The Cu redistribution line (RDL) in advanced fan-out (FO) packages is approaching 1-2 μm or even a submicron-scale feature size for achieving high-density (input/output (I/O) number > 1000) packaging requirements. The downsizing trend of the Cu RDL gives rise to an increasing current density which will be greater than 105 A/cm2 in a 2μm/2μm line/space (L/S) trace. The electromigration reliability concerns caused by the electrical-thermal coupling effects may be raised accordingly. The present study reported the electromigration failure behavior and mechanism of a fine-pitch 2μm/2μm L/S Cu RDL, 20 μm in length and 3 μm in thickness, embedded in polyimide (PI) dielectric layer for advanced high-density FO packaging. The Cu RDL was stressed with a direct current with 53 mA (8.8 × 105 A/cm2) at 180°C in accordance with the JEDEC standard. After the thermal annealing pre-treatment at 230°C for 6 h, a porous Cu oxide layer accompanying a few nanovoids was formed on top of the Cu RDL. The electromigration test was found to accelerate the oxidation behavior and voiding issue with increasing current stressing time. A bilayer Cu oxide structure was formed upon electromigration, while the nanovoids formed at the oxide/RDL interface increased in number and enlarged in dimension. Large voids formed underneath the Cu RDL, at the Cu RDL/Ti adhesion layer interface, were also observed. These phenomena were responsible for the steady resistance increase under the electromigration test. Once the Cu RDL has been occupied with voids formed upon electromigration, the rapid reduction in the cross-sectional area will give rise to current crowding and thus induce an increasing RDL temperature. The abruptly rising temperature caused the local melting of the Cu RDL and disruption of the neighboring multilayers. The open circuit failure that occurred in the middle region of the Cu RDL, whereas not in the cathode region, suggests a prominent thermal effect induced failure mechanism under the electromigration test.