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What price impedance match?

Discussion in 'Electronic Design' started by spamme0, Aug 18, 2009.

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  1. spamme0

    spamme0 Guest

    There have been several threads recently about
    impedance matching PCB traces.

    But how do you get that impedance to match?

    You can do wonderful things with cad programs.
    Problem is that boards aren't made by cad programs.
    They're made with inconsistent materials and process variations.

    How accurately do you want your impedances controlled?
    What's the trace width specification from your board vendor?
    How about the thickness spec on each layer?
    The dielectric constant spec?

    If you add it all up, you may find you can't get there
    from here.

    If you want specified impedance, you have to specify it.
    And pay for the testing at the vendor and at incoming
    inspection...and provide test point access.

    Call up your vendor and tell them you've got 12 layers
    and want impedance control on six of them. See what
    that does to the cost quote. You might want to rethink
    your layup.

    If you specify it, it's gonna be expensive.
    If you don't, you're gonna get screwed.
    I've seen boards fail because the vendor switched
    to a fiberglass with tighter weave.
    I've seen boards fail because they used too much
    pressure when laminating the board stack.
    I've seen boards fail because a process engineer
    switched a copper layer from one piece of fiberglass
    to the adjacent fiberglass. Same (nominal) thickness everywhere,
    the board just didn't work.
    Also remember that purchasing agents get paid to reduce
    costs. If the original engineer is gone and there's nobody
    to ask, it's easy for them to save a few bucks on the board...
    and shut down the production line.

    Engineers need to sit down with the board vendor long
    before the first prototype is ever made.
    There's a lot more to it than pressing the "route" button
    on the cad program. It's an analog problem.

    I've not personally specified a board in 20 years.
    Today, if you had 100 ohm traces on a bunch of layers,
    What impedance variations would you expect over the life
    of the product due to pcb manufacturing tolerances?
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