What is the purpose of PCB fiducial?

Introduction to PCB Fiducial Marks

Printed Circuit Boards (PCBs) are essential components in modern electronic devices. They provide a platform for mounting and connecting various electronic components, enabling the device to function as intended. However, the manufacturing process of PCBs involves several critical steps that require high precision and accuracy. One of the crucial elements that ensure the accuracy of the pcb assembly process is the use of fiducial marks.

What are PCB Fiducial Marks?

PCB fiducial marks, also known as fiducials or fiducial markers, are small, circular, or crosshair-shaped copper features placed on the PCB substrate. These marks serve as reference points for the automated assembly machines, helping them to accurately position and orient the PCB during the component placement process.

Fiducial marks are typically placed in three or more non-collinear locations on the PCB, providing the machine with a clear reference frame. The most common fiducial mark shapes are:

  • Circular fiducials
  • Crosshair fiducials
  • Diamond fiducials

The Role of Fiducial Marks in PCB Assembly

The primary purpose of fiducial marks is to ensure the precise alignment of the PCB with the automated assembly machines. These machines, such as pick-and-place machines and solder paste printers, use vision systems to detect the fiducial marks and determine the exact position and orientation of the PCB.

By using fiducial marks, the assembly machines can:

  1. Compensate for any misalignment or rotation of the PCB during the loading process
  2. Accurately place components on the PCB surface
  3. Ensure consistent and reliable solder paste application
  4. Minimize the risk of assembly errors and improve overall product quality

Types of PCB Fiducial Marks

There are two main types of fiducial marks used in PCB assembly:

  1. Global Fiducials
  2. Local Fiducials

Global Fiducials

Global fiducials, also known as panel fiducials or board fiducials, are placed on the periphery of the PCB panel. These marks are used to establish the overall position and orientation of the PCB panel relative to the assembly machine’s coordinate system.

Global fiducials are typically larger than local fiducials, with a diameter ranging from 1 mm to 3 mm. They are placed in at least three non-collinear locations on the panel, providing a stable reference frame for the machine vision system.

Local Fiducials

Local fiducials, also called component fiducials, are smaller marks placed near specific components on the PCB. These marks are used to ensure the accurate placement of fine-pitch or high-precision components, such as Ball Grid Array (BGA) packages or microprocessors.

Local fiducials are usually smaller than global fiducials, with a diameter ranging from 0.5 mm to 1 mm. They are placed close to the target component, providing a localized reference point for the machine vision system.

Design Guidelines for PCB Fiducial Marks

To ensure the effectiveness of fiducial marks in the PCB assembly process, it is essential to follow certain design guidelines. These guidelines help to optimize the detection and recognition of the fiducial marks by the machine vision systems.

Size and Shape

  • Global fiducials should have a minimum diameter of 1 mm and a maximum diameter of 3 mm.
  • Local fiducials should have a minimum diameter of 0.5 mm and a maximum diameter of 1 mm.
  • Fiducial marks should be circular or crosshair-shaped for optimal detection by the machine vision systems.

Placement

  • Global fiducials should be placed in at least three non-collinear locations on the PCB panel.
  • Local fiducials should be placed as close as possible to the target component, without interfering with the component’s pads or solder mask.
  • Fiducial marks should be placed in areas free from other copper features, such as traces or vias, to avoid confusion for the machine vision system.

Clearance

  • A minimum clearance of 1 mm should be maintained between the fiducial mark and any other copper feature on the PCB.
  • The area surrounding the fiducial mark should be free from solder mask or silkscreen, ensuring a clear view for the machine vision system.

Solder Mask and Silkscreen

  • Fiducial marks should not be covered by solder mask or silkscreen, as this can interfere with the machine vision system’s ability to detect the marks accurately.
  • If a fiducial mark must be covered by solder mask due to design constraints, ensure that the solder mask opening is at least 50% larger than the fiducial mark itself.

Benefits of Using PCB Fiducial Marks

The use of fiducial marks in PCB assembly offers several significant benefits, contributing to improved product quality, reliability, and manufacturing efficiency.

Improved Alignment Accuracy

Fiducial marks enable automated assembly machines to accurately align the PCB panel and individual components, reducing the risk of misalignment-related defects. This improved alignment accuracy is particularly crucial for high-density PCBs with fine-pitch components.

Increased Manufacturing Efficiency

By providing a reliable reference system for the automated assembly machines, fiducial marks help to streamline the manufacturing process. This leads to faster assembly times, reduced setup times, and increased overall production efficiency.

Reduced Assembly Errors

Fiducial marks minimize the occurrence of assembly errors, such as component misplacement or rotation. By ensuring accurate component positioning, fiducial marks help to improve the overall quality and reliability of the assembled PCBs.

Cost Savings

The use of fiducial marks contributes to cost savings in PCB manufacturing by reducing the need for manual intervention, minimizing rework, and improving production yields. These benefits ultimately lead to lower manufacturing costs and improved profitability.

Common Challenges and Solutions

Despite the numerous benefits of using fiducial marks in PCB assembly, there are some common challenges that designers and manufacturers may face. Here are a few of these challenges and their potential solutions:

Insufficient Clearance

Challenge: Insufficient clearance between the fiducial mark and other copper features can interfere with the machine vision system’s ability to detect the mark accurately.

Solution: Ensure that the fiducial mark has a minimum clearance of 1 mm from any other copper feature on the PCB. If space is limited, consider using smaller fiducial marks or relocating the marks to a more suitable location.

Solder Mask Coverage

Challenge: Solder mask coverage over the fiducial mark can obscure the mark and hinder accurate detection by the machine vision system.

Solution: Avoid placing solder mask over the fiducial marks whenever possible. If solder mask coverage is necessary, ensure that the solder mask opening is at least 50% larger than the fiducial mark to maintain visibility.

Poor Contrast

Challenge: Poor contrast between the fiducial mark and the surrounding PCB surface can make it difficult for the machine vision system to detect the mark accurately.

Solution: Use a copper finish that provides good contrast against the PCB substrate, such as bare copper or ENIG (Electroless Nickel Immersion Gold). Avoid using finishes that may blend with the substrate, such as OSP (Organic Solderability Preservative).

Frequently Asked Questions (FAQ)

  1. What is the minimum size for a global fiducial mark?
  2. The minimum size for a global fiducial mark is typically 1 mm in diameter.

  3. How many global fiducial marks should be placed on a PCB panel?

  4. It is recommended to place at least three global fiducial marks in non-collinear locations on the PCB panel.

  5. Can fiducial marks be covered by solder mask?

  6. It is best to avoid covering fiducial marks with solder mask. However, if necessary, ensure that the solder mask opening is at least 50% larger than the fiducial mark.

  7. What is the purpose of local fiducial marks?

  8. Local fiducial marks are used to ensure the accurate placement of fine-pitch or high-precision components, such as BGA packages or microprocessors.

  9. How does the use of fiducial marks contribute to cost savings in PCB manufacturing?

  10. Fiducial marks help to reduce the need for manual intervention, minimize rework, and improve production yields, ultimately leading to lower manufacturing costs and improved profitability.

Conclusion

PCB fiducial marks play a crucial role in ensuring the accuracy and reliability of the PCB assembly process. By providing a reference system for automated assembly machines, fiducial marks enable precise alignment, component placement, and solder paste application. This, in turn, leads to improved product quality, increased manufacturing efficiency, and reduced assembly errors.

To maximize the effectiveness of fiducial marks, it is essential to follow design guidelines related to size, shape, placement, clearance, and solder mask coverage. By adhering to these guidelines and addressing common challenges, designers and manufacturers can optimize the use of fiducial marks in their PCB assembly processes.

As PCB technology continues to advance, with increasing component density and miniaturization, the importance of fiducial marks in ensuring accurate and reliable assembly will only continue to grow. By understanding the purpose, types, and design considerations of PCB fiducial marks, designers and manufacturers can leverage these essential features to improve their products and streamline their manufacturing processes.