FR4 PCB Material: Complete Guide in 2022

May 05, 2022

FR4 is the common used material in PCB manufacturing. It’s versatile material and have a balance between cost and performance. That is the reason why FR4 material is widely applied in single-sided, double-sided, and multi-layered boards.

In this article, you will find out more about:

What does FR-4 Stand for?
Beneficial Properties of FR-4 Substrate
Limitations of FR-4 Circuit Board Materials
Alternative PCB Materials for FR4
Tips to Select the Right FR-4 Material

What Does FR-4 Stand for?

FR-4 is technically woven fiberglass-reinforced sheets, which were epoxy laminated. For you to not be mistaken, FR-4 is neither core nor a prepreg as it has not been copper foil plated and laminated with other layers yet. So this material is the most basic, rigid layer intended for further processing – substrate. Do not confuse it with FR-4-based cores.

FR-4 is a standard rating by National Electrical Manufacturers Association) (NEMA). FR – is simply flame retardant here, which indicates its compliance with the UL94V-0 standard about the inflammability of plastic materials. And 4 – the exact type in the category of epoxy reinforced materials. It means that the laminate is woven glass fiber-based.

What are the Beneficial Properties of FR-4 Substrate?

The popularity of FR-4 is well-founded on the following significant advantages:

Is a low-cost material.
Offers a high strength-to-weight ratio.
Is moderately moisture-resistant.
Good dielectric properties leading to high energy efficiency.
Neutral yellow to green color that is suitable for most applications.
Robust dimensional stability.
Good thermal management.
Great mechanical properties.

The detailed properties may not be as impressive as ones of high-cost PCBs. But still, FR-4 offers a high cost to performance ratio making it popular among manufacturers of low and medium-cost electronics worldwide.

Examples of FR-4

Speaking about other materials in the epoxy reinforced category, there are several types:

- Standard FR-4. A typical laminated material that has a heat resistance of 140-150°C.
- FR-1, FR-2, FR-3. All these differ from FR-4 in electrical conductivity, heat resistance, thermal expansion level, and other properties. For example, FR-3 is an epoxy resin binder reinforced substrate, while FR-2 uses phenolic resin. Other types of epoxy make such substrates less versatile and not that popular. But they are typically even more affordable.
- CEM-1 and CEM-3. They are epoxy synthetic resin-reinforced paper cores composed of laminates. CEM-1 offers a better bending strength, while CEM-3 uses a “fly” type of glass fabric and performs better in double-side and multi-layer PCBs.
- High TG FR4 (FT-5). Nearly the same as standard FR-4 with the higher transition of glass (TG).
- High CTI FR4. Nearly the same as standard FR-4, with the Comparative Tracking Index having a value higher than 600 Volts.
- FR-4 base with no laminated copper. While all the other types are substrates for thin copper foil to be platted, such FR-4 does not have this coating. It makes it perfect for insulations plates or for supporting PCBs with an additional, rigid layer.

Limitations of FR-4 Circuit Board Materials

Despite all the above-mentioned strengths, FR-4 has substantial limitations, essential of which are the following:

High-temperature operating environments. It includes all electronic devices that heat up to a higher degree than 120°C during their operation. FR-4 can withstand up to 130°C but will deteriorate quickly. And it will immediately stop functioning once the temperature exceeds this level.
Lead-free manufacturing. Restriction of Hazardous Substances (RoHS) directive obligates to limit the use of soldering of lead soldering where possible. Unfortunately, there is no lead-free soldering that performs well if applied to FR-4 material. Alternatively, FR-4 will not withstand the heating up to 250°C required while applying lead-free soldering.
High-frequency electronics. Naturally, FR-4 does not perform well as a component of radio and broadcast electronics. High-frequency signals affect FR-4 impedance stability making it malfunction and showing unreliable signal integrity.

How is FR-4 Used in PCBs?

Primarily, FR-4 serves as the base of any PCB. It is a foundation material for production, and it is laminated/plated with thin copper foil layers under high temperatures and pressure. Depending on the PCBs purpose, one or both sides of FR-4 sheet material can be laminated. This copper will be etched further to expose pads and routes – electrical connections. So most of FR-4 will be left uncovered.

FR-4 is the most rigid layer that defines most of the mechanical characteristics of PCBs, like its ability to bend and withstand external stresses, tensile strength, durability, etc. Prepregs can comprise one or multiple sheets of FR-4 mixed with insulation layers, copper foil, etc.

Alternative PCB Materials

FR-4 is nearly the most popular material, but definitely not the only choice. Let’s compare it to two alternative laminates:

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Factor	FR-4	Rogers	Teflon
Versatility	Extremely versatile, but is a good choice for most of the low and medium-cost electronics only	Nearly the only choice for high radio frequency applications and is a great choice for most expensive electronics	Moderately versatile, and is a good choice for some specific types of low and medium-cost electronics
Complexity of production	Difficult in production and can rarely be fixed. Is expensive to rework	Difficult in production, and can rarely be fixed. It is expensive to rework	Mostly simply in production and can rarely be fixed. It is expensive to rework
Resistance	Has superior chemical resistance to acids, solvents, and petroleum products. Moderate corrosion resistance, depending on the finish. Is wear-resistant but not hydrophobic	Has superior chemical resistance to acids, solvents, and petroleum products. High corrosion resistance	Chemically inert, and cannot be damaged by most solvents and organic and inorganic acids. High corrosion resistance. Is wear-resistant and hydrophobic
Electrical properties	Impedance dielectric constant equals 4.5. The dissipation factor is not that low. Conductivity is moderate	The dielectric constant can vary in the range 2.5 to 11 making impedance stability adjustable. The dissipation factor is low. Conductivity is high	The dissipation factor is low making it more energy-efficient. Conductivity is moderate, and the dielectric constant equals 2.1
Mechanical properties	Exceptionally durable and strong but not lightweight. Withstand external stresses	Offers a great strength-to-weight ratio. It is a high-density material and is not intended to be bent or withstand high external stresses	Durable and strong, but still can be bent and flexed even at low temperatures. Withstand external stresses
Thermal properties	Moderate thermal management variation. Slightly expanded, but suits most electronics that operate in a wide array of environments overall.	Low thermal management variation. Does not expand, but does not suit electronics that operate in high-temperature environments overall.	Moderate thermal management variation. Slightly expands but suits some electronics that operate in high-temperature environments overall
Scope of application	Most of the widespread low and medium-cost electronics. Does not suit any radio frequency applications	High-cost and technologically advanced electronics. High radio frequency applications	Low and medium-cost electronics that are intended for multiple operating environments. Some simple radio frequency applications
Cost	Comparatively low-cost material	One of the most expensive materials	Comparatively affordable materials

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Besides the above-mentioned, you may be offered the following types of laminates/substrates, which are not in the FR category, to use for your PCB productions: RF-35 Taconic material, which is a ceramic one and performs well for microwaves only. And Polymide substrate, which is a middle ground between Rogers and Teflon in the matter of their properties, applications, and costs.

Tips to Select the Right FR-4 Material

Perhaps you are stuck with choosing a more suitable material from the epoxy reinforced category. Or not sure which brand offers a better FR-4. Either way, you may find these tips and advice to be useful:

Do not try to find the cheapest option on the market, and always pay attention to customer reviews about a seller.
Know the properties of the perfect fit to your needs, and pick the most suitable one among epoxy reinforced materials.
Pay particular attention to FR-4 thickness, and choose thinner ones only when space constraints in electronic devices are projected.
Avoid choosing thinner FR-4 materials for PCB design that features grooves, or the board can fracture.
Always follow the simple dependency – lightweight electronics require lightweight boards, for which thinner FR-4 is needed, and vice versa.
Pay attention to FR-4 mechanical properties specified by the seller as they tend to fluctuate significantly from brand to brand.
Consider design requirements prior to choosing FR-4, as it will impact the choice of an appropriate thickness.