What Are Castellated PCBs?

Printed circuit boards form the foundation of almost every modern electronic device and one of their most versatile variations is the castellated PCB. Recognised by the small half-holes that line their edges, castellated boards are designed for modular integration, allowing one board to be soldered directly onto another. These edge connections provide both mechanical strength and electrical continuity, making them ideal for compact modules such as wireless transmitters, embedded controllers and Internet of Things (IoT) devices.

As an IPC Certification Centre, A.R.T. trains engineers and manufacturers to work confidently with such designs, combining theoretical knowledge with practical inspection and soldering expertise. Understanding how a castellated PCB functions, and how it differs from conventional layouts, is an essential part of maintaining reliability in high-density assembly environments.

A castellated PCB uses plated holes drilled near its outer edge that are subsequently milled to expose a semicircular cross-section. This creates conductive pads along the board’s perimeter, which can then be aligned and soldered onto another PCB. The result is a secure electrical and mechanical bond between the two boards.

The process begins with the same fundamentals as a plated-through hole, except the holes are intentionally cut in half during profiling. Proper plating thickness and alignment are critical to ensure a uniform copper wall that supports consistent solder wetting. If the milling is off-centre or the plating is too thin, it can lead to poor connectivity or delamination during reflow.

The design advantages of a castellated PCB make it a preferred choice for modular electronics. Because the half-holes act as ready-made connection points, designers can integrate sub-modules, such as Wi-Fi or Bluetooth chips, directly onto a host board without the need for complex connectors.

For manufacturers, this means easier prototyping, simplified assembly and quicker replacement of faulty or upgraded sub-boards. Inspection and rework are also easier, as solder joints are visible and accessible from the side. A.R.T. emphasises these benefits during IPC training, teaching participants to identify proper solder fillets and avoid common defects such as solder bridging.

Manufacturing a castellated PCB demands precise process control to maintain alignment and plating quality. The production stages include drilling and plating the through-holes to create conductive barrels, then milling the board’s edge so each hole is neatly halved to expose the plated interior. Surface finishes such as ENIG or HASL are then applied to protect solderability and ensure consistent joint formation.

Each manufacturing stage directly influences overall reliability. Issues such as uneven plating or inaccurate milling can cause solder defects, trapped flux and intermittent electrical connections. Through its IPC-accredited programmes, A.R.T. trains engineers to identify and correct these faults, ensuring that every castellated PCB meets the highest inspection and performance standards.

Designing a castellated PCB involves balancing electrical, mechanical and manufacturing requirements. The hole pitch must align precisely with the pads on the receiving board, while pad size and solder paste volume must support proper fillet formation.

Routing clearances should also accommodate features such as traces and copper pours to prevent accidental bridging. A poorly designed layout can create solder wicking, voiding or open circuits. Designers must ensure that the solder mask defines clean pad edges without overexposure, as excessive mask pullback may weaken joints.

A.R.T.’s training in PCB Design reinforces how these details affect reliability, especially in multi-layer or high-frequency assemblies where impedance control and component density are critical.

Although robust in design, a castellated PCB can still present manufacturing challenges. Common issues include incomplete plating, misaligned milling, voids within the solder joint or surface contamination that prevents full wetting.

Visual inspection is often the first step, using magnification to check that solder wraps evenly around each castellated edge. Advanced methods, such as X-ray inspection or cross-sectional analysis, can verify internal plating quality and solder penetration.

IPC-A-600 and IPC-A-610 standards define acceptance criteria for such assemblies, outlining the minimum plating coverage and solder fillet shape required for Class 2 and Class 3 electronics. These guidelines also help technicians diagnose and prevent PCB soldering issues during inspection and rework.

Castellated PCBs are used across an expanding range of industries. In consumer electronics, it forms the connection backbone for wireless modules such as GPS, Wi-Fi and Bluetooth transceivers. In automotive and aerospace applications, castellated edges are employed for sensor and control interfaces that demand both compactness and high reliability.

Because castellated designs simplify module integration, they are widely used in prototyping and low-volume builds where flexibility is essential. They also facilitate modular design in IoT networks, where devices must be easily replaceable or upgradeable without redesigning the entire system.

Assembly and rework of a castellated PCB require steady hands and a clear understanding of solder dynamics. Excess heat or solder volume can damage thin edge plating, while insufficient reflow may leave cold joints.

Through its IPC-accredited training programmes, A.R.T. teaches proper techniques using both soldering stations and controlled reflow soldering environments. Participants learn how to identify surface finishes, apply flux and monitor solder flow across the exposed copper edges that define a castellated design.

Such hands-on experience prepares technicians for real-world scenarios, ensuring that every solder joint meets defined quality standards and that assemblies remain durable in service.

A castellated PCB combines the functionality of a connector with the simplicity of a standard circuit board. Its half-hole edges create strong electrical connections while reducing space and assembly time. However, achieving consistent quality demands precision at every stage, from drilling and plating to solder inspection and rework.

By understanding the structure, advantages and design requirements of castellated PCBs, engineers can produce reliable modules that integrate seamlessly into complex systems.

At A.R.T., our commitment to IPC standards throughout each of our practical training courses helps to ensure that technicians can handle even the most intricate assemblies with confidence. For manufacturers across aerospace, defence and consumer electronics, this expertise remains essential to maintaining reliability in today’s competitive production environments.