A Proposed Uniaxial Anisotropic Dielectric Measurement Technique

Abstract

The popular printed circuit board (PCB) material, FR4 is composed of a glass fiber weave embedded in epoxy. Because the epoxy and the glass fiber have different dielectric constants, the dielectric constant experienced by electric field in the substrate depends on the direction of the electric field. Error in knowledge of this anisotropy causes error in the electromagnetic (EM) analysis of any circuits being designed. An "average" dielectric constant can be used, however the correct average depends on the specific field configuration and thus on the specific circuit layout. As such, both accurate knowledge of dielectric anisotropy and the ability to perform EM analysis including anisotropy is important. This paper proposes a new dual mode resonator for measurement of dielectric anisotropy, specifically uniaxial anisotropy where there is one dielectric constant for electric field perpendicular to the substrate surface, and a second dielectric constant for all electric field tangential to the substrate surface. The technique proposes to compare EM analysis results to measurements to determine the anisotropy. In this paper, we describe and simulate the proposed approach and illustrate evaluation of the expected error.