Plated through-hole half-holes, commonly known as castellated holes or castellated vias, are plated holes placed along the edge of a printed circuit board and cut through their center during board separation or edge routing. The result is a row of exposed, semicircular or "C"-shaped plated holes along the PCB edge.
Unlike a conventional through-hole, which remains enclosed by the PCB material, a half-hole exposes part of the plated barrel at the board edge. This exposed copper acts as a solderable connection point when the module is mounted directly onto another PCB. The structure is widely used on compact wireless modules, sensor modules, embedded controllers, and other PCB assemblies that require a board-to-board connection without a separate connector.
What Is a PTH Half-Hole?
A PTH half-hole begins as a PTH hole in PCB. During fabrication, the hole wall is plated with copper so that it can provide electrical continuity between the relevant PCB layers. The board edge is then routed, milled, or otherwise formed through the hole, removing approximately half of the barrel and exposing the remaining plated section.
The exposed copper surface forms a solderable "pin" along the module edge. When the module is placed on the host board, the half-hole aligns with a corresponding land pattern. Solder then bonds the module edge to the host PCB, providing both electrical continuity and a mechanical attachment.
In practical PCB design, the half-hole is therefore more than a modified via. It is an edge connection structure that combines the functions of a plated contact, a module pin, and a board-to-board solder joint. The finished result depends on the accuracy of the hole location, copper plating, board contour, surface finish, and assembly process.
Primary Design Functions
1. Direct Board-to-Board Module Connection
The most important purpose of a half-hole is to allow a PCB module to be soldered directly to a host board. Wireless modules, including Wi-Fi and Bluetooth modules, are typical examples. Instead of using a pin header, socket, or dedicated board-to-board connector, the module edge itself provides the required connection points.
This approach simplifies the interconnect structure. The half-holes serve as module "pins," while the host PCB provides the corresponding surface-mount pads. Once soldered, the connection carries power, ground, control signals, and high-speed or RF signals as required by the module design.
Eliminating an intermediate connector can also reduce the number of components and assembly operations. It may reduce the overall package height and remove connector-related tolerance issues. However, the half-hole connection must be designed with adequate solder coverage and mechanical support because the solder joint is directly exposed at the module edge.
2. Higher-Density Product Integration
Connectors and pin headers occupy valuable board area and often require additional clearance around the mating structure. A castellated edge connection can reduce this space and help create a more compact product. This is particularly useful in portable electronics, compact control boards, industrial sensor nodes, and other products with strict mechanical constraints.
The module can be positioned close to the host PCB surface, allowing the complete assembly to fit into a smaller enclosure. The reduction in connector height can also simplify enclosure design. Nevertheless, the final mechanical stack-up must still account for solder thickness, component keep-out areas, inspection access, and rework clearance.
3. Testing, Programming, and Module-Level Production
In some designs, half-holes provide test or programming access. A module can be powered, programmed, or functionally tested as an independent PCB before it is assembled onto the main board. This can improve production flexibility because defective modules may be identified before they are installed in the final product.
The same edge connection can also support fixture-based testing. A test fixture can contact the exposed plated surfaces or mate the module with a temporary carrier board. After verification, the module can be soldered to the final host PCB. The exact test arrangement depends on the module design and the required access to power, ground, programming, and communication signals.
Typical Applications
Half-hole connections are commonly used on modules that must be integrated into a larger system without a separate connector. Typical examples include:
- Wi-Fi and Bluetooth communication modules
- Wireless transceiver and RF modules
- Microcontroller and embedded control modules
- Sensor and data-acquisition modules
- Power-management and interface modules
- Compact development boards and production subassemblies
The module may be mounted parallel to the host PCB as a surface-mounted subassembly, or it may be positioned in another orientation when the mechanical design requires it. In every case, the edge geometry and host-board land pattern must be designed together.
PCB Layout Considerations
Hole Position Relative to the Board Edge
The distance from the center of each plated hole to the final board edge must be controlled accurately. The contour must pass through the intended location so that the finished hole exposes the correct amount of plated copper. If the edge is too far from the hole center, too little of the barrel is exposed. If it cuts too deeply, the remaining copper may be reduced and the mechanical integrity of the connection may be weakened.
In the described design approach, the hole edge is typically aligned with the board edge or retains only a very small margin, approximately 0.1–0.2 mm. The actual value must be confirmed with the PCB fabricator because routing tolerance, hole tolerance, copper thickness, and the selected manufacturing process all affect the final geometry.
Land Pattern and Pad Extension
The host-board land pattern should provide sufficient soldering area for the exposed half-holes. Compared with a conventional PTH arrangement, the associated pad may need to extend beyond the nominal connection region to ensure adequate overlap with the module edge. A typical design consideration is an extension of approximately 0.3–0.5 mm, but this value is case-specific and must be checked against the module drawing and assembly capability.
The solder pad should not be enlarged without considering neighboring pads and exposed copper. Excessive solder volume can increase the risk of bridging, while an undersized pad can produce insufficient wetting or a weak joint. The module footprint and the host-board footprint should therefore be reviewed as one complete interface rather than as independent designs.
Spacing Between Adjacent Half-Holes
The spacing between neighboring half-holes must support the selected soldering process. If the spacing is too small, solder may bridge between adjacent contacts, especially when the stencil deposits excessive paste or when the exposed copper geometry varies along the board edge.
Spacing also affects inspection and rework. Adequate separation makes it easier to identify solder bridges, insufficient wetting, and void-related defects. The final pitch should be evaluated together with the half-hole diameter, pad width, solder-mask definition, stencil aperture, and assembly tolerances.
Manufacturing Process Considerations
Drilling, Plating, and Board Contouring
The manufacturing sequence is critical to half-hole quality. A commonly used process completes drilling and copper plating before the final edge contour is produced by routing, CNC milling, or another board-forming operation. Cutting the plated hole after plating exposes the plated barrel at the edge.
During edge routing, the tool can pull or tear copper if the process is not properly controlled. This may leave burrs, rough edges, copper lifting, or partial separation of the plated layer. The fabricator must therefore control the routing path, tool condition, cutting parameters, and board support method.
V-scoring or V-cut processing may be suitable for some board-separation structures, but it is not interchangeable with every castellated-hole geometry. The selected method should match the required edge profile and the location of the plated holes. The manufacturing drawing should clearly define the finished board edge and the required half-hole condition.
Plating Integrity
After the board edge is formed, the exposed hole wall must retain a continuous copper layer without peeling or delamination. Any break in the plating can increase contact resistance, interrupt electrical continuity, or reduce solder-joint reliability.
The exposed edge should also be free of excessive burrs and loose copper particles. A clean, consistent edge improves solder wetting and reduces the risk of contamination during assembly. Inspection should include the plated barrel, the final contour, copper adhesion, and the consistency of the exposed semicircular profile.
Soldering and Assembly Precautions
Stencil Design and Solder-Paste Volume
Stencil apertures must align accurately with the half-hole land pattern. Solder-paste volume requires careful control because the connection is located at the module edge. Excess paste can cause bridging between adjacent contacts, while insufficient paste can lead to incomplete wetting, voids, or an electrically open joint.
The stencil design should be evaluated together with the module placement tolerance and the actual exposed copper geometry. A footprint that appears correct in the CAD database may still produce uneven solder deposition if the half-hole contour or board edge is not consistent.
Reflow Temperature Profile
The reflow profile must provide enough thermal energy for the solder to wet the exposed plated surface and the host-board pad. At the same time, the module and the host PCB may have different thermal masses, so the temperature distribution should be considered across the entire assembly.
Insufficient heating can produce poor wetting or an unreliable joint. Excessive or uneven heating may affect the module, nearby components, or the board material. The reflow profile should therefore be verified for the specific module and assembly configuration rather than selected solely from the host board.
Mechanical Strength
A half-hole solder connection does not provide the same mechanical structure as a complete through-hole connection with copper surrounding the entire barrel. The module may therefore be more vulnerable to peeling or edge loading, particularly if it is installed in a location exposed to vibration, bending, insertion force, or repeated mechanical impact.
Mechanical supports, mounting features, or additional soldered connections may be needed when the module is subject to significant stress. The half-hole interface should not be treated as a substitute for structural hardware when the product requires high mechanical robustness.
Reliability and Rework
The exposed copper at the board edge is more vulnerable to oxidation and contamination than copper fully protected by solder mask or a finished pad structure. A suitable surface finish, such as electroless nickel immersion gold or OSP, can help preserve solderability. The selected finish should be compatible with the assembly process and the required storage conditions.
Rework also requires special attention. During module removal, repeated heating and mechanical separation can lift the host-board pad or damage the plated half-hole. The design should provide sufficient rework allowance, and the repair procedure should avoid applying excessive force to the module edge.
Overall, a PTH half-hole is a compact and practical interface for direct PCB-to-PCB integration. Its benefits include connector elimination, higher packaging density, and convenient module-level testing. Reliable performance depends on coordinated control of the hole position, board contour, copper plating, surface finish, footprint, stencil, reflow profile, and mechanical environment.
