Mechanical Behavior and Microstructure Evolution in Lead Free Solders Subjected to Mechanical Cycling at Elevated Temperatures

Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. These stresses and strains are induced by mismatches in coefficients of thermal expansion, and lead to damage accumulation that contributes to crack initiation, crack propagation, and eventually to failure. In our previous paper at ECTC 2020, we investigated the accumulation of damage in several solder materials (SAC305, SAC +Bi, and SAC +Bi-Ni-Sb) during mechanical cycling at room temperature. Circular cross-sectioned solder specimens were first reflowed, and these samples were then mechanically cycled for various durations using a Micro-Mechanical tester. Cyclic stress-strain, monotonic stress-strain, and creep tests were then conducted on the prior cycled samples. The cyclic stress-strain curves obtained were studied to observe the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with mechanical cycling. The monotonic stress-strain and creep test data were plotted, and several mechanical properties were characterized for various levels of cycling. Using the data from these tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. All of the mechanical cyclic testing in our prior work and the work of others has been performed at room temperature (T=25
^°
C), and thus cannot be easily extended to thermal cycling applications. In our current work, we have extended our prior study to examine mechanical cycling at elevated temperature. In particular, we have examined lead free solders subjected to high temperature mechanical cycling at T=100
^°
C and T=125 °C. Two solder alloys, SAC305 and SAC +Bi (SAC_Q), have been investigated. In this paper, we report on the findings for SAC305 cycled at T=100
^°
C, and compare to the analogous results obtained for cycling at T=25
^°
C. Initially, small uniaxial cylindrical samples were prepared and reflowed in a reflow oven. These specimens were then mechanically cycled for various durations at T=100 °C. The measured cyclic stress-strain curves were then used to characterize the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with high temperature mechanical cycling. As expected, the SAC305 samples cycled at elevated temperature demonstrated a lower peak stress and smaller loop area relative to the results for analogous specimens cycled at room temperature and the same strain range. Uniaxial tensile tests and creep tests at room temperature were also conducted on specimens that had been previously mechanically cycled for various durations (e.g 0, 50, 100, 300, 600 cycles) at high temperature. This allowed us to study the degradation of the constitutive behavior of the solder alloys that occurred during the high temperature mechanical cycling due to the fatigue damage that builds up in the specimens. In particular, the degradations of the elastic modulus, yield strength, ultimate strength, and the secondary creep strain rate with duration of high temperature mechanical cycling were evaluated. These values were compared to the analogous results obtained for room temperature cycling in our previous study. As expected, the SAC305 samples cycled at 100 °C proved to have larger damage accumulation and degradations in mechanical properties relative to the analogous samples cycled at room temperature. The degradations in the properties that occur during mechanical cycling were also correlated with the corresponding changes in the microstructure of the specimens. Rectangular cross-sectioned samples of the two lead free solder alloys were polished and selected regions indented to track the changes in the microstructure of a fixed region with mechanical cycling at T=100
^°
C. The observed microstructure evolution during cycling included IMC coarsening as well as micro crack initiation and growth. Using the results of this study, we are working to develop better fatigue criteria for lead free solders which are subjected to variable temperature applications.