What are Castellated Holes in PCB Design?

Castellated holes, also known as castellation holes or castellations, are a type of plated through hole located on the edges of a printed circuit board (PCB). These semi-circular holes are formed by drilling or routing through the copper pads, resulting in a series of “half-moon” shaped holes that resemble the battlements of a castle wall, hence the term “castellated.”

Castellated holes serve several purposes in PCB design and production:

1. They allow for easy and secure attachment of the PCB to other boards or components using solder.
2. They provide a means for electrical connection between the PCB and other devices.
3. They facilitate the manufacturing process by providing reference points for cutting or breaking the PCB into smaller sections.

Advantages of Using Castellated Holes

Castellated holes offer several advantages over traditional edge connectors or pins:

• Cost-effective: Castellated holes eliminate the need for additional connectors, reducing the overall cost of the PCB.
• Space-saving: By utilizing the edges of the PCB, castellated holes help to minimize the footprint of the board, making it ideal for compact designs.
• Simplified assembly: Castellated holes allow for easy attachment of the PCB to other components using solder, simplifying the assembly process.
• Improved reliability: The plated through holes provide a strong mechanical and electrical connection, enhancing the reliability of the PCB.

Design Considerations for Castellated Holes

When incorporating castellated holes into your PCB design, there are several factors to consider to ensure optimal performance and manufacturability.

Hole Size and Spacing

The size and spacing of castellated holes depend on the specific requirements of your project. However, there are some general guidelines to follow:

• Hole diameter: The diameter of the castellated hole should be large enough to accommodate the desired amount of solder for a strong connection. A typical hole diameter ranges from 0.5mm to 1.0mm.
• Pad diameter: The copper pad surrounding the castellated hole should be slightly larger than the hole diameter to ensure proper plating and soldering. A typical pad diameter is 0.2mm to 0.5mm larger than the hole diameter.
• Spacing: The spacing between castellated holes should be sufficient to maintain the structural integrity of the PCB and prevent solder bridging. A minimum spacing of 0.5mm to 1.0mm is recommended.

Copper Thickness and Plating

The copper thickness and plating of castellated holes are crucial factors in ensuring a reliable electrical and mechanical connection.

• Copper thickness: The copper thickness of the pads and traces leading to the castellated holes should be sufficient to handle the desired current and provide mechanical strength. A minimum copper thickness of 1 oz (35 µm) is recommended.
• Plating: Castellated holes should be plated with a suitable material, such as gold or tin, to protect against oxidation and enhance solderability. The plating thickness should be appropriate for the application, typically ranging from 0.5 µm to 2 µm.

Solder Mask and Silkscreen

Proper application of solder mask and silkscreen around castellated holes is essential for successful soldering and identification.

• Solder mask: A solder mask should be applied around the castellated holes to prevent solder bridging and improve the aesthetic appearance of the PCB. The solder mask opening should be slightly larger than the copper pad to ensure proper coverage.
• Silkscreen: Silkscreen markings can be used to identify the castellated holes and provide assembly instructions. The silkscreen should be legible and positioned away from the holes to avoid interference with soldering.

PCB Manufacturing Process for Castellated Holes

The manufacturing process for PCBs with castellated holes involves several steps to ensure proper formation and plating of the holes.

Drilling or Routing

Castellated holes are typically formed by drilling or routing through the copper pads on the edges of the PCB. The choice between drilling and routing depends on the desired shape and size of the holes.

• Drilling: Drilling is suitable for circular castellated holes and involves using a drill bit to create the holes. The drill bit diameter should be slightly smaller than the desired hole diameter to accommodate plating.
• Routing: Routing is ideal for creating non-circular or larger castellated holes. A CNC router is used to remove the excess material around the copper pads, forming the desired hole shape.

Plating

After the castellated holes are formed, they must be plated to ensure proper electrical connection and protect against oxidation. The plating process typically involves the following steps:

1. Cleaning: The PCB is cleaned to remove any dirt, grease, or debris that may interfere with the plating process.
2. Activation: The exposed copper surfaces are activated to improve the adhesion of the plating material.
3. Electroless copper: A thin layer of electroless copper is deposited on the activated surfaces to provide a conductive base for electroplating.
4. Electroplating: The desired plating material, such as gold or tin, is electroplated onto the electroless copper layer to the specified thickness.
5. Inspection: The plated castellated holes are inspected for quality and adherence to the design specifications.

Solder Mask Application

Once the castellated holes are plated, a solder mask is applied to the PCB to protect against solder bridging and improve the aesthetic appearance. The solder mask application process involves the following steps:

1. Solder mask application: A liquid or dry film solder mask is applied to the PCB, covering the areas around the castellated holes.
2. Exposure: The solder mask is exposed to UV light through a photomask, hardening the exposed areas.
3. Development: The unexposed solder mask is removed using a developer solution, leaving openings around the castellated holes and other desired areas.
4. Curing: The solder mask is cured using heat to ensure proper adhesion and durability.

Best Practices for Using Castellated Holes

To ensure the best performance and reliability of your PCB with castellated holes, follow these best practices:

1. Follow the recommended design guidelines for hole size, spacing, and copper thickness to ensure proper plating and soldering.
2. Use appropriate plating materials and thicknesses for your application to protect against oxidation and enhance solderability.
3. Apply solder mask and silkscreen correctly to prevent solder bridging and improve identification.
4. Work closely with your PCB manufacturer to ensure that the castellated holes are formed and plated according to your specifications.
5. Conduct thorough testing and inspection of the assembled PCB to verify the integrity of the castellated hole connections.

Frequently Asked Questions (FAQ)

1. What is the primary purpose of castellated holes in PCB design?
   Castellated holes serve as a means for electrical connection and mechanical attachment between the PCB and other components or boards.

2. What is the typical diameter range for castellated holes?
   The typical diameter range for castellated holes is 0.5mm to 1.0mm.

3. What is the minimum recommended spacing between castellated holes?
   The minimum recommended spacing between castellated holes is 0.5mm to 1.0mm, depending on the hole diameter.

4. What plating materials are commonly used for castellated holes?
   Common plating materials for castellated holes include gold and tin, which protect against oxidation and enhance solderability.

5. Why is solder mask application important for castellated holes?
   Solder mask application around castellated holes is important to prevent solder bridging and improve the aesthetic appearance of the PCB.

By understanding the design considerations, manufacturing processes, and best practices for using castellated holes in PCB production, you can create reliable and cost-effective PCBs that meet the requirements of your specific application.