PCB Tech - PCB manufacturer explain what is fr4 dielectric constant

FR-4 sheet is a double-sided copper-clad PCB sheet made of epoxy resin + glass cloth. The fr4 copper-clad sheet is commonly used, and the fr4 dielectric constant relative to air is 4.2-4.7. The fr4 dielectric constant changes with temperature, and the maximum change range can reach 20% in the temperature range of 0-70 degrees. The change of the dielectric constant will cause a 10% change in the line delay. The higher the temperature, the greater the delay. The dielectric constant also changes with the signal frequency. The higher the frequency, the smaller the fr4 dielectric constant. In general, the classic value of fr4 dielectric constant is 4.4. The dielectric constant changes with frequency as shown in the figure.

fr4 dielectric constant

The fr4 dielectric constant (Dk, Er) determines the speed at which the electric signal propagates in the medium. The propagation speed of the electrical signal is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal transmission speed. Let's make a vivid analogy, just like you are running on the beach. The depth of the water floods your ankles. The viscosity of the water is the dielectric constant. The more viscous the water, the higher the dielectric constant and the slower you run.

The dielectric constant is not very easy to measure or define. It is not only related to the characteristics of the medium, but also related to the test method, test frequency, and material state before and during the test. The dielectric constant also changes with the temperature. Some special materials take into account the temperature factor in the development. Humidity is also an important factor affecting the dielectric constant, because the dielectric constant of water is 70, and very little moisture will cause Significant changes.

FR-4 sheet dielectric loss: the energy loss caused by the insulating material under the action of an electric field due to the hysteresis effect of dielectric conductivity and dielectric polarization. Also called dielectric loss, referred to as dielectric loss. Under the action of an alternating electric field, the complementary angle δ of the included angle (power factor angle Φ) between the current phasor and the voltage phasor flowing in the dielectric is called the dielectric loss angle. The dielectric loss of fr4 sheet is generally 0.02, and the dielectric loss will increase with the increase of frequency.

TG value of fr4 sheet: also called glass transition temperature, generally 130 degree Celsius, 140 degree Celsius, 150 degree Celsius, 170 degree Celsius.

Conventional thickness of fr4 sheet

Commonly used thicknesses: 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.5mm, 1.6mm, 1.8mm, 2.0mm, the thickness error of the plate needs to be based on the production capacity of the fr4 plate factory Certainly.

Common copper thicknesses for fr4 copper clad laminates: 0.5 ounces, 1 ounces, 2 ounces, and other copper thicknesses are also available, which need to be determined by consulting ipcb.

Dispersion, an important optical effect, is also important in high-speed PCB and high-frequency PCB. In the PCB, different signals propagate at different speeds in the trace.

Like any other material, the dispersion of fr4 affects the traveling pulses and waves in the PCB trace. The physical principles describing dispersion are well known and can be used to develop analytical models of signal behavior in PCBs.

For those who may not remember their engineering or physics classes, the dielectric constant (and thus the refractive index) in a material is a function of the frequency of electromagnetic wave propagation. This is why a prism can be used to separate white light into rainbow colors. Similarly, the absorption rate of electromagnetic waves is also a function of electromagnetic wave frequency.

This will have many effects on the fr4 PCB. These effects are particularly important in high-speed PCB or high-frequency PCB applications. The change of fr4 dielectric constant with frequency is called dispersion, which causes different frequency components in the electrical pulse in the PCB trace to propagate at different speeds. In the case of positive dispersion (the dielectric constant increases with frequency), the higher frequency components reach the load later than the lower frequency components, and vice versa.

The digital pulse is actually just a superposition of analog waves, and the influence of dispersion on each frequency component is slightly different. fr4 happens to have negative dispersion in terms of signal propagation speed, but placing a laminate with positive dispersion on the substrate can compensate for signal distortion and reduce loss.

Most of the frequency spectrum (approximately 75%) in the digital pulse is concentrated between the switching frequency and the knee frequency. The knee frequency is approximately one third of the reciprocal of the signal rise time. A decent approximation only considers the dispersion at the switching frequency, but this approximation is only suitable for low and medium dispersion.

The loss tangent of fr4 also changes with frequency until it increases rapidly at about 100 KHz, and then increases steadily until about 100 GHz. Therefore, the attenuation is greater at higher frequencies, but the stretching caused by the digital pulse is not too severe. At lower frequencies and data rates, stretching is more important, which affects the mismatch tolerance of the trace length.

Compared with analog signals, PCB traces on fr4 tend to have higher losses than other PCB materials specifically used for analog signal applications in the GHz range. Therefore, fr4 boards for high-speed/high-frequency applications should include high-speed laminates to reduce losses and compensate for the inherent negative dispersion of fr4. In addition, you should use other materials specifically for RF applications.

Considering that the dispersion in the circuit model of the transmission line is completed on a per unit length basis. In other words, the important parameters for modeling a transmission line are the series resistance and series inductance of the conductor, the parallel conductance of the dielectric, and the capacitance between the conductor and its return path. The important point here is to consider the changes in shunt conductivity and fr4 dielectric constant with frequency.

The conductivity of fr4 material is divided into static component and frequency-dependent component, the latter of which is proportional to dielectric loss and frequency. At the same time, fr4 dielectric constant is inherently a function of frequency, which is due to the excitation of surface charges or dipole oscillations at lower frequencies, or the excitation of lattice vibrations and electronic transitions at high frequencies.

In terms of building a circuit model for the fr4 PCB, the total capacitance and parallel conductance must be determined at the frequency of the signal of interest on fr4. When modeling circuit behavior, these values must be included in the circuit model of the traces on the fr4 board. The calculations involved are basic, but getting the values wrong can cause your model to produce results that don’t match the actual situation.

Of course, you can use equations to analyze the transmission lines of each part of the circuit board, but you can also use a SPICE-based circuit simulator. You need to include the correct shunt conductance and capacitance values for the fr4 PCB substrate at the frequency you are interested in.

In addition, since you have determined the fr4 dielectric constant at the relevant frequency, you can include the correct value in the 3D field solver. This enables you to check radiation fields, which can cause signal integrity issues in the entire device or multi-board design.