Please consider attending the following: MATERIALS ENGINEERING SEMINAR "Nucleation, Growth, and Structure of Beta-Tin in Tin-Based Solder Alloys" By Thomas C. Reeve Purdue MSE Ph.D. Final Exam Advisor: Professor Carol A. Handwerker ABSTRACT Solidification of the β-Sn phase in Sn-based solder alloys is often unpredictable and difficult to induce, commencing heterogeneously from a highly-undercooled liquid. Additionally, the crystal structure of β-Sn is highly anisotropic and influences properties in the bulk material, such as elasticity, thermal expansion, resistivity, and diffusivity. Couple this intrinsic anisotropy with the difficulty in nucleation (often resulting in a relatively large β-Sn grain size), and the result is a high level of variability in solder joint mechanical and thermal properties. This research has aimed to enhance β-Sn nucleation and promote β-Sn grain refinement in Sn-based solders via the manipulation of alloy thermodynamics and solidification through the addition of micro-alloying elements, the influence of inoculating phases, and the introduction of growth-restricting solutes. In the first half of the work, micro-alloying additions of Al in Sn-Cu-Al and Sn-Ag-Cu-Al alloys were studied to examine the utility of Al micro-alloying additions for manipulating solder intermetallic compounds (IMC) and β-Sn grain structures. In these studies, additions of Al were shown to result in the formation of CuxAly (Cu33Al17 or Cu9Al4) IMCs. These CuxAly IMCs had an identifiable and preferred orientation relationship with the Cu6Sn5 phase. Both phases proved easily refined by rapid solidification techniques, and that refinement was maintainable during reflow cycling under specific compositional and thermal conditions. Despite the enhanced microstructural manipulation and control of IMCs demonstrated through this work, it was ultimately shown that the use of Al micro-alloying additions for refinement of β-Sn grain size was ineffective. The second half of the research was then devoted to identifying new inoculating phases for β-Sn. Based on crystallography and thermodynamics, the Sn-Ge alloying system was highlighted as a new possible solder alloy to potentially promote β-Sn grain refinement. Film deposition experiments revealed an orientation relationship between Ge-(111) single crystal substrates and liquid-state dewet pure Sn films. Density functional theory (DFT) models were utilized to examine this observed orientation relationship in comparison with the lattice parameter matching relationship between Sn and Ge that prompted the initial studies. Simulated interfacial binding energies of the different interface relations were calculated and discussed. This research has broadened the understanding of β-Sn nucleation in Pb-free solders, as well as impacted the field of Pb-free solder by applying solidification and phase transformations concepts, commonly implemented in large-scale castings, to the small-scale of microelectronics production. Date: Monday, April 9, 2018 Time: 9:30 A.M. Place: ARMS 3115