Evolution of Mechanical Properties and Microstructure of SAC305 Lead Free Solder Subjected to Mechanical Cycling and High Temperature Aging
Authors: Maruf Mahbub Alam , Mazumder Golam Rakib , Chakraborty Souvik , Suhling Jeffrey C. , Lall Pradeep 
Published: November 2024

Lead-free solder joints in electronic packaging are susceptible to damage and failure due to mechanical loading at high temperatures. For example, electronic components in car engines or power plant turbines may experience cyclic loading from vibration while also undergoing thermal cycling due to frequent power switching. The solder materials used to attach these components are subjected to cyclic stresses and strains caused by the mismatches of the coefficients of thermal expansion (CTE) of the materials in the assembly. Recent studies have examined mechanical property degradation and microstructure evolution in lead-free solder alloys subjected to isothermal aging. Other studies have examined damage accumulation and mechanical property degradation in lead-free solders due to isothermal mechanical cycling. However, there is no prior research on the combined effects of high-temperature isothermal aging and mechanical cycling on the mechanical properties and microstructural evolution of lead-free solder alloys. This study investigates the synergistic effects of these two environmental factors on the reliability of lead-free solder joints.

In our present study, we are examining the evolution in both mechanical properties and microstructural characteristics that arise when SAC305 solder is exposed to the combined influences of mechanical cycling-induced damage and isothermal aging. Within the scope of this research paper, various sequential applications of mechanical cycling and isothermal aging have been considered. Cylindrical uniaxial samples of SAC305 solder were prepared by solidification in glass tubes under a controlled reflow profile. Subsequently, these samples underwent preconditioning through sequential exposure to isothermal mechanical cycling and isothermal aging at high temperatures. We explored both orders of exposure, with one set of samples subjected first to isothermal aging and then undergoing mechanical cycling, while the other set experienced the reverse order of exposure — starting with mechanical cycling at a constant temperature, followed by isothermal aging. Isothermal aging conditions involved subjecting the samples to a temperature of T = 125 °C for 20 days, whereas mechanical cycling was conducted at room temperature with a total strain range of ε = ±0.006, encompassing various durations (e.g., 0, 50, 100, 200, and 300 cycles).

After subjecting the specimens to preconditioning through sequential mechanical cycling and aging, we conducted assessments of their mechanical performance using the nanoindentation testing technique. Specifically, we examined how different preconditioning sequences affected key mechanical properties such as elastic modulus, yield stress, and creep behavior, comparing these outcomes with those of pristine specimens. Notably, our findings demonstrated a high degree of sensitivity to the specific order in which mechanical cycling and aging were applied. The samples subjected to cycling followed by aging demonstrated more degradation in mechanical properties compared to those that underwent aging before cycling. Additionally, we employed Scanning Electron Microscopy (SEM) to visualize the microstructures of the preconditioned specimens and establish correlations between these observed microstructures and the mechanical behavior measurements.