I. Core Attributes and Scenario-Specific Parameters

　　Product Positioning

　　As the "central nervous system" of a smartphone, the motherboard PCB core carries the interconnection functions of all core modules, including the processor, memory, RF, and camera modules. It addresses the four major pain points of "high-density integration, high-frequency interference resistance, heat dissipation management, and thinness." It is compatible with various form factors, including candy-bar/foldable screens, 4G/5G (including Sub-6G/millimeter Wave), and AI-powered devices, and supports high-speed interface protocols such as DDR5, PCIe 4.0, and UFS 4.0. Key Electrical and Physical Parameters

　　Integration Density: Flagship models utilize 8-12-layer high-end HDI boards with line widths and spacings of 20-30μm and via diameters of 0.1-0.2mm, resulting in a 60% increase in wiring density per unit area compared to mid-range models.

　　High-Frequency Characteristics: Covering the full 700MHz-47GHz communication frequency band, RF link impedance control accuracy is ±5% (50Ω), differential signal impedance (such as USB 3.2) is 90Ω, and crosstalk attenuation is ≥50dB, preventing 5G signal interference with audio/touch functions.

　　Current Carrying Capacity: The battery power circuit utilizes 2oz (70μm) rolled copper, with local thickness increased to 3oz (105μm), supporting 5A fast charging current. Core power traces have a width of ≥0.2mm to meet the peak power consumption requirements of the processor.

　　Size Compatibility: Candy-bar motherboards are mostly sized 50mm×80mm-70mm×100mm, thickness ≤1.5mm; the bending area thickness of the folding screen rigid-flex board is ≤0.3mm, allowing for a 180° fold.

　　Core Material System

　　Substrate: Flagship models utilize high-Tg FR-4 (Tg ≥ 170°C) or high-speed materials (such as Megtron 6, dielectric constant 3.4-3.6). Millimeter-wave models offer optional ceramic-filled substrates. The foldable display utilizes a rigid-flex substrate (FR-4 rigid board + PI flexible board), with ALIVH build-up technology used in the flex area to enhance folding resistance.

　　Copper Foil: Rolled copper (ductility ≥ 15%) is used in the high-frequency RF area, while electrolytic copper is used in the digital signal area. ENIG (electroless immersion gold, thickness 0.1-0.2μm) is preferred for surface treatment, with a pad oxidation resistance lifespan of ≥ 12 months.

　　Solder Mask: Thin photosensitive green ink (8-12μm thickness) is used to support precise coverage of fine lines. Flexible solder mask ink is used in the folding area and has passed 100,000 flex cycles without cracking. II. Module Architecture and Layout Design

　　Core Functional Module Layout

　　Partitioned and Isolation Layout: ① Core Digital Area (Processor, DDR, UFS): Centrally located, using dam grounding for isolation; ② RF Area (Baseband Chip, Power Amplifier, Antenna Switch): Independently partitioned, ≥5mm from the digital area, with a metal shielding cavity; ③ Power Area (PMIC, Fast Charging Chip): Close to the battery port, ≥8mm from the RF area; ④ Peripheral Interface Area (USB, Camera): Layout along the PCB edge to reduce signal routing.

　　Key Module Adaptation: The processor (such as the Snapdragon 8 Gen3) uses a BGA package, with blind and buried vias in the fan-out area, ensuring a signal path of ≤2cm; the RF power amplifier (such as the Skyworks SKY58048-21) is ≤3cm away from the antenna interface to reduce insertion loss; the camera module interface uses differential pair routing with a 90Ω impedance match. Core Routing Rules

　　High-speed signals: DDR5 trace length difference ≤ 5mm, impedance controlled at 40Ω ± 10%; PCIe 4.0 trace length ≤ 15cm, spacing between adjacent traces ≥ 3 times the trace width to prevent signal crosstalk.

　　RF signals: Use microstrip or coplanar waveguide structures, with millimeter-wave paths isolated by grounding copper foil, and use arc transitions at bends (radius ≥ 3 times the trace width).

　　Grounding Design: Use a "digital ground + RF ground + analog ground" partitioning system, with each zone connected to the main ground plane through a single-point grounding point. Ground resistance ≤ 0.03Ω; an independent ground via array (0.5mm spacing) is provided in the RF zone. III. Key Design Technologies and Reliability Assurance

　　High-Density Integration and Anti-Interference Design

　　HDI Process Application: 2-4 level HDI is used, with buried and blind vias and resin-filled vias achieving interlayer interconnection, saving 40% wiring space compared to traditional through-holes. Some flagship models utilize SLP (Substrate-Like PCB) technology to further enhance integration.

　　Enhanced Shielding: The RF module utilizes an integrated metal shield (0.2mm thick, nickel-plated brass), with multiple solder points to the PCB ground pads. The camera interface cable utilizes a "double ground clip, one signal" routing scheme, improving crosstalk attenuation to 60dB.

　　Optimized Filtering: A common-mode inductor (such as the Murata DLW32SN series) is connected in series with the power input, and a 100pF high-frequency filter capacitor is connected in parallel with the RF front end to suppress conducted interference. Heat Dissipation and Environmental Resistance Design

　　Heat Dissipation Optimization: ① Component Level: A 20mm×20mm heatsink pad is provided beneath the processor BGA, with 12-16 0.3mm heatsink vias filled with thermal grease and connected to the vapor chamber. ② Board Level: The power supply area is covered with exposed copper (≥50mm²), with thermal conductivity conducted to the metal midframe via a thermal pad.

　　Foldable Screen Special Design: The rigid-flex interface uses a non-adhesive PI substrate, with traces at the bending axis running perpendicular to the folding direction. The copper foil thickness is reduced to 12μm, and has passed 100,000 bend tests (bending angle ±180°) without any circuit breakage.

　　Environmental Adaptability: Passed 30 high-temperature cycles from -40°C to 85°C, 1000 hours of 95% humidity at 40°C, and 48 hours of salt spray testing with no corrosion, meeting the requirements of outdoor use. Process and Testing Standards

　　Manufacturing Process: Laser drilling (aperture diameter 0.1-0.2mm), chemical immersion gold surface treatment, and fine line etching accuracy of ±3μm are used. B2it buried bump technology is used in the folding area to improve interlayer connection reliability.

　　Full-Process Testing: ① Electrical Testing: ICT testing covers 100% of network points, and flying probe testing shows impedance deviation ≤3%; ② Signal Integrity Testing: DDR5 eye diagram amplitude ≥ 200mV, PCIe 4.0 bit error rate ≤ 1e-12; ③ Reliability Testing: Candy-bar drop test (1.5m height, three times on each of the six sides) shows no functional failure. The on-resistance change is ≤10mΩ after 100,000 flexes of the folding screen. IV. Selection Recommendations and Scenario Compatibility

　　Flagship AI Smartphone Selection

　　Applicable Scenario: High-end models supporting 5G mmWave, AI computing power ≥200TOPS, and UFS 4.0 (such as the iPhone 16 Pro and Huawei Mate 70 Pro);

　　Core Configuration: 10-12 layers of high-level HDI (4th order), high-Tg FR-4 (Tg 180°C), rolled copper + ENIG, and support for SLP process;

　　Key Designs: Independent RF shielding cavity, optimized DDR5 signal matching, and processor heat dissipation via array;

　　Cost Range: ¥40-80/unit (mass production). Mid-range mainstream smartphone selection

　　Applicable scenarios: Sub-6G 5G, UFS 3.1, and mid-range models for daily audio and video use (such as the Xiaomi Mi 14 Youth Edition and the OPPO Reno 12);

　　Core configuration: 6-8 layers of HDI (Level 2), standard high-Tg FR-4 (Tg 170°C), electrolytic copper with OSP surface treatment;

　　Key design features: Simplified shielding design (shielding cover only for the RF area), conventional heat dissipation layout, and cost-prioritized routing;

　　Cost range: ¥15-30 per unit (mass production). Foldable Smartphone Selection

　　Applicable Scenarios: Foldable devices with inward/outward folding configurations and multi-tasking support (such as the Samsung Galaxy Z Fold6 and Honor Magic V3);

　　Core Configuration: 8-layer rigid-flex PCB (FR-4 rigid + PI flexible), ALIVH/B2it build-up technology, rolled copper + ENIG;

　　Key Designs: No component routing in the flex zone, flexible solder mask, and stress relief in the rigid-flex transition zone;

　　Cost Range: ¥80-150 per unit (mass production).

　　Tips for Avoiding Pitfalls: ① Flagship models using substrates with a Tg < 170°C can easily deform at high temperatures, increasing RF signal attenuation by 20%; ② Electrolytic copper used in the flex zone of foldable screens is prone to fracture, requiring the use of rolled copper; ③ Failure to ground the RF and digital zones will result in a 30% drop in 5G download speeds (measured speeds return to normal when the gap is ≥ 5mm).