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The Influence of Anisotropy on Thermal Cycling, Electromigration, and Mechanical Property Variability in Tin Based Solder Joints

April 14, 2017 | 11:00 AM – 12:30 PM
Nedderman Hall 100 | Seminar Flyer

Seminar Speaker

Thomas R. Bieler, Ph.D.

Professor, Materials Science and Engineering, Michigan State University

Abstract

The thermal expansion, diffusion, resistivity, elastic, and plastic anisotropy of tin in lead-free solder joints makes interpretation of variability in experimental observations of solder joint microstructural evolution, damage, and failure processes challenging. EBSP mapping and 3-D volumetric measurements of evolving tin microstructure in ex-situ and in-situ thermal cycling and electromigration experiments are compared. Orientation changes due to electromigration are very small, though the relative volume fractions of the three orientations present in cyclically twinned joints can vary, implying mobile boundaries. In contrast, thermal cycling leads to orientation changes resulting from thermal strains and development of low angle and high angle boundaries. The interactions of grain orientation on resistivity, Cu migration and intermetallic growth within the solder joint indicate that the anisotropy of resistivity and diffusion interact with each other in complex but understandable ways that depend on the crystal orientations and anisotropy.

Consequently, isotropic modeling of solder joint microstructural evolution and deformation is inappropriate for predicting detailed deformation history of a particular joint, because most joints tend to be single crystal or multicrystal joints that amplify effects of anisotropy. To model anisotropic behavior, elasto-plastic crystal plasticity based modeling is needed, which requires knowledge of the slip resistance on each slip system family. There are 14 families of slip and mechanical twin systems that provide 56 distinct shear deformation systems. Shear experiments on 32 joints characterized using EBSD mapping and observations of slip traces were used to obtain over 100 observations of slip activity, from which an assessment of slip resistance on each family of slip systems was extracted. These values are roughly consistent with the atomistic simulation of slip resistance by Kinoshita et al. (2012).

Progress in developing a crystal plasticity modeling framework for tin based solder is summarized, showing that a first-generation rate-independent crystal plasticity finite element model can simulate the heterogeneous shear deformation of both single crystal and beach-ball microstructures. Tensile experiments on single crystal joint specimens provide the means to calibrate a first-generation rate-independent model. An in-situ synchrotron tensile deformation experiment of a polycrystal provides a data set that can be used to validate future crystal plasticity models that include rate and temperature dependence. This experiment shows that some grains are more likely to split into subgrains than other orientations, and the local stress tensors in each subgrain can be resolved.

Bio

Thomas R. Bieler earned a B.A. in applied mechanics at University of California at San Diego in 1978 followed by an M.S. in ceramic engineering at the University of Washington in 1980. He worked for five years at Sandia National Laboratory in Livermore on high rate deformation and completed his Ph.D. in Materials Science (with a minor in continuum mechanics) at University of California at Davis in 1989. Since then, he has been at Michigan State University. His research focuses on characterization of mesoscale deformation mechanisms and plasticity modeling in titanium based alloys, tin in the context of lead-free solder joints, and high purity niobium used in superconducting particle accelerator cavities. With colleagues, he has published more han 300 papers, with more than 4,700 citations and an h-index (ISI) of 37. He is active in ABET Engineering Accreditation, and he is an organizer or co-organizer of many recent symposia involving heterogeneous deformation related to grain or phase boundaries.