This paper presents analysis, investigation, and design of a new transition between CPW GaAs chip and Aluminum MS motherboard with single via interconnect and one common CPW-ground. Finite-Difference Time-Domain method "FDTD" is used as our computational electromagnetic technique. The FDTD Gaussian pulse responses of the package in presence and absence of interconnect characterized its electromagnetic features using the Fourier Transformation. In frequency domain, these responses give an accurate computation of the scattering parameters of the discontinuity over a wide band of frequencies. The main objective of this paper is to use these parameters to investigate and evaluate the overall package performance as function of substrate type of chip and motherboard as well as interconnect geometry. The interconnect geometry has a great impact on the overall package performance. Optimization of the package performance
can be achieved by reconfiguring the interconnect geometry. Excellent S-parameters of the package have been obtained up to 30 GHz; S11/S22 and S21/S12 are of order -20 dB and – 0.4 dB respectively.