Digging Deeper into Dielectric Constant for PCB Materials

The dielectric constant is the first number most engineers consider when sorting through different printed-circuit-board (PCB) materials. It is the number that guides the search, when needing a circuit-board material with a high dielectric constant or when a design requires a PCB material with a low dielectric constant. But what does the dielectric constant really mean? And how can it affect a design if it is not “the right” dielectric constant? What happens to the circuit if it is designed and fabricated on a PCB material with a high dielectric constant and it should have been one with a low dielectric constant?

Many engineers are taught that a circuit material’s dielectric constant, or relative permittivity or Dk as it is also known, is a fixed value for a given material. In truth, the Dk values listed by materials manufacturers on their data sheets are numbers from a specific test method at a specific frequency, and the value will change under different conditions. For this reason, ROGERS Corp. lists Dk values for their materials in reference to specific test methods, but will also provide “Design Dk” values for the materials that represent Dk values for the material under different conditions and values that can be used when designing or modeling a circuit, as in a commercial computer-aided-engineering (CAE) software program.

Several earlier blogs in this series have explored the meaning of the dielectric constant for PCB materials, including the different types of measurements that PCB materials suppliers use to determine the dielectric constants of their materials. The dielectric-constant values of the materials inevitably compare to the dielectric constant of a vacuum, which is one, or the dielectric constant of dry air, which is close to one, which are standard references for understanding dielectric constant. But unlike a vacuum, the measured values of dielectric constant for a PCB material can differ, depending upon the measurement method and the frequency of the measurements.

In practical terms, the dielectric constant of any PCB material will be higher than a vacuum. The size of a circuit’s structure, such as a resonator, will be determined by the wavelength of the frequency of interest. The size of the circuit structure for a given wavelength decreases as the dielectric constant increases. The dielectric constant is often defined by a capacitor formed with a dielectric material and the number of electrons or charges the capacitor can store for a given voltage. Materials with higher dielectric constants can store more charge than materials with lower dielectric constants. In theory, for equivalent performance, circuit dimensions can be made smaller by using a circuit material with a higher dielectric constant.

Numerous articles on selecting PCB materials, including some by this author, have suggested the use of circuit materials with higher-dielectric-constant values to miniaturize a circuit, such as an amplifier.[1] Is the process of miniaturizing an RF/microwave/millimeter-wave circuit simply a matter of switching to a higher dielectric-constant PCB material? Is it that easy? Or are other tradeoffs involved with a move to a higher dielectric constant?

Selecting a PCB material is not simply a matter of choosing a material based on one parameter, such as dielectric constant. If such was the case, circuit designers seeking larger circuits would start with PCB materials having a dielectric constant of 3 or less, while engineers in need of miniaturization would work with PCB materials having a dielectric constant of 10.2 or higher. As will be seen when a future blog takes a closer look at the dissipation factor (Df), choosing a PCB material requires more than just one material parameter. Yes, materials with higher dielectric constants can yield smaller circuits for a given operating frequency or bandwidth but, depending upon the requirements for the circuit or circuits to be fabricated on the material, a smart material choice should consider the dielectric constant along with other key material parameters for that application.

For example, in the article on amplifier design, a jump was not made simply from a low-dielectric-constant material with a value of 3 to a circuit material with a dielectric constant of 10.2. A material with a higher dielectric constant of 6.15 was used (RO4360™ thermoset material from Rogers Corp.) in place of circuit materials with lower dielectric constants, to achieve the circuit miniaturization. The quality associated with the dielectric constant, such as the dielectric-constant tolerance (variations in dielectric constant regarding lot-to-lot consistency) and dielectric-constant variations with temperature, should also be part of the process of evaluating different materials for dielectric constant. For miniaturizing the amplifier, consideration was also given to other important material parameters, such as thermal conductivity. Since an amplifier circuit will generate heat, and improperly managed heat can cause mechanical and electrical variations in a PCB material, the material’s thermal conductivity was considered at least as important as the dielectric constant and its related parameters.

The dielectric constant of a PCB material is formed by the component materials, such as woven glass, ceramic, or polytetrafluoroethylene (PTFE). Circuit designers tend to plan around a specific dielectric constant for material, but should never lose sight of the fact that a circuit material’s Dk is not constant: it will change with temperature and over frequency, and even with the thickness of the material. Any comparison of different PCB materials can never assume constant Dk. It can also not assume that other material parameters remain constant when switching to a material with a higher Dk value.

Most PCB materials are anisotropic, which means that the Dk value is different for different axes of the material. Material data sheets typically provide information on such parameters as Dk tolerance and variations in Dk with temperature. In addition, Rogers Corp. has performed extensive evaluations on its circuit materials and offers curves of dielectric constant versus frequency for most of its materials, from RF typically through lower millimeter-wave frequencies.

Among other things, Rogers’ investigations into material properties have revealed that working with Dk should never be taken for granted. Dk can vary with temperature and frequency, but it can also vary in unexpected, nonlinear ways, especially at lower frequencies. Although Rogers has invested a great deal of time and effort into developing “Design Dk” values for its materials, to represent the dielectric characteristics of the circuit materials under real-world conditions, the nonlinear behavior of dielectric constant can be difficult to track for many circuit materials.