  Ad-hoc interface PCBs are today the standard connection between cryogenic
cabling and quantum chips. Besides low-loss and
low-temperature-dependent-dielectric-permittivity materials, FR4 provides a
low-cost solution for fabrication of cryogenic PCBs. Here, we report on an
effective way to evaluate the dielectric performance of a FR4 laminate used as
substrate for cryogenic microwave PCBs. We designed a coplanar waveguide
{\lambda}/2 open-circuit series resonator and we fabricated the PCB using a
low-cost manufacturing process. Such a geometry allows to exploit the resonance
peak of the resonator to measure the variation of the complex dielectric
permittivity as a function of the temperature. Resonance peak frequency and
magnitude were used as sensing parameters for the real part of dielectric
permittivity and dielectric loss tangent, respectively. We estimated a 9 %
reduction of the real part of the dielectric permittivity and a 70 % reduction
of the dielectric loss tangent in the temperature range from 300 to 4 K. The
proposed approach can be immediately extended to the detection of cryogenic
temperature-dependent dielectric performance of any kind on substrate.
