Castellated PCBs: Process Challenges and Standard Design Guidelines
With the continuous iteration of consumer electronics, industrial control equipment, and wireless communication technology, electronic devices are evolving toward miniaturization, high integration, lightweight and multi-functional performance. To simplify circuit design, shorten R&D cycles and improve product compatibility, modular PCB design has become a mainstream industry trend.
Wireless communication modules such as Bluetooth, Wi-Fi, and NB-IoT, as core auxiliary components of electronic products, are usually manufactured as independent sub-boards and mounted on main PCBs through a special welding process — the castellated hole process. PCBs with edge castellated structures are widely adopted in modular electronic products due to their convenient assembly, stable electrical connection and space-saving advantages. This article elaborates on the basic definition, manufacturing challenges, standardized design specifications and panelization rules of castellated PCBs.
What Are Castellated Holes?
Castellated holes, also known as metalized half-holes, refer to a row of specially processed metalized half-hole structures distributed on the edge of a PCB. Different from conventional full through-holes, these holes are cut along the PCB outline, retaining half of the complete metalized hole wall on the board side.
Castellated PCBs are mostly used as daughter boards and can be directly soldered to the main PCB without additional connectors or pin headers. The intact copper plating on the half-hole wall ensures reliable electrical conduction and mechanical bonding between the sub-board and the main board. This structure is highly suitable for small-sized modular boards, sensor modules and portable electronic components, effectively optimizing product structure and reducing assembly difficulty.
Core Manufacturing Challenges of Castellated Holes
Although castellated hole structures bring great convenience for PCB assembly, their manufacturing process is far more complex than that of standard PCBs, with strict requirements on process precision and production control. The core difficulty lies in the forming and post-processing of metalized half-holes.
The conventional production flow of castellated holes includes two key steps: integral drilling of full round holes and hole wall copper plating, followed by edge milling to cut the holes into half-hole structures. In the milling process, the high-speed rotating spindle will produce uneven force on the hole wall copper plating. When the milling tool moves to the stressed blind area of the hole wall, the copper layer loses substrate support, which easily causes common defects such as copper burrs, copper peeling, residual copper scraps and hole wall warping.
These manufacturing defects will bring serious hidden dangers to subsequent SMT assembly and product use. Residual copper burrs may lead to poor solder joint fusion and unstable electrical connection, while excessive copper residues can cause pin short circuits and circuit failure, directly affecting the yield and service life of electronic products.
To solve the above process pain points, standardized secondary drilling and milling processes are adopted in mass production. The segmented processing method effectively retains the integrity of the hole wall copper plating, eliminates burrs and residues, and realizes stable and batch production of high-quality castellated PCBs.
Standard Design Requirements for Castellated PCBs
To ensure producibility and assembly reliability of castellated PCBs, designers must follow unified dimensional standards and pad design specifications. Unreasonable design parameters will lead to production failure or defective products. The detailed dimensional requirements are as follows:
| Design Dimension Parameter | Minimum Standard Value | Remarks |
|---|---|---|
| Single PCB Board Size | 10 mm × 10 mm | Minimum size for mass production |
| Distance from Board Edge to Castellated Pad | 1.0 mm | Avoid edge damage during milling |
| Castellated Hole Drill Diameter | 0.5 mm | Ensure complete metalized hole wall |
| Hole-to-Hole Clearance | 0.5 mm | Prevent adjacent hole short circuits |
| Annular Ring of Castellated Hole | 0.25 mm (Absolute minimum: 0.18 mm) | Sizes below the limit require special order notes |
Pad Shape and Placement Rules
Castellated holes support round and oval hole designs, matching round or square solder pads. All solder pads must be arranged inside the PCB outline to ensure effective welding area. It is forbidden to design only one-third of the hole structure inside the board; at minimum, the castellated hole must be centered on the board outline centerline.
For oval castellated holes parallel to the board edge, at least half of the hole structure must penetrate into the PCB interior. In addition, if the length of the oval castellated hole exceeds or equals 5 mm, the ENIG surface finish process is mandatory to improve solderability and corrosion resistance.
Panelization Guidelines for Castellated PCBs
Panelization is an essential process for batch PCB production, which improves production efficiency and reduces material loss. Castellated PCBs can adopt mainstream panelization methods, but there are strict process restrictions for edge processing.
The single-board size of castellated PCBs in panel production must not be less than 10 mm, and the overall panel size also needs to meet the 10 mm minimum standard. V-CUT and mouse bite (break-away tab) panelization are both optional, but there is a key process taboo: V-CUT grooving is strictly prohibited on castellated hole edges.
The V-CUT tool will generate strong pulling and friction force on the edge copper layer during grooving, which is easy to cause peeling and warping of the castellated hole wall copper plating, resulting in scrapped products. For all edges with castellated holes, CNC milling must be used for shaping and separation. V-CUT is only applicable to non-castellated edges of the PCB. For four-sided castellated PCBs, mouse bite panelization with full CNC milling separation is the most reliable production solution.
Cost Factors of Castellated PCBs
Castellated PCBs belong to special-process circuit boards, and their production cost is higher than standard PCBs for two main reasons. First, the manufacturing process is more complicated: based on conventional drilling and plating processes, additional secondary milling and trimming procedures are required, increasing production man-hours and equipment loss. Second, most castellated modular boards are small in size, which raises the unit production cost compared with conventional large-size PCBs.
All non-standard special processes and size limitations will lead to differentiated pricing. If designers have special requirements for pad spacing, annular ring size or processing technology, they need to mark relevant notes when placing orders and confirm production file adjustments with the manufacturer in advance.
Conclusion
Castellated PCB technology provides a simple and efficient assembly solution for modular electronic products, which is widely used in wireless communication, intelligent sensing, wearable devices and other fields. However, the special half-hole metalizing and milling process puts forward higher requirements for design and production. Following standardized dimensional parameters, pad layout rules and panelization process taboos is the key to ensuring high-yield and high-reliability castellated PCBs.
Professional process control and standardized design can effectively avoid common defects such as copper burrs and copper peeling, ensuring stable electrical performance and mechanical strength of modular board welding. For customized castellated PCB production, it is essential to cooperate with manufacturers with mature process capabilities to guarantee product quality and delivery efficiency.
About US
Founded in 2012, JKRGLO strives to build a one-stop platform for the electronic industry chain. By integrating PCB manufacturing, component procurement and PCB assembly services, we enable digital PCBA processing. With increasing investment in innovation and digital systems, we have achieved rapid growth and emerged as a leading PCB and PCBA manufacturer in the industry, capable of rapidly producing high-reliability and cost-effective products.
