Design Guidelines for PCB Using FR4 Substrates

Material substrate concerns are necessary while designing electrical circuit boards. This is due to the fact that printed circuit boards cannot be manufactured without the appropriate PCB materials. When developing and producing circuit boards, the substrate material must be taken into account just as much as the copper layers, silkscreen, and surface polish. Generally speaking, non-conductive materials like porcelain, Marlon, FR4, and the like must be used for PCB substrates. Depending on its intended use, the material can be selected. If certain core and material-related parameters are not selected correctly, the circuit may exhibit unexpected behavior. We shall learn what applications the FR4 is appropriate for in this article today. This will inform you of certain FR4 PCB design recommendations.

What is FR4?

Flame Retardant 4 or FR4, is a grade designation for a type of material used in the production of PCBs. It is primarily made of fibreglass that has been woven and sealed with epoxy resin. It offers excellent electrical insulation and mechanical strength. The "4" in FR4 sets it apart from earlier varieties of flame-retardant materials. That is why it is now the most widely used material due to its superior properties. As fiber-reinforced materials and synthetic resins were being produced in the middle of the 20th century, FR4 was created.

As said earlier "FR" designation indicates that the material is flame retardant, so it is appropriate for many different demanding uses. The material has dielectric constant (Dk) between 4.2 and 4.8, depending on frequency. Dissipation factor (Df) is around 0.02 at 1 MHz which makes it suitable for general-purpose electronic designs.

Classification and Properties of FR4:

FR4 boards are categorized by thickness, material source, electrical qualities, and thermal characteristics in the PCB industry. FR4 has a normal thickness of 1.6 mm, however there are other variations of 0.5 mm and 2.36 mm. The normal range of copper thickness is 18 µm to 140 µm, contingent on design specifications.

The glass transition temperature (Tg), which controls the material's behaviour under heat, is one of the thermal properties that are most important in the FR4 classification. FR4 comes in three different temperature ranges:

• Low Tg (130–140°C)
• Standard Tg (150–160°C)
• High Tg (>170°C)

Because of their better resistance to heat and moisture, the high Tg materials are recommended. FR4 starts to break down at temperatures over 180°C. Additionally, FR4 has a low coefficient of thermal expansion (CTE). Which makes it perfect for situations where heat is a concern.

7 Design Guidelines for Using FR4 in PCB Designs

1)  Stackup Design and Layer Planning

A well-structured layer stackup enhances both signal integrity and thermal management. It is very common that due to wrong stackup in PCB we will face issues related to signal reflections. We have discussed the HDI stackup, for better signal integrity and lower EMI, see the derailed article from here. By the way some common stackups include:

• 2-layer PCBs for simple designs.
• 4 to 8-layer stackups for more complex, high-density circuits.

2)  Trace Width and Impedance Control

For controlled impedance designs calculate trace widths based on FR4’s dielectric properties. Usually we have to match the trace impedance with output and input port, if not there will be signal reflection causing EMI problems. Standard online calculators or PCB design software can help determine, we also have a JLCPCB impedance calculator for this. To know more about  impedance control in PCBs, see our recent article on this topic. For Example: Microstrip and stripline configurations should be carefully designed based on the stackup to maintain consistent signal quality.

3) Thermal Considerations

Although FR4 is not thermally conductive but some good strategies can be used to make it more heat conductive. For example we can use thermal vias, copper pours and heat sinks to remove local heat. For components with significant heat output we may consider using metal-core PCBs (MCPCBs). The FR4 is best suited in power electronics circuits, if you are using onboard heatsink to dissipate heat, yet there are other methods also as shared above which can be implemented to reduce the thermal effects. Better the thermals of a PCB ensures its long life and durability.

4) Via Design and Aspect Ratio

The aspect ratio (board thickness to drilled hole diameter) should be kept within 8:1 to 10:1 for reliability via plating. Use via stitching for ground planes and thermal vias under power-hungry components for improved heat dissipation. Avoid unnecessary via stubs in high-speed designs by using backdrilling if necessary. We have covered some recent articles on placement of a thermal via and types of Via in PCB design, See from here. [link]

5) Clearances and Spacing

Clearance is very important, if designing for high speed or high frequency applications. This is basically the space through which the electromagnetic data flows. Any retardation in flow of EM waves directly causes issues like crosstalk. Follow IPC-2221 guidelines for spacing:

• Track to Track: ≥ 0.15 mm (6 mil) for standard fabrication.
• Pad to Pad: Based on component footprint and assembly process.
• Edge Clearance: Leave greater than 0.3 mm space for routing near PCB edges.

For high-voltage circuits we can increase the clearance even more, according to applicable safety standards.

6) Placement and Routing

We can group components logically to minimize trace lengths and avoid trace crossing. For Example: analog and digital should be routed at least 4-5 mm away. Other methods can be by placing the decoupling capacitors close to IC power pins and keeping differential pairs tightly coupled. By matching the trace lengths of differential pairs. And using the ground pours and guard traces for high-speed digital circuits to provide them noise coupling.

7) Signal Integrity Considerations

While FR4 supports high-speed digital designs up to a point. But after that due its loss tangent the signal attenuation increases when going in a few GHz. Although what we can do is, follow some rules such as: for critical signal paths, keep traces short and use matched impedance. For frequencies above 3-4 GHz, just switch to some other low loss material such as Rogers and PTFE.

Conclusion

Designing PCBs with FR4 substrates offers a balance of performance and cost. To sum up what we have with FR4 is:

• Dielectric Constant (Dk): Its high around 4.2 to 4.8 (varies with frequency)
• Loss Tangent (Df): Its in mid but depend on frequency 0.02 at 1 MHz
• Thermal Conductivity: Around 0.3 W/mK (poor for thermal dissipation)

FR4 material is popular for PCB manufacturing and assembly. FR4 has varied attributes encompassing a wide range of temperature and frequencies. For standard applications, the low cost of the FR4 materials also acts as an attraction.