The Effect of Bismuth Content on Mechanical Property Evolution of SAC+Bi Lead Free Solders Subjected to Long Term Thermal Exposures

Abstract

Solder joints in electronic assemblies are frequently exposed to thermal cycling environments in their service life or during accelerated life testing where temperature variations occur from very low to high temperature. In our recent papers, the mechanical behavior evolutions occurring in SAC305 and SAC+3%Bi (SAC_Q) lead free solders have been characterized for up to 20 days of exposure to four different thermal profiles including isothermal aging, slow thermal cycling, thermal shock, and thermal ramping. In the current investigation, we have extended our prior study to examine several different SAC+Bi solder alloys with various bismuth contents. In particular, a family of SAC+Bi alloys with 1%, 2%, and 3% Bi were studied with four different thermal exposure profiles (isothermal aging, slow thermal cycling, thermal shock, and thermal ramping). The primary objective of this study was to determine how much bismuth is needed in the lead-free alloy to mitigate microstructure and material property evolutions during thermal exposuresUniaxial miniature bulk specimens were prepared for the three SAC+Bi alloys using a controlled reflow profile. After fabrication, the samples were then preconditioned by thermal exposure under stress-free conditions for various durations up to 100 days. After preconditioning via thermal exposures, the samples were subjected to stress-strain testing to measure their mechanical properties including effective elastic modulus, and Ultimate Tensile Strength (UTS).For the miniature bulk solder samples of each SAC+Bi alloy, the evolutions of the mechanical properties for each thermal exposure profile were characterized as a function of the duration of thermal exposure. For all of the alloys, the degradations for the slow thermal cycling exposure were the largest, while those for isothermal aging were surprisingly the smallest. Increasing the Bi content of the SAC+Bi alloy led to increased mitigation of thermal degradation effects for all of the exposure profiles. Reduced microstructural evolution in the SAC+Bi alloy samples was found to be the major reason for the improved resistance to mechanical behavior changes. The tensile strength results for samples with 2% Bi and 3% were nearly the same, suggesting that lower bismuth content could be sufficient for many applications.