Castellated Hole Design: PCB Castellation for Modules

Introduction

The core of modern PCB design lies in efficiency, scalability, and modularity. Driven by the IoT boom alongside advancements in consumer electronics and industrial control technologies, there is a growing market demand for flexibly designed and easily assembled devices. It is precisely within this context that castellated hole technology (also referred to as PCB castellation or semi-plated holes) has emerged, bringing transformative changes to every stage from prototyping to mass production.

Castellations have transformed how engineers mount one PCB onto another.The castellated hole process now enables direct soldering of modules onto the main PCB or larger printed circuit board, replacing traditional interconnection methods relying on connectors and wires. This innovation fundamentally streamlines the assembly process while enhancing the efficiency and reliability of surface mounting. In high-volume manufacturing and complex PCB layouts - such as those found in Raspberry Pi Pico or custom wireless communication modules - adopting castellated holes not only facilitates rapid development but also ensures stable electrical connections and mechanical robustness.

What are Castellated Holes on a PCB?

A castellated hole is a unique, semi-circular via that's partially exposed along the edge of a PCB. These holes are typically plated through-holes that, through CNC milling or routing, are sliced so that only half of the hole remains, exposed at the board’s edge. This creates what’s commonly called a half-hole, half-plated hole, semi-plated hole, or half-cut hole.

Castellations allow a module to act like a large, surface mount device. The module is designed with holes along its edge (often matching the standard pitch of plated through-holes), and these holes are then soldered onto pads on the main board—perfectly aligning sub-circuits for seamless integration.

Key Features

• Semi-Plated Structure: Each hole is only partly embedded in the PCB, with one edge openly exposed.
• Surface Soldering: The module and the board are joined by soldering these half-holes to mating pads.
• Plated Edge: Internal copper plating, just like a normal via, ensures proper electrical connection even though the via is open to the board edge.
• Space Efficiency: Castellated holes simplify mounting, especially where space is limited or vertical profile needs minimizing.

Evolution and Purpose of Castellated Holes

The use of castellations marks a significant evolution in PCB assembly process and module-based product design. In the field of electronic interconnection technology, early solutions were heavily reliant on through-hole components and large connectors. Today, driven by the strong trends of miniaturization and modularity, more efficient solutions are continuously advancing.

Why Castellated Holes?

• Efficient Module Assembly: Easily solder wireless communication modules, RF modules, or any custom PCB module onto carrier boards.
• Mass Production: Sub-circuits can be mass-produced as separate modules, then integrated onto main boards using castellation in the final assembly.
• Rapid Product Iteration: Replace or upgrade a module without reworking the main board.
• Space Constraints: This solution is an ideal choice for high-end consumer electronics and industrial control applications where PCB space is severely limited.
• Improved Signal Performance: Plated edges and direct soldering reduce resistance and potential signal loss compared to connector-based assemblies.

Types of PCB Castellations

PCB castellation can be tailored for various mounting and assembly needs:

Full Castellations

These are plated through-holes that are cut exactly in half, used along the edge of a PCB. They provide robust mechanical support and maximum electrical contact, commonly found in power modules and industrial PCBs.

Partial Holes

Sometimes, only a section of the via is exposed at the edge, known as a partial hole. This approach is used when layout constraints or the number of connections dictate space-saving techniques without sacrificing electrical connectivity.

Staggered/Interleaved Castellations

A zigzag or alternating pattern of holes, often used in HDI circuit boards or when there’s a need to increase the pin density along the edge. This technique is essential in communication PCBs, or for breakout boards with multiple signal types.

Castellation Configurations and Mounting Techniques

The key parameters of castellated holes (quantity, spacing, arrangement) are not fixed but are determined by the design specifications of the final application.

Single-Row Castellations

Most commonly, a single row of castellated holes is aligned along the edge of the module. The number of holes depends on the functions needed—more pins for complex processes, fewer for simple breakouts.

Double-Row or Interleaved Patterns

Staggered or double-row castellated hole layouts optimize grounding references and signal paths, providing fundamental assurance for the integrity of high-speed signals (such as USB, HDMI, and RF). This represents a core design methodology for enhancing the performance of high-end circuit boards.

• Mounting Tips: The spacing design of castellated holes must strictly correspond to the pad pitch on the main PCB, which is a precondition for achieving precise alignment and robust assembly.

Mechanical Mounting Holes

In addition to castellated holes, standard mounting holes (unplated or fully plated) may be included for additional mechanical retention, particularly for modules handling vibration or physical stresses in industrial or automotive environments.

How are Castellated Holes Manufactured?

Manufacturing high-quality castellated holes on PCBs involves several specialized PCB fabrication steps:

• Drilling and Plating: Plated through-holes are drilled near the board edge and copper-plated to ensure electrical connectivity.
• Routing and Milling: CNC milling cuts away the outer edge of the PCB, exposing the semi-plated hole to create the castellated edge.
• Quality Control: Ensuring no copper burrs, maintaining the annular ring size, and avoiding peeling on exposed copper. Inspection for alignment and clean finish is crucial.
• Solder Mask and Surface Finish: Prevent mask creep over holes, and specify surface finish (ENIG, HASL, etc.) per design guidelines for the assembly process.

Manufacturing Table Example:

Design Guidelines and Best Practices

Top-quality PCB design and reliable module-to-main-board assembly depend on adhering to proven design guidelines for castellated holes in PCB projects:

Core Design Guidelines

1. Minimum Hole Size: 0.5 mm to 1.2 mm is standard for castellation, depending on signal/power needs.
2. Edge Clearance: Maintain at least 1.0 mm from the board edge to other features or copper pours to avoid shorts.
3. Annular Ring: Minimum of 0.25 mm width around each hole for robust plating and solder-wicking.
4. Pad Shape and Placement: At least half of each pad/plate should remain on the PCB after routing.
5. Spacing and Pitch: Space holes according to module requirements and the main board's pad layout; proper spacing avoids bridging and facilitates automated PCB assembly.
6. Mechanical Reinforcement: For modules subject to mechanical stress, use additional mounting holes and thicker copper layers.
7. Solder Mask Clearance: Apply sufficient clearance in your PCB layout so that no solder mask covers or partially obscures the castellated edges or half-holes.

Additional Design PCB Tips

• For multi-row or staggered castellation (common in Raspberry Pi add-ons or HDI boards), verify that the PCB layout software supports complex hole configurations “along the edge.”
• In high-frequency or wireless communication modules, design ground castellations between signal lines to minimize noise and maximize signal integrity.
• Test alignment by printing a 1:1 scale copy of the PCB layout and manually fitting components or test boards before finalizing your design.

Practical Engineering Tips

• Reflow Assembly: Favor reflow soldering with a professionally designed stencil when possible—this increases consistency, especially when a high pin count along the edge is involved, such as with Raspberry Pi Pico or other advanced modules.
• Manual Soldering: Use a fine-tipped, temperature-controlled iron and plenty of flux for clean joints on half-plated holes.
• Mechanical Support: For larger or heavier modules, combine castellated edges with mounting holes to reduce strain on solder joints.
• Inspection: Use a strong magnifier or microscope to check for solder bridges or cold joints after assembly, especially on densely packed communication PCBs.
• Testing: Always perform continuity and function testing on every castellation, not just on visual inspection. Sensitive circuits (like Bluetooth or Wi-Fi modules) require pristine connections.

Applications of Castellated Holes

The variety of applications for castellated holes and PCB castellation is staggering, reaching far beyond hobbyist boards:

• Wireless Communication Modules: GSM, Bluetooth, Zigbee, and Wi-Fi breakouts are soldered onto larger PCBs—enabling rapid, connectorless expansion in consumer and industrial IoT.
• Industrial Control and BMS: Castellated modules simplify scalable PCB design for multi-board battery management systems, relay boards, and sensor arrays.
• Raspberry Pi and Pico Ecosystem: Add-ons for small computers, including communication, display, and sensor boards, mount directly using castellation and mounting holes—no header pins required.
• Prototyping and Education: Quickly swap sub-circuits for product development or classroom projects.
• Consumer Electronics: In high-end devices, castellation enables ever-more-compact PCBs with fewer connectors and greater reliability.

Limitations, Pitfalls, and Solutions

While castellated holes enable modularity and fast integration, they introduce specific considerations:

• Mechanical fragility: Modules relying only on soldered half-holes risk damage from vibration or repeated stress. Solution: combine with mechanical mounting holes, or plate the PCB edge for extra resilience.
• Solder bridging: Fine pitch PCB modules can be hard to solder by hand. Solution: use reflow and test for bridges on all unique holes.
• Assembly precision: Misalignment can lead to failed connections. Solution: use alignment holes or silkscreen guides and invest in proper jigs for mass assembly.
• Not suitable for high current: Use normal via or full through-holes for power delivery paths, reserving castellation for signal lines.

Castellated Holes vs. Standard PCB Holes

Cost, Scale, and Industry Trends

While PCB castellation incurs slightly higher unit pricing from extra CNC milling and finishing, its advantages in modularity, speed of assembly, and main PCB real estate savings far outweigh initial costs—especially as sub-circuits can be mass-produced. The assembly process is also dramatically shortened, since mounting holes and connectors are reduced or eliminated altogether.

In the PCB industry, increasing numbers of communication modules, consumer electronics, and IoT devices rely on castellation for fast “plug-and-play” product launches and easy version control of firmware or hardware. Many PCB board houses now offer special castellation services for prototyping and volume fabrication, making this technique accessible to both startups and enterprise-level teams.

FAQs: Castellated Holes and PCB Castellation

Q: Can castellated holes be used for high-power signals?

A: For low-to-moderate current applications, castellated holes suffice; for high current (2A), supplement with plated through-hole or edge-plated pads.

Q: What PCB design tool supports castellation?

A: All major EDA/PCB design platforms (Altium, Eagle, KiCad, etc.) can lay out semi-plated holes and board edges; use mechanical-layer drawings for precision.

Q: Should I use castellation or headers for PCB module mounting?

A: Choose castellation when space is limited, miniaturization is critical, or for SMT-based assembly lines. Use headers for easy hand-assembly or repeated mating/unmating.

Q: How many holes should a module have?

A: The number of holes depends on signal and power/GND needs; always follow proper spacing and IPC design guidelines for reliability.

Q: Are castellation designs suitable for consumer and industrial electronics?

A: Absolutely—high-end consumer electronics, industrial control systems, and even wireless communication modules increasingly use castellated edges for robust integration.

Summary: Why Castellation Is Here to Stay

As an innovative interconnection technology, PCB castellated holes combine the compactness of surface-mount design with the robustness of plated through-holes, providing engineers with a mature and reliable flexible solution. This excellence in module installation, functional expansion, and manufacturable sub-circuit production has established it as an exemplary process driving rapid development in IoT, modular devices, and consumer electronics.