Anatomy of a PCB Design > Parts > Footprints (Land Patterns)

Footprint Fiducials are small reference markers or alignment points included in the design of a PCB’s footprint to aid automated assembly machines, such as pick-and-place equipment or optical inspection systems. Fiducials serve as visual reference points that help machines accurately align and place components during the assembly process.

Purpose and Importance of Footprint Fiducials

•Footprint Fiducialsare small reference markers or alignment points included in the design of a PCB’s footprint to aid automated assembly machines, such as pick-and-place equipment or optical inspection systems. Fiducials serve as visual reference points that help machines accurately align and place components during the assembly process.

•Alignment and Orientation: Fiducials provide precise reference points that machines use to understand the exact location and orientation of the PCB or specific components. This ensures accurate placement and soldering of components.

•Compensating for Misalignment and Distortion: During the assembly process, the PCB may shift or warp slightly. Fiducials help compensate for these distortions by providing a fixed reference for recalibration.

•Component-Specific Fiducials: For complex or fine-pitch components (e.g., BGAs or QFNs), fiducials are often added directly on the component’s footprint. This helps ensure the machine places the component with high precision relative to its own footprint, especially when dealing with very small pad spacing or high pin counts.

Types of Fiducials

There are two primary types of fiducials used in PCB design:

•Global Fiducials: Placed on the corners or edges of the PCB, global fiducials provide reference points for the entire board. They help in aligning the entire PCB relative to the assembly machine.

•Local (Footprint) Fiducials: Placed within or near the footprint of a specific component. These fiducials help ensure the accurate placement of that particular component, especially for complex components like BGAs, QFNs, or components with fine-pitch leads.

Characteristics of Footprint Fiducials

•Shape: Fiducials are typically designed as circular pads with a solid copper area. The most common shape is a simple circle, though sometimes crosshairs or targets are used for better optical recognition.

•Size: The diameter of a fiducial marker typically ranges from 1mm to 3mm, with a clear annular ring or “keep-out” area surrounding it (usually around 2mm or more). The fiducial size and clear area help the machine’s vision system distinguish the marker from surrounding features.

•Placement: For component-specific fiducials, they are usually placed at known positions around the footprint, often on diagonal corners of the component to allow the machine to determine both location and orientation.

•Surface Finish: Fiducials are usually left as exposed copper (e.g., with a surface finish like HASL, ENIG, or OSP) and are not covered by solder mask. This provides good contrast for optical recognition systems.

When to Use Footprint Fiducials

•Fine-Pitch Components: Footprint fiducials are critical for components with fine-pitch leads (e.g., BGAs, QFPs, and QFNs), where even a slight misalignment can cause shorts or open connections.

•High-Density Boards: For PCBs with high component density, local fiducials ensure precise placement within a specific region, reducing the risk of misalignment due to board warping or shifting.

•Optical Inspection and Testing: Fiducials are also used during Automated Optical Inspection (AOI) and automated testing to verify that the PCB and its components are correctly aligned and oriented.

Example of Footprint Fiducials on a PCB

Consider a BGA (Ball Grid Array) footprint with hundreds of tiny solder balls arranged in a grid. To ensure the BGA is placed correctly, two or more fiducials may be placed on opposite corners of the BGA’s footprint. These fiducials provide reference points for the machine to accurately align the BGA with its pad array on the PCB.

Design Guidelines for Footprint Fiducials

•Consistent Shape and Size: Use uniform fiducial shapes and sizes throughout the PCB for consistency in recognition.

•Clear Area Around Fiducial: Ensure that the fiducial is surrounded by a “keep-out” zone where no other traces, pads, or silkscreen elements are present. This helps the machine’s camera detect the fiducial accurately.

•Avoid Solder Mask Coverage: Fiducials should not be covered by solder mask or silkscreen, as this can interfere with their visibility to optical systems.

In summary, footprint fiducials are essential features for ensuring precision and reliability during the automated assembly and inspection of PCBs, particularly for complex components and high-density designs. They help mitigate alignment errors and contribute to the overall manufacturability and quality of the PCB.