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PCB Footprint Question

Discussion in 'PCB Layout, Design and Manufacture' started by DD234, Mar 31, 2016.

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  1. DD234


    Mar 31, 2016
    Hi Everyone,

    I am working on creating a PCB footprint based on the dimensions from the datasheet for the connector I'm using. I'm a bit confused about whether I should design the footprint using the dimensions shown in mm or the dimensions shown in inches? Which would be better to use? Is there a way to tell if the manufacturer designed the footprint using mm or inches as their unit of measure? I attached an image from the datasheet of the PCB footprint for the connector to this message.

    Thanks for your help!

    Attached Files:

  2. dorke


    Jun 20, 2015
    Welcome to EP.
    In inches ,
    most footprints of devices are in Inches and so is this connector( 0.1" spacing).
    And so are PCB wire traces widths in mils.
    The manufacture gave both for convenience only.
    Aren't most other devices on your board in inches?

  3. AnalogKid


    Jun 10, 2015
    The pin spacings are what matter. If they are in reasonably nice, round numbers in inches, then go with that. If you have a part that is native in mm, like a metric header with pins that are 2.00 mm apart or 0.7874", go with metric. Everything about the connector body, seating plane, hold-down tabs, etc. may be important, but not as important as the pin spacing. Also, all of that other stuff frequently has more loose tolerances.

    If your CAD program can handle mixing metric and English library parts in the same design, this is an even stronger reason *not* to create a metric part with English dimensions (and vice versa). My program does the conversions to 6 decimal places, something I'm sure not going to do by hand.

  4. garublador


    Oct 14, 2014
    My guess is the tolerance of what the board house can accomplish (and what a pick and place machine can accomplish for SMT parts) will be looser than the rounding errors between metric and English units. I agree that using the pin spacing that's "native" to the part like AnalogKid is suggesting is best, but functionally it won't make a difference.

    FWIW, that part looks like it was designed using metric, so I'd go with that if I were making the part. The 0.1" and 0.05" spacings are pretty standard, which is why those metric numbers don't look even, but those conversions have essentially no rounding error so it won't matter which one you use. If you have some reason to use one over the other I'd stick with that.
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