A transmission line for high-frequency differential signals and having a transforming impedance is formed into a substrate. The transmission line is comprised of a slot, the opposing surfaces of which carry a conductive surface capable of carrying electrical signals. The conductive surface on the opposing surfaces is gradually receded along a length of the slot. An equivalent amount of metallization is applied on the substrate's surface and electrically continuous with conductive surfaces on the slot's opposing sidewalls. The metallization on the substrate's surface provide solder lands. Dielectric in the slot prevents solder wicking.