60-GHz Substrate Materials Characterization Using the Covered Transmission-Line Method

At millimeter-wave (mm-wave) frequencies, antennas and systems are highly influenced by the electromagnetic characteristics of packaging materials. Accurate knowledge of the complex permittivity of packaging materials is vital for the optimal and robust design of such systems. In this paper, the covered transmission-line method is used to determine the complex permittivity at 60 GHz for several common FR-4 and FR-5 packaging materials. Measurements show that packaging materials must be accurately characterized at high frequencies because their properties can appreciably differ from those at lower frequencies. Moreover, depending on the fabrication process, different thickness samples of the same material can have different permittivity values. The complex permittivity variation versus temperature is also analyzed. We show that, at low frequencies, this variation is negligible, but at mm-wave frequencies, it can represent up to a 5% variation in the relative permittivity and have a severe impact on the loss tangent. The covered transmission-line method is explained in detail and its accuracy and measurable permittivity range are also discussed.