Effect of electric field and Sn grain orientation on Cu consumption in Sn/Cu solder joint

Excessive consumption of Cu metal pads during electromigration is one of the most common failure modes for the flip-chip Sn/Cu solder joint, which is found to be closely related to Sn grain orientation and current density. In this paper, Cu consumption mechanism was investigated by employing the density functional theory, considering the effects of electric field and Sn grain orientation. The adsorption energy of Cu atom on Sn (1 0 0) and (0 0 1) surface, penetration energy from the surface to body, diffusion energy in the bulk and electronic structures were analyzed. It is found that Cu atoms were energetically favorable at the hollow sites of Sn (1 0 0) and (0 0 1) surfaces. Moreover, the binding could be enhanced by increasing electrical field intensity (F). The Cu diffusion activation energies along a-axis and c-axis of the Sn grain were 0.83 eV and 0.17 eV, respectively. While they decreased with the increase of F. Electronic structure analysis show that the charge transfer and electron redistribution of Sn grains under the F mainly contributed to strengthened binding and decreased activation energy of diffusion.