Fine Microstructured In–Sn–Bi Solder for Adhesion on a Flexible PET Substrate: Its Effect on Superplasticity and Toughness

A novel In–Sn–Bi solder with a low electrical resistivity of 14.3 × 10–6 Ω cm and a melting temperature of 99.3 °C was produced for use in adhesive joining on a flexible poly(ethylene terephthalate) substrate. We determined that the fine microstructure of the In-based solder (which had an average phase size of 62.2 nm) strongly influenced its superplasticity and toughness at diffusive temperatures of 55–85 °C because the late-forming BiIn intermetallic compound (IMC) suppressed the growth of two other IMCs, In3Sn and In0.2Sn0.8, which formed earlier in the soldering process. Thus, an elongation of 858.3% and toughness of 36.0 MPa were obtained at a temperature of 85 °C and a strain rate of 0.0020 s–1. However, due to phase boundary fracturing, the phase-refined solder exhibited a slightly more brittle nature (with an elongation of 74.3%) at room temperature compared with a standard In–Sn solder consisting only of the In3Sn and In0.2Sn0.8 IMCs, which had a slightly larger phase size of 84.9 nm and higher ductility (with an elongation of 80.7%). In terms of superplastic deformation, the conventional fracture system based on the Hall–Petch effect transformed into phase boundary sliding at the solder operating temperature, significantly enhancing ductility.