Circuit materials are exposed to high temperatures for short periods of time during processing and for longer periods of time during use. The effects of long-term high temperatures are often seen as oxidation accumulation on PCB thermoset dielectric materials, which can cause a shift in the Dk of the circuit material. Determining how sensitive a circuit material is to the effects of long-term aging requires precise and careful testing, as different measurement methods and techniques can yield very different measurement results when characterizing high-frequency circuit materials. Selecting the right test method, whether it is material-based or circuit-based testing, provides reliable data when modeling Dk and circuits.