Facets Formation of Ag₃Sn Intermetallic in Sn-Bi-Ag Alloys: An EBSD and First-Principles Study

Ag₃Sn intermetallic growth within Sn-Bi-Ag alloys during relatively slow cooling rates has a pronounced effect on the performance and reliability of solder joints. In this study, we combined experimental electron backscatter diffraction (EBSD) analysis with first-principles calculations to explore the crystal growth and faceting mechanisms of Ag₃Sn intermetallic in solidifying Sn-xBi-1Ag (x = 10, 57 wt.%) alloys. Using EBSD techniques, accommodated for a pseudo-hexagonal setting of Ag₃Sn to avoid pseudo-symmetry, and subsequent lattice transformation to an orthorhombic structure (a = 5.97 Å, b = 4.78 Å, c = 5.18 Å, Pmmn), this study reveals predominant (001)_orth facets in Sn-10Bi-1Ag, and both (001)_orth and more frequent (010)_orth facets in Sn-57Bi-1Ag. Stability assessments of various crystal surfaces through first-principles calculations found the (010)_orth surface to be most stable, followed by the (001)_orth. Variations in stable facets between the two alloys may result from energy minimization influenced by atomic attachment at the liquid/solid interface, dynamic non-equilibrium solidification conditions, Bi concentration, and growth twinning. These findings enhance the understanding of intermetallic compound growth in solder alloys, with implications for solder joint technology improvement.