A flat cable (100) includes an insulative carrier (20) extending along a front-to-back direction, a set of signal conductors (10) held by the insulative carrier, and a metal grid layer (30) attached to the insulative carrier. The insulative carrier has a top face facing upwardly and a bottom face facing downwardly. The insulative carrier defines a set of receiving passageways (210) disposed along a transverse direction perpendicular to the front-to-back direction. The set of signal conductors extend along the front-to-back direction and have different pitches along the transverse direction. The metal grid layer is attached to the top face or the bottom face. The metal grid layer has different densities along the front-to-back direction in order to make the impedance of the flat cable consistent along the front-to-back direction.