The effect of static and dynamic aging on fatigue behavior of Sn3.0Ag0.5Cu solder alloy

摘要

In microelectronic assemblies, solder joints serve as interconnection between different packaging levels and are an important cause for the failure of microelectronic products. Sn-Ag-Cu solder alloys became important after lead-based solder alloys were caused to be discarded by regulations in European Union and Japan. However, the constitutive behavior of Sn-Ag-Cu alloys is not as well understood as lead-based solder alloys, and many studies confirm the aging of these alloys with time. The aging of Sn-Ag-Cu alloys and its effect on mechanical behavior challenges the reliability prediction of microelectronic assemblies. In this study, the effect of pretest isothermal aging and in-test aging on the fatigue behavior of Sn3.0Ag0.5Cu alloy are examined using the microstructurally adaptive creep model (MACM) and the maximum entropy fracture model (MEFM).;In this thesis, first, the development of microstructurally adaptive creep model is reviewed. Compared to traditional constitutive models, this model considers the effect of thermal history. Two microstructural parameters, the average Ag3Sn particles size and the average primary-Sn cell size are identified as critical parameters and incorporated into a modified Dorn creep form, which can describe both climb-controlled and glide-controlled dislocation motions.;Next, the maximum entropy fracture model is discussed and compared to traditional fatigue fracture model. The MEFM utilizes the damage accumulation parameter, which connects the accumulated damage to the accumulated inelastic dissipation. This parameter is independent of sample geometry, test temperature and strain rate.;Later, using MACM and MEFM, the extraction of the damage accumulation parameters is presented. The creep models for different aging conditions are constructed first based on microstructural characterization. The damage accumulation parameters of 25 celsius and 100 celsius tests are fit using MEFM. The parameters are presumed different for the two conditions because of the different aging states of the material.;The concepts of static aging and dynamic aging are introduced and utilized to describe pretest aging and in-test aging. In 25 celsius test, with longer static aging, the damage accumulation parameter is smaller, indicating a faster fatigue damage accumulation. Through the relationship between damage accumulation parameter and the average primary-Sn cell size, the influence of microstructural evolution introduced by static aging on fatigue behavior is confirmed. In 100 celsius tests, the effect of dynamic aging is captured by the change of damage accumulation parameter in experiments. Comparing the damage accumulation parameters from 25 celsius and 100 celsius tests, during test, further aging of Sn3.0Ag0.5Cu microstructure occurs, degrading fatigue behavior until microstructural evolution is completed.;Finally, the thesis is summarized and future work to better characterize the relationship between fatigue behavior and microstructure is put forward. The proposed work includes building a dynamic aging model and microstructural evolution model.