Investigating the preferential growth of Bi grains in Sn-Bi based solder under thermal aging

Sn-Bi based solders have attracted widespread attention due to their promising applications in heterogeneous integration and three-dimensional microelectronic packaging. However, the low melting point of Sn-Bi based solder means their high homologous temperature during service, leading to fast coarsening of the microstructure, which can significantly impair the reliability of electron devices. It has been found that Bi grains in Sn-Bi based solder interconnects grow significantly and exhibit a preferred orientation under thermal loading conditions, but the mechanisms underneath the preferential growth of Bi grains is yet to be understood. In this work, a phase field model incorporating the thermal stress effect is developed and employed to investigate the dynamic evolution of the microstructure of Sn-Bi solder under thermal aging, and to capture the morphology changes of the grains with different orientations. It is demonstrated that Bi grains with c-axis parallel to the z-axis preferentially exist in eutectic Sn-Bi solder under thermal aging, and Bi grains with a large angle between the c-axis and the z-axis are swallowed by the preferred grains. Moreover, there is a competitive evolution between grains with different orientations in the polycrystalline eutectic Sn-Bi solder, due to the minimization of system elastic strain energy. Further mechanism analysis elucidates that the Bi grain orientation preference in Sn-Bi solders is caused by the dependence of their Young's modulus and coefficient of thermal expansion on the grain orientation, the favored grains possess a lower strain energy compared to other grains.