How many Fiducials on PCB for best placement

"IPC-2221 - Generic Standard On Printed Board Design" talks to fiducials. Anther take on fiducials says they shall meet the following requirements. * Make fiducials 1 - 1.5 mm diameter circle. * Cover fiducials with tin. * Keep solder mask opening at least 1mm greater diameter than fiducial. * Keep the distance from any fiducial to any land, trace or hole a minimum of 1mm. * Keep silk screen markings a minimum distance of 1.5mm from any fiducial. * Keep a minimum of 4 mm distance from fiducials to any board edge. * Place fiducials on each side of board that contains SMT components. * Ideally, use 3 fiducials for each side of the board with SMT. * Place 3 fiducials to lie on two lines that intersect at a right angle. * Use a minimum of 2 fiducials for each side of the board with SMT. * Locate fiducials as far apart as possible within all restrictions stated above. * Place fiducials on each individual board, rather than side rails or panel break-aways. * Further, place a set of fiducials on each panel array in the break-aways using above restrictions.

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Here's some recommendations from various companies: * http://www.sibex.com/pcbguidelines.pdf

* http://www.carltonindustriesonline.com/Docs/Fiducial.doc

* http://www.tracktronics.com.au/pdf/PicknPlace_Fiducials.pdf

* http://microtronix.co.za/pcb_requirements/fiducial_mark_requirements.htm

Your mileage may vary

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