PCB Stackup Design Considerations for Intel® FPGAs

ID 683883
Date 6/28/2017
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1.2.1.1. Relative Dielectric Constant

The relative dielectric constant (εr) is a measure of a material’s ability to be polarized by an electric field and store electrostatic energy, as well as its ability to facilitate signal propagation. The relative dielectric constant varies with frequency. Material datasheets often refer to this parameter as Dk. Low εr or Dk materials make excellent insulators and are good for isolating signals on adjacent layers.

From a board designer’s perspective, the benefits of a lower dielectric constant include the following:
  • Better insulation
  • Faster signal propagation through the material
  • Higher trace impedance for a given trace geometry
  • Smaller stray trace capacitance

PCB manufacturers typically characterize εr at 1 MHz. However, the dielectric constant of a material is generally a function of its frequency. As frequency increases, εr is reduced. Because of this non-uniformity, a transmission line’s impedance can vary from its calculated value at 1 MHz, causing it to differ from what a propagating signal actually sees on the channel. As board designer, you should get the Dk value at high frequency range for using the high data rates signal on the board.

Because digital signals are comprised of many frequencies (harmonics), the resulting difference in impedance that is presented to the different harmonics can cause signal loss from both reflections and phase distortion (phase jitter) arising from the different harmonics arriving at the destination at different times.

Always choose lower εr material with a flat frequency response whenever possible for best signal performance and to minimize signal distortion and phase jitter.