Fiducial marks are recognition marks, created in the form of small pads on the PCB board to act as reference points for PCB Stencil alignment and for automated component placement by Pick and Place machines. During the PCB Assembly process these points are used by automated machines to ensure correct positioning of the board. These points are also measurable and act as a frame of reference during SMT component placement.

When an operator loads the PCB board for printing or component placement, the machine identifies the fiducial marks on the PCB and uses image recognition to match the exact location. This way, the automatic machine aligns itself based on the fiducial marks. 

The addition of fiducial marks is a part of PCB circuit artwork. It means that the PCB circuit pattern and fiducial marks should be etched at the same step. The desired base material of the fiducial mark should be copper protected by an anti-oxidation coating. The fiducial locations are added to the stencil and pick & drop data file. 

It is always recommended to create three fiducial marks on the PCB since it helps the automated machine to identify whether the board is loaded in the right direction or not. Two fiducial marks are also acceptable.

Types of Fiducial marks:

There are two types of fiducial marks available based on their location on the PCB board:

• Global Fiducial Marks: They are created to differentiate the PCB circuit pattern from the panel datum. They should be located based on three point grid system, where the lower left fiducial is located at the 0,0 point while the other two   at the positive X and Y axis. Also, called panel fiducial, if multiple boards are required to be processed at the same time.
• Local Fiducial Marks: These marks are used to locate the position of an individual component that requires more precision.

Key Guidelines for creating fiducial marks on a PCB:

While adding the fiducial marks, the following points should be considered:

• These marks should be added only on the top or bottom copper layers of the PCB.
• They should not be covered by the solder mask since the automated system needs to identify them and covering the same with solder mask will make it impossible to track.
• For better performance, a high degree of contrast between the surface of the fiducial mark and the adjacent PCB board base material is desired.
• Usually, the fiducial marks are round in shape, but square or diamond shapes are also acceptable.
• To achieve better precision, the fiducial marks should be placed as far from each other as possible on the PCB.
• The diameter of a fiducial mark should lie between 1 mm (0.040 in) to 3 mm (0.120 in). The marks on the same PCB should not vary in size by more than 25 microns (0.001 in).