The geometrical effects in a model coupled with microstructural evolution and mechanical behavior for small-scale solder joints

The effects of stress, size, geometry, and composition on the microstructure in small-scale Sn-Pb solder joints are investigated by using a model coupled with microstructural evolution and mechanical behavior. The growth of both the Pb-rich phase and the Sn-rich phase along the boundaries of the solder joints is observed under an external shear stress and boundary constraints on diffusion. This result indicates that the microstructure in small-scaled solder joints is sensitive to stress. In addition, the heterogeneity of microstructure tends to increase with the shrinking of solder joint size. In particular, the size of the Sn-rich phase is observed to be nearly 1/5 of the standoff height in the solder joint with 1.5 μm pad size. The size effect on the microstructural heterogeneity is more pronounced in the hourglass-shaped joints than in the barrel-shaped counterparts. Furthermore, a larger volume fraction of the well-connected Sn-rich phase is found in the solder joints of a higher content of Sn. The heterogeneous microstructures exert influences on the stress distribution in the joints because the stress concentrates in the Sn-rich phase. The correlation between stress and microstructure is stronger in the hourglass geometry.