Bonding Mechanism and Physical Properties of Flexible Graphite Film Joints Soldered with Sn-Based Solder

Abstract:

Flexible Graphite Film (FGF) has great application potential in the field of flexible electronic thermal management, but its inherent poor wettability and layered structure restrict reliable joining. In this paper, an ultrasonic-assisted plating–brazing composite process is proposed; using pure Sn and Sn9Zn solders, low-temperature joining of FGF is realized. The ultrasonic plating process is systematically optimized, and the bonding mechanism at the Sn/FGF interface is investigated by first-principles calculations. The microstructure and properties of joints with and without ultrasonic assistance are comparatively analyzed. The results show that ultrasonic plating can form a uniform coating on the FGF surface; for pure Sn solder the optimum ultrasonic plating time is 45 s, temperature 320 °C, power 640 W, whereas for Sn9Zn solder the optimum time is 35 s, temperature 280 °C, power 800 W. Ultrasonic brazing significantly improves interfacial bonding and promotes the formation of a mechanical interlocking structure between solder and FGF. First-principles calculations reveal that Sn atoms lose electrons and C atoms gain electrons at the interface; the C–Sn bond population is between 0.03 and 0.06. Combined with the calculated work of separation, comprehensive analysis indicates that Sn/FGF interfacial bonding is dominated by van der Waals forces and weak chemical bonding induced by charge transfer, accompanied by weak covalent–ionic mixed bonding. In terms of joint performance, joints ultrasonically brazed for 3 s with pure Sn solder exhibit the best electrical conductivity, superior to joints ultrasonically brazed for 5 s with Sn9Zn solder; the electrical conductivities are 3.49×10₆ ^{S/m and 2.25×10₆} S/m, respectively, and the stability in bending tests is better than that of joints without ultrasonic assistance. In heat dissipation, Sn9Zn-brazed joints are overall superior to pure Sn joints; because of secondary ultrasonic application during joining, 5 s Sn9Zn joints brazed without ultrasonic assistance display the optimum thermal conductivity and thermal diffusivity.