Understanding Fiducial PCB: Reference marks for easy assembly

Printed circuit boards are made up of insulating material with conductive pathways etched onto the surface. The board combines electronic components, such as resistors, capacitors, and integrated circuits, in various electronic devices. However, manufacturing and assembly can be complex and challenging, requiring precise alignment of components to ensure the proper functioning of the electronic device. 

Fiducials are commonly made of copper and are typically placed near the corners of a PCB. They are often added to a PCB design as a separate layer, and they are usually given a unique identifier such as a letter or number to make them easier to locate and distinguish from other features on the PCB. Once the fiducials have been added to the PCB design, they will be included in the manufacturing files.

In Altium Designer, the process of adding fiducials to your PCB design is straightforward and user-friendly. With the "Fiducial" option located conveniently under the "Place" menu in the toolbar, the designer can easily select the desired fiducial shape and size from the list of available options. 

Once selected, the designer can simply click on the locations where the fiducials should be placed on the printed circuit board. This streamlined process allows designers to efficiently add fiducials to their PCB designs, thereby facilitating the accurate alignment of components during the assembly process and ensuring optimal functionality of the finished product.

Fiducials are small but important markers placed on the printed circuit boards to assist in the assembling process. This article explores the role of fiducials in PCB manufacturing and assembling, including their importance and placement. The readers will also have a comprehensive understanding of the fiducial marks on the PCB for the reliability and efficiency of electronic devices. 

What are Fiducials?

Fiducials are reference marks on a printed circuit board that is placed during its manufacturing process. This mark is typically circular or square in shape with a contrasting color, sometimes red, to the surrounding PCB material. They are used to assist in the alignment and placement of the components during the assembly process on the printed circuit board. 

Manufacturers recommend adding fiducials to the PCB design to achieve better assembly results. Assembly of components on the PCB without fiducial markers on the PCB can result in a high percentage of boards being rejected and hours spent fixing the faults from manual soldering. The need for a fiducial market on a PCB design is important, even if the supplier has machine assembly.

Fiducial markers are made up of copper that act as a reference point for the pick and place of the components on the PCB. They also help assembly machines recognize the PCB's orientation and its surface mount components with packages with tiny pitches like QFP, BGAs, and QFNs. 

In addition to assisting with automated assembly, fiducials can also be used for visual inspection of the PCB. The markers can be captured by a camera during the inspection and used as reference points to ensure that components are correctly aligned and soldered.

Fiducials can be placed at various locations on the PCB, including at the corners, in the center, or near specific components that require precise positioning. The number and placement of fiducials depend on the specific requirements of the PCB and the assembly process.

Recommended reading: Silkscreen on a PCB: What is it?

Importance of using Fiducial PCB

Even though fiducial marks are small, they are important for picking and placing by the assembly machines. Here are some of the following reasons why every designer should have fiducial markers on the PCB:

1. Precision component placement: As a reference point, the automated machine can ensure that the components are placed precisely in the correct location on the PCB. This is important for PCB with small components or tight surface space, where slight misalignment can cause functional issues or failure. 

2. Inspection accuracy: The manufacturers can verify if the components are correctly aligned and soldered. The fiducial marks enable visual inspection of the PCB after the assembly process. This can become important for high-reliability applications like aerospace and military electronics. 

3. Saving costs: The reference marks reduce the risk of error and defects, which can lead to cost savings. Otherwise, the inaccuracies can even cause the scrapping of the entire PCB. The reduction of errors through fiducial PCBs can improve manufacturing efficiency. 

Manual assembly of components on the PCB

Recommended reading: Detailed Guide to Cleaning Printed Circuit Board

Placement of Fiducial Marks

There are several steps and observations which are important while adding fiducial marks on the PCB:

1. Fiducial marks can be added to the top or bottom copper layers and should not be covered by a solder mask. 

2. The visual inspection team will have difficulty finding the reference points if a solder mask covers the fiducial marks.

3. The fiducial marks can be round, square, or diamond in shape. However, the majority of the reference points are circular.

4. They should not have copper nearby as it can cause the camera from inspection tools not to recognize the fiducial marks. 

Some general guidelines are commonly followed for the placement of fiducial marks, and it can vary depending on the project:

1. There are at least two fiducial marks on the PCB, one on each side of the board. There can be more than two, depending on the complexity and size of the board.

2. The fiducial points are placed on the opposite corners of the PCB, as this provides optimized reference points for automated assembly machines.

3. The fiducial points should be placed sufficiently far from other components and the board's edge to avoid interference during the assembly process. 

4. The fiducial marks should have orientation in the same direction on both sides of the PCB to ensure consistency and accuracy. 

Recommended reading: What is a Multilayer PCB?

Key Takeaways

1. Fiducials are reference marks on a printed circuit board that are typically circular or square in shape with a contrasting color to the surrounding PCB material.

2. Manufacturers recommend adding fiducials to the PCB design to achieve better assembly results. 

3. Even though fiducial marks are small, they are important for picking and placing by the assembly machines.

4. Some of the reasons every designer should have fiducial markers on the PCB are precision component placement, inspection accuracy, and saving costs.

5. There are at least two fiducial marks on the PCB, one on each side of the board, with sufficient distance from other components and the board's edge to avoid interference during the assembly process. 

References

1. Altium. Are Fiducial Marker Placements on PCBs Still Necessary with Modern Manufacturing Capabilities? https://resources.altium.com/. Accessed on March 06, 2023.

2. PCB Unlimited. Fiducial Data in PCB Design. https://www.pcbunlimited.com/. Accessed on March 06, 2023.

3. Worthington Assembly. Working With a Manufacturer - What Are Fiducials and Why Are They Useful? https://www.worthingtonassembly.com/. Accessed on March 06, 2023.

4. WellPCB. Fiducial PCB: Design and Function. https://www.wellpcb.com/. Accessed on March 06, 2023.

5. Tempo Automation. PCB Fiducial Markers & Why They Are Essential to Manufacturing. https://www.tempoautomation.com/. Accessed on March 06, 2023.

6. Cadence Design System. Why Fiducial Marker Placement is Crucial for Automated Assembly of PCBs. https://resources.pcb.cadence.com/. Accessed on March 06, 2023.

7. Solutions Manufactuing Inc. Fiducial Mark Design Guidelines. https://www.solutionsmfg.com/. Accessed on March 06, 2023.