Each dielectric material has its own relative dielectric constant (DK) and dissipation factor/loss tangent (DF) that are contributing factors to line impedance (Z0), signal propagation delay (TPD), and signal loss (a). As DK increases, impedance decreases, while the signal propagation delay and signal loss increase, and vice-versa. A typical DK found on PCBs range from 3.4 to 4.6. The propagation delay in a given dielectric material is constant and is not affected by any other board parameters such as layer height, conductor width, or conductor spacing. Propagation delay is affected by frequency, but the effect is minimal with regards to typical memory speeds. Signal loss is also affected by frequency, with the loss increasing with increasing frequency.
This Equation shows the calculation for propagation delay (TPD), with DK as the dielectric constant, and c as the speed of light in free space (2.998 x 108 m/s or 1.180 x 1010 in/s).
The associations in Table: Relationship of DK to Impedance, Propagation Delay. and Signal Loss show the effect of the dielectric constant (DK) on the impedance (Z0), propagation delay (TPD), and signal loss (a).