GRAIN-SCALE ANISOTROPIC STUDY OF TENSILE VS. SHEAR MECHANICAL CONSTITUTIVE AND FATIGUE BEHAVIOR IN OLIGOCRYSTALLINE SAC305 SOLDER JOINTS

Abstract

Solder joints in microelectronic assemblies experience a multiaxial combination of cyclic extensional and shear loads due to combinations of thermal expansion mismatch and flexure of printed circuit assemblies (PCAs) during thermal cycling or during vibrational loading of constrained PCAs. Although, a significant amount of research has been conducted to study cyclic fatigue failures of solder joints under pure-shear loading, most of the current literature on cyclic tensile loading of solders is on long dog-boned monolithic solder coupons. Unfortunately, such coupon specimens do not capture the critical interactions between key micro-scale morphological features (such as grain orientation, grain boundaries, intermetallic compounds [IMCs] and substrates) that are believed to play important roles in the fatigue of functional solder joints under life-cycle loading. Therefore, Part I of this study uses a combination of experiments and finite element analysis to investigate the differences in mechanisms of cyclic fatigue damage in Sn-3.0Ag-0.5Cu (SAC305) few-grained (oligocrystalline) microscale solder joints under shear, tensile and multiaxial loading modes at room temperature. Cyclic fatigue durability test results indicate that tensile loads are more detrimental compared to shear loads. Tensile vs. shear loading modes are found to cause distinctly different combinations of interfacial damage vs. internal damage in the bulk of the solder (transgranular and intergranular damage), which correlates with the differences observed in the resulting fatigue durability. The test results also confirm that this type of multimodal fatigue damage cannot be modeled with the traditional approach of a power-law dependence on the cyclic amplitude of equivalent deviatoric strain. Instead, multiaxial fatigue damage results are seen to be affected not only by the cyclic equivalent strain amplitudes, but also by the severity of the stress-triaxiality, as hypothesized in models such as Chaboche model.Estimating the true deviatoric strains and triaxiality ratios at the failure sites is not a trivial task in typical oligocrystalline SAC305 solder joints, because the strong anisotropy of the individual grains - and the interactions of such grains with surrounding grains as well as with the interfacial boundaries - make the strain field unique in each joint. Thus, the current approach of modeling solder joints as homogenous isotropic structures, are clearly inadequate because they fail to capture the true grain-scale stress fields at the failure sites. The joint-to-joint variation in the grain morphology leads to variability in fatigue damage accumulation rates under cyclic loading. Part II of this study thus focuses on grain-scale study of the fatigue results presented in Part I, by: (a) characterizing multi-scale anisotropic elastic-plastic properties of SAC305 single crystals, using a hybrid combination of experiments and finite element simulation, (b) applying a grain-scale parametric study to explain the variability seen in Part I, in the bimodal fatigue failures under multiaxial cyclic loading. The anisotropic elastic-plastic properties in Part IIa were determined by conducting monotonic tensile and shear tests on SAC305 single crystal specimens. The anisotropic elastic behavior is modeled using anisotropic elastic stiffness constants for SAC305, whereas anisotropic plasticity is modeled using Hill’s potential in conjunction with a Holloman-type power-law plastic constitutive model. Microstructurally motivated scaling factors are empirically developed, to assess the effect of dendritic and eutectic microstructural features on single-crystal stress-strain properties. This facilitates extrapolation of constitutive properties across different cooling rates and different isothermal aging protocols. Additional empirical scaling factors are also developed to account for the influence of characteristic grain sizes and grain aspect ratios (relative to principal loading directions). The parametric study in Part IIb, was conducted using the anisotropic properties of Part IIa, to quantify the effect of grain anisotropy on variability in cyclic mechanical fatigue curves of SAC305 solder. This study demonstrates an efficient computational approach for determining variability in mechanical response and fatigue behavior of Sn-rich solder joints, thereby reducing the time and costs associated with physical testing.