Effects of Grain Size and Orientation on Mechanical Response of Lead Free Solders

Abstract

In this study, the effects of grain size and orientation on mechanical behaviors of lead free solders were evaluated by measuring the sub-grain size and determining the plane orientation of the grains. Sn-Ag-Cu (SAC) solder alloys and solder joints (SAC 105, SAC 305, SACX) were chosen for investigation. By using polarized light microscopy, Ball Grid Array (BGA) solder joints were found to typically be comprised of a single grain or just a few grains. To quantitatively measure the sub-grain size, three methods including the intercept method, the planimetric method, and the electron backscatter diffraction (EBSD) method were applied to examine the microstructure of SAC solders. The results indicated that the sub-grain size increased during aging, which could be a cause of aging-induced degradation of mechanical properties of SAC solders. In the second part of this work, the grain orientations in solder specimens and joints were characterized by the EBSD technique. Inverse pole figures (IPFs) were generated and used to calculate the Euler angles and crystallographic orientations of the grains. Based on the determination of the crystallographic orientations, the elastic modulus values were determined in the global coordinate directions of the joints. Simple finite element models were also developed to show the importance of the grain structure and orientation. The results revealed that in-plane shear stress concentrations were induced along the grain boundaries and the stress distributions were dependent on the grain sizes and orientations in the SAC solder joints.