A method and structure for preventing integrated circuit failure due to electromigration and time dependent dielectric breakdown is disclosed. A randomly patterned metal cap layer is selectively formed on the metal interconnect lines (typically copper (Cu)) with an interspace distance between metal cap segments that is less than the critical length (for short-length effects). Since the diffusivity is lower for the Cu/metal cap interface than for the Cu/dielectric cap interface, the region with a metal cap serves as a diffusion barrier.