The results of stress tests on 50 μm-pitch Pb-free microbump interconnect 3D Si structures are summarized. It was found that thermal processes alone compete strongly with electromigration at stress currents of 50 and 100 mA. Typical test sites exhibited a gradual increase in relative resistance increase over time. While large changes in bump interconnection resistance did occur for some sites, the interconnections were resistant to full open electrical failure, even after subjected to mechanical shock following long-term EM stress. Underfilled samples were more robust than non-underfilled samples. Overall, the results suggest that for application in 3D Si stacks, microbumps can support higher power densities than those used currently with standard pitch bump interconnections. © 2012 IEEE.