Characterization of Constitutive Equation of Sn-Bi by Studying Creep Behavior of Flip Chip Solder Joints

With the development of lead-free solder, Sn-Bi solder is becoming more widely used in industry due to its unique properties. One of the advantages of the Sn-Bi solder is that the low melting temperature of the eutectic Sn-Bi solder will reduce the reflow cost. Moreover, the coefficient of thermal expansion of Sn-Bi solder is around 15 ppm/C, which is close to the substrate material and will help improve the board level reliability during thermal cycling. In the study of board level reliability, the constitutive equation of solder is used to describe the nonlinear behavior of solder joints during creep. However, limited datasheet is provided for the material properties of Sn-Bi solder. One of the methods is to extract the creep testing data results and obtain the constants in governing equation by curve fitting. However, this method requires mathematical simplification and previous studies are mostly focused on bulk samples. In this study, the eutectic Sn42Bi58 solder balls are tested under different constant loadings. Using 2D digital image correlation (DIC) technique, the displacement data as well as the creep rates can be obtained. As a result, the constitutive equation of Sn-Bi solder is obtained in the form of Garofalo model by nonlinear regression of the experimental data. Finite element modeling is performed based on the acquired constants, the good agreement with experimental data validates the fitted results in the constitutive equation.