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Thoughtful PCB array and panel design can reduce line stoppage, improve changeover time and reduce material scrap. For the purpose of this discussion, a panel is an array of PCBs which may or may not include scrap material.
Fiducial location and design are good aids to prevent mistakes, but may still result in line stoppage. In this figure, the round features with square mask perimeter are fiducials. The square feature with round mask is a PCB bad mark.
Figure 1: A simple panel array of the top side and the mirrored horizontal view of bottom side components if applicable.
If you have fiducial recognition issues, making the shape of the mask opening different than the fiducial feature may help the vision system discern the target feature easier. Locating the bad mark at the PCB center consistently helps the line operator identify the bad mark without supporting drawings and reduces camera movement. Locating fiducials off center of the PCB will help prevent a print and/or place error but it will still stop the line upon the error.
Figure 2: The scored PCB array with the top row rotated 180°.
Figure 3: One row with top-side up and the second row mirrored vertically.
Figure 4: An array with a combination of mirroring and rotations. Within the four PCB orientations are the top side mirrored horizontally and vertically.