The micro-mechanism for the effect of Sn grain orientation on substrate consumption in Sn solder joints

Abstract

The reliability of flip-chip M/Sn solder joint strongly depends on Sn grain orientation, since it affects the dissolution of metal pads and the growth of intermetallic compounds. In this paper, the micro-mechanism of the effect of Sn grain orientation on the diffusion of M atom (=Cu, Ni, Ag, Au or In) in Sn solder joints was investigated by employing the density functional theory. The adsorption energy of M atom on Sn(100) and (001) surfaces, the energy of M atom penetrating from the Sn surface to Sn body (penetration energy, Epe) and the energy of M atom diffusing in the bulk Sn (diffusion energy) were analyzed, as well as the corresponding electronic structures for different steps. It is found that the M atoms were energetically favorable at the hollow sites of Sn(100) and (001) surfaces. Moreover, the adsorption energy of M/Sn(100) and M/Sn(001) were obtained. The M atom was more strongly adsorbed to Sn(001) than to Sn(100). The Epe for each M atom along a-axis of Sn grain was greater than the corresponding value that along c-axis. Moreover, the Epe increased gradually with the M atom changing from Ni to In. Electronic structure analysis shows that the Epe change can be attributed to three factors, including different atomic radius from Ni to In, difference in interatomic repulsive resistance between a and c-axis, and different hybrid orbitals between M 3d and Sn 2p from Ni to In. At last, the results show that M atoms could migrate easily in bulk Sn.