Low-thermal-budget (180 degrees C for 15 sec) Cu pillar to Cu pillar bonding with Pd passivation under the atmosphere is developed without any planarization pretreatment before the bonding process. The bonded structure is investigated with material analysis, electrical measurement and reliability test. The results show that although the Cu pillar has a high roughness surface due to the electroplating process with a high deposition rate, Cu atoms can still diffuse and connect through the passivation layer and perform the low-thermal-budget Cu pillar direct bonding. Low specific contact resistance and stable resistance of daisy chain shown in the reliability test reveal the excellent bonding quality and integrity. This Cu-Cu bonding method is therefore favorable for chip stacking technology development in 3D packaging domain.