msepostdoc-list Seminar Reminder - Koppes, Mon. 7/16/12, 9:30 AM, ARMS Rm 3115

[Stacey, Lisa A]

MATERIALS SCIENCE AND ENGINEERING
SEMINAR


Effect of Cyrystallographic Orientation on Hillock Formation in
Thermally Cycled Large Grain Tin Films

by

John Koppes
Ph.D. Final Exam

Advisor
Professor C. Handwerker

ABSTRACT

Tin whiskers and hillocks grow spontaneously from the surfaces of polycrystalline Sn films at room temperature.  Whiskers can grow long enough to cause short circuits in electronic devices.  We hypothesized that the anisotropies of the crystal structure lead to locally high strain energies that are relieved by the growth of whiskers and hillocks.  This research studies hillock formations on large grain Sn-alloy films relative to the crystallographic orientations of the adjacent grains.  Large grain films were produced by solidifying 96.5wt% Sn - 3wt% Ag - 0.5wt% Cu solder alloy on a Cu substrate.  These surface defects (hillocks) grew predominately at grain boundaries during thermal cycling.  The formation of the surface defects between two grains created a pseudo-bi-crystal sample geometry, making it ideal for studying surface defects relative to the local crystallographic orientations and the grains' corresponding anisotropic properties.

The crystallographic orientations of the grains were studied with Electron Backscatter Diffraction (EBSD) and Laue micro-diffraction at the Lawrence Berkeley National Laboratory Advanced Light Source. Local orientation studies of the surface defects and the surrounding grains indicated that the surface defects nucleated and grew with low dislocation densities.  In addition, the linear surface defect densities along the grain boundaries were measured and observed to change as a function of orientation.  The change in linear defect density with respect to orientation was due, in part, to the anisotropy of the coefficient of thermal expansion of β-Sn.  In addition, it was important to account for elastic anisotropies.  The elastic stresses, strains, and strain energy densities of the microstructures were determined with Object Oriented Finite element analysis.  The simulations indicated that during thermal cycling the local stresses exceeded the yield strength.  As a result, the highest linear defect densities did not occur at orientations with the highest local strain energies.  Instead, the highest local strain energies were accommodated by other plastic deformation mechanisms, indicating that surface defect formation is not only affected by local strain energy densities but also the local strain rates.  Therefore, the propensity of specific grain orientations to form surface defects will change as a function of the thermal cycling process.

Date:   Monday, July 16, 2012

Time:        9:30 AM
Place:   ARMS 3115


Lisa Stacey
Secretary/Development Assistant
Purdue University
School of Materials Engineering
765/494-4100

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