Mixing Rule
Mathematical mixing rules for composite materials calculate the effective dielectric constant of a substrate based on the proportions of its constituents. The lichtenecker equation uses a logarithmic approach to predict how resin and glass fibers combine to determine the final electrical properties of a laminate. It allows manufacturers to estimate the permittivity of a board without building a sample for every possible glass to resin ratio.
Calculation Accuracy
The formula sums the products of the volume fraction of each material and the logarithm of its dielectric constant. Using the lichtenecker equation provides a more accurate result for PCB materials than a simple linear average because it accounts for the complex interaction of the fields at the material boundaries. This model works best when the materials are mixed uniformly and the particles are small compared to the wavelength of the signals.
Material Design
Designers rely on these calculations to create stack-ups that meet specific impedance targets while using standard prepreg and core materials. Adjusting the resin content allows the fabricator to fine-tune the dielectric constant to match the needs of high-speed digital or radio frequency circuits. The equation is a standard tool in the development of low-loss laminates where even small changes in the glass weave can impact performance.
Knowing the effective permittivity from the start reduces the number of trial builds needed to achieve the final design goals.
