A significant reliability problem with the baseline alloy SAC305 is the deterioration of its mechanical properties and change its microstructure after aging. In addition to aging, multiple factors can contribute to the early failure of the solder joint at different levels. Research has shown that shear testing with a high strain rate accelerates the failure mechanism of a solder joint. In addition, solder joints subjected to high-stress levels deformed considerably leading to failure. On the other hand, several studies have revealed that doped, or micro-alloyed solder joints exhibited better results in terms of mechanical properties, such as an increase in ultimate shear strength and ultimate tensile strength, as well as a stabilization of the microstructure after aging. Typical dopants include nickel (Ni), bismuth (Bi), indium (In), and antimony (Sb). Therefore, this study examines the effects of aging on the microstructure and the mechanical properties for different solder alloys (SnAgCu, SnAgCu-3Bi, and SnAgCu-6Bi ) and the reliability of solder joints under realistic applications.
The Instron 5948 Micro Tester is used to perform shear and creep tests for individual solder joints by using customized experimental setup. The evolution of the microstructure is assessed by using both optical microscope and scanning electron microscopy.
In the first part of this research, the effect of Bi content on the shear properties of different solder alloys (SnAgCu, SnAgCu-3Bi, and SnAgCu-6Bi) is investigated. Four aging periods (0, 10, 100, 1000 hrs.) at 150°C are considered in the study. Test samples were subjected to shear tests at room temperature. Extension Vs Strain data is collected and analyzed to study the effect of different factors such as Bi content, aging times, and shear strain rate on the reliability of solder alloys. Results show that doping alloys with Bi improves the shear properties and mitigates the effect of aging. Three failure mechanisms were noticed including ductile failure, brittle failure, and a combination of ductile and brittle failures. In the second part of this research, the effect of Bi content on the creep properties of different solder alloys (SnAgCu, SnAgCu-3Bi, and SnAgCu-6Bi) is investigated. A micro indentation is performed on individual solder joints under different stress levels using a customized test schematic.Solder joints were subjected to creep tests at room temperature. Extension Vs Time has been collected and the creep rate has been analyzed for each test specimen. Results demonstrate that SAC-Bi alloys had less creep rate compared with SAC305 due to the solid solution phenomena of Bi in the Sn lattice. Moreover,the deformation zone for SAC305 is significantly larger than that of SAC-Bi alloys.