I. Core Attributes and Scenario-Specific Parameters

　　Product Positioning

　　As the "micro-hub" of a smartwatch, the high-density PCB core interconnects key components such as the processor, biosensor, and RF module. It addresses four key challenges: ultra-compact integration, multi-signal interference mitigation, low-power adaptability, and compatibility with various form factors. It is compatible with various form factors, including round/square, sports/health monitoring, children's/flagship devices, and supports Bluetooth 5.3, Wi-Fi 6, NFC, and PPG/ECG biosensor protocols.

　　Key Electrical and Physical Parameters

　　Integration Density: Flagship models utilize 6-8 layer HDI boards with line widths and spacings of 15-25μm and via diameters of 0.08-0.15mm, resulting in a 120% increase in wiring density per unit area compared to traditional watches.

　　High Frequency Characteristics: Covering the 2.4GHz (Bluetooth/Wi-Fi), 13.56MHz (NFC), and 10GHz and above (future RF expansion) frequency bands, RF link impedance control accuracy of ±3% (50Ω), and biosensor signal crosstalk attenuation ≥55dB, preventing wireless signal interference with heart rate monitoring.

　　Current Carrying Capacity: The battery power circuit utilizes 1oz (35μm) rolled copper, with the fast charging circuit partially thickened to 1.5oz (52μm), supporting 1-2A charging currents. Core power traces have a width of ≥0.15mm to meet the peak power requirements of processors (such as the Snapdragon W5 Gen2).

　　Size Compatible: Round watch PCBs Diameter ≤ 30mm, square watch ≤ 30mm x 40mm, overall thickness ≤ 0.8mm; special-shaped models use a rigid-flex structure with a bending area thickness ≤ 0.2mm to accommodate the curvature of the watch case.

　　Core Material System

　　Base Material: Flagship models utilize high-Tg FR-4 (Tg ≥ 150°C) or high-speed materials (such as Megtron 4, dielectric constant 3.6-3.8). Health monitoring models utilize optional low-CTE base materials. Special-shaped structures utilize a rigid-flex base material (FR-4 rigid board + PI flexible board). ALIVH build-up technology is used in the curved areas to enhance fold resistance.

　　Copper Foil: Rolled copper (ductility ≥ 20%) is used in the RF area, and electrolytic copper is used in the digital area. ENIG (Electronic Immersion Gold), 0.1μm thickness, is preferred for surface treatment. Pad oxidation resistance is guaranteed for ≥ 18 months.

　　Solder Mask: Ultra-thin photosensitive green ink (5-8μm thickness) is used to support fine line coverage. Flexible solder mask ink is used in the curved areas, and has passed 50,000 flex cycles without cracking.

　　II. Module Architecture and Layout Design

　　Core Functional Module Layout

　　Partitioned Isolation Layout: ① Core Digital Area (Processor, Memory): Centrally and compactly arranged, with dam ground isolation; ② RF Area (Bluetooth Chip, Antenna): Independently positioned edge-to-edge, ≥3mm from the biosensor area. Flagship models can embed the antenna into the back cover substrate to avoid sensor interference; ③ Biosensor Area (PPG/ECG Module): Close to the back window, with a separate ground domain for isolation; ④ Power Area (PMIC, Charge Management): Close to the battery interface, with a distance of ≥4mm from the RF area.

　　Key Module Adaptation: The processor (e.g., Microchip SAM L22) uses a QFN package, with blind and buried vias in the fan-out area, ensuring a signal path length of ≤1.5cm; the biosensor (e.g., Max30102) and ADC chip are ≤2cm apart, with differential routing to reduce noise; the RF antenna and power amplifier chip are ≤2cm apart to minimize insertion loss.

　　Core Wiring Rules

　　High-speed signals: Bluetooth 5.3 differential trace length difference ≤ 2mm, impedance controlled to 90Ω ± 5%; SPI control signal trace length ≤ 5cm, spacing between adjacent traces ≥ 2 times the trace width;

　　Sensor signals: PPG analog signals use a "double ground clip and one signal" routing scheme, with a trace width of 0.12-0.15mm and a distance of ≥ 1mm from power traces;

　　Grounding Design: Use a "digital ground + RF ground + sensor ground" partitioning scheme, with each zone connected through a single-point ground connection. The sensor ground resistance is ≤ 0.05Ω, and an array of ground vias (0.3mm spacing) is provided in the RF zone.

　　III. Key Design Technologies and Reliability Assurance

　　High-Density Integration and Anti-Interference Design

　　HDI Process Application: 2-3 level HDI is used, with buried blind and resin-filled vias achieving interlayer interconnection, saving 50% wiring space compared to traditional through-holes. Some flagship models utilize SLP (Substrate-Like PCB) technology to further reduce size.

　　Enhanced Shielding: The RF module utilizes a 0.15mm thick nickel-plated brass shield, welded to the PCB ground pads at multiple points. A ground isolation ring is provided around the biosensor area, improving crosstalk attenuation to 60dB.

　　Optimized Filtering: A miniature common-mode inductor (such as the Murata LQW series) is connected in series with the power input, and a 220pF high-frequency filter capacitor is connected in parallel with the sensor signal front end to suppress conducted interference.

　　Heat Dissipation and Special-Shaped Structure Design

　　Heat Dissipation Optimization: ① Device Level: A 10mm×10mm exposed copper foil is placed above the processor, and a diamond/copper composite substrate (with twice the thermal conductivity of traditional substrates) is used to accelerate heat dissipation. ② Board Level: A micro-thermal pad is installed in the power supply area to transfer heat to the metal case.

　　Special-Shaped Adaptation: Circular watch PCBs utilize curved routing, with components 1mm away from the case edge. No components are placed in the rigid-flex interface, with traces perpendicular to the bending axis. The copper foil thickness is reduced to 12μm, and the device has passed 50,000-cycle bend tests.

　　Environmental Adaptability: Tested with 20 cycles of high and low temperatures (-20°C to 70°C) and 500 hours of 90% humidity (40°C). Waterproof rating: IP68 (10 meters deep for 2 hours). The radiation rating of the children's model complies with the "Shenzhen Children's Smartwatch Technical Standard."

　　Process and Testing Standards

　　Manufacturing Process: Laser cutting of contours (tolerance ±0.01mm), laser drilling (aperture 0.08mm), and fine line etching accuracy ±2μm. Surface treatment utilizes an ENIG+OSP composite process, ensuring both weldability and oxidation resistance.

　　Full-Process Testing: ① Electrical Testing: ICT testing covers 100% of network points, flying probe testing impedance deviation ≤2%; ② Sensor Performance Testing: PPG heart rate detection error ≤2 beats/minute, ECG signal-to-noise ratio ≥60dB; ③ Reliability Testing: 1.2m drop test (two times on each of the six sides) with no functional failure, and insulation resistance ≥100MΩ after waterproof testing.

　　IV. Model Selection Recommendations and Scenario Compatibility

　　Flagship Health Monitoring Model

　　Applicable Scenario: High-end models supporting ECG, blood oxygen, and body temperature monitoring (such as the Apple Watch Ultra 2 and Huawei Watch GT 4 Pro);

　　Core Configuration: 8-layer HDI (3rd order), high-Tg FR-4 (Tg 160°C), rolled copper + ENIG, SLP process, diamond/copper heat sink substrate;

　　Key Design: Independent sensor shielding area, RF rear shell embedded layout, multi-dimensional ground isolation;

　　Cost Range: ¥25-50/unit (mass production)

　　Mid-Range Sports Model

　　Applicable Scenarios: Basic health monitoring, long-lasting sports models (such as the Xiaomi Watch S3 and Amazfit GTS 4);

　　Core Configuration: 6-layer HDI (2nd order), standard high-Tg FR-4 (Tg 150°C), electrolytic copper + OSP, conventional heat dissipation layout;

　　Key Designs: Simplified shielding (shielding cover only for the RF area), integrated sensor module, and cost-prioritized routing;

　　Cost Range: ¥10-20 per unit (mass production).

　　Entry-Level Children's Model

　　Applicable Scenarios: Children's watches primarily featuring positioning and calling functions (such as the Xiaotiancai Q2A);

　　Core Configuration: 4-layer board, standard FR-4 (Tg 130°C), 1oz electrolytic copper, no independent shielding layer;

　　Key Designs: Integrated and compact layout, enhanced waterproofing, and radiation-compliant design;

　　Cost Range: ¥5-12 per unit (mass production);

　　Tips to avoid pitfalls: ① If the flagship model misuses a substrate with Tg<150℃, it will easily deform during charging, causing sensor signal drift (error increased by 15%); ② If the biosensor area is not independently grounded, the false alarm rate of heart rate detection will increase by 25%, and strict zone grounding is required; ③ When the distance between the RF antenna and the PPG module is <3mm, wireless transmission will interfere with blood oxygen detection. When the actual measured distance is ≥4mm, the accuracy is restored to 98%.