In this paper, we present the outcomes of a specific software designed for the ESTEC project No. AO-5086: New investigations of RF breakdown in microwave transmission Lines. The objective of the project is to develop a software tool capable to predict the multipactor and corona breakdown onsets in shielded microstrip devices, paying special attention to elements like connectors, bridges or ribbons. In the first step, the electromagnetic fields of the devices are computed using an efficient implementation of a volume/surface integral equation technique. Secondly, the high power breakdown onsets are determined, employing accurate and realistic models for the electron trajectories and the secondary electron emissions. Also, the corona discharge breakdown level is computed by means of the numerical solution of the free electron density continuity equation.