Anisotropic Plastic Constitutive Properties of SAC305 Single Crystal Solder Joints

Micro-scale Sn-Ag-Cu (SAC) solder interconnects have oligocrystalline grain structure with one to few grains in each solder joint. As well documented in the literature, SAC solder joint consisting of 96.5% β-Sn is highly anisotropic due to the inherently anisotropic mechanical behavior of β-Sn. Therefore, each joint exhibits a unique mechanical response. However, due to the complexities in the quantification of microstructure and finite element (FE) modeling methodology, engineers typically model solder joints as homogenous isotropic structures with directionally averaged mechanical properties. These approximations cause inaccurate prediction of strain levels in the solder and in turn leads to uncertainties in lifetime predictions. A key challenge in grain-scale anisotropic modeling of solder joints, is the lack of widely accepted anisotropic inelastic mechanical properties of solder grains in the literature. The goal of this paper is to determine rate-independent plastic constitutive behavior of Anisotropic SAC305 single grains. Monotonic tensile and shear tests are conducted at room temperature on a set of single-grain SAC305 solder joints. The grain structure for each test specimen is characterized with EBSD and finite element modeling is used to iteratively extract model constants for Hill-Holloman continuum plasticity model, which utilizes Hill’s anisotropic yield criterion along with a Holloman Power-Law plasticity model to represent each grain. Plastic deformation in the grain boundaries is ignored.