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The effects of floating conductors in three-dimensional (3-D) packaging structures are investigated. The simulations are based on a frequency-dependent integral equation formulation for the calculation of the magnetoquasistatic current distribution in complex interconnect structures. The calculated current distributions are used to develop an inductance/resistance equivalent circuit representation of the package that can be used as a subcircuit in SPICE for simultaneous switching noise calculations. The model is frequency dependent and captures the effect that multiple solid/meshed ground/power planes, pins, vias and traces have on overall package inductive performance. The impact of floating planes, such as a heat spreader under a ball grid array (BGA) structure, on the mutual inductances of the structure is demonstrated. The frequency dependence of the floating plane effects is examined also. The behavior of the induced eddy currents in the floating plane is investigated also, particularly in the vicinity of interconnect discontinuities. Switching noise results obtained using SPICE simulation and the generated inductance/resistance equivalent package models show the effects of floating planes on switching noise.