Leaky wave excitation on three-dimensional, via-fed single and coupled microstrip interconnects is studied. Closed-form asymptotic expressions for the fields associated with the interconnect are derived and are applied in the traveling/standing wave and leaky wave regimes, both of which lead to radiation. The leaky wave beam angle is found to correspond to the usual two-dimensional ray optics leakage angle for long interconnects, as expected, and depends on interconnect length, spacing, and excitation for shorter interconnects. Comparisons with full-wave results are shown for the case of via-fed coupled interconnects.