We studied electromigration (EM) lifetimes and void growth at low cumulative failure probability. We carried out EM test in damascene Cu lines using sudden-death type test structures. Its cumulative failure probability ranges from 0.005 to 90%. To investigate the void growth behaviour, Cu microstructures was investigated. EM lifetime shows correlation with the void nucleation site and the void volume. In addition, the nucleation site is affected by the characteristics of grain boundary near vias. .