Liquid Metal-Based Solder Composite with Ultralow Reflow Temperature for Surface Mounting on Flexible Printed Circuits

In this study, an amalgamation-inspired, low-temperature liquid metal (LM)-based soldering composite was developed by incorporating copper powder into a Ga–In–Sn alloy. The GaInSn/Cu composite undergoes spontaneous solidification at a low temperature of approximately 70 °C through interfacial chemical reactions, forming intermetallic compounds (Ga–Cu and Sn–Cu) and an In–Sn–Ga solid solution. Differential scanning calorimetry (DSC) analysis revealed that, after solidification, the melting point of the GaInSn/Cu solder composite increased from 61.4 to 106.3 °C, confirming the successful prevention of liquid solder leakage. Meanwhile, X-ray diffraction (XRD) analysis verified the formation of intermetallic phases. Guided by the Ga–In–Sn ternary phase diagram, the LM composition is optimized to enhance mechanical performance, resulting in a cured shear strength of 19.5 MPa─approximately 100 times higher than the preadjustment condition. The composite also exhibits excellent electrical performance, with a postcure bulk resistivity of 2.13 × 10–7 Ω·m. When applied to stretchable conductive circuits on PET substrates, the solder demonstrates strong bending durability, maintaining resistance variation below 3% at bending radii above 7.5 mm. Application tests further show successful bonding of a flexible circuit to an EMG sensor module on a sports leg sleeve, with stable signal collection during movement. Owing to its low solidification temperature (∼70 °C) and superior mechanical and electrical properties, the GaInSn/Cu solder composite is highly suitable for surface-mount assembly of flexible printed circuit boards (FPCBs), minimizing thermal damage and advancing the development of next-generation flexible electronics.