Ficiduals, Best Practice

We use a 40mil circle with the solder mask backed off another 40 mils (120mil circle) beyond that. Works good for us but not sure if that is an industry standard or not.

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There are several shapes that are standard - circle, triangle, two rectangles, cross... I personally have the impression that most people use circle. Anyway, all systems on the market should be able to recognize the standard shapes. Most imprtant is where you put your fiducials - pannel or circuit? Depending on what you use accuracy of the machines may vary.

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According to IPC-7351A, the optimum fiducial is a solid circle with a minimum of 1mm diameter, but can be up to 3mm. There should be a solder mask clearance a minimum of 2x the radius of the copper circle. So the minimum fiducial size is 0.040" with a 0.080" mask opening. Also the background under all fiducials should be the same with no traces running under them or differences in ground plane to provide consistent contrast. Be sure to add local fiducials on opposite corners of all fine pitch IC's. I find that our equipment picks up a 0.060" fiducial more reliably than 0.040", but you have to weigh this against available board space.

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Thanks all of you for this information.

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