A flip-chip interconnect is proposed to achieve good electrical performance applicable to millimeter-wave applications. By Au-Au thermocompression technique, the transition structure provides continuity of characteristic impedance from the on-carrier CPW line to the on-chip microstrip line, as well as smooth current flow. Parameter optimization further indicates that the tapered reference ground connection is insensitive to the frequency response. Characterization of the structure is conducted by the THURREFLECT-LINE (TRL) calibration technique to test the flipchip interconnects up to 50GHz. Measurement results show that return loss is better than 15dB and insertion loss smaller than 1.7dB up to 50GHz. The broadband transition structure is suitable for high frequency application without any external matching network.