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We report electromigration induced failure on lead-free micro bumps in three-dimensional integrated circuits samples with chip on chip configuration. Compared to flip chip solder joints, micro bumps of chip-on-chip samples exhibit better electromigration resistance and are able to withstand a higher current density. No exhibited electromigration-induced failure was observed when current density was below 2 × 104 A/cm2. A threshold current density to trigger electromigration in chip-on-chip samples was found to be 3.43 × 104 A/cm2. When current density was higher than 7.5 × 104 A/cm2 at an ambient temperature of 150 °C, no void propagation through whole bump opening was found; instead, electromigration induced voids were observed at the cathode side of Al trace. [ABSTRACT FROM AUTHOR]