Investigation of Low-Melting Sn-Bi-In Solder Joints for Flexible Interconnects

Flexible packaging is an emerging technology in electronic device packaging, addressing the limitations of traditional methods in achieving lightweight and flexible designs. By replacing rigid substrates with flexible ones, flexible packaging enhances flexibility and bending performance, essential for the design and manufacturing of advanced electronic products. A critical aspect of flexible packaging is controlling the packaging temperature to protect components and prevent substrate damage. In our research, we developed a low-melting-point Sn-Bi-In solder alloy for flexible packaging applications. This solder alloy, designed for bonding flexible copper-clad PI films, exhibited a melting point of approximately 82°C, as determined by TG-DSC analysis. Microstructural analysis using XRD and SEM revealed a eutectic structure of (In, Bi) Sn4 and BiIn. Uniaxial tensile tests showed the solder alloy had a tensile strength of about 32 MPa, indicating good ductility. The wetting angle of the solder on a pure Cu substrate was measured at 66°, suggesting it could effectively wet and spread on both the Cu substrate and the copper-clad PI film. To analyze interface bonding strength and microstructure, we designed a sandwich structure of Cu/solder alloy/copper-clad PI flexible joints. This structure allowed for detailed examination of joint properties. The connection between the solder and the substrate was achieved at temperatures above 110°C, with the solder's flowability increasing with temperature, resulting in a reduced width of the solder joint. Shear experiments showed that the solder alloy could form reliable joint connections with Cu at 120°C. Our study demonstrates the potential of flexible packaging in electronic device manufacturing, particularly through the use of low-melting-point solder alloys for bonding flexible substrates. These findings contribute to the advancement of flexible packaging techniques and their application in developing flexible electronic devices.