Thermal impedance is a combination of thermal resistance and thermal contact resistance. This is often measured using a TIM Tester (ASTM D5470), pictured above. With this data, apparent thermal conductivity can be calculated; however, while thermal impedance can be a useful property, apparent thermal conductivity can be a misleading value. TIM Testers require very specific pressure requirements, temperature differentials, and sample thickness measurement, which makes it difficult to achieve data at representative conditions. Furthermore, as a TIM Tester is a steady state method, it reports apparent thermal conductivity for the sample on average, meaning it is unable to detect uneven properties. This is especially important for heterogeneous TIMs, as particle settling can cause noticeable differences in thermal conductivity from the top and bottom of the sample. These uneven properties can cause performance and safety issues if they are not detected. Learn more about thermal impedance here.