In situ investigation of SnAgCu solder alloy microstructure

In situ X-ray diffraction experiments, using synchrotron radiation, were employed to analyze microstructure evolution of the 96.5Sn3Ag0.5Cu (wt.%)—SAC305 lead-free solder alloy during heating (30–240 °C), isothermal dwell (240 °C) and cooling (240–30 °C). The special emphasis was placed on the study of the melting and solidification processes, explaining formation, distribution and the order of crystallization of the crystal phases (β-Sn, intermetallic compounds) in the solder alloy. Furthermore, thermal expansion behaviour of the main constituent phase β-Sn was analyzed prior to melting and after the consequent solidification.

Research highlights▶ In situ X-ray diffraction investigation enabled detailed analysis of the melting and solidification process of the SAC305 alloy. ▶ It was found that the SAC305 solder melts at 230 °C. When cooling from 240 °C the SAC305 alloy solidifies at the temperature of 214 °C. During solidification β-Sn and Cu6Sn5 is also formed. Formation of Ag3Sn occurs at 206 °C and the remaining amount of alloy crystallizes approximately at 160 °C. ▶ Furthermore, observation of the thermal expansion behaviour of the β-Sn tetragonal unit cell revealed linear dependence of the unit cell volume on temperature. The unit cell parameters a and c also increase linearly with the temperature. Despite the fact that the c parameter is substantially smaller than parameter a, it exhibits a significantly higher linear thermal expansion coefficient. Comparison between data obtained during heating and cooling indicates that the thermal expansion coefficient is slightly greater in the case of cooling.