Effect of Bismuth Content on the Mechanical Cyclic Properties of SAC+Bi Lead Free Solders

Abstract

During the transition from tin-lead solders to lead-free solders, Sn-Ag-Cu (SAC) alloys have become the most widely used lead free alloys for the various levels of interconnects in an electronic package. To improve the thermal cycling reliability of SAC alloys, new doped $\text{SAC} +\mathrm{X}$ alloys have been developed, were X = Bismuth is often added to provide enhanced strength and improved aging resistance. Our prior work has shown that the use of $\text{SAC}+\text{Bi}$ solders can improve the reliability of the electronic package. However, there have been only limited studies regarding the cyclic mechanical properties of lead free SAC solder alloys with added bismuth content. In this paper, the authors have quantified the evolution of the properties of $\text{SAC} +\text{Bi}$ lead free solder joints subjected to isothermal mechanical cycling. Various levels of bismuth content have been studied including 1%, 2%, and 3%, and the results have been compared to previous studies performed on SAC305. Cylindrical solder specimens were prepared and reflowed with a reflow profile similar to that utilized in industry SMT joints. The formed samples were then mechanically cycled at room temperature using five different total strain ranges including 0.003, 0.005, 0.007, 0.009, and 0.012. The obtained cyclic stress-strain curves were recorded up to the 20th cycle, where the hysteresis loops became somewhat stable from one cycle to the other. The obtained hysteresis loops were then analyzed to evaluate the plastic work accumulated, the peak stress, and the plastic strain range for each of the total strain ranges considered. The plastic work dissipated per cycle is often an important parameter in predicting solder joint reliability with models such as the Morrow/Darveaux models. For each solder alloy, the obtained plastic work was correlated with the applied total strain range to develop correlations between the two quantities. Using the obtained results, specimens of the various $\text{SAC} +\text{Bi}$ alloys were then mechanically cycled to failure using the same initial plastic work (hysteresis loop area) in the first cycle. Failure was considered to have occurred with a 50% drop of the applied load. The cyclic stress-strain loops obtained from the fatigue failure cyclic experiments were then studied to measure the change in the cyclic properties for different bismuth contents. This approach removed the limitations from previous fatigue life studies where a constant strain range was used across all the alloys, which resulted in various hysteresis loop sizes. Rectangular cross-sectioned and polished samples of all the alloys were also cycled at the same corresponding strain ranges to study the change in microstructure of the alloys with mechanical cycling. This has helped us gain a better understanding on how the bismuth percentage in an alloy affects the microstructure evolution during mechanical cycling.