A material’s thermal impedance is the sum of all contact resistance and the material’s inherent thermal resistance. Thermal impedance depends upon the material size, shape and orientation. In essence, one cannot compare thermal impedance across materials without knowing the part’s thickness and testing conditions. To illustrate an aspect of this, the graph shown here is of the Phase Change Material T557 where thermal impedance is measured as pressure increases. As the pressure increases, the T557 overcomes the contact resistance as the material better fills in the air voids from surface irregularities and/or surface warp. Therefore, with increased pressure, the material’s thermal impedance decreases, making for improved heat transfer. Thermal impedance values will change with test conditions. It is important to understanding the test conditions when comparing one material to another.