Impacts of Anisotropic Permittivity on PCB Trace and Via Modeling

The dielectric material properties in multilayered PCBs must be modeled correctly to simulate highspeed signals to produce eye diagrams and ultimately calculate accurate bit-error-rates. Typically, the dielectric properties are modeled as isotropic quantities where the properties are the same in any physical direction. In practice, the materials act differently in different directions and this is known as anisotropic behavior. The anisotropy can come from the dielectric material itself or from the manufacturing process of creating the layered structure of a PCB with a dielectric composed of fiberglass fabric and epoxy resin leading to a difference of the permittivity between the horizontal directions and the vertical direction. PCB traces and vias are analyzed using full-wave simulation in the paper to quantify the impacts of anisotropic dielectrics on the signal performance. It is observed that anisotropy has large influence on loss, impedance, resonances and can increase the far-end cross-talk dramatically.