The annealing behavior of electroplated copper deposits is reported in this study. The copper deposits were electroplated with a current density of 0.7 A/cm 2 at 53 โข C in a sulfuric acid bath containing various concentrations of thiourea. The hardness values of the copper deposits were measured before and after annealing, and the differential scanning calorimetry (DSC) diagrams of the as-electroplated copper deposits were recorded. An improvement of the softening resistance of the copper deposits was observed when the bath contained thiourea โฅ3 mg/L. By adding thiourea in the plating bath, smaller grain size of the copper deposits can be achieved. As thiourea content increased โฅ3 mg/L, the twin boundary of the copper deposits was significantly increased, and many sulfur-rich particles were deposited along the grain boundaries and a few within the grains of the deposit. These sulfur-rich particles are capable of impeding migration of the grain boundaries and improving the softening resistance of the copper deposits during annealing. The aforementioned microstructures of the copper deposits were examined with transmission electron microscope (TEM) integrated with energy-dispersive X-ray spectrometer (EDX) for chemical composition analysis.