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What is the FR-4 Material in PCB Fabrication?


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The “FR” in FR4 indicates “flame retardant.” FR-4 is a fiberglass-reinforced, epoxy-laminated sheet used in the production of printed circuit boards. It does not catch fire easily and may extinguish flames on its own. This material is utilized in electronics because it is safe and reliable for PCB manufacturing.

Composition of FR-4

FR4, often known as glass-reinforced epoxy, is made up of two major components: woven fiberglass cloth and epoxy resin. A typical FR4 PCB comprises a FR4 PCB material sandwiched between two copper sheets.

  • Fiberglass cloth offers rigidity and strength. FR4 is made up of E-glass fibers, which are robust, low in electrical conductivity, and can endure high temperatures and chemical exposure.
  • Epoxy Resin holds fiberglass threads together. It provides electrical insulation and resistance to heat. When heated, the resin sets up, resulting in a robust, stiff, and chemically resistant shell.

The normal composition is 60% fiberglass and 40% epoxy resin by volume. This mixture gives FR4 its strength and insulation from electricity, making it excellent for printed circuit boards.

FR1 - CEM1 - FR4 Composition
FR1 - CEM1 - FR4 Composition

FR4 offers a strong bond to copper foil and low water absorption, making it ideal for standard uses. FR4 is used as a copper-coated substrate for simple and double-layer PCBs, as well as a prepreg and core for layered PCBs.

Properties of FR-4

Flame Retardant

FR4 stands for “flame retardant” and assures the UL94V-0 fire resistance grade. This means that it does not promote flame propagation and swiftly melts away flames. To make FR4 fire-resistant, it undergoes treatment with organic or inorganic flame retardants. Aluminum trihydrate (ATH) is one of the materials used in inorganic treatments.

  1. Self-Extinguishing – When the flame goes out, FR4 stops burning, preventing flames from growing in electronic equipment.
  2. Safety Standards – FR4 materials adhere to severe fire safety rules, making electrical gadgets secure to use.
  3. Material Integrity – FR4 retains its structure in the presence of heat or flames and generates no harmful fumes, hence increasing the reliability and security of electronic devices.

FR4 is a standard of NEMA(National Electrical Manufacturers Association) for glass-reinforced epoxy resin laminate that ensures it fulfills the flame resistance requirement UL94V-0. To be UL94 V0 compliant, a material must self-extinguish within 10 seconds of a vertical flame test and not emit firing drops.

This standard is essential for engineers to understand how flammable a substance is. If a material passes the UL94 V0 test and is UL-certified, it is suitable for use in applications requiring fire safety. This makes FR4 a safe choice for PCBs in terms of fire resistance.

Mechanical Properties

FR4’s mechanical properties are important for the strength and dependability of PCBs. Tensile strength is the greatest amount of strain that a material can withstand when pulled or pressed before breaking.

A megapascal (MPa) is a unit of pressure that is widely used in technology and research. One megapascal is equivalent to one million pascals, the single unit of pressure. This great strength means that the PCB can survive manufacturing, assembly, and operational forces without breaking.

Flexural strength refers to the ability to cope with displacement during stretching. FR4 has a high flexural strength, which ranges from 350 to 600 MPa. This feature is critical for PCBs that must bend or flex during assembly or operation, such as in flexible electronics.

The mechanical qualities of FR4 are critical for producing sturdy and dependable PCBs. Tensile strength is an indication of how much strain a material can withstand before breaking when pushed or pressed. Flexural strength refers to the ability to withstand bending. FR4 has a high flexural strength making it ideal for PCBs that must bend during assembly or usage, such as in electronic devices.

Electrical Properties

The electrical characteristics of FR4 are critical for signal integrity and frequency control in PCBs.

  • Dielectric constant (Dk)

The dielectric constant (Dk) measures how well a material can store electrical energy. FR4’s Dk ranges from 3.8 to 4.8, depending on the grade and composition.

A lower Dk is better for high-frequency applications because it reduces signal loss and crosstalk. This helps in maintaining clear and accurate signals. For high-speed digital and RF circuits, controlling the Dk is essential for reliable performance.

It allows for precise control of signal speed and reduces signal distortion, ensuring cleaner and more reliable transmission. FR4’s stable Dk value also helps maintain signal timing, reducing the risk of timing errors in high-speed applications.

  • Dissipation Factor (Df)

The Dissipation Factor (Df) determines how much electrical energy a material dissipates as heat. A smaller dissipation factor is preferable for applications that use high frequencies to maintain signal integrity.

Low Moisture Absorption

The FR4 substance is resistant to absorbing moisture, allowing it to maintain its shape and size regardless of humid environments. This makes it ideal for usage in electronics and electrical parts that are exposed to dampness.

FR4’s capacity to resist water absorption is significant. In controlled experiments, it absorbs little moisture, increasing in weight by only 0.10% after 24 hours of soaking in water.

These characteristics make FR4 suitable for use in harsh locations, such as ship engine rooms. Even when working with water, FR4 remains solid and reliable. Its moisture resistance, stable sizes at various temperatures, and outstanding electrical qualities make it a flexible and dependable material for a wide range of electronic applications.

FR4 Material
FR4 Material

The Grades of FR4 Material

FR-4 copper-clad laminates are essential materials in an extensive number of electronic products, with grades available to meet a variety of price and performance needs. These laminates are utilized in a wide range of industries, including the military, interaction, computers, digital circuits, industrial instruments, and automotive circuits, to ensure reliable and high-quality performance.

Each grade, from high-performance A1 to low-cost B, has been created for a specific purpose while balancing price and performance. Here are the grades of FR-4:

  • FR-4 A1 Grade Copper Clad Laminate
  • FR-4 A2 Grade Copper Clad Laminate
  • FR-4 A3-Level Copper Clad Laminates
  • FR-4 AB1 Grade Copper Clad Laminate
  • FR-4 AB2 Grade Copper Clad Laminate
  • FR-4 AB 3 Grade Copper Clad Laminate
  • FR-4 B Grade Copper Clad Laminate
  • FR-4 Halogen-Free Copper Clad Laminate

A1 grade provides high performance in demanding applications such as the aerospace, military and the automotive sector. A2 and A3 groups deliver satisfactory performance for common gadgets and home appliances, with A3 being less costly.

AB1, AB2, and AB3 grades are designed for lower-end applications with a focus on price. B grade is ideal for less demanding industries and has the lowest price. Halogen-free laminates are an environmentally safe solution for excellent applications in a variety of sectors.

FR-4 in PCB Fabrication

FR4 PCB design: It is critical for the results, security, and cost of electronic devices. Material selection, stackup design, and signal integrity consideration are the most important factors when using FR4 in PCB.

Material Selection: Choosing the right FR4 material depends on factors like operating temperature, power dissipation, frequency performance, and cost. For high temperatures or power, a high Tg FR4 grade is better. High-frequency applications need FR4 with a lower dielectric constant and dissipation factor to reduce signal loss. Cost also matters, balancing performance and budget.


Stackup Design: The stack-up design of a PCB includes organizing its layers, including copper and insulating layers. The dielectric constant of FR4 affects impedance, influencing signal integrity. Proper trace widths, spacing, and layer distances are essential to achieving the desired impedance. Thermal conductivity is important for heat dissipation, requiring more techniques like heat sinks or thermal vias. Mechanical features such as tensile and flexural strength ensure that the PCB can survive production and operational stress.

Signal integrity and EMC: The features of FR4 have an impact on signal integrity as well as electrical compatibility. Signal integrity refers to the accuracy of electrical signals as determined by the dielectric constant, dissipation factor, and trace geometry. EMC ensures that the gadget neither causes nor is harmed by electromagnetic interference (EMI). Techniques to improve signal integrity and EMC with FR4 include impedance control, ground planes, decoupling capacitors, differential signaling, and EMI shielding.

How is FR-4 Used in PCBs?

FR-4 is the base of PCB and it acts an insulator.This is the foundation upon which the manufacturing business constructs the circuit. In PCB fabrication, FR-4 serves as the main insulating layer, ensuring the PCB can control mechanical and thermal stress.

During preparation, the FR4 board is laminated with several layers of copper foil using heat and epoxy. This copper creates the circuits in the result and might cover one or both sides, based on how the board is designed.

FR4 Thickness
FR4 Thickness

The Thickness of FR-4

When choosing the thickness of FR4, many factors must be considered. For limited PCB space, thinner boards are better, aiding in the miniaturization of electronic products. Lighter boards reduce the weight, contributing to lower transportation costs and appealing to consumers.

The thickness of FR4 affects impedance matching by determining the dielectric thickness, which impacts capacitance. Thin boards offer flexibility, which is beneficial for applications requiring bending, but they might not be suitable for products that experience frequent stress.

The thickness must also accommodate through-hole components with varying pin lengths to ensure functionality. Common FR4 thicknesses range from 0.3mm to 2.0mm, with typical copper thicknesses being 0.5oz, 1oz, and 2oz.

Rigid Multilayer FR4 SMT PCB Assembly
Rigid Multilayer FR4 SMT PCB Assembly

Limitations of FR4 Circuit Board Materials

While FR4 circuit boards are used, they have limitations to consider:

1. Insulating Stability – FR4 is good for insulation but can be damaged by stress, high temperatures, or excessive current, leading to circuit failure.

2. Controlled Impedance – Unlike high-speed materials, FR4’s dielectric constant varies more with frequency, making it challenging to maintain precise impedance in designs.

3. Signal Loss – FR4 boards may not be ideal for high-frequency applications due to higher signal loss compared to specialized materials.

4. Temperature Stability – FR4 boards have limits on how much heat they can handle; exceeding these limits can damage the material.


FR4 is a popular material for PCB manufacturing due to its strength, flame retardancy, and insulation from electricity qualities. It is made of fiberglass fabric and epoxy resin, which provides rigidity and durability while also protecting the safety of electrical gadgets. FR4’s ability to handle a wide range of applications, from ordinary to high-frequency circuits, makes it important, despite its limits in regulated impedance and temperature sensitivity. Understanding these factors aids in refining PCB designs for reliable operation and cost-effectiveness in electrical goods.

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Irene Shi
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