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 Fiducial and Datum 

Last post Wed, Oct 9 2013 8:11 PM by redwire. 2 replies.
Started by TiBo 09 Oct 2013 07:58 AM. Topic has 2 replies and 598 views
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  • Wed, Oct 9 2013 7:58 AM

    • TiBo
    • Not Ranked
    • Joined on Wed, Sep 26 2012
    • Posts 4
    • Points 165
    Fiducial and Datum Reply

    Hi all,

    I really need help on this issue, I have been using the fiducials ( at least 2 on my boards ) for a long time for dimensions and tolerances.

    I put at least 2 fiducials on the board, one on the left bottom and one on right top corner of my board. Then assign the board surface as Datum A, bott left one as Datum B, and the top right one as Datum C. My Datum B is also the origin of my board and I dimension everything from Datum B using Datum dimensioning.

    We have a guy in our group keep insisting that I can not make a Fiducial a Datum, only a Hole can be a Datum and if I use datum, the two fiducials on the board have to be on the same axis x or y ( can not be at opposite corner ) 

    My thought is in general, you can use a hole, a fiducial, an edge of board as Datum ..., since fiducial is just a round 40 mils SMT pad, all other SMT footprint on the board will have a tight and proportional tolerance to it as I use the fiducial as my reference point.

    Please help, I really want your opinion about this. All feedbacks are appreciated.

    Best regards,



    • Post Points: 35
  • Wed, Oct 9 2013 8:11 AM

    • chads108
    • Top 50 Contributor
    • Joined on Thu, Mar 29 2012
    • Plano, TX
    • Posts 193
    • Points 3,385
    Re: Fiducial and Datum Reply

     We almost always use fiducials as our datums.  Haven't had an issue yet.

    • Post Points: 5
  • Wed, Oct 9 2013 8:11 PM

    • redwire
    • Top 10 Contributor
    • Joined on Thu, Jul 17 2008
    • Allen, TX
    • Posts 880
    • Points 13,550
    Re: Fiducial and Datum Reply

     Technically a fiducial should *not* be made a datum as it's not a verifiable measurement point on a mechanical reference table.  A hole is the best datum.  Next a corner can be used as long as it's a verifiable corner -- and not one created off-board as an intersection of sides.

    A fiducial is ok for SMT reference as the optical scanner will use it to reference other pads on the board.

     So if you're trying to pass a dimension verification for mechanical reasons always use a hole for datum.  HTH

    • Post Points: 5
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Started by TiBo at 09 Oct 2013 07:58 AM. Topic has 2 replies.