Where are fiducials placed in PCB?

What are Fiducials and Why are They Important?

Fiducials, also known as Fiducial Markers or alignment marks, are small copper pads or other features placed on a printed circuit board (PCB) to serve as precise reference points. They play a crucial role in the automated assembly process of PCBs, ensuring accurate placement and orientation of components.

Fiducials are typically round pads, but can also be crosses, diamonds, or other distinct shapes that are easily recognizable by machine vision systems. These markers are placed at strategic locations on the PCB, usually in the corners or along the edges of the board.

The primary purpose of fiducials is to provide a common reference point for the pick-and-place machines and other automated assembly equipment. By using these markers, the machines can accurately locate and orient the PCB, ensuring that components are placed precisely where they are intended to be. This is especially important for surface mount technology (SMT) components, which require a high degree of placement accuracy.

In addition to enabling precise component placement, fiducials also help in the following ways:

1. Compensating for PCB distortion or shrinkage during the manufacturing process
2. Aligning multi-layer PCBs during the lamination process
3. Facilitating automated optical inspection (AOI) of assembled PCBs

Fiducial placement Guidelines

To ensure optimal performance and reliability of fiducials, it is essential to follow certain guidelines when placing them on a PCB. Here are some key considerations:

Number and Location of Fiducials

The number and location of fiducials on a PCB depend on various factors, such as the size of the board, the complexity of the design, and the specific requirements of the assembly process. However, there are some general guidelines to follow:

• Use a minimum of three fiducials for each PCB
• Place fiducials near the corners of the board, forming a triangle
• Ensure that fiducials are not obstructed by components or other features
• Keep fiducials at least 5mm away from the edges of the board
• Space fiducials evenly across the board to minimize the impact of local distortions

Here’s an example of recommended fiducial placement on a rectangular PCB:

Fiducial	Location
F1	Top-left corner, 5mm from edges
F2	Top-right corner, 5mm from edges
F3	Bottom-left corner, 5mm from edges

Size and Shape of Fiducials

The size and shape of fiducials can vary depending on the specific requirements of the assembly process and the capabilities of the machine vision system. However, there are some general guidelines to follow:

• Use round fiducials with a diameter between 1mm and 3mm
• Ensure that fiducials are large enough to be easily detected by the machine vision system
• Avoid using shapes that can be easily confused with other features on the PCB, such as vias or mounting holes

Here’s a table comparing common fiducial sizes and their suitability for different PCB sizes:

Fiducial Diameter	Suitable PCB Size
1mm	Small to medium-sized PCBs
1.5mm	Medium to large-sized PCBs
2mm	Large-sized PCBs

Clearance and Solder Mask

To ensure that fiducials are easily detectable by machine vision systems, it is important to provide adequate clearance around them and to use the appropriate solder mask settings. Here are some guidelines to follow:

• Provide a minimum clearance of 1mm around each fiducial, free of components, traces, and other features
• Use a solder mask opening that is slightly larger than the fiducial itself, typically 0.5mm larger in diameter
• Ensure that the solder mask color contrasts well with the fiducial color (usually copper) for easy detection

Here’s an illustration of a fiducial with proper clearance and solder mask opening:

       Solder Mask Opening
       |                 |
       |                 |
       |     Fiducial    |
       |                 |
       |                 |
       |_________________|

        Clearance Area

Fiducial Design Considerations

In addition to placement guidelines, there are several design considerations to keep in mind when creating fiducials for your PCB.

Fiducial Material and Finish

Fiducials are typically made of the same copper material as the rest of the PCB traces and pads. However, the finish of the fiducials can affect their detectability by machine vision systems. Here are some common finishes and their suitability for fiducials:

Finish	Suitability for Fiducials
HASL	Poor, uneven surface
ENIG	Good, flat and reflective
OSP	Fair, may tarnish over time
Immersion Silver	Good, flat and reflective
Immersion Tin	Good, flat and reflective

It is generally recommended to use a flat, reflective finish such as ENIG or Immersion Silver for fiducials to ensure optimal detectability.

Fiducial Symmetry and Orientation

To minimize the impact of PCB distortion and ensure accurate alignment, it is important to design fiducials with symmetry and consistent orientation in mind. Here are some guidelines to follow:

• Use symmetrical shapes for fiducials, such as circles or crosses
• Ensure that all fiducials on a PCB have the same orientation (e.g., all point in the same direction)
• Avoid using asymmetrical shapes or orienting fiducials differently, as this can lead to alignment errors

Unique Fiducial Patterns

In some cases, it may be beneficial to use unique fiducial patterns to help machine vision systems distinguish between different types of PCBs or to provide additional alignment information. Here are some examples of unique fiducial patterns:

• Using a combination of different shapes (e.g., circles and diamonds)
• Varying the size of fiducials across the PCB
• Creating specific patterns or arrangements of fiducials

However, it is important to use unique fiducial patterns sparingly and only when necessary, as they can increase the complexity of the machine vision system and potentially lead to alignment errors if not implemented correctly.

Frequently Asked Questions (FAQ)

1. Can I use vias or mounting holes as fiducials?

While it is technically possible to use vias or mounting holes as fiducials, it is generally not recommended. These features may not have the same detectability and accuracy as dedicated fiducials, and they can be easily confused with other similar features on the PCB.

2. How do I ensure that my fiducials are compatible with my assembly partner’s equipment?

To ensure compatibility, it is important to communicate with your assembly partner early in the design process. Provide them with information about your fiducial design, including size, shape, and placement, and ask for their feedback and recommendations. They may have specific requirements or preferences based on their equipment and processes.

3. Can I use different fiducial sizes and shapes on the same PCB?

While it is possible to use different fiducial sizes and shapes on the same PCB, it is generally recommended to keep them consistent. Using the same size and shape for all fiducials on a board helps to ensure accurate alignment and reduces the complexity of the machine vision system.

4. What should I do if my PCB design doesn’t have enough space for proper fiducial placement?

If your PCB design is too dense or complex to allow for proper fiducial placement, you may need to make some design adjustments. This could include rearranging components, reducing the size of certain features, or even increasing the overall size of the PCB. If space is still limited, you may need to prioritize the placement of fiducials in the most critical areas of the board and work closely with your assembly partner to ensure adequate alignment.

5. Can I use non-copper materials for fiducials, such as silkscreen or solder mask?

While it is technically possible to use non-copper materials for fiducials, such as silkscreen or solder mask, it is generally not recommended. These materials may not have the same reflectivity and contrast as copper, which can make them more difficult for machine vision systems to detect accurately. Stick with copper fiducials and use appropriate finishes and solder mask openings to ensure optimal detectability.

Conclusion

Fiducials play a critical role in the accurate assembly of PCBs, serving as reference points for automated assembly equipment. By following proper placement guidelines and design considerations, you can ensure that your fiducials are effective and compatible with your assembly partner’s processes.

Remember to:
– Use a minimum of three fiducials per PCB, placed near the corners
– Choose appropriate fiducial sizes and shapes based on your PCB size and assembly requirements
– Provide adequate clearance and solder mask openings around fiducials
– Use symmetrical shapes and consistent orientation for fiducials
– Communicate with your assembly partner early in the design process to ensure compatibility

By implementing these best practices, you can help to ensure the quality, reliability, and manufacturability of your PCB designs.