Solid-State Diffusion of Bi in Sn: Effects of Anisotropy, Temperature, and High Diffusivity Pathways
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It has been determined that bismuth (Bi) stabilizes the mechanical properties of lead-free solder alloys over time and improves alloy reliability. This is caused by homogenization of the microstructure, driven by solid-state diffusion of Bi in the β-Sn matrix. It was previously determined that β-Sn grain orientation has little effect on the diffusivity at 125°C, and even low angle grain boundaries may convolute the relationship between orientation and diffusivity. In this work, several additional temperatures were considered, and similar effects of orientation on the diffusivity of Bi in Sn were observed. The simulation technique used to estimate diffusivity was applied to the previously discussed polycrystalline diffusion data, and temperature was found to strongly affect the ratio between lattice and polycrystalline diffusivity, due to the active nature of Sn-based systems. Finally, Harrison Type B kinetics were considered to study the effects of grain boundaries on the diffusivity of Bi in polycrystalline Sn.
The authors would like to thank Dr. Yanan Liu from the Department of Earth Sciences at the University of Toronto for assistance with EPMA, as well as Harlan Kuntz from the Toronto Nanofabrication Center for support with sputter deposition. Finally, we acknowledge Diana Vucevic for annotating the stereographic projection for β-Sn, which aided in determining sample orientation indices. Financial assistance from the Department of Materials Science and Engineering, Ontario Graduate Scholarship, and the Refined Manufacturing Acceleration Process (ReMAP) are greatly appreciated.
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