Redistribution layer (RDL) composing of polyimide (PI) dielectric layer and electro-chemical deposited (ECD) Cu trace is a critical part for wafer level packaging (WLP). One concern of this multi-layered film structure is the wafer warpage induced during the process, which poses threats to automatic handling, 3-D integration and device reliability. In this paper, the warpage origination during the WLP process was identified and analyzed by experiments and simulations. The wafer warpage evolution during the WLP process was measured by a Multi-beam Optical Sensor system. We found that the cure shrinkage of PI has little effect on the warpage, however, it is mainly caused by the coefficient of thermal expansion (CTE) mismatch between the deposited materials. The ECD Cu trace in RDL accounted for a substantial proportion to the total wafer warpage and lead to a hysteresis response during the thermal processes indicating plastic deformation has taken place. For in-depth understanding, the plastic behavior of ECD Cu film was investigated and the kinematic hardening plastic model was established. Finally, the stresses distribution in RDL structure was simulated by numerical method and the influence of ECD Cu trace pattern on the wafer warpage was evaluated.