The dielectric constant, also known as the permittivity, is a key electrical parameter that measures a material’s ability to store electrical energy in an electric field; it essentially reflects the response characteristics of the material’s degree of polarisation to the electric field. In everyday engineering applications, the term ‘Dk’ refers to the relative dielectric constant, i.e. the ratio of a material’s energy storage capacity to that of a vacuum (air can be approximated as a vacuum, with a dielectric constant of approximately 1.0006).
For high frequency PCBs, the substrate serves as the medium for signal transmission; the magnitude, stability and frequency adaptability of its dielectric constant directly determine the transmission behaviour of signals on the board. In high-frequency scenarios where the frequency is ≥1 GHz, this influence is significantly amplified, becoming a critical factor that cannot be overlooked.
The core objective of high frequency PCBs is to achieve distortion-free transmission of high-speed signals, and the dielectric constant (Dk) is the primary factor in controlling this process. The propagation speed of signals within the PCB substrate follows a fixed formula: Signal propagation speed ≈ Speed of light ÷ Dielectric constant (εr). This relationship directly reveals the fundamental link between Dk and signal velocity.
The lower the Dk value, the weaker the material’s polarisation, the less energy is lost from the electric field, and the closer the signal propagation speed is to that in a vacuum; conversely, the higher the Dk value, the more electric field energy is consumed during the polarisation process, and the slower the signal propagation speed.
The stability of the dielectric constant (Dk) directly determines the precision of impedance control in high frequency PCBs, and impedance consistency is a key guarantee for high-speed signal transmission. The characteristic impedance of a transmission line is a crucial parameter determining signal reflection and transmission efficiency, and its value is closely related to the dielectric constant—the higher the Dk, the more concentrated the electric field is between the transmission line and the ground plane, resulting in a larger equivalent capacitance and a lower characteristic impedance; conversely, the lower the Dk, the higher the characteristic impedance.
More critically, even minute fluctuations in the Dk value can cause significant changes in impedance. Industry experience indicates that a 1% change in Dk corresponds to an approximate 0.5% change in characteristic impedance; such fluctuations are sufficient to cause serious signal issues in high-frequency scenarios.
High frequency signals (such as 32 Gbps and above) demand extremely high consistency in characteristic impedance, typically requiring control within tolerance ranges of 50 Ω ± 1 Ω or 100 Ω ± 2 Ω. If Dk fluctuates excessively (e.g., exceeding ±3%), it will result in impedance discontinuities, causing signal reflections and standing waves during transmission, which in turn lead to signal distortion, increased bit error rates, and may even render the equipment inoperable.
For example, in a PCIe 5.0 graphics card PCB, if the substrate Dk fluctuates from 4.6 to 4.8 (a change of 4.3%), the impedance will decrease by approximately 2.15%, falling from 50Ω to 48.9Ω. This exceeds the tolerance range, directly leading to a deterioration in signal integrity and preventing the graphics card from performing as intended. Consequently, for high frequency PCB manufacturers, maintaining the stability of the substrate’s Dk is not only key to enhancing product competitiveness but also a crucial demonstration of production capability.
The dielectric constant (Dk) also has a direct impact on signal integrity in high frequency PCBs, with distributed capacitance playing a particularly significant role. Distributed capacitance is an inherent capacitive effect between transmission lines and components, the magnitude of which is directly related to the dielectric constant: the higher the Dk, the greater the energy storage capacity of the dielectric layer, and the greater the distributed capacitance per unit length.
An increase in distributed capacitance leads to three major issues: firstly, increased delay; the greater the distributed capacitance, the longer the signal charging and discharging time, which further exacerbates signal delay and affects the synchronisation of high-speed signals; secondly, increased crosstalk; the coupling capacitance between adjacent transmission lines causes signals to interfere with one another.
The higher the Dk, the greater the coupling capacitance and the more severe the crosstalk. Particularly in high-density, high-frequency PCB designs, crosstalk is one of the primary factors affecting signal quality; Thirdly, increased power consumption. The charging and discharging processes of distributed capacitance consume additional electrical energy; in high-frequency, high-speed scenarios, this loss increases significantly, not only raising the energy consumption of equipment but also potentially shortening the service life of the PCB due to heat generation.
Different high-frequency application scenarios have distinct requirements for dielectric constant (Dk). Selecting the correct Dk value is key to ensuring high frequency PCBs meet the demands of specific scenarios, and it is also a crucial prerequisite for PCB manufacturers to provide precise solutions. Combining industry standards with practical application experience, the selection of Dk for high frequency PCBs must align with the operating frequency, signal speed and environmental conditions of the specific application, balancing performance and cost to avoid arbitrary selection.
In high-speed digital circuit applications, such as PCIe 4.0/5.0/6.0, DDR4/DDR5 and Ethernet 802.3ck, the higher the signal speed, the stricter the requirements for Dk consistency. PCIe 5.0 (32 Gbps) requires the Dk tolerance of the material to be controlled within ±2%, whilst PCIe 6.0 (64 Gbps) requires a tolerance of ≤±1.5%. Typically, materials with Dk = 3.0–4.8 are selected; for mid-to-low-end applications, conventional FR-4 modified materials with Dk = 4.2–4.8 may be used, whilst high-end applications require low-dielectric materials with Dk = 3.0–3.8 (such as FR-4 low-Dk modified boards or PPO resin boards).
In high-frequency communication applications such as 5G base stations and radar, particularly in the millimetre-wave band (28 GHz and above), not only is a low Dk value required (typically ≤3.0), but Dk must also remain stable across a wide frequency range with fluctuations of <5%. This must be combined with a low dissipation factor (Df) to minimise dielectric loss during signal transmission and prevent excessive signal attenuation.
It is worth noting that the standardisation of dielectric constant (Dk) testing directly impacts the selection and performance control of high frequency PCBs. According to the CSTM group standard T/CSTM 00907-2022, ‘Test Methods for High-Frequency Substrate Materials’, the frequency range for dielectric constant testing of high frequency substrates must be between 1 GHz and 40 GHz, with a temperature range of -50°C to 150°C. Prior to testing, samples must be pre-conditioned for 24 hours in a standard environment at 23±1°C and 50±2% relative humidity to eliminate the influence of moisture and stress on the test results.
Currently, there are three commonly used testing methods in the industry: the resonant cavity method (laboratory-grade precision measurement, suitable for the R&D stage), the transmission line method (rapid testing commonly used in factories, suitable for mass production inspection), and the parallel-plate capacitance method (low-cost rough assessment, suitable for initial comparisons). The accuracy and suitability of these methods vary; PCB manufacturers must select the appropriate testing method based on their specific requirements to ensure the accuracy of the Dk parameter.
The importance of the dielectric constant (Dk) for high frequency PCBs extends throughout the entire process of design, material selection, production and application. It not only determines signal transmission speed, impedance control accuracy and signal integrity, but also determines whether high frequency PCBs can meet the demands of different scenarios and achieve steady development amidst fierce market competition. With the continuous evolution of high-frequency and high-speed technologies, the requirements for controlling the dielectric constant (Dk) will become increasingly stringent, and prioritising and controlling Dk will become the inevitable path for the PCB manufacturing industry to achieve high-quality development.
