1. Fiducials

What Are Fiducial Marks?

Fiducial marks are precise, optically recognizable reference points printed onto the surface of a PCB. These marks play a vital role in automated assembly by enabling vision systems to accurately determine the board's position, orientation, and alignment during component placement.

For optimal performance, it is recommended to include three fiducials on each individual PCB as well as on the panel it resides within. This tri-point approach ensures accurate alignment in both the X/Y axis and rotational orientation, minimizing placement errors in high-speed assembly environments.

Each fiducial should feature a clear solder mask opening to create a high-contrast region easily identifiable by machine vision systems. Overlapping solder mask can interfere with recognition and reduce placement accuracy.

The preferred fiducial geometry is a solid circular pad with a diameter between 1 mm and 3 mm. To maintain system reliability, all fiducials on a given board should be uniform in size, with a tolerance no greater than 25 microns.

To prevent visual interference, maintain a clearance zone—at least equal to the fiducial diameter—between each fiducial and any adjacent silkscreen, copper, or text elements. Clean isolation ensures reliable detection during production.

Why Use Three Fiducials?

Using exactly three fiducials allows the assembly system to define the board's location and angular orientation. Adding a fourth fiducial can introduce potential confusion if the board is inverted or mirrored during handling.

Each fiducial should include a solder mask relief to maintain alignment precision. For panel-level fiducials, position them a minimum of 4 mm from the panel edge to avoid obstruction by conveyor clamps or tooling fixtures.

Properly designed fiducials support higher yields, faster setup times, and more accurate SMT/SMD placement, making them a critical element in scalable, high-reliability production workflows.

Types of Fiducial Marks

Fiducial marks are typically categorized into two types: Global Fiducials and Local Fiducials.

Global Fiducials are used to identify the orientation of the entire board or panel. They are positioned on the corners of the panel or PCB to assist with board-level registration during pick-and-place processes.

Local Fiducials are integrated directly into the footprints of fine-pitch components (such as QFPs or BGAs). These ensure precise alignment during placement, particularly when tolerances are tight and the margin for error is low.

Components with pitches of 0.5 mm or smaller should include two local fiducials within their footprint to support accurate targeting by the placement head.

Acceptable fiducial finishes include bare copper, anti-oxidation coatings (OSP), nickel, tin plating, or solder-coated finishes such as HASL. Select a single finish type and apply it consistently to ensure uniform optical recognition across the board.