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