Structure-dependent dielectric constant in thin laminate substrates

This paper describes an experimental method for extracting the frequency-dependent dielectric constant of board material. The method is applied to thin laminate substrates used for SiP substrates at frequencies up to 10GHz. Cross-section measurements are used to determine layer thicknesses and etched linewidths in the experimental samples. The extraction method uses a combination of electromagnetic simulation and measurement to extract the dielectric constant from propagation delay measurements. Characteristics extracted from an example line are used to model a variety of other transmission line geometries with good agreement. For the example substrate used in this investigation, the dielectric constant was significantly lower than expected compared with the manufacturer's specifications. A possible reason for this is that in very thin boards, it is not always possible to achieve the same ratio of resin to reinforcement as in thicker boards with multiple layers of glass cloth. Consequently, the board composition in thin packages may not precisely match the average composition of more conventional, thicker boards. The delay-based results are compared with more conventional capacitance-based methods. The results in both cases are similar, but the capacitance method requires much more accurate determination of the layer thickness, which is difficult to achieve in thin laminates. Comparisons of odd- and even-mode propagation delays in differential transmission lines show no conclusive evidence of appreciable anisotropy in the board material studied. Modal dispersion is only weakly dependent on anisotropy, making experimental determination by this method difficult.