Dielectric Constant is the relative permittivity of a dielectric material (like FR-4, Polyimide) shorten to Dk, Er or ε. Constant determines the speed at which an electrical signal will travel in a dielectric material. Signal propagation speed is expressed relative to the speed of light in a vacuum, which is roughly 3.0x1010cm/sec. The dielectric constant of a hard vacuum (space) is defined as 1.00. Higher dielectric constants will result in slower signal propagation speed. Dielectric constant decreases slightly when frequency increasing. Take ITEQ IT-180A CCL as example. DK=4.4@1GHZ, 4.3@2GHZ, 4.1@5GHZ, 4.1@10GHZ. In high frequency PCB manufacturing, like Rogers PCB, it requires low Dk materials.
