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I. Lau, A. Hajian, F. Michler, G. Gold, F. Lurz, U. Schmid, K. Helmreich, R. Weigel, A Koelpin:
"Influence of the PCB Manufacturing Process on the Measurement Error of Planar Relative Permittivity Sensors Up To 100 GHz";
IEEE Transactions on Microwave Theory and Techniques, 67 (2019), 7; 2793 - 2804.

Accurate and precise knowledge of the relative
permittivity of printed circuit board (PCB) materials is essential
for the reliable design of high-frequency circuits. For simplicity
reasons, planar, resonant permittivity sensors, which are directly
integrated on the unknown PCB material, are widely used.
However, the sensors are affected by the nonidealities of the
copper-clad laminate and PCB manufacturing process, e.g.,
the difference in roughness between the top and bottom sides
of each metal layer. This paper analyzes the influence of these
nonidealities on the extracted relative permittivity values of
different sensor geometries in microstrip and substrate integrated
waveguide (SIW) technology up to 100 GHz. Microstrip
resonators are very sensitive against the investigated nonidealities.
Additional roughness measurements and more detailed
simulation models cannot noticeably reduce the uncertainties.
SIW cavity sensors are more robust, and simple modeling
approaches lead to low uncertainties smaller than 0.05 for the
whole frequency range from 10 to 100 GHz.

http://dx.doi.org/10.1109/TMTT.2019.2910114