Abstract
With the trend of developing wearable and flexible electronic devices, low temperature soldering process is preferred due to the temperature sensitivity of unconventional polymer-based substrates or other low heat budget material. On account of an adequate reflow temperature around 130°C, Bi-33In solder with 109°C eutectic melting temperature is a candidate with application potential on flexible electronic devices using PET as substrate. The Bi content successfully reduced reflow temperature of In-based solder and only dense layer-type Cu2In phase was revealed in Bi-33In/Cu solder joints. Cu2In, compared to Cu11In9 IMCs forming at In/Cu joints, is intrinsically more ductile and has a higher shear strength. Although no Bi-based IMCs were observed, Bi content altered the thermodynamic and kinetic properties of the solder joints. In addition, the advantageous layer-type morphology of Cu2In in Bi-33In/Cu alleviates stress concentration and acts as a diffusion barrier, as compared to the scallop-type Cu11In9 in In/Cu joints.