Flex circuits are suitable for very short runs, but it is difficult to shield them. They can be used as interconnects between boards within a system. If flex circuits are used, a system design engineer should closely couple the members of differential pairs where (S < W). Do not run signal pairs near the edges of the cable, as these are not balanced. Use a ground plane to establish the impedance. Use ground shield traces between the pairs if there is room. Connect these ground traces to the ground plane through vias at frequent intervals.