Comprehensive Analysis of Castellated (Half-Hole) PCB Structural Characteristics
With the trend toward miniaturization and high-density integration in electronic devices, castellated PCBs (also known as half-hole or plum-blossom-hole PCBs) have become key components in consumer electronics, industrial control, medical devices, and other fields, thanks to their core advantage of direct board-to-board connection without connectors.
The semi-cylindrical plated copper holes along the board edge not only solve the problems of bulky space occupation and high signal loss associated with traditional connectors, but also achieve breakthroughs in reliability and cost control. This article provides a comprehensive breakdown of castellated PCBs—from technical principles and manufacturing processes to design challenges and practical applications—helping engineers make accurate selections and successfully implement them in real products.
What Is a Castellated PCB? Core Definition and Characteristics
1.1 Basic Definition and Structural Principle
A castellated PCB is a type of circuit board in which a “semi-through conductive interface” is formed along the board edge through a combination of partial drilling, copper plating, and substrate removal. Its academic name is Castellated Holes PCB.
The core structural features include:
50%–70% of the plated copper layer is retained on the hole wall, with continuous copper coverage on the inner wall, forming a reliable conductive path;
One side of the substrate is precisely removed, exposing an arc-shaped copper surface that serves as the contact point for board-to-board soldering;
The design combines electrical conductivity (replacing connector pins) and mechanical positioning (nesting and fixation). During connection, reflow soldering is used to fuse and secure the half-hole copper surface to the pads of another PCB.
1.2 Core Characteristics
| Feature | Technical Specification | Test Standard | Industry Pain Point Addressed |
|---|---|---|---|
| Hole structure | Semi-cylindrical, located at board edge, with continuous copper plating on the hole wall | IPC-A-600G 2.4.1 | Prevents signal interruption during connection |
| Plating requirements | Hole wall copper thickness ≥ 25 μm; plating adhesion ≥ 1.5 N (no peeling in tape test) | IPC-6012 2.3.1 | Prevents plating delamination during long-term use |
| Dimensional tolerance | Hole diameter tolerance ≤ ±0.05 mm; hole position deviation ≤ ±0.03 mm | IPC-2221A 7.2 | Ensures precise board-to-board alignment and avoids cold solder joints |
| Surface finish | ENIG: nickel layer 5–8 μm, gold layer 0.05–0.1 μm; Immersion tin: tin layer 7–10 μm | IPC-4552 3.2 | Improves solderability; ENIG suitable for high-frequency applications |
| Mechanical strength | Bending strength ≥ 150 N/cm (1.6 mm board thickness); mating cycles ≥ 50 | MIL-STD-202G 211 | Suitable for vibration environments (e.g., automotive electronics) |
Why Use a “Half-Hole” Design?
As electronic devices continue to shrink (such as smartwatches and Bluetooth earphones), internal space has become extremely limited. Traditional interconnection methods using connectors plus wires occupy significant space and are prone to poor contact. The half-hole PCB design effectively addresses these issues.
1. Save Space and Enable More Compact Devices
In conventional designs, connecting a PCB to another module requires soldering a separate connector (such as a USB connector or pin header), which typically occupies 5–10 mm of space. In contrast, half-hole PCBs integrate the connection directly into the board edge, eliminating the need for extra space—essentially integrating the connector into the PCB itself.
For example, the control module of a smart fitness band may measure only 2 cm × 3 cm, leaving no room for a traditional connector. By using a castellated PCB, the edge half-holes can be directly inserted into the main board slot, achieving reliable connection without wasting space, allowing the device to be lighter and thinner.
Similarly, in Bluetooth earphone charging interface modules, a half-hole design can reduce module thickness by 2–3 mm, perfectly matching the compact earphone enclosure.
2. More Reliable Connections with Fewer Failure Points
Traditional connectors are independent components soldered onto the PCB, making them susceptible to cold solder joints or detachment. Moreover, the multiple contact points between connectors and sockets are prone to oxidation and wear over time, leading to poor contact.
In contrast, the half-holes of a castellated PCB are integrated into the board itself. The metallized holes directly contact the mating pads or slots, eliminating separate solder joints and reducing potential failure points by over 80%.
For example, industrial sensor PCBs often operate long-term in environments with vibration and dust. With traditional connectors, vibration may cause the connector to loosen or detach, interrupting data transmission. Castellated PCB edge connections eliminate the risk of loosening; even under continuous vibration, the contact between the half-holes and the slot remains stable, significantly reducing failure rates.
3. Lower Cost and Simplified Manufacturing Process
Traditional PCB interconnections involve three steps: PCB fabrication, connector procurement, and connector soldering. This not only incurs connector costs (a standard pin header typically costs 0.5–1 RMB per unit) but also adds extra processes and labor costs.
With castellated PCBs, the half-holes are formed during PCB manufacturing, eliminating the need to purchase connectors and perform additional soldering operations. This can save 1–2 RMB per board.
For products with annual production volumes in the millions (such as routers and smart plugs), saving just 1 RMB per board can reduce total costs by over 1 million RMB. Furthermore, simplified assembly processes can improve production efficiency by around 30%—instead of soldering connectors first and then assembling modules, manufacturers can directly insert the castellated boards, significantly shortening production time.
Castellated (Half-Hole) PCB Manufacturing Process
1 Complete Production Flow
| Process Step | Operation Details | Key Equipment | Quality Control Points | Common Issues & Solutions |
|---|---|---|---|---|
| 1. Base Material Cutting | Select FR-4 (general applications), Rogers 4350B (high-frequency applications), or flexible PI (bendable applications). Cutting dimensional tolerance ≤ ±0.1 mm | CNC cutting machine | No burrs, no warpage of substrate | Warping: Apply pre-baking treatment (120 °C / 2 hours) |
| 2. Drilling | CNC drilling with spindle speed 30,000–50,000 rpm, feed rate 50–100 mm/min; full through-holes (φ1.0–6.0 mm) | High-precision CNC drilling machine (accuracy ±0.01 mm) | Smooth hole walls, no burrs or carbon residue | Carbon residue: Increase spindle speed; use water-soluble cutting fluid |
| 3. Electroless Copper Deposition | Degreasing (60 °C / 5 min) → Micro-etching (NaPSO₃ solution, 30 s) → Catalyzation (PdCl₂ solution, 2 min) → Electroless copper plating (45 °C, deposition rate 0.5 μm/min); final copper thickness 5–7 μm | Automatic electroless copper plating line | 100% hole wall copper coverage, no voids | Voids: Optimize copper bath concentration; extend plating time |
| 4. Pattern Transfer | Exposure (UV wavelength 365 nm, energy 80–100 mJ/cm²) → Development (Na₂CO₃ solution, 1% concentration, 30 s) → Electroplating (Copper bath: 2 A/dm², 60 min; Tin bath: 1 A/dm², 30 min); final copper thickness 25–30 μm, tin thickness 7–10 μm | Automatic electroplating line | Trace accuracy ≤ ±0.02 mm; uniform plating | Uneven plating: Adjust agitation speed; optimize rack design |
| 5. Castellated Hole Formation | Two processes: ① CNC milling: φ1.0 mm tungsten steel end mill, 40,000 rpm, feed rate 30 mm/min; milling along a position 0.5× hole diameter outside the hole center to retain half hole wall. ② Die punching: Precision die, punching pressure 5–10 MPa, positioning accuracy ±0.03 mm | CNC milling machine / Die punching machine | No burrs on half-hole wall; no copper delamination | Burrs: Add post-milling deburring (nylon brushing + chemical deburring) |
| 6. Etching & Post-Processing | Etching (CuCl₂ solution, etch rate 2 μm/min) → Solder mask (silkscreen printing, thickness 10–20 μm) → Legend printing → Inspection (AOI + X-ray) | Automatic etching line, AOI inspection equipment | Accurate solder mask openings (deviation ≤ ±0.03 mm); no shorts/opens | Solder mask misalignment: Optimize screen alignment; improve exposure accuracy |
2 .In-Depth Comparison of Castellated Hole Forming Processes
| Process Dimension | CNC Milling | Die Punching | Practical Selection Recommendation |
|---|---|---|---|
| Precision | Hole diameter tolerance ±0.05 mm; hole wall roughness Ra ≤ 0.8 μm | Hole diameter tolerance ±0.1 mm; hole wall roughness Ra ≤ 1.2 μm | CNC preferred for high-precision applications such as medical and military |
| Efficiency | Single-sided board processing time: 30 s / panel (10 castellated holes); changeover time 5 min | Single-sided board processing time: 1 s / panel; changeover time 30 min | Die punching for mass production (>100k pcs); CNC for small batches (<10k pcs) |
| Tooling Cost | No mold cost; tool wear cost approx. 0.1 RMB / board | Mold development cost USD 5,000–15,000 per set; mold life approx. 1 million cycles | CNC is more cost-effective for orders <50k pcs |
| Applicable Hole Diameter | Minimum hole diameter 0.4 mm (board thickness ≤ 1.0 mm) | Minimum hole diameter 0.6 mm | Micro-hole designs (<0.6 mm) require CNC |
| Edge Quality | No compression damage; excellent copper integrity | Minor compression marks possible (probability <3%) | CNC recommended for high-frequency, signal-sensitive applications |
| Typical Customers | Medical device manufacturers (e.g., Mindray), defense industry companies | Consumer electronics manufacturers (e.g., Xiaomi, OPPO) | Decide based on product positioning and order volume |
Applications of Castellated (Half-Hole) PCBs
The core advantage of castellated PCBs lies in miniaturized interconnection, making them especially suitable for devices with limited space and high requirements for connection reliability. Typical applications include:
1. Network Communication Equipment: Router Modules, Switch Interface Boards
Wireless modules and Gigabit Ethernet interface modules inside routers are widely implemented using castellated PCBs.
For example, a router’s 5G wireless module typically measures only 3 cm × 4 cm. By inserting the castellated PCB directly into the mainboard slot, space is saved while ensuring stable transmission of high-speed network signals. If traditional connectors were used, signal attenuation could occur during transmission, negatively affecting network speed.
2. Wearable Devices: Smart Bands, Smartwatches, Bluetooth Earphones
These devices feature extremely compact form factors (a smartwatch mainboard typically has an area of only about 5 cm²), leaving no room for conventional connectors. Castellated PCBs are an ideal solution.
For instance, the heart-rate sensor module of a smartwatch can be connected to the mainboard via castellated holes, allowing the module thickness to be controlled within 1 mm, perfectly fitting into the slim device enclosure. Moreover, castellated connections are highly reliable and will not suffer from poor contact due to wrist movement.
3. Industrial Sensors: Temperature, Pressure, and Displacement Sensors
Industrial sensors are required to operate for long periods in harsh environments such as vibration, high temperature, and dust, and are often installed in narrow mechanical spaces.
The edge-connection method of castellated PCBs eliminates the risk of loosening, ensuring stable sensor data transmission. At the same time, the absence of additional connectors reduces gaps through which dust and moisture could enter, significantly improving the sensor’s water and dust resistance.
4. Consumer Electronics Accessories: Wireless Charging Modules, Bluetooth Adapters
For example, in smartphone wireless charging pads, the internal control module often uses a castellated PCB, with the half-holes directly connected to the charging coil. This design reduces overall module thickness (down to below 0.5 mm) while ensuring stable charging current transmission.
Similarly, in USB Bluetooth adapters, the internal Bluetooth module is connected to the USB interface board via castellated holes, enabling the adapter to be as compact as a USB flash drive.
Castellated PCB vs. Standard Through-Hole PCB vs. Blind/Buried Via PCB
| Comparison Dimension | Castellated PCB | Standard Through-Hole PCB | Blind/Buried Via PCB | Selection Guidance |
|---|---|---|---|---|
| Hole Location | Board edge only | Anywhere on board | Inner layers / surface layers (non-through) | Castellated: board-to-board connection; Through-hole: interlayer conduction; Blind/Buried: high-density internal routing |
| Core Function | Board-to-board connection + mechanical fixation | Electrical interlayer connection | Internal signal interconnection (saves surface space) | — |
| Manufacturing Process | Drilling → Plating → Milling / Punching | Drilling → Plating → Etching | Laser drilling → Plating → Lamination | Castellated process is the most complex and costly |
| Cost Level | 20–30% higher than standard through-hole | Baseline (100%) | 50–80% higher than standard through-hole | Cost-sensitive designs choose through-hole; high-density designs choose blind/buried |
| Precision Requirement | Strict (±0.05 mm) | Moderate (±0.1 mm) | Very strict (±0.02 mm) | Medical and military prefer castellated / blind vias |
| Signal Performance | Low high-frequency loss (up to 5 GHz) | Moderate high-frequency loss | Lowest high-frequency loss (10 GHz+) | 5G and radar prefer blind vias; consumer electronics prefer castellated |
How to Choose a Reliable Castellated PCB Supplier?
1. Core Evaluation Criteria
(1) Technical Capability Assessment
| Evaluation Item | Qualified Standard | Excellent Standard | Verification Method |
|---|---|---|---|
| Machining accuracy | Hole tolerance ±0.05 mm; position deviation ±0.03 mm | Hole tolerance ±0.03 mm; position deviation ±0.02 mm | CNC equipment model list (e.g., Mitsubishi MV2400), inspection reports |
| Plating control | Copper thickness ≥25 μm; adhesion ≥1.5 N | Copper thickness 25–30 μm; adhesion ≥2.0 N | Plating thickness reports (XRF), tape test videos |
| High-frequency capability | Dielectric constant deviation ≤±5% (Rogers materials) | Dielectric constant deviation ≤±3% | Impedance test reports (TDR) |
(2) Quality Assurance System
Certifications: ISO 9001 (basic), ISO 13485 (medical devices), AS9100 (aerospace);
Inspection Equipment: AOI automatic optical inspection (100% coverage), X-ray inspection (hole wall void detection), impedance testers (for high-frequency applications);
Quality Control Flow: Incoming inspection (IQC) → In-process inspection (IPQC) → Final inspection (FQC) → Outgoing inspection (OQC), with defect rate controlled at PPM < 50.
(3) Service Support Capability
Pre-sales: DFM design consultation (hole layout, material selection), response time ≤ 2 hours;
In-production: Real-time production progress updates (twice-weekly reports), abnormal issue resolution ≤ 24 hours;
After-sales: 3-month warranty (free repair for non-human damage), lifetime technical support.
2. Key Points for On-Site Supplier Audits
Production equipment: Availability of high-precision CNC milling machines (e.g., DMG MORI), automatic plating lines, AOI inspection systems;
Process documentation: Complete castellated PCB SOPs and quality control plans (QCP);
Customer cases: Experience with high-end industries such as medical, military, and automotive electronics (e.g., Huawei, Mindray);
Production capacity: Monthly output ≥ 500,000 pcs; sample lead time ≤ 3 days; mass production lead time ≤ 7 days.
3. Recommended Supplier
Leadsintec
Technical strengths: 20 Mitsubishi CNC milling machines; machining accuracy ±0.03 mm; high-frequency castellated PCB impedance control within ±3%;
Quality certifications: ISO 9001, ISO 13485, AS9100; medical-grade products passed biocompatibility testing;
Service assurance: Free DFM optimization, 3-day sample delivery, 7-day mass production delivery, lifetime technical support;
Customer cases: Castellated PCBs for Mindray glucose meters and Huawei 5G modules, with defect rates controlled at PPM < 30.
Conclusion
As a core technology enabling miniaturization and high-density integration, castellated (half-hole) PCBs have proven their technical advantages across consumer electronics, industrial control, and medical device applications.
By thoroughly understanding their definitions, characteristics, manufacturing processes, and design specifications—and by selecting appropriate fabrication methods and suppliers based on real application scenarios—manufacturers can significantly improve product reliability, reduce costs, and shorten development cycles.
If you require customized castellated PCB solutions (for high-frequency, medical, or military applications), or need DFM optimization and cost evaluation, you are welcome to contact Leadsintec for free technical consultation and sample testing.
