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width of the trace and printed circuit board

Discussion in 'Electronic Design' started by john, Mar 28, 2007.

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  1. john

    john Guest


    I have following questions

    1. How can I determine the size of the trace? What parameters should
    be considered before determine the size of the trace?

    2. I am using the formula k / ( 1+ (D/H) ^ 2 ) . How can I calculate
    the value of "K "?

  2. Tom Bruhns

    Tom Bruhns Guest

    1. Measure it?? Or do you mean to ask how you can determine what
    width trace you should use for a particular application? If it's the
    latter, it depends on several things: do you need a particular
    impedance? How much current will it carry? What temperature rise can
    you tolerate? How much voltage drop can you stand? What is the PC
    process you are using capable of?

    2. Is K the same as k? If not, what is it? What is k? What are D
    and H? What does the formula claim to determine?

    You may well find answers to your questions with a Google search about
    printed circuit trace width. For example, this was one of the early
    hits of such a search:
    It may well answer at least your first question.

  3. Noway2

    Noway2 Guest

    If what you are considering is how wide to make digital traces, you need
    to also take into account the ability of the drivers to drive them. The
    impedance of the trace and hence the current required by the driver is
    a function of the trace geometry: width versus height above the
    reference plane.

    As a rule of thumb and unless there are other considerations, I try to
    keep the digital traces on the order of 75 Ohms though I don't worry too
    much about it. I chose 75 Ohms mostly on the advice of another engineer
    who's opinion and abilities I respect. When I asked him to review a PCB
    layout for me, he commented that my 50 Ohms traces might be a little low
    for some drivers to handle, though most modern stuff could, and I might
    want to consider going a bit higher. Based on the standard core
    thickness used by the PCB vendor I use I typically keep my digital
    traces at .008", which gives me an appropriate impedance and is still
    reliably producible.

    The capabilities of the PCB vendor are another factor to keep in mind,
    as is the cost of your design choices.
  4. Robert Baer

    Robert Baer Guest

    I normally use a magnifier with a graticule...
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