Printed Circuit Board Material Characterization at Millikelvin Temperatures for Cryogenic Microwave Circuit Design

Cryogenic microstrip designs are required for integration in quantum computers. However, precise substrate characterization is necessary for more complex microwave circuit design but has not been throughly performed in the millikelvin (mK) range. This work presents a cryogenic calibration-free method for the characterization of printed circuit board (PCB) materials for microwave circuit design in cryogenic environments. The relative permittivity and the attenuation coefficient were determined using microstrip lines (MSLs) and ring resonators down to 40 mK. It is shown that at 40 mK the relative permittivity of the common and widely available Rogers RO4350B RF substrate only changes by 1 % and that the attenuation coefficient at 40 mK is reduced by 42 % compared to room temperature, proving its suitability for microwave PCB design for mK temperatures.