Non-destructive Observation of Void Formation Due to Electromigration in Solder Microbump by 3D xray

Due to the shrinkage of solder microbumps diameter for higher I/Os, the failure analysis of electromigration (EM) in solder microbumps is more and more important for 3D IC packaging. The non-destructive observation method through 3D X-ray was adopted to study voids formation and necking during EM. A daisy-chain test vehicle with the number of 400 solder microbumps was under current density of
$8\times 10^{4}\ \mathrm{A}/\text{cm}^{2}$
at 150 °C. The structure of solder microbump was Cu/Sn2.3Ag/Ni/Cu. With the non-destructive observation method, the evolution of EM failures can be analyzed when the resistance change was 0%, 5%, 10%, and 20%. Therefore, the process of void formation was easily to be discovered through different view angles of computed tomography (CT). However, resolution of 3D X-ray was about a micrometer, destructive analysis of cross-section BEI was done after EM test. By comparison of CT images and BEI, the limitation of CT images could be known. Additionally, some solder microbumps were not damaged seriously due to the effect of Sn grain orientation. Through EBSD analysis, the orientation of Sn grain was measured. The results show that when the c-axis of residual solder was nearly perpendicular to electron current direction, EM damage can be retarded.