Z-Axis Permittivity

Impedance Modeling

Designers use this value in their calculations to set the width and spacing of the copper traces to achieve the target impedance. The z-axis permittivity is typically higher than the values measured in the horizontal directions because of the orientation of the glass fibers. Accurate modeling of the vertical capacitance is necessary for any board that carries high-speed signals or sensitive radio frequency circuits.

Anisotropic Nature

Recognizing that the dielectric constant is not the same in all directions is a fundamental part of modern high-frequency design. Z-axis permittivity provides the baseline for most signal integrity simulations, but the other directions must also be considered for a complete analysis. The difference between the vertical and horizontal permittivity is a result of the manufacturing process used to create the laminate sheets.

Knowing this value allows for more precise control of signal timing and reduces the number of design iterations needed for a new product.