How to Properly Place Fiducial Marks on PCBs: A Step-by-Step Guide

In circuit board design, Fiducial marks help manufacturing and pick-and-place machines accurately identify the direction and position of the circuit board, further ensuring placement precision. This article in this case will provide a detailed guide on how to create fiducial marks in design software and the proper methods for placing them on the PCB.

Composition and Design Method of Fiducial Marks

Fiducial Marks consist primarily of two parts: the marking point (or feature point) and the clearance area.

Design Steps:

  1. In design software, a fiducial mark can be created by adding a pad and a solder mask layer. Typically, the design features a solid circle with a diameter of 1mm, and the solder mask layer is designed as a window with a diameter of 3mm. Moreover, the center of the marking point is located 3.85mm from the edge of the board.
  2. The pad diameter should comply with standards (IPC-2221), meaning the solder mask layer’s radius is 2-3 times the radius of the pad.
Steps to create fiducial mask
Steps to create fiducial mask (Image source: online)

Marking Point:

  • Typically designed as a solid circle, the recommended diameter is 1.0 mm for most production scenarios.
The recommended diameter (1.0 mm) for marking point
The recommended diameter (1.0 mm) for marking point (Image source: online)

Clearance Area:

  • A clearance area around the fiducial mark should be free of other circuit features or markings.
  • The radius of the clearance area should be at least twice the radius of the fiducial mark. When the radius of the clearance area reaches three times of the radius of the fiducial mark, the recognition performance of the machine is even better.
Example of Clearance Area
Example of Clearance Area (Image source: online)

Local Fiducial Marks

Local Fiducials are used for component positioning:

  • Placement: Place near the component that is typically at its diagonal corners.
  • Function: Ensure precise alignment of the component during the pick-and-place process.

Global Fiducial Mark

Global Fiducials are used for overall PCB positioning:

Quantity and Distribution:

  • Three Fiducial Marks: Arrange them in a triangular layout, furthermore, ensuring they are as far apart as possible.
  • Four Fiducial Marks: The layout should form an irregular trapezoid, with points distributed as far as possible along the longest diagonal.

Distance to the Edge:

  • The distance from each fiducial mark to the edge of the PCB must be greater than 3.85 mm (measured from the center of the fiducial mark to the PCB edge).
  • Fiducial marks need to be located completely within the board to avoid being obscured by the fixture.
Example of showing the distance from the edge of the fiducial mark to the PCB edge should be greater than 3.85mm
Example of showing the distance from the edge of the fiducial mark to the PCB edge should be greater than 3.85mm (Image source: online)

Panel Fiducial Marks

To accommodate the 3mm-width clamp edge (rails) of certain pick-and-place machines and ensure accurate recognition of fiducial marks, it is recommended to set the distance from the mark center to the board edge at 3.85mm. When using PCBA services, we will handle all fiducial mark setups on the tooling strip.

Key Considerations:

  1. Missing Tooling Strip: If the original design file does not include a tooling strip, we will automatically add a 5mm-wide tooling strip according to SMT process requirements. Additionally, This strip will include fiducial marks and tooling holes.
  2. Insufficient Tooling Strip Width: If the original design file contains a tooling strip less than 5mm in width, we will extend it to 5mm and add fiducial marks and tooling holes accordingly.
Example of the tooling strip and fiducial mark
Example of the tooling strip and fiducial mark (Image source: online)

Avoid Symmetrical Placement

  • Problem with Symmetrical Fiducial Marks

If only two fiducial marks are symmetrically positioned (e.g., equivalence of the top-left and bottom-right corners), the placement machine may fail to identify the correct orientation after a 180° rotation of the PCB. Consequently, this could result in incorrect component placement.

  • Solution

Adopt an asymmetrical layout for the fiducial marks is to ensure that the machine can accurately detect the PCB’s orientation after all. Alternatively, increase the number of fiducial marks to four, ensuring they form a non-symmetrical configuration, such as an irregular trapezoid.

Placement Considerations

  1. Positional Accuracy: Fiducial mark needs to be within the recognition range of the device camera and always visible during the placement process.
  2. Board Layout: Avoid overlapping fiducial marks with key circuit features or other markings to ensure the integrity of the clearance area.
  3. Production Verification: Verifing the recognizability and positioning accuracy of the fiducial marks through production testing after design completion.
When the PCB enters the machine, the fixture clamp secures the edges of the board. If the fiducial mark is obstructed, it will fail to serve its positioning function
When the PCB enters the machine, the fixture clamp secures the edges of the board. If the fiducial mark is obstructed, it will fail to serve its positioning function (Image source: online)

Some images are sourced online. Please contact us for removal if any copyright concerns arise.


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