What are Castellated Holes on a PCB?

Castellated Holes or Castellations are indentations created in the form of semi-plated holes on the edges of the PCB boards. These Castellations help to mount one PCB board on top of another during assembly. Depending upon the application, instead of half holes, they may also look like a small or larger portion of a broken circle. Such holes are designed to provide proper alignment between the boards while soldering. This mounting process takes place with the help of castellated holes and called board-to-board soldering. Castellated holes are created on various PCB modules (Bluetooth or Wi-Fi modules), these modules can then be used as independent parts to be mounted onto another board during the assembly process.

Through Hole Technology (THT) and Surface Mount Technology are commonly used during PCB assembly process. Apart from these, board-to-board soldering is used, when there is a need to mount a board on the top of another PCB. Castellations serve as a link between the module and the board onto which the module has to be soldered. 

Why do we use Castellated holes?

By creating these holes on the board edges, they can be used to replicate certain parts of a PCB circuit. For example, if there is a circuit that includes an inverter, filter, or feedback loops. These small sub-circuits can be mass-produced, tested and can be soldered on to the main PCB that contains other parts of the circuit whenever required.

Use of Castellated Holes: 

• Castellated PCBs can be used as breakout boards for a particular portion of the larger PCB.
• These holes give ease to change the Pin layout of the component as per the user requirement.
• Integrated modules can be produced on a single PCB board using castellation, which can be used further into another assembly during production.
• PCBs with castellated holes can be mounted easily to another PCB during final production.
• They are used to combine two boards to validate the solder joint quality.
• They are used for breakout boards or small modules like Wi-Fi modules.
• Castellated holes also help to create wireless PCB to PCB links.

Castellated Holes Design

The castellated holes/ vias are created using a normal via process, which includes Drilling holes and copper plating. These holes look like semi-circles on the edges of the board since they are cut through to form a partial hole or half hole to create an opening on the board edges. Castellated holes can be created in various ways:

Half Holes

Holes with Smaller cut-in Half Holes

Holes with smaller lead-in holes for post-processing: They are designed for connector applications.

Recommended Specification for Castellated Holes 

There are some design attributes for castellated holes to be followed:

• Size: Use the largest possible size available.
• Surface Finish: Depends on the intended application of the board. ENIG finish is recommended.
• Pad Design: It is recommended to use the largest possible pad on both Top and Bottom.
• Number of Holes: Number of holes depends upon the design. Always go for an optimum number of holes otherwise it will make the alignment and assembly process difficult.

Applications of Castellated holes

Castellated holes on PCBs are used in a wide range of industries such as telecommunication, computer application, industrial control, power, automobile and high-end consumer electronics, and many more.