If you know what a prism is, you have an idea of how it disperses light. Most circuit boards use Fr4 as the dielectric material, impacting electrical signal dispersion. Dispersion is also vital in high-speed and high-frequency PCBs because electrical pulses move at different velocities on the traces.
Therefore, if you are designing such a PCB, you need to understand the properties of FR4. This includes the FR4 dielectric constant, which helps develop analytical models showing the signal behavior.
We have looked at the dielectric constant property in detail below, so take a look!
Types of Dielectric Material
There are three kinds of material categories based on electrical conductivity. These are wires (conductors), semiconductors, and insulators. Dielectrics like FR4 act more like electrical insulators because they inhibit electricity transmission.
However, dielectrics fall under two types with very different properties. These are:
Active dielectrics accept electricity when subjected to an external electrical field and take the flow of work from them. They can adapt to store this electrical energy and include ferroelectrics, piezo-electrics, etc.
Passive dielectrics restrict the flow of electrical energy and act precisely like insulators. They include materials like glass, mica, FR4, etc.
Properties of Dielectric Material
- Non-metallic with a high impedance
- High activation energy that exceeds 3eV
- Exceptionally highly bonded electrons to the nucleus
- Extremely low conductivity due to a lack of electrons
- Possible to anticipate the dielectric nature/polarizing behavior using the allowability rating
- The dielectric constant can quantify the dielectric polarization intensity
FR4 Dispersion and Dielectric Constant
Insulators, dielectrics, and all other materials possess some electrical permittivity, which is a term defining two things:
- The speed at which electrical signals propagate through materials
- The quantity of charge that the material can hold at a given time.
For reference, the electrical permittivity of vacuum or free space is:
Ɛ0 = 8.854 x 10-12 Farads/meter
FR4 Dielectric Constant (Dk)
The Dk of FR4 ranges from about 3.8 - 4.8 (4.3 average). But this constant depends on the resin content, thickness, roughness of the copper foil, and glass weave style.
Factors like trace and plane arrangement on the PCB base material determine the Dk for pulses propagating in interconnects.
Similarly, the dielectric constant and trace geometry affect pulses on the surface of coplanar waveguides or microstrip traces.
Therefore, you need to determine the propagation velocity and impedance accurately. To do this, your printed circuit board design software should have a stack-up utility design with the following data for precise impedance and velocity determination.
- Flammability rating
- Dielectric constant
- Standard thickness
- Breakdown field strength
- Glass transition temperature
FR-4 Dielectric Constant (Dk) Dissipation
FR-4 has its limitations, especially in RF applications, because the material's dielectric stability might suffer on the high frequencies. Also, FR-4 has a high dissipation factor with additional insertion losses in the 1 - 15 GHz microwave frequencies.
A Microwave mixer printed circuit.
The traces placed on FR4-laminate materials also experience more significant attenuation at radio frequencies.
Aside from that, FR-4 thickness does affect the circuit board's Dk and the impedance matching needed for RF PCBs.
That said, some high-performing FR-4 materials offer better reliability because they can handle several lamination cycles.
It is worth noting that:
- Balancing the FR-4 dielectric constant with the board's trace width and laminate thickness is not easy. However, the right stack-up manager might help to produce precise propagation delay and impedance calculations
- FR-4 dielectric constant measurement is not easy because the results vary depending on the method of measurement
- Standard trace geometry impedance calculations (like strip lines) require precise wideband Debye dispersion models
Difference Between FR-4 Material and Rogers Material
- Even though their use is the same, Rogers and FR-4 materials have many differences that suit different applications. These differences include:
- Price: FR-4 material is cheaper than Rogers' laminates
- Dielectric constants: It is lower in FR-4 (about 4.3 - 4.4) and higher in Rogers (6.14 - 11)
- High Frequencies: Rogers is excellent with high frequencies, making it ideal for RF circuit boards
An unassembled radiofrequency PCB
- Dissipation factor: FR-4 has a higher dissipation factor than Rogers' material, leading to higher signal loss
- Dk numbers in impedance constancy: Rogers material provides a broader spectrum than FR-4
- Fluctuation: Rogers materials have fewer fluctuations in temperature regulation than FR-4
Mechanical and Thermal Properties of FR4
Aside from the dielectric constant, it would help to consider the thermal and mechanical properties of the laminate material. The properties of FR-4 material depend on the temperature, so ensure you consider the temperature stability.
Here is a summary of the properties of the material.
Limitations of FR4 Circuit Board Materials
FR-4 presents these problems when used in a high-speed PCB.
FR-4 material is an excellent insulator but deteriorates if heat, voltage, or power exceeds certain limits. Therefore, the material will start to conduct electricity in such situations, resulting in failures.
Unlike high-speed board materials, FR-4 does not offer a uniform Dk. Its tolerance goes up to 10%, while that of the high-speed materials is less than 2%. When in use, these variations bring up challenges while maintaining the impedance values, so they are not the ideal choice for controlled impedance boards.
FR4 has a dissipation factor of 0.020, exceeding 0.004 for high-frequency materials. Therefore, it results in more signal losses, making it less ideal for high-frequency applications.
Additionally, the Df of FR-4 material goes higher as the frequency increases, leading to significant losses.
Lastly, FR-4 material is not ideal for devices exposed to high temperatures because its Tg is relatively low. Therefore, it does not support lead-free soldering because the reflow temperatures far exceed what FR4 can handle.
The PCB soldering process at a factory
Tips to Select The Right FR4 Material
When choosing an FR4 substrate, use these tips to help you pick the suitable material.
- Avoid using thin materials for PCBs with grooves.
A V-groove, V-cut PCB
- Pick a base material that provides a uniform Dk across a wide range of frequencies. Dk variations will affect the parasitic capacitance between a trace and any nearby conductor, including the reference conductor.
- Select a high-performance FR-4 material like Isola 370HR for applications that exceed an operating temperature of 150°C. Such materials have low expansion rates and better thermal performance than standard FR4.
A burned PCB
IPC-A-600 Standard for FR4 Materials
Weave exposure refers to the state where the unbroken woven cloth fibers remain uncovered. It is acceptable in class 1, 2, and 3 boards, provided the space between the conductors meets the minimum spacing requirements.
Weave texture is a condition where the weave pattern is visible on the surface, but the woven cloth fibers remain covered in the resin. It is acceptable in all classes.
Measling is the discrete white spots that form on the base material. Even though measling is acceptable in all high voltage applications, it might decrease the PCBs' overall performance.
A series of white, connected spots or crosses on the base material create the crazing phenomenon. It indicates a separation of the glass cloth fibers from the connecting weave junctions.
The phenomenon is acceptable in class 1 boards provided the space between the conductive patterns is not less than the minimum conductor spacing.
For classes 2 & 3:
- The space between adjacent crazing patterns should not exceed 50% of the distance between adjacent conductive patterns
- Board edge crazing has zero effect on the minimum space between the edge and the conductive pattern
Delamination refers to the separation of space between a circuit board's plane. On the other hand, a blister is a delamination with localized swelling. The acceptability conditions are:
- The blister/delamination should not be less than 25% of the distance between conductors
- Delamination should not be closer to the board's edge
- The PCB area affected by delamination should not exceed 1% of the total area
Class 2 & 3
- All class 1 conditions
- The distance between conductive patterns must not be less than the minimum conductor spacing
Foreign materials are conductive or non-conductive substances detected on the laminate. Translucent materials are acceptable in all classes, provided they do not affect the board's thermal and electrical properties.
FR4 is the most commonly used laminate material for PCB manufacturing due to its affordability, durability, water resistance, and insulation. However, it is not the best for high-frequency or high-temperature conditions.
You need alternative materials like Rogers for such conditions, and we can build circuit boards using the best material for your project. Contact us today to get more information.