Establishment Of Constitutive Model Of SnPb And Life Prediction Of SMT Solder Joint

Numerical simulation is proved to be one of the best tools to analyze the stress and strain evolution in SnPb solder joint and predict its fatigue life under environment cycling temperature. In the simulation, the selection of constitutive model of SnPb and life prediction method determine the accuracy of finite element simulation results directly, so it needs deeply theoretic study. In this paper, a viscous-mixed hardening constitutive model of SnPb is established, which can describe the strain hardening, creep deformation and high temperature softening characteristics of SnPb.Firstly, the mixed hardening constitutive equation has been used to describe the isotropic hardening characteristic and Bauschinger Effect under cyclic loading. The incremental constitutive relations are built based on Power Exponential hardening model and Ziegler bilinear hardening model, respectively. The three kinds of constitutive relations, including isotropic hardening equation,kinetic hardening equation and mixed hardening equation have been completed by users'subroutine of MARC written in Fortran. In addition, User's subroutine has been compiled to define the creep deformation performance of SnPb based on Darveaux steady creeping equation. Afterward, FEM calculation process to solve creep deformation problem is given.Then based on Darveaux life prediction model of solder joint, the average strain energy density of the whole solder joint is chosen as the dominant mechanical factor for fatigue life prediction of SMT solder joint. The high constraint of the tiny joint and the effect of meshing density on the stress-strain field have been considered, and information of unit such as strain energy density, volume is abstracted by subroutine program.Finally, the influence of constitutive models of SnPb and shape of the solder joints on the mechanical behavior and fatigue life is studied. The simulated results of stress-strain field evolution in SMT solder joint show that the root of solder joint between the component and substrate gap is the weakest zone with a high stress concentration. The distribution of stress and strain within the solder joint under temperature cycling is dynamic and relative with the temperature history. As the strain energy density field remains about the same, it's proved to be an appropriate dominant mechanical factor to judge the location where the crack begins and expands.It shows that the constitutive model of SnPb has a great impact on the life prediction results of solder joint. When the ideal elastic-plastic constitutive model is used to define the mechanical behavior of SnPb, life of the solder joint is the longest. When creep deformation is taken into account, the nonlinear deformation is much larger. Accordingly, the calculated life of solder joint is slightly lower. When the viscous-mixed hardening constitutive model is used, the solder joint shows an obvious strain hardening characteristic and the improved stress leads to much larger creeping deformation. Synthetically the strain energy density increases significantly that life of the solder joint is comparatively the shortest. With most factors affecting SnPb deformation considered, the simulated result can be used as reference for reliability design of solder joint. With quantity of solder increasing and the shape changing from concave to convex, the average strain density gets lower and fatigue life increases.