You will understand how a prism disperses light if you have an idea of what it is. Majority of circuit boards make use of the FR4 material as its dielectric material because it impacts the dispersion of electrical signals. Furthermore, dispersion is important in high-frequency and high-speed printed circuit boards. This is because electrical pulses usually move at various velocities on traces.
This is why when designing a printed circuit board, there is a need to comprehend the features and properties of the FR4 material. These include the FR4 permittivity or relative permittivity that helps in developing the analytical models that show the behavior of the signal.
Understanding FR4 Relative Permittivity and Dispersion
Dielectrics, insulators, as well as other materials feature some form of electrical permittivity. This term defines two things:
- First is the speed whereby the electrical signals propagates via the materials
- Amount of charge, which a material will be able to hold at a specific point in time
FR4 Relative Permittivity
The FR4 permittivity value usually falls between 3.8 and 4.8. On average it falls on 4.3. However, fr4 permittivity usually depends on the substrate’s thickness, resin content, copper foil roughness, as well as the style of the glass weave.
Furthermore, factors such as plane and trace arrangement on the base material of the printed circuit board helps in determining the fr4 relative permittivity for pulses that propagate in interconnects. Likewise, the trace geometry and fr4 relative permittivity affects the pulses that are present on the microstrip traces or coplanar waveguides surfaces.
Therefore, you must determine the impedance and velocity of the propagation accurately. To achieve this, your software for PCB design must possess the stack up utility design having the data below for precise velocity and impedance determination.
- Dielectric constant
- Flammability rating
- Standard thickness
- Temperature for glass transition
- Strength of breakdown field
FR4 Relative Permittivity and Reliability
The variations in the FR4 relative permittivity could raise some issues. FR4 permittivity varies with the substrate’s thickness, resin content, copper foil roughness, as well as the style of the glass weave. The FR4 insulator’s stability is usually subject to some deterioration when influenced by extreme heat, high voltage, or high power. As this temperature experiences some variation, the FR4 permittivity or dielectric constant becomes unstable. This leads to instabilities in impedance in the board. Furthermore, the FR4 permittivity variations pose some serious challenges or issues when utilized for controlling the impedance boards.
Furthermore, FR4 relative permittivity is dependent on frequency and provides values that are non-uniform in high-frequency or high-speed board applications. Also, the circuit’s characteristics as well as board’s performance fluctuates when there are variations in the dielectric constant, which aren’t preferable in all high-frequency applications.
Analyzing FR4 Relative Permittivity
The fr4 relative permittivity is a very sensitive parameter varying with signal frequencies, texture differences, and dimensional changes, etc. With top analysis and design tools, you can analyze the FR4 relative permittivity variation of a PCB material with respect to varying conditions. With these tools, you will be able to diagnose the fr4 relative permittivity variation effects on high frequency and high speed signals.
FR4 Permittivity Dissipation
There are some limitations that go with using FR4, most especially in the RF applications. This is because the dielectric stability of the material may suffer during high frequencies. In addition, FR4 features a dissipation factor that is high with extra insertion losses in microwave frequencies falling between 1 and 15 GHz. These traces which are placed on these FR4 laminate materials experience greater significant attenuations during radio frequencies.
Apart from this, the thickness of the fr4 affects the relative permittivity of the circuit board as well as the required impedance matching for the radio frequency PCBs.
This is why some highly performing fr4 materials will provide better reliability due to their ability to handle different lamination cycles.
Note that:
- The balancing of the FR4 permittivity with the laminate thickness and trace width if the board isn’t easy. Moreover, using the appropriate stack up manager may aid in producing accurate propagation delays as well as impedance calculations.
- The measurement of the fr4 dielectric constant isn’t easy because there are differences in the result, which depends on the measurement method.
Limitations of Using the FR4 Material
When you use the FR4 Permittivity material in high speed printed circuit boards, you may experience these problems.
In contrast to the board materials of high speed, fr4 doesn’t deliver uniform relative permittivity. Its tolerance value increases by 10%, while the high speed board material is below 2%. Whenever in use, the variations will bring some challenges up during the maintenance of the value of impedance. Therefore, this choice isn’t the best choice for the controlled impedance circuit boards.
Temperature Stability
Using the FR4 material isn’t great for any device that experiences exposure to extreme or high temperatures. This is because it has a relatively low Tg value. Therefore, lead-free soldering isn’t supported because the temperature for reflow far surpasses what the FR4 material will be able to handle.
Signal Losses
The dissipation factor of the FR4 permittivity material is 0.020. This exceeds the 0.004 value meant for all high-frequency materials. So, this will only lead to more losses in signal, thereby making it inappropriate for the high frequency applications.
In addition, the FR4 material’s dissipation factor will increase as frequency increases, which results in great losses.
Insulating Stability
The FR4 material is a great insulator. However, it deteriorates when power, voltage, or heat surpasses certain limits. This is why the material would start conducting electricity in these situations, which causes failures.
Conclusion
The FR4 material is mostly used as a laminate material during PCB manufacturing because of its insulation, durability, affordability, as well as water resistance. Moreover, it isn’t the most appropriate for high-temperature or high-frequency conditions.
However, fr4 permittivity usually depends on the substrate’s thickness, resin content, copper foil roughness, as well as the style of the glass weave. Furthermore, factors such as plane and trace arrangement on the base material of the printed circuit board helps in determining the fr4 relative permittivity for pulses that propagate in interconnects.
