The thermal impedance Z_th(j*omega) has been calculated numerically using the boundary element method for a structure consisting of a silicon chip glued on a ceramic substrate. The interface between the chip and the substrate is modelled by an interface contact resistance r_c (in Km^2/W) so that the non perfect thermal contact can be taken into account. If the thermal impedance Z_th(jomega) is represented as a Nyquist plot (Im[Z_th] versus Re[Z_th] with omega as parameter), mainly two circular arcs are observed. The arc corresponding to the higher frequencies is found to be almost independent from r_c. The low frequency part on the other hand is largely influenced by r_c. In DC conditions (omega=0) our results correspond to those published in the literature: the thermal resistance of the structure increases linearly with r_c. From our simulations it has been found that similar conclusions can be drawn for the real and imaginary part of Z_th at a fixed frequency. In experimental measurements this offers the advantage that one does not need to wait till the entire structure is warmed up and has reached thermal steady state conditions.