this one... Any failure to attain true position on any component certainly wasn't any fault of mine...!! That would be like saying 2 + 2 doesn't always equal 4... ! Numbers on a print are just that... numbers. A component is either to the print or it is not.

    Material? Yes.

    Method? Sure.

    Drill? Certainly.

    Complexity? Shouldn't.

    I'm guessing this might be part of a homework assignment and board policy says "no helping out with homework." Or something like that. ...but I think I was vague enough that we'll be O.K. on this one.

    Good Luck,
    Bob

  3. #3
    Administrator Kelly Bramble's Avatar
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    The biggest contributors of True-Positon (As-built) failure I'm aware of are:

    The positional tolerance specification is out of the manufacturing process capability for the given material. For example, a Position tolerance of .001 for a .250-20 UNC threaded hole feature in Aluminum alloy.

    The designer/engineer failed to run the math on the perspective mating components such that even if the features are manufactured within position tolerance specification the parts will not mate due to misalignment or interference.

    Improper datum specification on the perspective mating parts.

    Of course there are manufacturing failures in setup, tool selection, process selection, machine failure, etc...

  4. #4
    Technical Fellow
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    Unless I am misunderstanding the question, to me it points more to manufacturing tolerances of the machine doing the work.

    I recall many years back designing a test bed for around 1200 chips (I think) on a Silicon wafer (6" or 8" diameter, I forget, I am old ). Each chip had maybe ten test points that had to be hit with pinpoint (0.0003") accuracy and repeatability. It didn't matter which style of chips were on the wafer, we programmed the computer to how many and where the test-pads were relative to each chip's datum point and let it run. The Datum was optically detected for each chip. Probably one in four or five-hundred Datums, the machine had to seek a few tenths. But the test-pads were never missed.

    Using servo motors, it was almost a blur to watch operating. And, as far as I can recall we never had positioning errors severe enough to warrant questioning the method. In fact I don't recall experiencing positioning errors.

    Soooo, from that, I stand by the machine design and manufacture providing wear and tear is compensated (repaired) for earlier enough before it becomes a problem.

  5. #5
    Associate Engineer
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    Bob,


    Vague enough and thank you.

  6. #6
    Associate Engineer
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    Great stuff. So many places to go wrong. Also, who ever runs a real First Article / First Piece through to completion before beginning the production run? Just does not happen in my world.

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