Controlling Heterogeneous Stress Relaxation in Tin Films: Whiskers, Grain Boundary Sliding, and Beyond

Purdue University, West Lafayette IN

Investigators

Abstract

Non-Technical Abstract In microelectronics some of the materials are inherently unstable because they are being used at temperatures close to their melting temperatures. One such material is tin in solder joints: Tin atoms are able to move around (diffuse) relatively quickly at room temperature in response to changes in their environment. Of particular concern is the formation of long Tin whiskers in response to stresses normally occuring in microelectronics. Long Tin filaments can grow spontaneously from surfaces of thin Sn films and can reach lengths of several millimeters. Such whiskers can bridge adjacent contacts and cause short circuits leading to electronic system failures. The question is how to stop them from forming. A new strategy is needed to better understand and specifically to mitigate failure due to whisker growth in Sn films. In particular, forming whiskers is only one of the ways in which the atoms in thin Sn films can respond to stresses. This new strategy needs to take into account the contributions of other processes to relaxing stresses in thin films and to learn how to manipulate them to keep whiskers from forming. The goal of this project is to develop models, numerical simulations and critical experiments at the microscopic scale to quantify the different contributions of these processes in these films. The proposed computational effort is a step forward in reducing the reliance on experimentation to develop new materials or to improve reliability in existing ones. Achieving this goal for materials design requires the development of new predictive simulation tools and training the next generation work force on the use of these advanced tools. The data and simulation tools developed in this project will be broadly available through the NSF supported nanoHUB.org with open access to the materials community including researchers in industry and academy and educators. Demonstration tools for education will be deployed in nanoHUB.org and integrated in the Engineering curriculum at Purdue and will be accessible for universities and industry everywhere. The work proposed provides an excellent opportunity to train graduate students and undergraduates in the integration of materials science and engineering experimental and computational techniques, in developing cross-disciplinary approaches, and in working as members of a multi-disciplinary international research team. Techical Abstract Heterogeneous microstructure-induced stresses that drive stress relaxation are linked to a wide range of failure mechanisms in thin metal films. Whisker and hillock formation are known responses of thin metal films to residual stresses but others include yielding, diffusional and dislocation-mediated creep, grain boundary sliding, cracking, delamination, surface roughening, extrusion-intrusion formation, recrystallization and grain growth. The relative contributions of these multiple operations to stress relaxation frequently switch as stress distributions and microstructures evolve in a dynamic and complex process. A strategy to better understand their changing contributions and specifically to mitigate failure due to whisker growth in Sn films needs to take into account the contributions of these mechanisms to identify: i) the local conditions under which surface grains form whiskers and influence their rate of growth, and ii) what other mechanisms compete with or accelerate whisker formation and growth. The goal of this project is to develop models, numerical simulations and experiments at the microscopic scale to study deformation-microstructure relationships to relax residual stresses in thin Sn films during cyclic bending and thermal cycling, two configurations where multiple processes operate. We propose to develop a framework with simulations of these simultaneous processes with experiments designed to explore the different contributions of these mechanisms. While the proposed framework could be applied to a variety of thin film systems, Sn films not only display a wide range of phenomena that will demonstrate its capabilities, but will also provide the opportunity to test its usefulness in developing mitigation strategies to inhibit tin whisker formation. While the understanding of local stress relaxation processes in thin films and small-scale structures has grown significantly over the past decade, a strategy to examine multiple simultaneous processes, such as dislocation generation, recrystallization, creep, and whisker formation, is still developing. The recent observation that, in Sn films, whiskers nucleate and grow along some grain boundaries during thermal cycling and cyclic bending with other dislocation and diffusion processes being evident offers the opportunity to explore this stress-microstructure-deformation space. The numerical simulations and experiments proposed in this effort are a step forward in answering questions, such as where and how do grains nucleate to form whiskers and hillocks, what local conditions affect their rates of growth and how does whisker growth competes or collaborates with other stress relaxation mechanisms.