Step phenomenon of intermetallic compounds thickness during laser soldering dependence on laser power

The thickness of CuSn intermetallic compounds (IMCs) plays a crucial role in determining the quality of laser soldering joints. However, there is a lack of research on laser soldering with wire feeding, which is a relatively complicated process of melting and spreading. In this study, we investigated the effect of laser power on IMCs thickness, the maximum temperature of the molten pool, and the spreading behaviors of solder. The results revealed that the thickness of Cu6Sn5 IMCs could be divided into three stages. Notably, a step phenomenon was observed, which is a stable stage between the non-wetting stage and the rapid linear increase stage. Since the phenomenon could not be accurately explained by the temperature-time theory, the actual laser absorptivity (only 20 %–25 %) was calculated during laser soldering by establishing a quantitative relationship between the spreading behaviors and the laser reflectivity. Furthermore, we established the linear relationship (R2 = 98.6 %) between the actual absorptivity and IMCs volume. Finally, the formation mechanism of the step phenomenon is rationally explained in combination with the spreading process of the solder. The findings are of great significance for accurately controlling IMCs thickness, expanding the process window, and enhancing the quality stability of solder joints.