Millimeter-Wave Complex Permittivity of Silica/Alumina-Filled Epoxy-Molding Compounds

Composite materials made of micron-sized oxide particle fillers in an epoxy resin matrix that can be molded into desired shapes are widely used for packaging radiofrequency and microwave-integrated circuits (ICs). To potentially employ these materials with millimeter-wave ICs (MMICs), quantitative knowledge of the composites’ dielectric properties across a broad millimeter-wave band is necessary. Here, we present non-destructive measurements of the complex relative permittivity, ε r = ε ′ + jε ″, on some possible MMIC packaging composites consisting of silica and/or alumina microsphere fillers dispersed in an epoxy matrix. Measurements using phase-sensitive transmission over the WR3 and WR5 frequency bands (140 to 325 GHz) show that ε ′ ranged from 3.6 for pure silica filler to 7.2 for pure alumina filler, with very little frequency dispersion. In all materials, the loss tangent tan δ = ε ″/ ε ′ was between 0.01 and 0.02 in this frequency range. An analysis using the theory of two-component composites is used to extract the real permittivity of the epoxy resin. The results could be used to model performance of packaged MMICs and to design composites having a tailored value of ε ′.