Hi readers! I hope you are having a great day and finding something new to learn. Today, the topic of our discourse is PCB Castellation and Castellated Holes.
Printed Circuit Boards (PCBs) are the core of nearly all electronic devices and are a structured platform on which components are mounted and interconnected. As electronics are moving toward more miniaturization, modularization, and mass production, PCB technologies must advance to be able to support these new requirements. Part of this advancement is the use of PCB castellations or castellated holes, a technology that supports small, modular board-to-board connections.
Castellated holes are tiny, plated half-holes on the border of a PCB. They allow one PCB, a module, or daughterboard, to be soldered onto another directly, much like a surface-mount device. This process eliminates connectors and enables easy integration of pre-tested, reusable modules within a system. Castellated PCBs find extensive use in Wi-Fi and Bluetooth modules, DC-DC power supplies, and sensor interfaces, where space is limited and utmost reliability is needed.
By having a simple yet robust technique of stacking or soldering PCBs along with each other, castellated holes enhance design flexibility without adding complexity to assembly, thus emerging as a highly desired solution for prototyping as well as volume production.
In this article, you will learn about PCB Castellation, its characteristics, manufacturing process, design guidelines, applications, and quality testing. Let’s dive into in-depth details.
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PCB castellation is the term used for producing half-plated holes along the periphery of a PCB. Holes are drilled and plated as regular vias, but subsequently partially routed off when milling the outer profile of the board. The consequence is a half-circular hole cutting through the edge of the PCB, with the copper plating exposed. These notched holes look like the battlements on a castle, hence the name "castellation".
As compared to conventional vias, castellated holes have both a mechanical and electrical purpose, allowing soldering of the PCB module onto a higher motherboard with consistent electrical connections.
Castellated holes, otherwise known as half-holes, are special features along the edge of some PCBs that allow for complete electrical and mechanical connections with other boards. They are made by plating through-holes (PTHs) along the edge of a board and then milling or routing them in half during the last profiling phase. This leaves a series of half-semi-circular notches that reveal the internal copper plating along the edge of a board.
These packages are often utilized in surface-mount modules like RF transceivers, sensor breakouts, and power management modules, in which a sub-board has to be mounted directly on a host PCB.
Every castellated hole contains a conductive lining of copper, electroplated for superior electrical continuity and solderability. The copper wall assures that when the module is reflow soldered onto the base PCB, there is a strong electrical bridge between the two boards. Proper plating is important—thin or uneven plating can lead to poor connections or increased resistance.
The half-hole shape provides self-centering on placement. The shape naturally seats onto the respective SMT pad on the base PCB, and reflow solder surface tension assists in centering the module. The design also provides maximum contact area, enhancing the integrity and stability of the solder joint.
The castellated edge serves as a functional terminal, similar to a standard surface-mount lead. Solder paste is placed on the pads of the receiving PCB, and during reflow, the solder flows up the copper surface of the castellation to form a secure connection.
Castellated holes enable a plug-and-play design philosophy, as pre-validated modules can be reused in various product lines. This speeds development and minimizes the risk of board-level design flaws.
Casting away the use of connectors or sockets, castellated PCBs conserve precious board space. This is extremely important for applications such as wearables, IoT nodes, drones, and handheld devices, where real estate is precious.
These PCBs are well-suited to standard SMT assembly methods, pick-and-place and reflow soldering, so they're great for automated manufacturing environments.
There is easy access for visual inspection along the exposed edges, and occasionally, electrical probing. Modules can be tested by engineers before soldering and retested afterwards without specialized test arrangements.
Production of high-quality castellated holes involves precise execution of a number of accurate manufacturing processes. Castellated holes are not simply edge embellishments; they are functional electrical and mechanical interfaces. Their manufacture, therefore, must be a controlled process to provide structural integrity and best solderability.
The operation is initiated in the same manner as conventional plated-through-hole (PTH) production. Drilling machines with precision drill holes through the PCB at specific locations close to the board edge. The holes are drilled into the required layers and chemically cleaned to make the walls of the holes ready for metal deposition. The second operation is electroplating, in which copper is deposited along the walls of every hole, forming a conductive cylinder electrically interconnecting the inner layers of the board.
After plating is finished, the PCB is milled or routed to establish the final shape of the board. In this process, the outer edge of the PCB is cut, specifically cutting through the holes that have already been drilled and plated. This routing is a very precise process that must be executed so as not to destroy the copper plating or distort the unbalanced hole exposure. This produces a series of half-holes or semi-anchored notches that create the castellation.
Accurate fixturing and CNC calibration are critical during this phase. A millimeter-sized misalignment will lead to non-uniform copper exposure, decreasing the efficiency of the solder joint or weakening the mechanical interlock between the host board and module.
To prepare the board for soldering, selective solder mask coating is applied, typically excluding the castellated edges. Exposing these edges facilitates improved wetting of solder in assembly. Surface finish is applied to shield exposed copper and enhance solderability. Typical finishes are:
ENIG (Electroless Nickel Immersion Gold) for excellent flatness and oxidation resistance
HASL (Hot Air Solder Leveling) for economy
OSP (Organic Solderability Preservatives) for use in lead-free applications
Castellated PCBs are inspected and tested thoroughly after fabrication. Automated Optical Inspection (AOI) verifies plating integrity and hole exposure. In high-reliability industries like medical, aerospace, and defense, the X-ray inspection can also be applied to identify internal plating or misalignment defects. The tests verify that the process of castellation is in compliance with both electrical and mechanical reliability requirements.
Castellated PCB design involves close attention to electrical and mechanical considerations to achieve long-term performance and manufacturability. These guidelines are used to optimize reliability and functionality:
Parameter |
Guidelines |
Hole size |
0.3–0.8 mm for proper plating and edge integrity |
Hole spacing |
≥ 0.5 mm to prevent solder bridging and milling issues |
Pad alignment |
Match base PCB pads with castellated holes for strong solder joints |
Thickness of copper |
1 oz standard; 2 oz for higher durability and current loads |
Mechanical reinforcement |
Add mounting holes or connectors for mechanical stability |
Castellated holes are usually between 0.3 mm and 0.8 mm in diameter. Holes under 0.3 mm can experience inferior plating quality, causing poor electrical connection. Larger holes above 0.8 mm will use up more board space and can weaken the module's edge strength. Selecting a suitable size is pivotal in trading off compactness against ruggedness.
Keeping a proper spacing between holes minimum of 0.5 mm is prevents solder bridging during the reflow and provides mechanical integrity of the board edge. Insufficient spacing is also making PCB milling challenging, with potential edge chipping or delamination.
The castellated holes on the module must be directly over the surface-mount pads on the base PCB. This assures strong, consistent solder joints during assembly. Misalignment results in cold joints, open circuits, or elevated resistance.
1 oz thickness of copper is generally standard enough, but employing 2 oz copper in high-current or mechanically stressful applications improves thermal performance and mechanical strength, particularly at the solder joint.
To provide mechanical reinforcement for heavier or larger modules, use mounting holes, board-edge connectors, or stiffeners. These elements reduce stress on the castellated joints, particularly under vibration or cycling.
The reliability of castellated PCBs requires the following quality assurance processes:
X-ray Inspection: To check for voids or defects in the plating.
Solderability Testing: To ensure that exposed copper interfaces are properly finished.
Functional Testing: As modules, castellated PCBs must be fully functionally tested before mounting onto a host PCB.
Also, the manufacturing companies utilize Design for Manufacturability (DFM) tools to check the location, alignment, and spacing of the castellated holes during the design process.
Castellated PCBs find widespread usage in a large number of areas of electronics due to their small size, solderability, and rigid interconnection. The major areas where these PCBs are applied are given below:
Castellated holes are typical in wireless communication modules such as Wi-Fi, Bluetooth, LoRa, and ZigBee transceivers. They are typically designed to be small and compact to be embedded within systems. Castellated build eliminates the use of big headers or connectors, which can be directly soldered on a main PCB, saving space and money.
Voltage regulator modules like buck and boost converters are typically available in castellated PCB configurations. These mini boards undertake tasks like voltage conversion and power filtering, and their castellated edges make them easily integrate into main designs without much space, making them appropriate for battery-operated and space-limited applications.
The majority of microcontroller breakout boards on ESP8266, ESP32, and STM32 use castellated PCBs to allow simple soldering onto a host board. Prototyping, IoT development, or permanent low-profile mounting in an application is all especially suitable for this.
Sensor modules with temperature, humidity, motion, light, or gas sensors typically use castellated edges to facilitate integration. Sensing capability can be easily incorporated into PCBs by designers without requiring a change in the base design.
Castellated boards are handy in test instrumentation where signals or transitory circuits should be connected. They give connections in a safe manner without the need to permanently attach, therefore ideally suited for debug, firmware flashing, or hardware validation stages.
PCB castellation is an extremely efficient design construct that helps meet increasing demands for compact, modular, and scalable electronic systems. By incorporating semi-circular plated-through holes at the edges of a PCB, designers can convert sub-circuits into surface-mount modules that are solderable with ease. Not only does this ease complex function integration, such as wireless communication, power regulation, and sensing, but it also helps ensure a cleaner, connector-free design with cost and space savings.
Castellated PCBs are now critical in high-speed industries with demanding needs for rapid prototyping, simple assembly, and miniaturization. They are used in RF modules and power converters, microcontroller development boards, and sensor platforms, to name a few applications.
Though precise design and accurate fabrication are needed to guarantee structural integrity and sound soldering, the advantages far outweigh the difficulties. With electronics progressing toward more modular architectures and compact footprints, PCB castellation will continue to be a useful tool in both prototype and production settings, enabling designers to create with flexibility and velocity.
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