Fiducials are reference marks on a PCB used to align and calibrate automated assembly equipment. These markers serve as visual targets for aligning the board during pick-and-place, solder paste printing, and automated optical inspection processes.
Fiducials are typically circular copper pads, sometimes with a solder mask clearance, and are not connected to any circuit. Their size, shape, and location are standardized to ensure easy recognition by optical sensors.
Types of fiducials:
- Global fiducials: These markers are placed at the edges of a PCB, and they provide a reference to ensure the board’s alignment with the assembly equipment.
- Local fiducials: Located near fine-pitch components (such as BGAs or QFNs), local fiducials assist in the precise alignment of specific parts, compensating for localized distortions or warping.
Fiducial design guidelines:
- Make sure the shape of the fiducials is circular (1.0–2.0 mm in diameter) with a solder mask clearance of at least 0.5 mm larger than the pad diameter.
- Place at least three global fiducials asymmetrically on the board to prevent alignment errors.
- Position local fiducials diagonally, close to critical components.
- Keep a clearance of at least 2 mm around fiducials. They should be free from silkscreen, copper traces, and masks to avoid false positives during machine detection.
Fiducials play a critical role in accurately aligning printed boards and components during the automated assembly process.