Importance and Limitations of Modeling Parasitic Capacitance between Package and PCB for Power Bus Noise and Radiation

Power and ground planes on multilayer PCBs can effectively radiate electromagnetic fields excited by the IC simultaneous switching noise. The high frequency electromagnetic radiation is often calculated from the electric field along the edge of the PCB, which can be estimated with a cavity model using magnetic walls. The excitation of the cavity modes is related to the via current passing through the power bus planes at the interconnection between IC package and PCB. Usually the attention is focused on the differential-mode current of the package pins, but in the present paper it is shown that the common-mode current flowing out from package pins plays a very important role in the excitation of cavity modes, and its neglect implies a fatal underestimation of the electromagnetic radiation from the power bus planes in some circumstances. A second important contribute to the radiation is given by the common mode current on the pins, together with the current flowing on the PCB ground plane. With the proposed equivalent circuit, the effectiveness of decoupling inductors depending on their location and on the value of the parasitic capacitance is studied. key words: common mode, parasitic capacitance, package, simultaneous switching noise