First Principles Investigation of Stability, Structure & Properties of Sn-Bi Based Alloys

Development of electronic packaging consisting of 2D, and 3D die-package enhanced interconnects and sensitive optoelectronic components has necessitated a hierarchy of solder materials. This is to accommodate for the different melting points, adhesion characteristics, thermal expansion, and mechanical properties required at each stage. However, there is still a considerable knowledge gap in understanding the structural details at the atomic level of the stable alloy compositions and how thermodynamics drives the stability of the solid solutions, especially for Sn-based alloys. For example, a quick review of the available literature yields a convex hull for the binary SnBi alloy with positive theoretical energies of formation, revealing other thermodynamic drivers playing a significant role in stabilizing the solutions. We use a hybrid approach of first-principles simulation and machine learning to identify thermodynamically and mechanically optimal alloys for low-temperature solder applications. Our study extends the structural database for SnBi-based alloys, mainly focusing on low-concentration Bi systems, and sets a routine for screening stable binary/ternary alloys. We calculate the stability of Sn-Bi, Sn-Cu, Sn-Ga, Sn-Zn binary alloys, and Sn-Ag-Cu (SAC) alloys and calculate their mechanical properties to evaluate their thermodynamic and mechanical stability using first principles. We expect this will contribute to the structural database of SnBi-based alloys, particularly regarding the low-concentration Bi systems in SnBi alloys and other potential candidates for low-temperature solder applications.