Wafer level metal bonding involving copper material is widely used to achieve 3D functional integration of ICs and ensure effective packaging sealing for various applications. In this paper we focus on thermocompression bonding technology where temperature and pressure are used in parallel to assist the bonding process. More specifically a broad range of conditions was explored and interesting results were observed and are reported. Indeed, despite a relatively high roughness, the presence of a native oxide and the lack of surface preparation, there still exists a process window where wafer level bonding is allowed. In these conditions, limiting the bonding mechanisms to basic copper diffusion is no longer satisfactory. In this study, a specific scenario inspired by both wafer bonding and metal welding state of the art is put forward. Accordingly, pure copper diffusion through the bonding interface is lined with plastic deformation and metallic oxide fracture. In addition, polycrystalline film deformation due to thermomechanical stress is highlighted and grain growth and voiding formation are observed and confirmed.