Small but mighty: fiducial marks and their purpose
A fiducial is basically a round solder mask opening with round bare copper in the center. Fiducials are reference points on a Printed Circuit Board (PCB) for automated equipment. Its full name is “Fiducial Marker”. Fiducials help machines recognize where an object is in its space.
Figure 1: Fiducial. Source: EPR
A helping hand
When assembling and controlling PCBs fiducials ensureour stencil printing machine, pick and place machine, and AOI (automated optical inspection) machine can recognize where exactly the PCB is in the machine.
During assembly of the PCB, the pick and place machine uses fiducial marks as reference points on the PCB to position the Surface Mount Components (packages). If components are placed on double sided PCB designs, it is necessary to place fiducials on both sides of the PCB.
Assembly machines use cameras to locate fiducial marks, and then adjust component placement based on the exact position of the board.
Figure 2: Global and local fiducials. Source: Accutron Inc.
Don’t put all your eggs in one basket
The whole process typically requires more than one fiducial mark in a non-reversible pattern. With the first two fiducials (diagonal placement) the machine can recognize where the PCB is at its X and Y position and also how skewed the PCB is in the clamps. The machine is able to measure the rotation angle and compensate all of the placements accordingly so the components will be placed properly. Finally, the third fiducial helps the machine to compensate for any shrink or stretch of the PCB.
If you have BGA, QFN, QFP packages with a very small pitch on the layout, it is very useful to position one fiducial mark – a local fiducial – near every one of those packages/footprints. These components require a more precise location.
Panelization
It is recommended – and sometimes necessary – to place fiducial markers near panel corners. These fiducials are called panel fiducials. It also depends on the kind of assembly machine how many fiducials you have to place. At EPR we prefer three fiducials.
Figure 3: Standard PCB panel features. Source: Norcott Technologies
A proper size
Normally fiducials have a 1mm diameter of bare copper and 2mm solder mask opening. The fiducials are placed as a component during the layout design.
Figure 4: Fiducial sizes. Source: Accutron Inc.
