01 What Is a Fiducial Mark?

A fiducial mark (also known as a reference point) provides a measurable position reference for placement equipment using the principle of optical alignment, enabling the machine to achieve precise positioning during operation. It is the first coordinate of SMT placement accuracy.

02 Why Can't We Do Without Fiducial Marks?

The pick-and-place machine scans the fiducial marks on the PCB and, together with the placement program, calculates the placement coordinate of every component. If there are no reference fiducial marks on the board, the machine has no way to determine where each component should be placed — neither placement accuracy nor soldering yield can be guaranteed.

03 Fiducial Mark Design Requirements (Three Hard Rules)

① Dimensions

Place 3–4 solid pads with a diameter of 1.0mm on the copper layer, and open a window with a diameter of 3.0mm in the solder mask. No traces or other elements are allowed inside the opening, as they may interfere with fiducial recognition.

② Quantity & Polarization (Foolproofing)

Use 3–4 fiducial marks on the board. If 4 fiducials are used, they must **NOT** be arranged in a rectangle — one of them must be offset to prevent the PCB from being mistaken for a 180°-rotated orientation (foolproof design). The placement machine only reads the fiducial coordinates and does not care about the pad positions of the components on the board. For high-precision ICs and BGAs, additional fiducial marks can be added at the diagonal corners of the component to further enhance placement accuracy.

③ Coverage / Placement Range

Fiducial marks should be placed as close to the four corners of the board as possible, maximizing the coverage area of the fiducials to ensure that all components to be placed on the board fall within the reference coverage zone.

04 What If You Forget to Add Fiducial Marks?

Always check whether the PCB has fiducial marks before placing an SMT order! If they were omitted, you can add tooling edges to the PCB before production and place the fiducial marks on these edges as a remedy. Once PCB production has started or been completed, fiducial marks can no longer be added.

05 Should Fiducial Marks Be Added Even on Through-Hole-Only Boards?

Even if the board is populated entirely with through-hole components, it is still recommended to add fiducial marks — they also serve as the coordinate reference during assembly and inspection.

06 Local Fiducial Marks for High-Precision Components

When a board contains large, high-pin-count, or fine-pitch components (e.g., BGA, QFN, precision modules), global fiducial marks alone may not be able to meet their extremely high placement accuracy requirements. In this case, local fiducial marks are of critical importance. They provide a dedicated positioning reference for a single critical component and are key to ensuring soldering yield.

- Positioning & Layout — Diagonal Principle: Place one local fiducial at each diagonal corner of the critical component (bottom-left + top-right, or top-left + bottom-right). Two points define a line, giving the placement head the reference to calculate the component's precise position and rotation angle. A single point can only provide coordinates; it cannot correct angular deviation.

- Distance & Clearance: Place the local fiducials as close as possible to the component corner, while maintaining sufficient clearance (typically recommended to be **greater than 2.5mm**) to reduce reference errors caused by PCB expansion, contraction, and warpage. The closer the distance, the smaller the relative displacement between the component and the fiducial, and the more accurate the positioning.

- Stricter Background Requirements: The keep-out zone around a local fiducial must be absolutely clean. Not only must there be no traces, but large copper pours must also be avoided. A fiducial etched over a large copper area may have a non-uniform background (e.g., thermal relief), which can cause the machine to miscalculate the recognition center. The background must be bare substrate completely free of copper to ensure the highest contrast.

- Consistent Sizing: All local fiducials on the same board should be identical in size, but they may differ from the global fiducials. The recommended size is also **1.0mm**.

Quick Reference — Core Parameters

Summary

Getting the fiducial marks right is the starting point for a high SMT yield. From global references to local references, every single dot safeguards "precise placement." Juehui Weiye Circuit (www.szjhpcba.com/pcba.html) specializes in small-to-medium batch PCBA customization and upholds a "zero-defect" engineering standard to support high-end manufacturing scenarios such as medical, defense communications, 5G communications, and AI high-speed servers.