As electromigration failure continues to be one of the most severe reliability issues in Cu interconnects, understanding the mechanism of electromigration-induced void evolution in dual in-laid Cu interconnect structures is imperative to make process and design changes for achieving stringent reliability requirements in next technology generation interconnects. Electromigration-induced void evolution mechanisms in dual in-laid Cu interconnect structures having different geometries such as upper/lower layer and interconnect tree were studied by in-situ SEM and Monte Carlo simulation. In-situ SEM studies on various dual in-laid interconnect structures showed void migration and agglomeration along the Cu/cap interface in case of Cu/SiN x capped structures. Although the principle mechanism of void migration along the Cu/cap interface was the same in all structures, the subsequent void agglomeration location and void shape evolution at the cathode region was dependent on the structural differences with respect to Cu/cap interface for several geometries. A phenomenological model assisted by Monte Carlo simulations, which considers redistribution of heterogeneously nucleated voids and/or pre-existing vacancy clusters at the Cu/dielectric cap interface during electromigration was proposed to explain qualitatively the electromigration-induced void evolution observed during in-situ SEM as well as in various other reported studies. The mechanism of electromigration-induced void evolution in several dual in-laid Cu interconnect structures as well as their behavior during conventional package-level electromigration tests can be clearly discerned based on in-situ SEM investigations and the proposed model.