What are PCB Castellations?

Castellations on a PCB, also known as Castellated Holes or castellated edges, are a type of edge connection feature used in printed circuit board (PCB) design. These castellations are essentially plated-through holes (PTHs) that are cut in half along the edge of the PCB, creating a series of semi-circular pads. The term “castellation” comes from the resemblance of these half-holes to the architectural features found on top of castle walls.

PCB castellations serve as an alternative to traditional connectors, offering several advantages in terms of space savings, cost reduction, and manufacturing simplicity. They enable direct board-to-board connections, allowing multiple PCBs to be easily joined together without the need for additional connectors or cables.

Advantages of PCB Castellations

1. Space Savings: Castellations eliminate the need for bulky connectors, saving valuable PCB real estate.
2. Cost Reduction: By removing the requirement for separate connectors, castellations can reduce overall component costs.
3. Simplified Manufacturing: Castellations are created during the standard PCB Fabrication process, requiring no additional assembly steps.
4. Increased Reliability: Direct board-to-board connections using castellations minimize potential points of failure compared to traditional connectors.
5. Improved Signal Integrity: Castellations offer shorter signal paths, reducing the risk of signal degradation or interference.

How are PCB Castellations Created?

The process of creating PCB castellations involves a few key steps during the PCB manufacturing process:

1. PCB Design: The PCB layout is designed with castellations incorporated along the desired edges. The castellation holes are typically placed at regular intervals and are sized to accommodate the required electrical and mechanical connections.

2. Drilling: During the PCB fabrication process, the castellation holes are drilled along with other vias and through-holes on the board. The drilling process creates complete holes that extend through the entire thickness of the PCB.

3. Plating: After drilling, the PCB undergoes a plating process, where a conductive layer (usually copper) is deposited onto the walls of the drilled holes, including the castellation holes. This plating creates an electrical connection between the top and bottom layers of the PCB.

4. Routing: Once the PCB has been plated, it undergoes a routing process, where the board outline and any required cutouts are machined. For castellations, the routing process cuts through the center of the plated castellation holes, effectively splitting them in half. This creates the distinctive semi-circular pads along the edge of the PCB.

5. Finishing: The PCB may undergo additional finishing processes, such as solder mask application and surface finish (e.g., HASL, ENIG, or IAg), to protect the exposed copper and enhance solderability.

Designing with PCB Castellations

When incorporating castellations into a PCB design, several factors should be considered to ensure optimal performance and manufacturability:

Castellation Size and Spacing

The size and spacing of castellations depend on the specific requirements of the application, such as the number of connections needed, the current carrying capacity, and the mechanical stability of the board-to-board connection. Common castellation hole sizes range from 0.5mm to 2.0mm in diameter, with a typical spacing of 1.0mm to 2.54mm between the centers of adjacent castellations.

Mechanical Considerations

Castellations not only provide electrical connections but also contribute to the mechanical stability of the board-to-board connection. The number and placement of castellations should be carefully considered to ensure adequate mechanical support, especially in applications subject to vibration or physical stress. Additionally, the castellation design should allow for proper alignment and mating between the connected PCBs.

Electrical Considerations

When designing with castellations, it is essential to consider the electrical requirements of the connected signals. The size and spacing of the castellations should be chosen to accommodate the required current carrying capacity and to minimize signal integrity issues, such as crosstalk or impedance mismatches. It is also important to ensure proper grounding and shielding techniques are employed to maintain signal integrity.

Manufacturing Constraints

PCB Manufacturers have specific guidelines and constraints related to castellation design. These may include minimum hole sizes, spacing requirements, and tolerances for the plating and routing processes. It is crucial to consult with the chosen PCB manufacturer early in the design process to ensure the castellation design is compatible with their manufacturing capabilities and to avoid potential issues during fabrication.

Applications of PCB Castellations

PCB castellations find use in a wide range of applications across various industries. Some common applications include:

1. Board-to-Board Connections: Castellations enable direct board-to-board connections, allowing multiple PCBs to be stacked or connected side-by-side. This is particularly useful in space-constrained designs or when creating modular electronic systems.

2. Mezzanine Boards: Castellations are often used to connect mezzanine boards, which are secondary PCBs mounted parallel to a main board. Mezzanine boards can add functionality or expand the capabilities of the main board without increasing its footprint.

3. Edge Connectors: Castellations can serve as edge connectors, allowing a PCB to be plugged into a socket or another board. This approach is commonly used in card-edge connectors for expansion cards or modules.

4. RF and High-Speed Designs: Castellations are well-suited for RF and high-speed applications, as they provide short, direct signal paths between boards. This minimizes signal degradation and allows for better control over impedance matching.

5. Wearable Electronics: In wearable electronic devices, castellations enable the creation of slim, compact designs by eliminating the need for bulky connectors. They also provide a more robust connection compared to flexible printed circuits (FPCs) or ribbon cables.

6. Automotive Electronics: Castellations are used in automotive electronic systems, where reliability and space savings are critical. They can be found in applications such as instrument clusters, infotainment systems, and electronic control units (ECUs).

PCB Castellation Manufacturing Considerations

When manufacturing PCBs with castellations, there are several key considerations to ensure a successful outcome:

Material Selection

The choice of PCB material is important when incorporating castellations. The material should have good mechanical strength to withstand the stresses of the board-to-board connection and the PCB routing process. FR-4, a common PCB substrate material, is well-suited for castellated designs due to its excellent mechanical and electrical properties.

Plating Thickness

The plating thickness of the castellation holes is critical for ensuring reliable electrical connections and mechanical stability. Insufficient plating thickness can lead to weak connections or mechanical failures, while excessive plating can cause difficulties during the routing process. A typical plating thickness for castellations ranges from 25μm to 50μm, depending on the specific requirements of the application.

Routing Accuracy

Accurate routing of the castellation holes is essential for achieving the desired semi-circular pad shape and ensuring proper alignment between mating PCBs. High-precision routing equipment and strict process controls are necessary to maintain the required tolerances. Misaligned or poorly routed castellations can lead to connection issues or mechanical weakness.

Solder Mask Application

The solder mask application process for PCBs with castellations requires special attention. The solder mask should be applied in a way that exposes the castellation pads while protecting the surrounding areas. Proper solder mask coverage helps to prevent solder bridging and ensures reliable soldering during the assembly process.

Inspection and Testing

After fabrication, PCBs with castellations should undergo thorough inspection and testing to verify their quality and functionality. Visual inspection can help identify any defects in the castellation shape, plating, or solder mask coverage. Electrical testing, such as continuity and insulation resistance tests, ensures the integrity of the electrical connections. Mechanical testing may also be necessary to validate the strength and reliability of the board-to-board connection.

FAQ

1. Q: What are the advantages of using PCB castellations over traditional connectors?
   A: PCB castellations offer several advantages, including space savings, cost reduction, simplified manufacturing, increased reliability, and improved signal integrity. They eliminate the need for separate connectors, reducing component costs and saving PCB real estate. Castellations also provide direct board-to-board connections, minimizing potential points of failure and offering shorter signal paths.

2. Q: How are PCB castellations created during the manufacturing process?
   A: PCB castellations are created through a series of steps during the PCB fabrication process. The PCB layout is designed with castellation holes incorporated along the desired edges. These holes are drilled and plated along with other vias and through-holes. During the routing process, the castellation holes are cut in half, creating semi-circular pads along the edge of the PCB.

3. Q: What factors should be considered when designing PCBs with castellations?
   A: When designing PCBs with castellations, several factors should be considered, including castellation size and spacing, mechanical stability, electrical requirements, and manufacturing constraints. The size and spacing of castellations should accommodate the required electrical and mechanical connections. Proper grounding and shielding techniques should be employed to maintain signal integrity. It is important to consult with the PCB manufacturer to ensure compatibility with their manufacturing capabilities.

4. Q: In what applications are PCB castellations commonly used?
   A: PCB castellations are commonly used in applications such as board-to-board connections, mezzanine boards, edge connectors, RF and high-speed designs, wearable electronics, and automotive electronics. They are particularly useful in space-constrained designs, modular electronic systems, and applications requiring robust and reliable connections.

5. Q: What are the key manufacturing considerations for PCBs with castellations?
   A: Key manufacturing considerations for PCBs with castellations include material selection, plating thickness, routing accuracy, solder mask application, and inspection and testing. The PCB material should have good mechanical strength, and the plating thickness should be appropriate for reliable electrical and mechanical connections. Accurate routing and proper solder mask coverage are essential for achieving the desired castellation shape and functionality. Thorough inspection and testing help ensure the quality and reliability of the fabricated PCBs.

Castellation Parameter	Typical Value Range
Hole Diameter	0.5mm – 2.0mm
Hole Spacing	1.0mm – 2.54mm
Plating Thickness	25μm – 50μm

In conclusion, PCB castellations offer a versatile and reliable solution for board-to-board connections in electronic design. By understanding the advantages, design considerations, manufacturing processes, and applications of castellations, designers can effectively incorporate this feature into their PCB layouts. Careful attention to material selection, plating thickness, routing accuracy, and inspection and testing ensures the successful fabrication of PCBs with castellations. As electronic devices continue to evolve and demand smaller, more compact designs, the use of PCB castellations is likely to grow, providing a robust and efficient means of connecting multiple PCBs in a wide range of applications.