RF modeling and design of flip-chip configurations of microwave devices on PCBs

In this paper, a new application of coplanar strip transmission lines in microwave flip-chip configurations, implemented with solder joints, is presented. Preliminary HFSS simulation results give reflection loss, -20xlog(S11), higher than 20 dB up to 40 GHz for flip-chip configurations of microwave devices on PCBs using coplanar strip lines, here S11 is the reflection coefficient S-parameter. The attenuation coefficient of different types of transmission lines with 50-ohm characteristic impedance, such as microstrip lines, coplanar waveguides, and coplanar strip lines on the GaAs chip and on the polymer-based board, were calculated up to 46 GHz and compared. For a complete flip-chip assembly of GaAs chip on a PCB, the total insertion loss is lower, 2.0 dB for frequencies up to 40 GHz. We have learned that, to improve performance further, the solder joints have to be simulated and designed as an integral part of the transmission line. To achieve high transmission over the solder joints, both impedance matching and geometrical matching are necessary. Geometrical matching means that the electrical field distribution of the microwave in the transmission line and that in the solder joints are similar. The geometrical matching requirement is not well recognized and thus is often ignored.