Abstract The thermal impedance Z th (j ω ) has been calculated numerically for a silicon chip glued on a ceramic substrate. The non perfect thermal contact is taken into account by modelling the chip-substrate interface as a thermal contact resistance r c . If Z th is represented as a Nyquist plot, mainly two circular arcs are observed. The high frequency arc is found to be almost independent from r c , whereas the low frequency part is largely influenced by r c . The thermal resistance R th = Z th (j ω = 0) increases linearly with r c , as known from the literature. Additionally, our simulations have shown that similar conclusions can be drawn for the real and imaginary part of Z th at a fixed frequency ω ≠ 0.