Dependence of electromigration damage on Sn grain orientation in Sn-Ag-Cu solder joints

The relationship between the "polarity effect" of electromigration on interfacial intermetallic growth and the Sn grain orientation was investigated in Sn-Ag-Cu solder joints. The growth of the interfacial intermetallic compound was found to depend strongly on the Sn grain orientation. When the c-axes of the Sn grains were closely aligned to the current flow direction, a severe polarity effect was observed in which the anode interface was covered with a thick layer of Cu-Sn intermetallic while the intermetallic layer at the cathode interface was almost depleted. If the c-axes of the Sn grains were rotated away from the current direction, the polarity effect became less pronounced. When the c-axis was perpendicular to the current direction, the polarity effect essentially disappeared. The difference in the intermetallic layer thickness between the anode and the cathode, Delta d, followed a parabolic function of the cosine of the angle, a, between the c-axis and the current direction. A kinetic model based on the highly anisotropic diffusion behavior of Cu in Sn was presented to explain the relationship between the electromigration polarity effect and Sn-grain orientation. The model showed good agreement with the experimental data. (C) 2013 AIP Publishing LLC.