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12 SMT Magazine • September 2016 In this fourth installment of the series, we will continue discussing the likely key processes engaged in tin whisker growth. These key pro- cesses include: • Grain boundary movement and grain growth • Energy dynamic of free surface • Role of recrystallization • Solubility and grain growth in response to external temperature • Lattice vs. grain boundary diffusion • Reaction and dynamic of intermetallic compounds • Crystal structure and defects In Parts 2 and 3, we have discussed the first five processes: grain boundary movement and grain growth; energy dynamic of free surface; solubility and grain growth in response to ex- ternal temperature; and the role of recrystalli- zation. Now, we will outline the next two pro- cesses—lattice vs. grain boundary diffusion, and reaction and dynamic of intermetallic compounds. Lattice vs. Grain Boundary Diffusion Diffusion is a key part of crystal growth pro- cess; and whisker phenomenon is primarily a diffusion-controlled process. Since diffusion is a necessary path to grow whiskers, slowing the rate of diffusion of tin intra-granularly or along grain boundaries should be an effective approach to slow down the whisker growth. The actual monitoring or controlling the processes between the lattice diffusion and grain boundary diffusion is a complex chal- lenge. Nonetheless, understanding the main factors between these two diffusion processes will help navigate the practical solutions. by Dr. Jennie S. Hwang H-TECHNOLOGIES GROUP The Theory Behind Tin Whisker Phenomena, Part 4 SMT PROSPECTS & PE RSPECTIVES

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