Determination of microwave material properties at cryogenic temperatures.

This article focuses on a method for accurately measuring the microwave conductivity and complex permittivity of printed circuit board (PCB) laminates over a wide temperature range (4 to 300 K), which is critical for cryogenic microwave electronics in quantum information processing systems. Utilizing a pair of balanced-type circular disk resonators (BCDRs) and a cryogenic calibration technique, the method enables broadband measurements from below 5 GHz to above 20 GHz and separates dielectric and conductor losses. Experimental results on two commercial PCB laminates show that dielectric loss decreases monotonically at cryogenic temperatures, while conductor loss is limited by the copper foil’s surface roughness; a Gradient Model effectively fits the frequency and temperature dependence of conductivity, allowing quantitative evaluation of interface roughness. These findings support the development of high-density, low-loss cryogenic circuits essential for quantum computing and related technologies.