Aging Induced Evolution of the Fatigue Behavior and Microstructure of Sn-Ag-Cu Lead Free Solders

Abstract

Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Cyclic loading leads to damage accumulation, crack initiation, crack propagation, and eventually to fatigue failure. On the microscopic level, aging leads to solder microstructural changes such as the coarsening of phases and grains. These microstructural changes are closely tied to the damage that occurs during cyclic mechanical loading. In the first part of this investigation, the effects of aging on the cyclic stress-strain and fatigue behavior of Sn-Ag-Cu (SAC) lead free solders have been studied. Cylindrical uniaxial lead free solder test specimens (SAC305 and SAC405) have been prepared and subjected to cyclic stress/strain loading for different aging conditions. Prior to testing, the specimens were aged (preconditioned) at 125 ºC or 25 ºC for various aging times up to one year. It has been observed that aging degrades the mechanical fatigue properties, and those degradations are much more significant at the first few days of aging. The effects of testing temperature, strain rate, and strain range on the cyclic stress-strain behavior of SAC305 lead free solder have also been examined, and the effects of aging on the cyclic stress-strain behavior have been quantified for the first time for different testing temperatures, strain rates, and plastic strain ranges. In the second part of this work, Scanning Electron Microscopy (SEM) has been utilized to examine aging induced microstructural changes occurring within lead free solders. Unlike many prior studies, fixed regions in the solder cross-sections were monitored throughout the aging process, rather than examining different samples and/or different regions after the various aging exposures. Nanoindentation marks were added to the cross-sections at certain locations to facilitate locating the fixed regions of interest in subsequent microscopy observations. After preparation, the samples were aged at T = 125 ºC, and the microstructures were observed and recorded in the selected regions after various aging exposures using SEM. With this approach, time-lapse imagery of the microstructure evolution in a particular region of a solder joint has been recorded as a function of the aging time. When placed together sequentially, these images can be used as frames to create experimentally recorded movies of the microstructural evolution in SAC solders exposed to aging. This approach has allowed the visualization of phenomena such as the coalescence and migration of intermetallic compounds (IMCs) and the coarsening of beta-Sn dendrites. The aging induced changes in number of IMCs, total area of all IMCs, average particle area, and average particle diameter have been quantified for several fixed regions in the solder samples. Finally, the effects of mechanical cycling on the cyclic stress-strain behavior and constitutive behavior of SAC305 lead free solder in fatigue testing have been explored. At the same time, effects of fatigue cycling on solder microstructure have been studied using SEM and polarized light microscope. The goal of this portion of the work was to better understand damage accumulation that occurs in the solder material during fatigue testing.