What are Fiducials in PCB Assembly?

Fiducials, also known as fiducial marks or fiducial points, are small, precise reference marks placed on printed circuit boards (PCBs) to help align and position components during the assembly process. These marks serve as visual cues for automated assembly machines, ensuring accurate placement of surface mount components, such as resistors, capacitors, and integrated circuits.

Fiducials are typically round, copper-plated pads or solder mask openings on the PCB surface. They are designed to be easily recognizable by machine vision systems, which use cameras to detect and locate the fiducials. By using these reference points, the assembly machines can determine the exact position and orientation of the PCB, allowing for precise component placement.

Types of Fiducials

There are two main types of fiducials used in PCB assembly:

1. Global Fiducials: These fiducials are used to establish the overall position and orientation of the PCB. They are typically placed near the corners of the board, providing a frame of reference for the entire assembly process. Global fiducials help align the PCB with the assembly machine’s coordinate system.

2. Local Fiducials: Local fiducials are used to provide reference points for specific components or groups of components. They are placed near critical components that require high placement accuracy, such as fine-pitch devices or ball grid array (BGA) packages. Local fiducials help the machine vision system fine-tune the placement of these components.

Fiducial placement Guidelines

To ensure optimal performance and accuracy during the PCB assembly process, follow these guidelines when placing fiducials on your PCB design:

1. Quantity and Location

• Use at least three global fiducials for each PCB, positioned near the corners of the board.
• Place local fiducials near critical components or component groups that require high placement accuracy.
• Ensure that fiducials are placed in areas free from other components, traces, or solder mask openings.

2. Size and Shape

• Fiducials should be round, with a diameter between 1.0 mm and 3.0 mm.
• The recommended size is 1.5 mm to 2.0 mm in diameter.
• Ensure that all fiducials on a PCB are the same size and shape for consistency.

3. Clearance and Spacing

• Provide a minimum clearance of 3.0 mm between fiducials and other components, traces, or solder mask openings.
• Maintain a minimum spacing of 10 mm between global fiducials.
• Keep local fiducials as close to the critical components as possible while adhering to the minimum clearance requirements.

4. Solder Mask and Copper

• Fiducials should be bare copper pads without solder mask coating to provide high contrast for machine vision systems.
• If using solder mask openings as fiducials, ensure that the openings are clean and free from solder mask residue.

5. Panelization

• When panelizing PCBs, include additional global fiducials on the panel frame to assist with panel alignment during assembly.
• Ensure that the panel fiducials do not interfere with the PCB Fiducials or other components.

Benefits of Proper Fiducial Placement

Proper fiducial placement offers several benefits in PCB assembly:

1. Improved Accuracy: Well-placed fiducials enable automated assembly machines to precisely locate and align components, reducing the risk of misalignment or placement errors.

2. Increased Efficiency: By providing clear reference points, fiducials help streamline the assembly process, reducing setup time and increasing throughput.

3. Enhanced Reliability: Accurate component placement, facilitated by fiducials, leads to better solder joint quality and overall PCB reliability.

4. Cost Savings: Proper fiducial placement minimizes the need for manual interventions and rework, reducing assembly costs and improving yield.

Common Fiducial Placement Mistakes

To avoid issues during PCB assembly, be aware of these common fiducial placement mistakes:

1. Insufficient Quantity: Failing to include enough global or local fiducials can lead to alignment difficulties and reduced placement accuracy.

2. Inconsistent Size or Shape: Using fiducials of varying sizes or shapes can confuse machine vision systems, leading to recognition problems and placement errors.

3. Inadequate Clearance: Placing fiducials too close to other components, traces, or solder mask openings can interfere with the machine vision system’s ability to detect the fiducials accurately.

4. Solder Mask Coating: Covering fiducials with solder mask can reduce their visibility and contrast, making it difficult for machine vision systems to locate them.

5. Placement in Congested Areas: Placing fiducials in areas with high component density or near the edges of the PCB can limit the machine vision system’s access to the fiducials, compromising alignment accuracy.

Frequently Asked Questions (FAQs)

1. Q: How many global fiducials should I use on my PCB?
   A: It is recommended to use at least three global fiducials, positioned near the corners of the PCB, to establish a reliable frame of reference for the assembly process.

2. Q: What is the recommended size for fiducials?
   A: Fiducials should be round, with a diameter between 1.0 mm and 3.0 mm. The recommended size is 1.5 mm to 2.0 mm in diameter.

3. Q: How close can I place fiducials to other components or traces?
   A: Provide a minimum clearance of 3.0 mm between fiducials and other components, traces, or solder mask openings to ensure proper detection by machine vision systems.

4. Q: Can I use solder mask openings as fiducials?
   A: Yes, solder mask openings can be used as fiducials, but ensure that the openings are clean and free from solder mask residue for optimal visibility.

5. Q: What happens if I don’t place fiducials on my PCB?
   A: Without fiducials, automated assembly machines may struggle to accurately align and place components on the PCB, leading to misalignment, placement errors, and reduced assembly efficiency.

Conclusion

Fiducial placement is a critical aspect of PCB design and assembly. By understanding the types of fiducials, following placement guidelines, and avoiding common mistakes, you can ensure that your PCBs are assembled accurately and efficiently. Proper fiducial placement improves component alignment, increases assembly speed, and enhances overall PCB reliability.

When designing your PCB, consider the quantity, location, size, shape, and clearance of fiducials. Collaborate with your assembly provider to ensure that your fiducial placement meets their specific requirements and guidelines. By taking the time to properly place fiducials on your PCB, you can streamline the assembly process, reduce costs, and improve the quality of your final product.