Crystal structure and grain formation mechanism of bismuth–indium particles generated by ultrasonic irradiation

The miniaturization and high functionalization of electronic devices are driving the micronization of electronic components. In order to effectively microjoin electronic components, it is necessary to micronize the particles used in the soldering paste. In this study, we utilized an ultrasonic irradiation method to produce fine metal particles, and examined the influence of ultrasonic irradiation on the solidification process of the soldering materials. Bismuth–indium (Bi–45wt.%In) solder melted in a high-boiling-point solvent was exposed to ultrasonic irradiation to form droplets. By irradiating these droplets with ultrasonic waves during solidification, the amount of BiIn₂ in the solid solder composition increased, while that of Bi₃In₅ decreased. In addition, submicron-sized agglomerations consisting of numerous small particles (~ 30 nm in diameter) were detected in the solvent; the In content of these clusters was ≥ 70 wt.%. It is thought that the amount of Bi₃In₅ generated decreased due to cavitation during solid–liquid phase coexistence, which caused the liquid-phase components with high In concentrations to separate from the droplets.