Table of Contents
In the design of modern electronics, achieving a compact size, high reliability and seamless module integration is more important than ever. Castellated holes, or plated half-holes, have become a practical solution for making direct board-to-board joining possible without using big connectors or making a mess with wires. They are mainly used in wireless modules, IoT gadgets, and embedded systems.
This article will help you understand what castellated holes are, the process of making them, their benefits, different uses, and what you need to consider while designing them for the assembly of PCB.
What are Castellated Holes in PCB?
Castellated holes, also known as half-holes or plated-through holes (PTH), are located along the edge of PCBs. They are commonly found features on the edge of the PCB, mainly the small modules. Castellations provide a link to mount the modules into the main boards during PCB assembly.
Structurally, castellated holes are semi-circular indentations created on the edges of PCB by drilling full vias during the fabrication process, and are precisely split in half. To ensure the electrical and mechanical connections during mounting, these holes are plated with copper or gold. By allowing the modules to be directly soldered into the main board, castellated PCBs reduce the need for bulky wire, connectors, and extra spaces.
How Castellated PCBs are Manufactured?
Unlike the regular holes produced by drilling, castellated holes follow a series of manufacturing procedures. They are as follows:
1. PCB Design and Layout
First of all, a schematic design is developed and then implemented in its layout. It provides the outline for the strategic placement of components and proper signal routing.
2. Material Selection
When the PCB design is finalized, the appropriate substrate material is selected, and an adequate copper thickness is determined for lamination. After the selection, the base is precisely cut according to the application requirements.
3. Drilling
In this step, perfectly aligned holes are drilled along the edge of the given substrate. The drilling must be done in such a way that the exact half of the holes sit inside the board area, whereas the other half sits on the waste material portion.
4. Plating
After the full holes are formed by drilling, the plating procedure is implemented. Plating refers to the deposition of a thin layer of copper on the walls of the holes. This procedure can be carried out by electroless chemical deposition or electroplating. It provides a conductive path between layers and prepares holes for soldering.
5. Specialised Edge Routing
It is the most critical step in manufacturing castellated hole PCBs. In this step, the plated holes are precisely milled through the centre of the hole, creating a vertical semi-circular copper channel at the edge. For precision, the manufacturer uses a specialized CNC routing bit or a double milling technique. If copper burrs are formed during milling, they are removed by using alkaline etching.
6. Testing and Inspection
Finally, the castellated PCBs are tested to ensure a flawless outcome. It includes the following tests:
- Visual Inspection: It is carried out by using high-magnification Automated Optical Inspection (AOI) to look for plating continuity and burr detection.
- Solderability Test: This test is performed to ensure that the soldering is accepted smoothly when the module is mounted to the main board.
- Dimensional Test: In this test, the diameter and position of the plated holes are verified as per the given design specifications. It should fit the modules during PCB assembly.
In addition, the castellated holes must meet the requirements of industry standards like IPC-7351, IPC-A-600 and IPC-6012.
Advantages of a Castellated PCB
- Better Signal Quality
Castellated PCBs provide a direct, vertical copper path that acts as a continuous copper path to the external module (WI-FI module, RF module) for smooth signal transition. It reduces the inductance and capacitance that are found in long wires and through-hole technology.
- Enhanced Reliability
The castellated hole provides an enhanced mechanical strength and better resistance to vibration due to vertical plating. Additionally, castellated PCBs can withstand frequent connections and disconnections.
- Compact Design
There is no need to use bulky wires and connectors. Therefore, castellated holes support a smaller and more compact design.
- Easier PCB Assembly
Castellated PCBs enable direct board-to-board mounting, allowing electrical connection through soldering, which reduces the need for additional hardware and connectors. This makes PCB assembly easier.
Common Use Cases of Castellated PCBs
Castellated holes are designed on different PCB modules for smooth operation. Some of the use cases of casellated circuit boards are:
Wireless Communication Modules
For easy integration and effective signal transition, wireless modules like WiFi, Bluetooth, LoRa and cellular modules are manufactured with castellated holes.
- Examples:
- ESP32-WROOM Module: Microcontroller integrated with Wi-Fi and Bluetooth module
- HC-05: Bluetooth Module
- RFM95/96 : Long Range (LoRa) module
- SIMCOM: Cellular Module
Consumer Electronics and Wearables
Due to the clean and compact outcome of castellation, many wearables and consumer electronics prefer castellated PCBs.
- Examples: GPS module in a smart watch, Biometric modules, etc.
Automotive and Industrial Device
Many automotive devices and industrial engineering applications use this technology due to a certain level of vibration-resistant capability of castellated holes.
- Examples: PLC modules, Motor Controller, ADAS sensors, etc.
Reliability and Failure Analysis of Castellated Holes in PCB
Since the modules are mounted onto the main board through castellated holes, the reliability of the given holes is a crucial factor in PCB assembly. Some of the failure cases of the castellation process are as follows:
Copper Burrs
Copper burrs are thin conductive copper protrusions that are created when the machine drags copper instead of cutting. These burrs can cause electrical defects and short-circuiting. Generally, burrs are formed if the CNC routing bit is dull. Therefore, specialized CNC routing bits and different deburring tools are used to avoid this problem.
Copper Peeling and Cracking
Copper peeling and cracking can be seen due to insufficient plating thickness and poor copper adhesion to the laminate. This risk increases when the V-cut tool is used instead of specialized CNC milling. Due to the damage, poor plating integrity occurs. As a result, circuit failure issues such as open circuits and signal degradation are created.
Misalignment
Misalignment occurs when the module is manually placed into the main board during PCB assembly. It is caused when the assembly process is carried out without proper footprints or process control, resulting in a weak mechanical bond and solder bridging. To ensure the proper alignment, fiducial marks must be placed on the module, and alignment jigs must be used for manual assembly.
Design Tips for Castellated PCBs
A reliable castellated PCB demands precision. Some common PCB design tips for castellated holes are described below:
Mechanical Specifications
Mechanically, the castellated holes must comply with the following specification for effective performance:
- Holes size and spacing
The hole size and spacing must be maintained in such a way that it prevents structural failure. According to the design standard, the minimum diameter of the hole should be 0.5 mm, and the spacing between two castellated holes must be at least 0.55mm. The smaller hole size needs advanced fabrication technology.
- Board Thickness
If the board is thin, it may cause cracking during milling or soldering. Generally, a board of thickness of 1mm or 1.6mm is preferred.
Placement and Routing
The castelalted holes must be placed away from components and the routing path to avoid interference during soldering and PCB assembly. The distance between castellated holes and the edge of boards must be maintained at a certain level to prevent mechanical stress and potential damage to the PCB.
Pad and Solder Mask Design
To prevent plating from peeling, the pad provides enough surface area for the board to hold the copper. Generally, the annular ring of 0.25 mm is maintained on both top and bottom copper layers. Similarly, the solder mask is slightly larger than the copper pad to ensure correct soldering without causing electrical shorts.
Surface Finish and Fabrication
According to the application of castellated holes, surface finish is carried out. The most popular surface finish technique is Electroless Nickel Immersion Gold (ENIG). It provides better solderability and resistance to corrosion. It is necessary to provide a burr-free finish to prevent shorts between adjacent pads.
Plating Integrity and Assembly Consideration
Plating integrity refers to the strength and thickness of copper inside the holes. The copper plating should not be torn out after milling. To maintain the plating integrity, electroplating is done according to the IPC Class 3 standard. This standard suggests the use of 25 to 35μm thick copper.
When to Consider the Alternatives to Castellated Hole
Although the castellated holes have a wide area of advantage and application, in some cases, alternatives may be used. In the following conditions, an alternative is preferred:
- High Cost Sensitivity
Castellation uses specialized CNC routing, a cleaner finish and precise alignment that increases the cost of manufacture. In high-volume production, Surface Mount Technology (SMT) is cheaper.
- Repairability
Desoldering the castellated holes requires professional skills in case of the requirement for repair. If the manufacturers or PCB suppliers lack skilled manpower and a professional rework station, castellated holes are not preferred.
- High mechanical Stress
As castellated holes are edge soldered, solder fatigue can be created if they can not withstand high mechanical stress and vibration. The difference in thermal coefficient between the module and carrier boards causes failure in castellated joints. In such cases, Through Hole Technology (THT) is preferred.
- Low complexity
For a simpler circuit, direct soldering is easier to implement than the castellated holes.
Conclusion
In summary, castellated holes serve as a reliable path for connecting modules to the main circuit board. To design an effective castellated PCB, it is necessary to utilize certain mechanical specifications, along with the maintenance of quality standards. Additionally, precision in hole placement and plating integrity are the most crucial factors for castellation. Ultimately, castellated holes provide a robust foundation for modules.
Wanna know more about PCB holes or looking for PCB manufacturing services? Contact us now >>
FAQs
A1: Castellated holes are U-shaped copper-plated holes at the edge of the module used for mounting modules to main boards, whereas edge plating includes copper plating at the vertical milled edge to shield Electromagnetic Interference (EMI) and provide proper grounding.
A2:
- Vias are the full holes located inside the PCB board, while castellated holes are the semi-circular holes present at the edge of the PCB.
- Vias are designed to create electrical connections between different layers of PCB, whereas castellated holes are designed to connect one board to another.
CONTACT US
English 
Italian 
Hungarian