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I've seen this in several designs. What purpose do these serve?

enter image description here

marked as duplicate by PeterJ, Asmyldof, Daniel Grillo, PlasmaHH, Dave Tweed May 20 '16 at 14:38

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    They're called "fiducials". – Dave Tweed May 20 '16 at 13:26
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    Another option (not seen in the picture) is unmounted components. Almost any PCB is overdesigned to allow flexibility during bringup or sometimes even different products on same PCB – Gregory Kornblum May 20 '16 at 13:32
  • @GregoryKornblum he actually specifies 'unconnected', unless we are speaking of a prototyping area of the pcb usually extra components are connected to something. – Vladimir Cravero May 20 '16 at 13:50
up vote 9 down vote accepted

They are Fiducial Marks and are often used for visual alignment of automated processes on PCB manufacturing and assembly.

These automated processes can vary from pick and place machines to visual inspection and flying probes.

To account for possible rotation of the board there will be at least 2 points on a PCB. In fine pitch projects there may be more fiducials near critical components. (ty Sean for pointing that out)

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    Typically there will be at least 2 per board (close to the corners), and sometimes additional ones next to fine-pitch components. – Sean Houlihane May 20 '16 at 14:07
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    On panelized small boards the fiducials may be on the tooling strips which are subsequently removed when individual boards are depanelized from the array. – Spehro Pefhany May 21 '16 at 8:10

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