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Type of Resource (primo): dissertations
Name: Personal
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Note: thesis: Thesis (Ph. D.)--Brown University, 2022
Genre (aat): theses
Abstract: Whiskers are electrically conducting filaments that form spontaneously in Sn systems and are a reliability risk in electronics. Despite being extensively studied, a fundamental understanding of their formation is currently lacking. Stress is generally believed to be the driving force behind their formation, but it is challenging to study them experimentally under a controlled state of stress. In this work, we designed a novel mechanical fixture to apply a controlled mechanical load on a small region of the Sn film surface. We simultaneously monitored the film area surrounding the pressurized region in a Scanning Electron Microscope to obtain nucleation and growth kinetics of the formed whiskers in real time. We developed a finite element model to analytically predict the stress state in the film and the average whisker growth rates. The model predictions alongside experimental observations suggest that whisker formation is governed by the generation of stress and stress gradients in the film driven by the application of the load. The results also suggest that the mechanisms controlling the generation and relaxation of stress, mainly the grain-boundary diffusion of Sn atoms and dislocation creep, are rapid and occur over sufficiently small timescales to drive whisker growth or immediately halt it, signifying the critical role of stress in whisker growth. A perplexing observation in the experiments was the abrupt stoppage in the growth of several whiskers despite a maintained state of stress. Sub-surface grain structures were studied using Focused Ion Beam cross-sectional measurements to investigate this observation. A hardening relation for increase in the threshold stress for growth with the whisker length was incorporated in the finite element model to explain the decrease in average whisker growth rates. To understand the role of nucleation and growth stages in the development of whiskers, which occur in tandem, we designed experiments to separate the two. We employed thermal strain to induce whisker nucleation in Sn film followed by mechanically stressing the film to drive the growth of whiskers. The results from this study suggest that nucleation is a significant barrier to whisker development in addition to a threshold stress barrier for their growth.
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