Ongoing changes in the electronics industry provide for an urgent need for a quantitative understanding of the behavior of SnAgCu solder alloys in both isothermal and thermal cycling. This kind of materials system is, however, also of considerable scientific interest in its own right. The tetragonal Sn crystal structure provides for extremely anisotropic properties, and the addition of a few percent of Ag leads to cyclic twinning structures with unique properties and behavior. This has particular consequences for overall sample dimensions well below a millimeter. Notably, thermomechanical properties vary strongly with various aspects of the microstructure and its evolution over time. The goal of the proposed research is to establish a quantitative understanding of the evolution of fatigue damage in high-Sn alloys with a few percent Ag and Cu under cyclic loading. Recent work has led to the formulation of a general hypothesis involving direct causal links between dislocation generation and recovery, the build-up of dislocation cell structures, the thermal and strain enhanced grain growth and coarsening of secondary precipitates, the formation and rotation of sub-grains, and the eventual initiation and growth of cracks. It is anticipated that much of this picture will apply to a number of other precipitate hardened metals as well. A systematic study will be conducted to validate and, if necessary, improve upon or revise the above hypothesis. Experimental efforts will focus on the in-depth characterization of dislocation cell structures and eventual sub-grains in selected high-Sn alloys before and after carefully designed combinations of thermomechanical loading. Cycling with varying amplitudes will be particularly effective for the characterization of the underlying mechanisms. Microstructure studies will rely heavily on cross polarized microscopy, scanning electron microscopy, electron backscatter diffraction, focused ion beam sectioning and transmission electron microscopy.
NON-TECHNICAL SUMMARY: Electronics are serving critical roles in ever more aspects of modern life and of the general functioning of society, and the premature failure of a microelectronics product may have consequences ranging from economical to loss of life. The inexpensive availability of many electronics products may have tremendous consequences in areas ranging from military preparedness and public safety to education, social services and the development of poorer countries around the world. The long term life of a product is most often limited by wear out of the solder joints that connect components to each other or to a printed circuit board. As the industry is forced to transition even high reliability products to environmentally friendly lead free solders there is an urgent need for a level of understanding of damage and failure mechanisms that has yet to be established. This is the focus of the proposed research. Results will help the industry design and manufacture more durable, reliable electronics at lower costs. Graduate and undergraduate students, including a number of minority students, will be trained on state of the art equipment and conduct experiments both at the university and in the laboratory of industrial partner Universal Instruments. At Universal Instruments the students will work closely with Ph.D. level industry engineers. Students will present results at regular meetings of a large industry consortium, as well as at regional conferences and at major scientific conferences. They will also publish their work in peer reviewed journals. Results and experiences will furthermore contribute to the ongoing development of undergraduate and graduate level courses.
