Parasitic Properties Understanding Impedances of Circuit Boards in More Detail

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Circuit boards are also, in a sense, passive components. Individual traces have a measurable resistance, inductance, and also capacitance. This depends heavily on the material used.

A double-sided Eurocard (160 mm × 100 mm) with a core thickness of 400 µm and a relative permittivity εr = 4.6 results in a parasitic capacitance of approximately 1.62 nF. This value can be calculated using the formula for plate capacitors:

with C = capacitance [F], ε₀ = electric field constant (approximately 8.854⋅10⁻¹² As/Vm), ε_r = relative permittivity of the material, A = plate area [m²], d = distance between the plates [m].

The Dielectric

Let’s take a closer look at the dielectric. It is a factor in the calculation of capacitance. Particularly important here is the dielectric constant (also called relative permittivity, ε_r). It is a material property that describes the attenuation of an electric field within a substance. It is measured relative to the attenuation in a vacuum (ε₀).

This constant indicates how well a material can store electrical energy in a field. Formally, it is the ratio of the permittivity of the material (ε_r) to the electric field constant (ε₀). The higher the dielectric constant, the more the electric field is reduced in the material and the more charge a capacitor can store at a given voltage. Typical values range from about 2 to 4 for plastics, around 80 for water, and up to several thousand for special ceramics used in the production of capacitors.

The Problem

When calculating the impedance of microstrip lines, the influence of the dielectric is particularly evident. A trace with a width of 3 mm on a 1.6 mm thick FR4 substrate over a ground plane achieves an impedance of approximately 49.5 Ω with a dielectric constant of ε_r = 4.4. Incidentally, FR4 stands for Flame Retardant 4. If instead a material like Rogers RO4730G3 is used, the impedance increases to about 58.6 Ω due to the lower dielectric constant of ε_r = 3.0. The advantage of such substrates lies not only in the higher impedance value but especially in their very low loss characteristics, stability, and homogeneity: unlike FR4, it is a hydrocarbon-based laminate filled with ceramic and hollow micro-particles, offering very stable electrical properties across a wide frequency range. Rogers RO4730G3 and the expected RO4725G3 material in Q1 2026, with an even lower dielectric constant of ε_r = 2.55, are specifically designed for high-frequency radio technology.

FR4 does not exhibit this property. Upon closer inspection, it becomes clear that it is composed of a fiberglass weave and epoxy resin. The fiberglass weave has a dielectric constant of approximately 5.8 to 6.7, while the epoxy resin ranges from about 3.0 to 3.4. This value is also highly temperature-dependent. FR4 is therefore a bulk average of these two components. The effective value depends on the resin-to-glass ratio. With a high resin content, ε_r decreases closer to 3.5 to 4.0. With a high glass content, ε_r increases toward 4.7 to 5.0.

For example, if a fine antenna or trace structure is located directly above a fiberglass area, it experiences a significantly higher effective dielectric constant locally than over an area predominantly consisting of resin. For the electromagnetic wave, this means that the propagation speed and thus the phase position differ in these zones. The antenna therefore does not "see" a uniform dielectric but rather a fine, fabric-induced structure with varying electrical properties and, in the worst case, a preferential alignment, also referred to as anisotropy. This inhomogeneity results in the effective electrical length not exactly matching the geometric design.

In practical operation, this manifests as shifts in the resonance frequency, altered impedance matching, and sometimes additional losses. The higher the frequency and the smaller the structures, the more pronounced these differences become. Therefore, FR4 is considered problematic in high-frequency technology and especially in antenna design, while homogeneous and lower-loss substrates such as PTFE, ceramic, or specialized RF laminate materials are typically used for precise applications.

Additionally, the impedance of a trace on FR4 does not behave consistently due to several factors. Impedance is primarily determined by the thickness of the substrate, trace width, and dielectric constant. If these parameters vary significantly from production batch to production batch, the impedance will shift.

Environmental influences, such as relatively high moisture absorption of the epoxy resin and the temperature dependency of the dielectric constant, also play a role.

Additionally, depending on whether the trace runs directly over a fiberglass strand or a pure resin area, tiny impedance jumps occur along the line, which can manifest at high frequencies in the form of reflections, increased attenuation, and, in some cases, unwanted crosstalk. While these effects are hardly relevant for low-frequency applications, they can significantly degrade signal quality in the GHz range and noticeably complicate the development of antennas or high-speed traces.

Dielectric Losses

As already mentioned, the behavior of the dielectric at high frequencies is particularly critical. While ideal materials would exhibit a constant dielectric constant across the entire frequency range, common substrates like FR4 show significant frequency dependency.

The cause lies in the molecular relaxation processes of the material. Polar molecules and molecular groups in the dielectric try to align themselves with the changing electric field. At low frequencies, these polarization mechanisms can fully follow the field changes. However, as the frequency increases, the time between field changes becomes shorter. Then, the inertia of the molecules prevents them from keeping up with the rapid changes. This delayed response of the molecular dipoles results in two effects. Firstly, the effective dielectric constant decreases with increasing frequency because fewer polarization mechanisms contribute to field shielding. Secondly, dielectric losses occur, as the energy used for the incomplete alignment of dipoles is converted into heat. These losses are characterized by the loss factor tan δ (tangent of the loss angle).

In practice, it becomes apparent that signal transmission is increasingly impaired as frequency rises. The frequency-dependent dielectric constant causes impedance fluctuations and phase shifts. The dielectric losses increase signal attenuation. Additionally, frequency dispersion affects different spectral components of a signal unequally.

High-quality RF laminates like Rogers RO4730G3 offer nearly constant electrical properties and low losses over a wide frequency range due to their homogeneous structure and optimized molecular composition. (mr)