Abstract: An electronic device may be provided with a flexible printed circuit and a rigid printed circuit mounted to the flexible printed circuit using a board-to-board (B2B) connector. The flexible printed circuit may include signal conductors coupled to one or more antennas on the rigid printed circuit through the B2B connector. A given one of the signal conductors may include a phase shifter segment on the flexible printed circuit and/or a thick impedance matching segment on the rigid printed circuit that help to form a smooth impedance transition from the flexible printed circuit to the rigid printed circuit and the antenna(s). The B2B connector may include signal contacts interleaved with a ground contacts. The B2B connector may include ground bars laterally surrounding the signal and ground contacts to maximize the strength of mechanical coupling between the flexible printed circuit and the rigid printed circuit.