What is Dielectric Constant (Relative Permittivity) in PCBs?

The Dielectric Constant (Dk), also known as the Relative Permittivity (Ɛr) of a laminate, is a measure of the capacitance or energy difference between a pair of conductors in the vicinity of the laminate and that pair of conductors in a vacuum. All materials have Dk values that are slightly higher than that of a vacuum, which is 1.0. In comparison to materials with lower Dk values, a laminate with higher Dk values can store more energy. 

The term "dielectric constant" denotes the ability of a dielectric material to store electrical energy. Permittivity becomes a complex quantity with real and imaginary components in AC fields. The real part (Ɛr) is a measure of the degree of polarisation, and the imaginary part (εi) is connected to dielectric losses. The value of Ɛr increases with the level of polarization and controls the speed at which the wave travels through a material. Frequency affects the dielectric constant. The Dk value decreases as frequency rises because polarisation mechanisms can no longer keep up with the rapidly shifting field. The imaginary part is always positive and stands for the energy lost or absorbed. 

tan δ = εi/Ɛr 

Following are some of the important properties of a Dielectric material:

• The majority of dielectric materials are non-metallic. As a result, these compounds have a high impedance.
• The activation energy is substantial and is greater than 3eV.
• The nucleus and electrons are tightly bound.
• Due to the absence of electrons, the conductivity is very low.
• Permeability - The allowability rating can be used to predict the polarising behavior or dielectric nature.
• The intensity of the dielectric polarisation is measured using the dielectric constant.

The dielectric properties of various components used in the fabrication and packaging of semiconductors play an important role in achieving the desired efficiency of circuit boards.

Low Dk PCB Materials

Since the EM wave is not confined to the traces and passes through the substrate when a signal propagates through a PCB substrate, the dielectric constant affects the signal integrity of the propagating wave. This can result in severe signal integrity issues. This effect will be more pronounced at higher frequencies.

The dielectric constant is inversely proportional to the speed of the propagating wave. As a result, a low Dk PCB material will offer the following benefits: 

• High-speed signals have lower transmission losses.
• Reduced Crosstalk between traces and vias with close spacing.
• Reduces the requirement for capacitive coupling in the circuit.
• Reduces the negative impact of mismatched trace lengths in parallel networks.

Despite all the benefits, low Dk PCB materials still have dispersion, which can cause digital signals to be distorted. They cost more than FR4 material as well. During the board fabrication process, they need higher pressures and temperatures, which may increase the cost. Rogers 4350B, RT Duroid 5880, and Isola-I speed are a few examples of high-speed materials.

High Dk PCB Materials 

Although we associate power integrity with constant voltage input, the dielectric constant also influences input power stability. The interplanar capacitance will increase if a high Dk material is positioned between the power and ground layers. A constant input voltage is maintained by the planes themselves, which serve as a sizable decoupling capacitor. Additionally, it reduces PDN impedance. The arrangement of conductors, dielectric substrate material, and capacitances in a PCB determines the PDN impedance. However, a higher Dk will cause more EMI problems, crosstalk, and dielectric loss. It is advised to choose a hybrid stack-up of high and low Dk PCB materials to balance power and signal integrity.

A higher dielectric substrate can be used to separate the power and ground layers, while a lower Dk substrate can be used to support the signal layer with less loss. A PCB material's relative permittivity (Dk) or dielectric constant (Ɛr) is typically between 3.5 and 5.5. A material must maintain a stable dielectric constant over a wide frequency range to be suitable for high-frequency applications.