The thermal impedance of several electronic packages has been measured experimentally. If the impedance is represented in a so called Nyquist plot, it was found that the results could be fitted very well with a few circular loops. In this paper the thermal diffusion equation has been solved numerically for different electronic packages. The technique is based on the boundary element method. Hence the thermal impedance could be calculated numerically. The Nyquist plot of these results could also be fitted with a few circular loops, as has been found in the experiments.