What is FR-4 in PCB?

A printed circuit board is a complex assembly part of various parts that work together to facilitate the electrical functionality of electronic devices. Among these parts, the PCB substrate plays a crucial role by providing mechanical support and electrical insulation. One of the most widely used substrate materials in PCB design is FR-4.

In this article, TechSparks aims to shed light on FR4 material, delving into its composition, properties, and the reasons behind its widespread adoption in PCB manufacturing. By understanding the advantages and characteristics of FR4, you can make informed decisions when selecting materials for your PCB projects.

Table of Contents

What is FR-4

FR4 material

We have already mentioned that FR4 is a common PCB substrate material, belonging to the category of epoxy boards or epoxy fiberglass laminates. It is composed of a fiberglass cloth that provides strength and durability, bonded together by epoxy as an adhesive for insulation purposes.

When utilizing FR4, different specifications can be chosen to suit specific needs. This includes options for colors such as white, black, green, blue, and more. In terms of material thickness, 1.6mm and 1.2mm are currently the most commonly used choices.

So why is it called “FR-4”?

The “FR” stands for “flame retardant”, indicating the material’s ability to self-extinguish after burning. The number “4” represents the grade code for flame-resistant materials. It is important to note that “FR-4” is not the material’s name but a grade designation commonly used in the industry, but we call it FR4 substrate material by habit.

Since “4” stands for grade, does that mean there are other grades?

Yes! FR include five grades of “1, 2, 3, 4, 5”, and they are slightly different in composition and usage scenarios.

  • FR1: Composed of phenolic resin and paper base, it possesses lower flame retardancy and mechanical strength. It is commonly utilized in simple applications due to its cost-effectiveness.
  • FR2: Similar to FR1, but with optimized paper manufacturing and modified resin formulation for improved mechanical and insulating properties.
  • FR3: Composed of phenolic resin and fiberglass substrate, it offers good mechanical strength and heat resistance.
  • FR4: Composed of glass fiber cloth and epoxy resin, it provides superior mechanical properties, insulation properties, electrical properties, and heat resistance compared to the previous grades. It is also more cost-effective than materials like ceramics and aluminum substrates, making it the most prevalent choice for PCB substrates.
  • FR5: Composed of glass fiber and epoxy resin, commonly employed in circuit board designs for high-frequency, high-speed, and high-temperature environments.

Why We Use FR-4 Substrate in PCB

FR4 material indeed offers a wide range of advantages and can be used not only as a PCB substrate material but also as a mechanical part, partition, and isolation material. Let’s discuss the advantages of FR4 as a PCB substrate material:

  • Process Compatibility: The number of circuit layers is a crucial consideration for designers during the PCB design process. However, not all substrate materials can accommodate the required number of layers. For instance, ceramic PCB typically has a limitation of up to 2 layers; Polyimide PCB is generally limited to 6 layers; Metal PCB is typically restricted to 4 layers. These limitations arise due to constraints in the manufacturing processes involved, including etching, welding, and assembly. In contrast, FR4 PCB can easily achieve a multi-layer circuit structure and are compatible with various processing techniques such as drilling, routing, etching, and component placement.
  • High Temperature Resistance: As mentioned earlier, FR-4 is a flame retardant material, offering an added advantage of high temperature resistance. The thermal conductivity of FR4 substrate typically ranges between 0.2 and 0.4 W/m-K, with variations depending on the material thickness and additives used. This allows the FR4 circuit board to operate within a temperature range of 130°C to 150°C while maintaining good electrical properties. It’s important to note that FR 4 material strikes a balance between price and performance and may not be the ideal choice for high-temperature applications. For such applications, materials like aluminum substrates are more suitable, especially in LED high-power applications.
  • High Reliability: FR-4 material offers excellent structural stability due to its fiberglass construction. This rigid material provides enhanced resistance to impacts and drops, making it advantageous in terms of physical durability. Furthermore, FR-4 has a low Coefficient of Linear Expansion (CTE) and Thermal Coefficient of Expansion (TCE) of 15-18 ppm/°C and 12-18 ppm/°C respectively. These values indicate excellent dimensional stability of the substrate material. Additionally, FR-4 demonstrates the ability to withstand electromagnetic interference and radiation interference, further contributing to its reliability. These factors collectively contribute to the long-term reliability of FR4 PCB board.
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