What is a Fiducial Mark
Fiducial marks or markers are circuit pattern recognition markers. Basically it is a round solder mask opening with round bare copper in the center. Bare copper has a smaller diameter than the solder mask opening. See picture 1.
Picture 1. Fiducial Mark
Pick and place machines use Fiducial marks as referent points on the PCB to position the Surface Mount Components (Packages) on the PCB during assembly of the Printed Circuit Board.
What is the size of fiducial marks?
It depends on assembly machines. Dimensions of 3.2mm solder mask opening diameter and 1.6mm diameter of bare copper or 2mm solder mask opening diameter and 1mm diameter of bare copper are excellent for almost all Assembly machines. Picture 2. shows dimensions of the fiducial marks.
Picture 2. Dimensions of the fiducial marks
How many and where to position them?
If you have SMD components on both sides of the PCB you need to place it on both sides top and bottom of your layout.
There aren’t strict rules for the number of fiducial marks. It is good if you position two fiducial marks on opposite corners of the PCB near corner edges. If you have BGA, QFN, QFP packages with small pitch on the layout, it is very useful to position one fiducial mark near every one of those packages/footprints. Picture 3. shows positions of the fiducial marks on the board with one QFN and one QFP package with small pitch.
Picture 3. Positions of the fiducial marks on the PCB
It is also helpful and sometimes necessarily to place four fiducial markers near panel corners. We recommend you to place it on your panel. It also depends on assembly machine. Picture 4. shows positions of the fiducial marks on the panel edge.
Picture 4. Positions of the fiducial marks on the panel edge
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