Mechanical Property Evolution in SAC+Bi Lead-Free Solders Subjected to Various Thermal Exposure Profiles

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. Due to the CTE mismatches of the assembly materials, cyclic temperature leads to damage accumulation due to shear fatigue in the solder joints. In addition, dwell periods at the high temperature extremes will cause thermal aging phenomena and additional microstructural evolution and material property degradation. Further aging effects can occur during the ramp periods between the low and high temperature extremes.While changes in solder materials during aging have been examined in detail in prior studies, there have been limited studies examining material evolution occurring during other thermal exposures such as thermal cycling and thermal shock. 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. The degradations in the mechanical properties (modulus, UTS, yield strength) were observed for both miniature bulk samples and solder joints, and then the results were compared for the different exposure profiles. For both bulk samples and joints, the largest changes were observed for the slow thermal cycling profile. In addition, the changes in the SAC+3%Bi solder samples were much smaller than those experienced in the SAC305 solder samples for all of the considered thermal profiles.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 exposures. Use of lower Bi content can lower solder cost and also increase reliability in high strain rate loadings such as shock/drop/vibration.Uniaxial 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. Several thermal exposure profiles from -40 C to 125 C were examined including: (1) isothermal aging at the high temperature extreme (aging), (2) 150 minute cycles with 45 minutes ramps and 30 minutes dwells (slow thermal cycling), (3) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps (thermal shock), and (4) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramping). After preconditioning via thermal exposure, the samples were tested to characterize their material behavior and microstructure evolutions. The uniaxial bulk specimens 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 and microstructure for each thermal exposure profile were characterized as a function of the duration of thermal exposure. Several comparisons were then made including: (1) comparing the observed mechanical properties evolutions for the three SAC+Bi alloys to each other, (2) comparing the microstructural evolutions for the three SAC+Bi alloys, and (3) comparing the relative severity of degradations in each alloy occurring for the four different thermal exposure profiles.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.