(Dr. Richard Neu, advisor)

"Thermomechanical Interaction in the Deformation and degradation of Solder Alloys"

Abstract

It is widely recognized that accelerated degradation is seen in metals when thermal cycling is coupled with mechanical cycling, known as thermomechanical fatigue, as opposed to strictly isothermal fatigue.  However, there have been no constitutive models generated that accurately predict the long-term thermomechanical behavior of solders using a microscale-based theory.  It is also well known that phenomena occurring on the microchemical level such as an elastic grain boundary sliding, microplastic work, and void generation most likely dominate the fatigue lifetime of electronic components where the volume of the material applied is of similar order of magnitude as that of the microconstructrual characteristics of the solder used.  The main thrust of this study is geared towards characterizing the thermomechanical fatigue of solder by separating the damage accumulated in the material during a thermomechanical cycle into superpositional components generated by mechanical work and thermal fatigue.  In particular, three types of tests are run: traction-free thermal cycling, uniaxial isothermal mechanical fatigue (at three separate temperatures), and uniaxial thermomechanical fatigue.  By utilizing the non-invasive method of examining the roughness before and after fatigue by the use of a confocal scanning laser microscope, it is then possible to make inferences regarding how much relative microdamage has been accumulated by each specimen by looking at its comparative roughness with respect to other specimens tested.  After examining the temperature dependance of the roughening, it is then possible to suggest which microstructural process or process dominate fatigue at a given temperature and make conclusions as to what terms would be useful in formulating an analytical constitutive model to more accurately describe this behavior.