Where are fiducials placed in PCB?

Introduction to Fiducial placement in PCB Design

Fiducials are small, precisely placed marks on a printed circuit board (PCB) that serve as reference points for automated assembly equipment. The proper placement of fiducials is crucial for ensuring accurate component placement and alignment during the PCB Assembly process. In this article, we will explore the various aspects of fiducial placement in PCB design, including best practices, common challenges, and the impact of proper fiducial placement on manufacturing efficiency and product quality.

What are Fiducials?

Fiducials, also known as fiducial markers or alignment marks, are small, circular, or crosshair-shaped features placed on a PCB. They are typically made of copper or solder mask openings and are used by automated assembly machines to accurately locate and orient the PCB during the pick-and-place process. Fiducials serve as reference points for the machine vision systems used in these machines, allowing them to precisely place components on the board.

Types of Fiducials

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

1. Global Fiducials: These are the primary reference points used by the assembly machines to determine the overall orientation and position of the PCB. Global fiducials are typically placed near the corners of the board, outside of the component placement area.

2. Local Fiducials: These are secondary reference points used for fine-tuning the placement of specific components or groups of components. Local fiducials are usually placed near critical components or in areas where high placement accuracy is required.

Importance of Proper Fiducial Placement

Accurate fiducial placement is essential for ensuring the quality and reliability of the assembLED PCB. Properly placed fiducials enable automated assembly equipment to precisely locate and place components on the board, minimizing the risk of misalignment or placement errors. This, in turn, reduces the likelihood of manufacturing defects, improves yield rates, and enhances the overall quality of the final product.

Moreover, well-placed fiducials can significantly increase the efficiency of the assembly process. By providing clear and reliable reference points for the machine vision systems, fiducials enable faster and more accurate component placement, reducing cycle times and increasing throughput.

Best Practices for Fiducial Placement

To ensure optimal results, consider the following best practices when placing fiducials on your PCB design:

1. Quantity and Distribution

• Use a minimum of three global fiducials, preferably located near the corners of the board.
• Place local fiducials near critical components or in areas requiring high placement accuracy.
• Ensure that fiducials are evenly distributed across the board to provide a stable reference for the machine vision system.

2. Size and Shape

• Use a consistent size and shape for all fiducials on the board.
• Fiducial diameters typically range from 1.0 mm to 2.0 mm, with 1.5 mm being a common choice.
• Circular fiducials are most common, but crosshair or square shapes can also be used.

3. Clearance and Spacing

• Provide sufficient clearance around fiducials to ensure they are easily distinguishable from surrounding features.
• Maintain a minimum spacing of 5 mm between fiducials and other components or features on the board.
• Avoid placing fiducials too close to the edges of the board, as this can interfere with the clamping mechanisms of the assembly machines.

4. Material and Contrast

• Use copper or solder mask openings for fiducials to provide good contrast against the PCB Substrate.
• Ensure that the fiducial material is compatible with the assembly process and does not introduce any contamination or reliability issues.

5. Solder Mask and Silkscreen

• Avoid covering fiducials with solder mask or silkscreen, as this can interfere with the machine vision system’s ability to detect them accurately.
• If necessary, use a clear solder mask opening or a non-solder mask defined (NSMD) pad for fiducials.

Common Challenges in Fiducial Placement

While proper fiducial placement is critical for successful PCB assembly, there are several challenges that designers may encounter:

1. Limited Board Space

In dense PCB designs, finding suitable locations for fiducials can be challenging due to the limited available space. Designers must strike a balance between optimal fiducial placement and efficient component layout.

2. Component Clearance

Ensuring adequate clearance between fiducials and nearby components can be difficult, particularly in areas with high component density. Designers must carefully consider the placement of fiducials to avoid interference with other board features.

3. Multiple Assembly Processes

If a PCB requires multiple assembly processes, such as surface mount technology (SMT) and through-hole assembly, designers must ensure that the fiducial placement is compatible with all processes involved.

4. Panelization

When designing panels containing multiple PCBs, designers must consider the placement of fiducials not only on individual boards but also on the panel itself. Panel fiducials are used to align the panel in the assembly machines and must be placed accurately to ensure proper registration.

Impact of Fiducial Placement on Manufacturing

The placement of fiducials has a direct impact on the efficiency and quality of the PCB manufacturing process. Properly placed fiducials contribute to:

1. Improved Placement Accuracy

Accurate fiducial placement enables automated assembly equipment to precisely locate and place components on the board, reducing the risk of misalignment or placement errors.

2. Increased Throughput

Well-placed fiducials allow for faster and more efficient component placement, reducing cycle times and increasing overall manufacturing throughput.

3. Reduced Defects and Higher Yield

By minimizing placement errors, proper fiducial placement helps reduce the occurrence of manufacturing defects, leading to higher yield rates and improved product quality.

4. Cost Savings

Efficient and accurate PCB assembly, enabled by proper fiducial placement, can result in significant cost savings by reducing scrap, rework, and production downtime.

Fiducial Placement Guidelines

To assist designers in achieving optimal fiducial placement, consider the following guidelines:

Aspect	Guideline
Quantity	Use a minimum of 3 global fiducials and additional local fiducials as needed
Size	Fiducial diameters should range from 1.0 mm to 2.0 mm, with 1.5 mm being common
Shape	Use circular fiducials for best results, but crosshair or square shapes are also acceptable
Clearance	Provide a minimum of 5 mm clearance between fiducials and other components or features
Spacing	Maintain a minimum spacing of 5 mm between fiducials
Material	Use copper or solder mask openings for good contrast against the PCB substrate
Solder Mask	Avoid covering fiducials with solder mask or silkscreen

Frequently Asked Questions (FAQ)

1. What is the purpose of fiducials in PCB design?

Fiducials serve as reference points for automated assembly equipment, enabling precise component placement and alignment during the PCB assembly process.

2. How many fiducials should be used on a PCB?

A minimum of three global fiducials should be used, preferably located near the corners of the board. Additional local fiducials can be placed near critical components or in areas requiring high placement accuracy.

3. What is the recommended size for fiducials?

Fiducial diameters typically range from 1.0 mm to 2.0 mm, with 1.5 mm being a common choice.

4. How much clearance should be provided around fiducials?

A minimum clearance of 5 mm should be maintained between fiducials and other components or features on the board.

5. Can fiducials be covered with solder mask or silkscreen?

No, fiducials should not be covered with solder mask or silkscreen, as this can interfere with the machine vision system’s ability to detect them accurately. If necessary, use a clear solder mask opening or a non-solder mask defined (NSMD) pad for fiducials.

Conclusion

Proper fiducial placement is a critical aspect of PCB design, ensuring accurate component placement, efficient assembly processes, and high-quality end products. By understanding the best practices, guidelines, and common challenges associated with fiducial placement, designers can optimize their PCB layouts for manufacturability and reliability.

Implementing the recommended fiducial placement strategies, such as using the appropriate quantity, size, and distribution of fiducials, maintaining adequate clearance and spacing, and selecting suitable materials and finishes, can significantly improve the efficiency and accuracy of the PCB assembly process.

By prioritizing proper fiducial placement in their designs, PCB designers can contribute to reduced manufacturing costs, increased throughput, and improved product quality, ultimately leading to greater customer satisfaction and business success.