High frequency signal transmission characteristics of cone bump interconnections

We evaluated high frequency signal transmission characteristics of coplanar waveguides fabricated using Au cone bump interconnections. Electroless Ni / Au bumps were used as counter-electrodes against the Au cone bumps. The signal transmission loss increased with the number of bump interconnections. At 40 GHz, the signal insertion loss due to the bump interconnections was 0.017 dB / bump with the 10 μm square size Ni / Au bumps. This value was small enough for application to radio-frequency devices. While, with the 5 μm square size Ni / Au bumps, open failure was observed between the bump interconnections. The open failure might be caused by decreasing of the contact pressure between the bumps, due to the mushroom-like shape of the Ni / Au bumps.