Material Constant
Dimensionless ratio of the electric field capacitance of a substrate material to that of a vacuum determines the capacitive load and signal speed of conductors embedded within the dielectric. Known as relative static permittivity, this value dictates how much electrostatic energy the laminate can store. It directly governs the characteristic impedance of printed circuit board traces and is a fundamental parameter in signal integrity simulations.
Impedance Influence
Choosing a dielectric with a lower relative static permittivity allows designers to use wider traces for a given impedance value, which helps to minimize conductor resistive losses. The propagation speed of signals on a circuit board trace is inversely proportional to the square root of this value. Consequently, materials with a high dielectric constant slow down the propagation of electrical signals and increase the parasitics of nearby traces.
In multi-layer board stackups, ensuring a uniform relative static permittivity across all dielectric layers prevents unpredictable impedance variations along the trace path. This uniformity is necessary for preserving the signal rise times of high-frequency digital channels.
Frequency Limitation
High-frequency signals reveal that the dielectric constant is not a static property but rather decreases as the signal frequency rises. Accurate high-speed designs therefore replace the relative static permittivity with a frequency-dependent value to capture the true phase velocity of the system.
