I. Core Attributes and Scenario-Specific Parameters

　　Product Positioning

　　This flexible printed circuit (FPC) is designed specifically for internal interconnection in TWS earbuds. It addresses three key challenges: wiring in confined spaces, dynamic bending reliability, and multi-signal interference mitigation. It acts as a "neural network" connecting the Bluetooth chip, microphone, speaker, battery, and touch control module. It is compatible with various earbud styles, including in-ear and semi-in-ear, and supports core features such as active noise cancellation (ANC) and touch control.

　　Key Physical and Electrical Parameters

　　Size and Weight: 25μm PI (polyimide) is the most common substrate thickness, with an overall thickness of ≤0.2mm and a single piece weighing <0.5g, suitable for headphone cavities of 3-5cm³.

　　Bending Performance: Dynamic bending radius ≥ 6 times the board thickness (e.g., a 0.2mm thick FPC has a minimum bending radius of 1.2mm). Passed 100,000 bend tests without circuit breakage (compliant with IPC-2223C dynamic bending standards), meeting the demands of opening and closing the charging case and wearing the earphones.

　　Signal Compatibility: Covers Bluetooth 2.4GHz RF signals, 20Hz-20kHz audio signals, and I2C/SPI control signals. Impedance controlled: 50Ω (RF) / 100Ω (differential audio), crosstalk attenuation ≥ 40dB.

　　Current Carrying Capacity: 1/2oz (17μm) rolled copper can carry 1A. Current meets the charging and power supply requirements of a single-cell lithium battery (3.7V).

　　Core Material System

　　Base Material: PI (temperature resistance -40°C-125°C, dimensional stability superior to PET) is preferred. High-end models use adhesive-free PI to reduce thickness.

　　Copper Foil: Rolled copper (RA) outperforms electrolytic copper (ED), with 30% greater ductility and less prone to breakage in the bending zone.

　　Cover Film: PI cover film + acrylic adhesive (low-overflow adhesive). The window opening is 0.05-0.15mm larger than the pad on each side to ensure soldering reliability.

　　Reinforcement Plate: 0.1-0.3mm thick FR4 is used for reinforcement in the connector area, and stainless steel reinforcement plates are used for component solder joints to enhance mechanical strength.

　　II. Internal Connection Architecture and Wiring Design

　　Core Module Connection Topology

　　Star-shaped interconnect structure: With the Bluetooth main control chip at the center, FPC branches connect the various modules. ① Audio branch: Connects the codec chip and the speaker (differential traces, length ≤ 3cm); ② Sensor branch: Connects 2-4 microphones (shielded, with a spacing of ≥0.5mm from the audio lines); ③ Power branch: Connects the battery protection board and charging contacts (trace width ≥0.3mm, reinforced with copper); ④ Control branch: Connects the contact electrodes and LED indicators (routed on the surface, avoiding the RF module).

　　Key layout principles: Components should be placed away from dynamic bending areas (such as the area corresponding to the charging case's hinge). Heavy components (such as the crystal oscillator) should be mounted on the reinforcement board and secured with glue.

　　Core Rules for Wiring in Bend Areas

　　Tracking Direction: Route wires perpendicular to the bend axis to reduce stress concentration and avoid parallel routing (which can easily lead to fatigue fracture).

　　Structural Taboos: No vias, pads, or hard components are allowed in the bend area. Use solid ground copper for the inner layer instead of a copper grid.

　　Transition Design: Use a circular transition (radius ≥ 0.5mm) and teardrop pads on both ends of the wire to enhance the connection strength.

　　Connector Adaptation Solution

　　Micro Connectors: Use a 0.4mm pitch board-to-board (BTB) connector on the motherboard side and a ZIF (zero insertion force) connector on the battery/sensor side, with a mating life of ≥ 500 cycles.

　　Direct Soldering: Use direct soldering on the FPC for fixed modules such as speakers and microphones to reduce connector space. Solder joints are protected with green wax.

　　III. Key Design Technologies and Reliability Assurance

　　Enhanced Anti-Interference Design

　　Spatial Isolation: The distance between the RF module (Bluetooth antenna) and the audio module is ≥1mm. The touch panel FPC utilizes a hollowed-out housing structure to increase the distance from the speaker (the through-hole diameter is 1.01-1.5 times that of the touch terminal).

　　Shielding Measures: The microphone signal line is wrapped with a "double ground wire." The RF area is covered with grounding copper foil and exposed with a window to facilitate the application of conductive adhesive to the housing.

　　Optimized Filtering: A 0402 package common-mode inductor is connected in series with the power input, and a 1000pF ceramic capacitor is connected in parallel with the audio interface to suppress high-frequency interference.

　　Process and Testing Standards

　　Manufacturing Process: Laser cutting of contoured surfaces (tolerance ±0.02mm), chemically etched gold (EIG) surface treatment (pad thickness 0.1-0.2μm) to prevent oxidation.

　　Reliability Testing: ① Flex Test: On-resistance change ≤10mΩ after 100,000 dynamic flex cycles (1Hz frequency, bend angle ±90°). ② Environmental Testing: 20 cycles of high and low temperatures from -40°C to 85°C, no debonding or cracking. ③ EMC Testing: Complies with EN 55032 Class B, RF radiation ≤40dBμV/m.

　　Failure Risk Mitigation

　　Stress Relief: A 2mm margin is left between the connector and the flex zone, and tear relief grooves are added to prevent edge tearing.

　　Thermal Management: A 2×2mm strip of exposed copper is placed on the FPC beneath the charging management chip, transferring heat to the earphone housing via a thermal pad.

　　IV. Selection Recommendations and Scenario Compatibility

　　High-End Noise-Canceling TWS

　　Applicable Scenario: Flagship models supporting active noise cancellation, spatial audio, and health monitoring (such as Huawei FreeBuds Pro 3 and Apple AirPods Pro 2);

　　Core Configuration: 4-layer FPC (2 signal layers + 1 ground layer + 1 shield layer), adhesive-free PI substrate, rolled copper with ENIG surface treatment, supporting 96kHz audio sampling;

　　Key Design: Microphone branch with independent shielding layer, hollow anti-interference structure in the touch area, and stress relief section in the bend area;

　　Cost Range: ¥8-15 per unit (mass production).

　　Cost-Effective Mainstream TWS Model Selection

　　Applicable Scenarios: Mid-range models with basic noise cancellation and touch controls (such as the Xiaomi Redmi Buds 5 and Edifier Lollipop Pro);

　　Core Configuration: 2-layer FPC, standard PI substrate, electrolytic copper + OSP surface treatment, support for 48kHz audio sampling;

　　Key Design: Simplified shielding (ground copper only in the RF area), low-cost BTB connector, and minimal bending area design;

　　Cost Range: ¥3-6 per cable (mass production).

　　Entry-Level Basic TWS Model Selection

　　Applicable Scenarios: Entry-level models without noise cancellation and basic call functions;

　　Core Configuration: 2-layer FPC, PET substrate (lower cost), 1/4oz copper foil, no independent shielding layer;

　　Key Design: Integrated wiring (reduced branching), omitting reinforcement plates (retained only at the connector), meeting static wiring requirements;

　　Cost Range: ¥1-2 per cable (mass production)

　　Tips to avoid pitfalls: ① Electrolytic copper is prohibited in dynamic bending areas (it is easy to break after being bent 5,000 times), and rolled copper is preferred; ② Ignoring the manufacturer's DFM specifications can easily lead to a sharp drop in yield (for example, some manufacturers cannot process line widths < 0.1mm); ③ Failure to design a grounding design will cause touch failure (actual measurements show that the false touch trigger rate of the ungrounded model increases by 20%).