Leakage phenomena of flip-chip structures on common GaAs and alumina main substrates are characterized using the spectral domain approach to reduce the possible chip-to-chip crosstald and transmission resonance. We have found taht the longitudinal section magnetic mode is dominant for the coplanar waveguide leakage andthe leakage can be suppreassed by properly managing the gap height and the main substrate thickness in addition to the dielectric constant. These calculation results will be helpful for designing and flip-chip packagaing of high-frequency integrated circuits.