Crystalline Orientation Effect of Single-Grain Sn-Rich Cu Pillar Bumps on Electromigration Lifetime

Electromigration (EM) is the critical issue for the micro-bumps in the advanced chip stacking technology. The diffusion of solute atoms, such as Cu and Ni, driven by high current density in the Sn-based solder bumps is the underlying process for the EM reliability issue, which is highly related to the crystalline orientation of the Sn matrix. Thus, if the single-Sn-grain structure can be produced in the Sn-based solder bump, the EM lifetime can be studied and correlated to their specific crystalline orientation (c-axis). In this present work, intriguingly, we found that a single-Sn-grain structure formed in the Sn1.5Ag0.1Cu solder matrix of Cu/Ni/Sn1.5Ag0.1Cu/electroless-Ni/electroless-Pd/immersion-Au bumps. As the c-axis of the single-grain solder bumps is aligning more vertically to the current-stressing direction, the EM lifetime of solder bumps would be prolonged. In contrast, the electroless-Ni layer under the solder bump with a c-axis more parallel to the current-stressing would be consumed more and form a porous Ni3P phase (with higher resistivity), which causes early EM failure. According to the present EM results, we found that the EM lifetime of the single-grain solder bumps correlates to their specific crystalline orientation (c-axis) under current stressing with
$4\times 10^{{4}}$
A/cm2 at
$140~^{\circ }$
C. Hence, we can conclude that the anisotropic property of single-grain solder matrix in diffusivity plays a key role in EM reliability and lifetime.