Prediction of Electrical Properties of Plain-Weave Fabric Composites for Printed Wiring Board Design

A mechanistic model is presented for predicting the effective dielectric constant and loss tangent of woven-fabric reinforced composites with low-loss constituents. A two-scale asymptotic homogenization scheme is used to predict the orthotropic effective properties. A three-dimensional unit-cell enclosing the characteristic periodic repeat pattern in the fabric weave is isolated and modeled mathematically. Electrostatic boundary value problems (BVP’s) are formulated in the unit-cell and are solved analytically to predict effective dielectric constant of the composite, using three-dimensional series-parallel reactance nets. Results are also verified numerically, using finite element methods. The effective dielectric constant and the effective loss tangent are then obtained, analogous to mechanical viscoelastic problems for low-loss materials. The predicted dielectric constant and loss tangent are compared with experimental results for E-glass/epoxy laminates. Frequency dependence of the effective dielectric constant and loss tangent is obtained from the corresponding behavior of the constituent materials. Trade-off studies are conducted to investigate the effect of the constituent material properties on orthotropic effective dielectric permittivity.