Simulations of the impact of single-grained lead-free solder joints on the reliability of ball Grid Array components

The microstructure of lead-free solder joints often consists of only one or a few randomly oriented tin grains as a result of a large degree of undercooling during solidification. Due to the severe anisotropy of single crystal Sn and the random nature of the microstructure, the stress state and microstructural evolution of each joint will be unique. The orientation of the c-axis of the Sn crystal will strongly affect localized strain induced by temperature cycling, influencing the recrystallization process and thus the damage evolution in the solder joints. This will have strong implications on the reliability of lead-free solder joints and the accuracy of the predictions made with the current standard practices in virtual qualification. As a result of this random distribution of single-grained solder joints, joints to fail first in BGAs are not necessarily only dependent on the distance to neutral point, but rather the combination of location and microstructure of individual joints. In this work, the effect of location and orientation of single-grained solder joints in a CTBGA208 lead-free package were investigated through thermo-mechanical finite-element simulation and were supported by experimental findings of the same package through failure analysis and microstructural investigations using electron backscatter diffraction (EBSD) as well as polarized light microscopy. Due to the lack of data on the material properties of single-grained Sn-based, Pb-free solder joints, the anisotropic elasticity of pure Sn has been combined with the isotropic viscoplastic constitutive models commonly used in the literature.