Ambient-atmosphere ultrasonic atomization of low-melting-point wood’s alloy powders: process optimization, atomization mechanism, and solderability

Abstract

Low-melting-point alloy powders are crucial for temperature-sensitive electronic packaging. However, preparing quaternary Wood’s alloy powders containing volatile elements (Pb, Cd) via traditional atomization often causes compositional deviation and oxidation. Herein, 13.3Sn–50Bi–26.7Pb–10Cd (wt%) Wood’s alloy powders were prepared using ambient-atmosphere ultrasonic atomization. The effects of ultrasonic amplitude, heating tank temperature, and vibration stage temperature on powder characteristics were systematically investigated. Furthermore, the atomization mechanism was studied using high-speed photography, and the solderability of the optimized powder on 304 stainless steel was evaluated. Results indicate that increasing ultrasonic amplitude from 20 to 30% regulates powder quality, decreasing the median diameter (D₅₀) from 77.78 to 71.07 μm, narrowing the size span from 2.29 to 1.96, and improving sphericity. An optimal heating tank temperature of 90–150 °C maintained low oxygen content (77–79 ppm) and good powder morphology, while temperatures above 150 °C increased oxygen content to 110 ppm and deteriorated sphericity. A room temperature vibration stage yielded the highest sphericity. Phase analysis confirmed the presence of β–Sn, Bi, Pb, Cd, and Pb₇Bi₃ intermetallics, with Pb₇Bi₃ acting as a solute sink for Bi to lower the melting temperature. High-speed imaging revealed that atomization is driven by the synergistic effect of Faraday capillary waves and acoustic cavitation, triggered by a critical liquid film thickness. Finally, joints soldered with the optimized powder exhibited an average shear strength of 25 ± 5.0 MPa, demonstrating excellent suitability for near-room temperature electronic interconnections.