Meaning
Changes in the relative permittivity of printed circuit board substrates across different frequencies, temperatures, or manufacturing lots disrupt the characteristic impedance of transmission lines. This phenomenon, which is called dielectric constant variation, causes signal degradation in high-speed digital and RF circuits. It establishes the limits for impedance control in aerospace and telecommunications boards.
Substrate Behavior
Commercial laminates consist of woven glass fiber reinforcements and epoxy resin matrices that possess distinct electrical properties. Because the glass has a different relative permittivity than the resin, the local permittivity varies depending on the fabric density and resin distribution. This nonuniformity alters the local phase velocity of signals traveling along different traces on the same layer.
Phase Distortion
As signal frequencies rise into the gigahertz range, non-uniform dielectric properties lead to differences in propagation delays across parallel traces. In differential signaling, this difference creates a shift in the arrival time of the positive and negative signals, which leads to phase skew and increases common-mode conversion. The converted signal then generates unwanted electromagnetic emissions that can violate regulatory standards.
Moreover, it distorts the receiver eye diagram.
Frequency Response
Permittivity decreases as the operating frequency increases because the polar molecules in the substrate cannot respond quickly to the alternating electromagnetic field. This decay is modeled during pre-layout simulations to prevent impedance mismatches.
