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What is FR4 PCB? A Complete Overview

Table of Contents

For many years, the FR4 PCB has been the most popular type of printed circuit board. It still is, even with the emergence of several other substrates. So, what is this material and what is it made of? For more about the FR4 PCB board, read its detailed overview below, starting with the definition.

What is FR4 PCB?

FR4 PCB means a type of printed circuit board that contains FR4 as its base and pre-preg material. FR4 full form is “flame retardant”. It means a material that will not spread a fire and that complies with the UL94V-0 standard for flame retardation.

The number “4”, on the other hand, is used to for the glass-reinforced resin that’s used to make FR4. It represents a standard that NEMA uses to denote the material based on its characteristic.

To make the PCB FR4, the resin is mixed with glass fibers and used to create sturdy sheets. For flame retardation, different compounds are added. These can be halogen-based, such as brominated types, or non-halogen.

In addition to offering flame retardation and excellent mechanical strength, the FR4 in PCB boards offers insulation and other properties such as ability to withstand high temperatures.

High Tg FR4 PCB for space application
High Tg FR4 PCB for space application
Resource: https://www.researchgate.net

FR4 PCB Material

FR4 PCB material is basically composed of the following: glass fibers, epoxy resin, and several other substances; the exact composition varies across different manufacturers. Generally, the use of different material compositions and structure gives rise to these types of FR4 PCBs.

Standard FR4

This is the conventional FR4 with basic properties. It’s also one of the most common when it comes to basic circuit board applications. Standard FR4 does not possess the high-end properties meant for more demanding conditions such as extreme temperatures and other environments.

High Tg FR4

Tg means glass transition, and Tg temperature the heat level at which a solid material begins to change state and become rubbery or soft. High TG FR4 means a PCB material that can withstand higher heat levels (of above 170°C), without changing shape.

High CTI FR4

CTI stands for Comparative Tracking Index. When used on PCBs, it refers to the ability by the laminate to resist the current flowing in the copper tracks or traces. A high CTI FR4, therefore, offers a higher resistive value for unwanted currents in case of leakages.

Halogen Free FR4

A halogen free FR4 PCB contains no halogen compounds, or contains them in very low (and negligible) quantities. This type of FR4 PCB material is becoming the norm today, given the toxic nature of halogens, and the shift toward safer circuit boards by users around the world.

Copperless FR4

Copper-less FR4 is a board that contains no copper layers or traces. Made of FR4, it has all its properties, but cannot conduct electricity. It’s, therefore mostly used for insulation or, more commonly, as a stiffener board in rigid flex circuit boards.

FR4 multilayer PCB
FR4 multilayer PCB
Resource: https://www.youtube.com/watch?v=Ukc_kc5OAhw

FR4 PCB Design

Depending on the required features and application needs, FR4 PCB board can be made in many different structures or designs; single sided, double sided, and multilayer circuit boards, among other options. These design options are explained in more detail below.

Single Sided FR4

This is most basic design of an FR4 PCB sheet. It comprises a single layer of copper backed by the FR4 core. Basic electronic products utilize this type of board, making it one of the most common. For applications that require packing more components, a double sided board is used.

FR4 Double Sided PCB

The double-sided PCB is more advanced. It consists of two conductive copper layers, with the FR4 laminate in between them. The two sides make the board more versatile, since manufacturers can place components on both surfaces. This design is useful in high-component density applications.

FR4 Multilayer PCB

The multilayer board is even more complex, being composed of more than one FR4 sheets. The number of layers is dependent on the application, and can range from 2 up to 16, or even more. To connect the different layers, tiny holes called vias are drilled through the board, and an array of other features included.

FR4 PCB material properties chart
FR4 PCB material properties chart
Resource: https://www.researchgate.net

FR4 PCB Material Properties

In addition to being flame retardant and offering good electrical insulation properties, printed circuits made from FR4 offer several different characteristics. These may or may not suit an application. Understanding them can, therefore, help you make the right choice. Important FR4 PCB properties are listed below:

Temperature Rating

FR4 material can withstand temperatures above 345° before decomposing. In terms of operating temperature, PCBs made from this material can work in a broad range of heat levels, typically between 50°C and 115°C. This property makes the material perfect for a wide range of application requirements.

Thermal Conductivity

FR4 does not offer good thermal conduction, with a conductivity level below 1W/mK (watts per Kelvin). This makes it a poor conductor with low ability to dissipate heat. However, the presence of copper in a PCB made of this material improves the conductivity by a small margin.

Dielectric Constant

Depending on various factors (composition, weave structure, amount of resin, etc.) FR4 PCB material can have a dielectric constant (Dk) between 3.3 and 4.8. These values are not constant, though, and change with rising signal frequencies. For example, the Dk value is 5 at 1 MHz, and higher when the frequency goes up.

Moisture Absorption

FR4 has a low moisture absorption rate. This gives it a high ability to resist damage by water. When immersed in water for twenty four hours, the absorption is measured at 0.01%. This makes it suitable for use in both dry and humid conditions.

Density

FR4 PCB density denotes its weight to volume ratio. The typical density of FR4 is 1.85/cm³. This makes it a lightweight but rigid material that can fit many different uses, from household electronic products to PBs for use in portable communication gadgets.

An example of thin FR4 PCB
An example of thin FR4 PCB
Resource: https://www.researchgate.net

FR4 PCB Thickness

Thickness can affect a circuit board’s performance. A laminate that is too thick, for instance, may cause signal problems. If too thin, it may not be strong enough for its electronic components or application. FR4 PCBs are built with standard thicknesses, or they are customized to fit requirements.

Standard Thickness

The standard FR4 circuit board is normally 1.57mm thick. Outside of this the measurement, the value can go up to 2.30 mm or even higher. There are also boards that are as thin as 0.78 mm, depending on the application needs and other requirements.

Thickness Tolerance

PPCB tolerance defines the allowable variation in the board’s dimensions, in this case, its thickness. Usually, this is calculated at + or – 10%. As an example, let’s say an FR4 PCB is needed to measure 2 millimeters with a tolerance of 0.05. On the upper limit, it may measure up to 2.05 mm, but not less than 1.95 mm thick on the lower side.

Black FR4 PCB, one of its many color options
Black FR4 PCB, one of its many color options
Resource: https://www.researchgate.net

Why Choose FR4 PCB?

FR4 is a popular material for printed circuit boards. It comes with a number of favorable characteristics that make it great for different PCB uses. Depending on the nature of your application or budget, you may want to choose it for the following reasons:

  • Low Cost: FR4 PCB material costs less than most other options such as metal substrates. This makes it an attractive choice when it comes to most PCB types, from the basic designs to complex, multilayered circuit boards.
  • Excellent Mechanical Strength: the FR4 PCB is a robust board that can hold heavy components. That’s in addition to being rigid enough for use in harsh conditions.
  • Moisture Resistant: resin bound, a FR4 board resists moisture, with a near zero absorption rate. So you can use it in humid conditions.
  • Flame Retardant: with a flammability rating of UL94-V0, an FR4 circuit board will not spread a fire in case of accidents, which makes it a safe material option.

FR4 has its limitations, too. It cannot effectively dissipate heat, for example, given its low thermal conductivity. It dielectric constant also changes at higher frequencies, which may not suit applications that require controlled-impedance.

FR4 alternatives include aluminum and other metal PCBs, ceramics, and polyimide, especially when it comes to flexible circuit boards, and CEM (composite Epoxy Material).

Conclusion

The FR4 PCB is the most common type of circuit board today. Being a low cost option and possessing an array of favorable properties, it makes a good laminate for a broad range of PCB uses. Common industries where circuit board using FR as the core and pre-preg dielectric include automotive, consumer electronics, and industrial systems.

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