What Are PCB Fiducials and Why Are They Important?

PCB fiducials are core automation features. Mastering their placement is, therefore, a fundamental part of designing printed circuit boards for manufacturing. Find out what they are, their benefits, and how to create them below.

What Are Fiducials in PCB Manufacturing?

PCB fiducials are alignment markers placed on the corners of a printed circuit board. Their presence creates reference points for automated systems, enhancing precision and increasing manufacturing yield.

For instance, a pick-and-place machine uses the markers to adjust itself in relation to the PCB being mounted. It ensures correct component placement, reducing errors and the need for rework.

A PCB has various other markings, including drill holes, etc. However, these cannot serve as fiducials. That’s because they are inconsistent in shapes, sizes, and coatings, with some featuring solder mask.

That said, the fiducials are typically copper. They should not be layered with anything that makes them less visible. This requirement ensures accuracy when using computer vision to identify them.

Why Are PCB Fiducials Important?

They enable the automated assembly of printed circuit boards. Processes that need the markers include solder paste printing, component mounting, and other manufacturing steps where machines must “see” the board.

The markers compensate for a PCB’s alignment that movement and shifting may cause. Without them, automated equipment would struggle to map PCBs, resulting in errors.

In addition to precision, fiducials help increase the production speed for bulk manufacturing. That’s besides reducing production costs by minimizing PCB assembly defects.

Types of PCB Fiducials

There are two major classifications for circuit board alignment markers: local and global. Let’s see the meaning and location of each type.

Global Fiducials

These help align the entire PCB or panel. Their location is usually on the opposite corners of individual circuit boards or arrays, with a minimum of two, one on either side. They are generally larger than local types.

Local Fiducials

Local types are those that designers place near specific components or a group of components. This placement positions them inside the circuit board.

They are usually placed diagonally on the opposite sides of the edges of the SMT component. Components that require local fiducials include BGAs, QFNs, and other IC packages.

It’s worth noting that modern machines are becoming more advanced, making local markers less critical. They identify component footprint and position with more precision than traditional systems.

How to Add Fiducials in PCB Design

Creating fiducial marks on PCB designs involves a few simple steps. First, determine the number of fiducials you intend to place, and then follow these steps.

• Open the copper layer file in the PCB design editor.
• Find the fiducials tool. Most programs provide it.
• Add the markers (typically round copper pads) with a similar shape for solder mask clearance.
• Ensure their correct placement for both position and angle.
• Add a clearance of about two to three times the fiducial diameter or other dimensions.
• Label the markers in the design file for proper identification.
• Perform a design rule check to confirm their placement and manufacturability. It helps to reduce errors in the succeeding production stages.

Key Guidelines When Creating PCB Fiducials

Errors are intolerable in PCB manufacturing. Designers must, therefore, understand the dos and don’ts when placing fiducials on PCB boards, as mistakes can result in flawed processes. Below are key guidelines for the features.  

Material

The material is usually similar to that of the traces. They are, therefore, usually copper and plated with the board’s surface finish (like HASL or ENIG) to prevent oxidation.

Automated systems visualize copper more easily, given the high contrast it creates with surrounding surfaces. It’s also more cost-effective to use copper pads for the markers.

Size and Shape

The most common and recommended shape for a global fiducial is a solid circle. While other shapes like squares, diamonds, or cross-hairs exist, solid circles are preferred by most vision systems for their consistency from any angle.

The copper pad is typically 1.0 mm to 1.5 mm in diameter. It requires a solder mask clearance (an area free of mask) around it, usually 2.0 mm to 3.0 mm in diameter, to provide high contrast for the camera.

Location and Clearance

Place fiducials on the opposite corners of the board and within the view of assembly equipment imagers. PCBs with components on either side require marking on both the upper and bottom sides.

Do not place the markers too close to the edges. Maintain a minimum of 0.5 mm solder mask clearance for better visibility and at least 5 mm between the markers and other components or edges.

There should also be a distance between the fiducial and other exposed copper features, such as pours and traces. Ensure around 2 mm for this requirement.

Other Considerations

The number of fiducials required depends largely on the complexity of your circuit board. While two fiducials can provide basic alignment, using three markers is considered best practice for enhancing accuracy.

Arranging three fiducials in an asymmetrical “L” shape allows the SMT machine to detect if a board has been accidentally rotated or loaded upside down—an error that four symmetrical corner fiducials might not catch. For exceptionally complex or large PCBs, four fiducials may be used to further stabilize the coordinate system.

PCB Fiducial Placement Mistakes to Avoid

Symmetrical Arrangement: Ideally, arrange fiducials asymmetrically (e.g., in an ‘L’ configuration). If fiducials are perfectly symmetrical (e.g., four corners), the machine may not detect if the board is loaded backward.

Coating with Solder Mask: Avoid concealing fiducials with solder mask. The cameras may not detect them as markers, leading to errors.

Inconsistent Shapes and Sizes: Use consistent features for the markers for easy identification by pick-and-place machines and other types of manufacturing equipment.

Insufficient Clearance: It affects visibility, increasing the chance of errors. Use correct distances between the markers and other features, including solder mask.

Similar Markings for Single PCBs and Panel: The array should have its own markers. Panel fiducials differ from those on single boards, both in size and placement.

Not Including Fiducials in Gerber Files: Ensure the features are correctly labeled for easy identification and manufacturing.

Not Matching the Markers with Manufacturer Systems. The machines a PCB manufacturer or assembler uses often dictate the kind of markers to use, including their size. Communicate with your vendor to ensure compatibility.

Conclusion

Fiducials are PCB markers for automated assembly needs. They help imaging systems to identify PCB placement and align correctly. Their location, size, and clearance are crucial for accurate alignment, requiring designers to understand their creation rules.