SURFACE-ENERGY-DRIVEN SELECTIVE SOLDERING OF SN-58BI ON HOLEY POLYIMIDE FOR LOW-TEMPERATURE, ROBUST FLEXIBLE INTERCONNECTS

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Reliable fine-pitch interconnects remain a critical challenge for next-generation flexible electronics, as conventional anisotropic conductive films (ACFs) suffer from stochastic particle distribution, unstable electrical contacts, and limited mechanical robustness under deformation. Herein, we report a low-temperature, surface-energy-driven selective soldering strategy that enables defect-free metallurgical interconnect formation on a hole-patterned polyimide (holey PI) platform. By exploiting the intrinsic surface-energy contrast between Cu pillars and the surrounding PI matrix, molten Sn–58Bi eutectic solder spontaneously wets and deposits exclusively on Cu surfaces through a simple dipping process, while solder spreading on PI is effectively suppressed. Thermodynamic analysis based on contact angle measurements, Owens–Wendt surface energy calculations, and Gibbs free wetting enthalpy confirms the strong energetic preference of Sn–58Bi for Cu over PI. Process optimization achieves highly uniform semispherical solder bumps with >98% yield within a few seconds at ~200 °C. Cross-sectional SEM/EDS reveals a refined eutectic lamellar microstructure and a thin intermetallic layer at the solder/Cu interface, ensuring robust metallurgical bonding. The resulting interconnects exhibit high shear strength up to 16.22 MPa and maintain stable electrical resistance (~0.023 Ω) after thermal aging (85 °C/85% RH) and cyclic bending (1,000 cycles). This work provides a scalable and reliable low-temperature interconnection route for flexible and thermally sensitive electronic systems.

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