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

Abstract

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.

Degradation of signal integrity due to package-common-mode resonance caused by external conductive noise in power supply system

“…For example, a BGA package and a PCB have a few pico-farads of capacitance and the package-common-mode resonance can occur in the range of GHz because the parasitic inductance at the ground connection is a few nano-henries. In our previous study, the authors showed that the package-common-mode resonance causes large radiated emission from a PCB and a package [7]. In this paper, we focus on the impact of the package-common-mode resonance on the immunity of an LSI.…”

Experimental verification of signal integrity deterioration due to package-common-mode resonance

“…1). This capacitance can resonate with a parasitic inductance [4], [5], and causes potential bounce on the planes [6]. In a case both planes are for ground, the antiresonance causes the potential fluctuation in phase on the power and ground lines; we call it "package-common-mode resonance".…”