Why Choose FR4 for Rigid PCB?

PCB is one of the most common and essential parts of countless electronic equipment in the market today. In addition, FR-4 is one of the most widely used materials used for building the PCB. But before we discuss why it is so, let’s take a step back to understand the basics.

What Is PCB?

PCB stands for Printed Circuit Board. At times you come across these thin green boards that have intricately designed brown copper wires and a few other components; it sounds familiar, right? Most probably, that is a PCB.

A PCB is made of a thin baseboard with copper wires printed on it. These wires act as connections between components you want to connect. All you need to do is solder the two components at different points based on your necessity, and you’d be set.

Types Of PCB:

There are usually three types of PCBs:

  1. Rigid PCB
  2. Flexible PCB
  3. Rigid-Flex PCB

Rigid PCB:

A rigid PCB is an inflexible and unfoldable PCB. Once built, it stays the same, and its shape cannot be modified. It is the largest manufactured type of PCB, and the best example of its application is the motherboard in PCs. These are traditional PCBs that are much cheaper than flexible PCBs.

Flexible PCB:

Flexible PCB or Flex PCB is almost the same as the rigid PCB, except its board is made with a much more malleable material. This enables them to bend in any shape or design while being used in the application.

Flex PCB is made to suit the design of the equipment in which it will be used and not vice versa, as in the case of rigid PCB.

Rigid-Flex PCB:

As the name suggests, rigid-flex PCB is a hybrid of both rigid and flex PCB. It has multiple layers of flexible substrates that are connected to a rigid PCB. It is usually designed in a way that it can be bent to fit any product.

Furthermore, its most significant advantage is that these rigid-flexes can be scaled in a product, and hence it is also known as dynamic flex.

Each of these types of PCB has three sub-types:

  1. Single-sided PCB
  2. Double-sided PCB
  3. Multilayer PCB

Single-sided PCB:

Single-sided PCBs allow you to mount conductive copper and components on only one side, while the other side is used for conducting wiring. In addition, since there’s only one layer where the components can be mounted, the density of the circuits is less.

Double-sided PCB:

One can mount conductive copper and components on both sides in a double-sided PCB instead of merely one. As a result, the circuits are much denser compared to those in a single-sided PCB.

Multilayer PCB:

A multilayer PCB must have at least two layers of conducting material, unlike a double-sided PCB. The circuits in this are highly dense and hence challenging to produce. However, it has several other advantages, such as space efficiency, reduced total weight, and many others.

Now that you have a brief idea about PCBs and their different types let’s understand why the FR4 is best suited for rigid and other types of PCB.

About FR4

FR4 stands for flame retardant, and 4 stands for a woven glass-reinforced epoxy resin. The former means that it is resistant to fire, while the latter describes the materials used to make an FR4 substrate.

FR4 isn’t a name but a category or material grade. There are several types of FR4 materials, and all those types need to be resins that are self-extinguishing. FR4 substrates are the most widely used materials for PCBs due to their high mechanical and insulating properties.

Characteristics Of FR4

  • FR4 has an excellent strength-to-weight ratio.
  • It is considered one of the strongest materials out there for building or making a printed circuit board.
  • It acts as an insulator while being used in PCBs and is usually used in the core of a PCB.
  • Its most significant properties include resistance to fire and water. These help keep any circuits safe from possible disasters.
  • Furthermore, this material is highly flexible compared to others and hence is pretty helpful for different types of PCBs.
  • The FR4 material grade performs well in hot and humid conditions and isn’t affected by the environment.
  • FR4 is impact resistant which is yet another quality that makes it appropriate for usage in PCBs.

Structure Of FR4 And How It Is Made

FR4 broadly consists of two components. First is the epoxy resin that acts as an adhesive, and second is a glass fiber cloth of electronic grade used as a reinforcing material.

FR4 is made from multiple sheets of prepreg. This prepreg consists of the glass fiber cloth that is impregnated with epoxy resin.

The glass fiber is made by weaving thin strands of glass fiber together, and in turn, these strands are made from tiny threads of glass twisted together. The epoxy resin is usually developed using epichlorohydrin and bisphenol-A.

Usually, a circuit board is made of 8 layers of prepreg and outer copper layers. All these layers are stacked up together and put into a long press, combining the entire structure into one fine laminate.


The qualities of FR4, such as near-zero water absorption, flame resistance, impact resistance, and more, make it perfect for PCBs.

PCBs are a crucial part of products responsible for their performance. So good quality materials and components must be used to accomplish high output with lower input.

FR4 in PCBs paves the way for high-quality circuits at a relatively lower cost when compared to other materials such as G10 grade and several others.

How To Choose The Right FR4 Material For Your PCB?

The most significant factor to consider while choosing an FR4 material is its thickness. The thickness affects the material’s various properties, and not choosing the right one would lead to dangerous hazards.

The thickness is usually measured in millimeters or inches, and the average thickness of an FR4 material is 1.6mm to 1.8mm.

While choosing the right thickness, there are several aspects to consider. Here are a few that can help you decide the optimum thickness for your products and design.

Space Efficiency

Products that have a significant space constraint need to have thinner FR4 material. For instance, FR4 used for USB connectors, Bluetooth accessories, and other equipment needs to be significantly thinner.

However, thinner materials are not limited to these. For projects which require a high-level intricacy circuitry and design, thin material is usually advisable.

The rationale behind this is that it would occupy less space and make it less heavy, thus keeping the overall product at an optimum mass.


It is also essential to understand what would be the usage of the PCB and the material. For instance, certain products in the medicine and pharmaceutical field require the products to be flexible. In cases like these, it is advisable to use a thinner FR4 material.

Design With Grooves

For any design that has too many grooves, it is better to use thicker material. Otherwise, there is a risk of the board breaking or damaging, leading to an irreparable product failure.


Any product, no matter where it is used and by whom, needs to be easily used. Products that are heavier always provide a poor user experience. So as much as possible, try to employ a thin material.

This leads to the product being lightweight, and not only is it easy to use and simple to transport and ship.


The impedance in the PCB is affected by the thickness of the board as well. The impedance increases progressively with the thickness. So based on your requirements, understand what would be the right amount of thickness and impedance for your circuit.

Thus these are some of the factors to consider while choosing an FR4 material to develop your PCBs.

When To Choose FR4?


FR4 is relatively cheaper when compared to other alternatives of high-frequency laminates. So choosing FR4 over them is appropriate when you want to reduce your cost but maintain the quality.


Its mechanical strength is impeccable and is one quality that sets it apart from several other laminates. Due to this, it is highly durable and can be used for products that would be used in rough conditions.

When Not To Choose FR4?

Though FR4 is widely used in PCB and perhaps the best material for them, there are a few scenarios when they shouldn’t be used. Here are a few of those scenarios.

Loss Of Signal

FR4 has a higher dissipation factor than its substitutes, resulting in a higher probability of signal loss. This will hinder the performance of the PCB that will be exposed to higher signal frequencies.

Furthermore, as the signal frequency increases, the dissipation factor of FR4 also increases, making it more of a challenge for you.

High Temperatures

Extremely high temperatures don’t bode well with the FR4 material since most of those materials have a low Tg. The Tg is called the glass transition temperature, and it is the temperature at which glass alters state.

Due to the low Tg, FR4 is not suitable for operating in exceedingly high temperatures such as near the engine.

Smaller Circuit Boards

Certain FR4 type materials won’t be suitable for smaller circuit boards depending on their size. This is because the smaller the circuit board, the smaller the transmission lines.

The constant dielectric needs to be higher to support these, but FR4 is known to have a low value for the constant. Hence it can’t be used for small circuit boards.

Industries Where Rigid PCB and FR4 Can Be Used

Rigid PCBs are used across various industries for a vast range of purposes. These purposes will be fulfilled successfully with the help of the FR4 material. The industries where FR4 Rigid PCB best used are as follows:

Computers And Electronic Gadgets

Needless to say, the IT industry is perhaps the biggest market for PCBs. It is used in computers, laptops, and almost every office machine that exists. Moreover, rigid PCBs are used in some important places, such as the motherboard of computers, and need FR4 material for efficient functioning.

Furthermore, rigid PCBs also use many other electronic gadgets and utilities such as Bluetooth devices, USB, etc., and other solid-state devices.

Medical And Pharmaceutical

In the medical and pharmaceutical industries, there are a large number of flexible PCBs being used. However, there is various equipment that uses rigid PCBs as well. A few examples are the tomography equipment, Magnetic Resonance Imaging (MRI) systems, and Electromyography (EMG) machines.

Essentially, rigid PCBs are used in machines of enormous size and ones that are not portable. Since these machines do not require much flexibility, it is best to use rigid PCBs. The FR4 material is also best suited for these kinds since you need it to be cost-effective and durable.


PCBs have a massive market in the automotive industry as well. With the advancement of technology, PCBs are being used for a variety of purposes. These include navigation, controlling temperatures, entertainment, and many more.

Though flex PCBs are often used, rigid PCBs and FR4 material are used in a few use cases too.


Aerospace is yet another mammoth industry where rigid PCBs are used. However, a challenge here is the high temperatures at which the PCBs have to perform. But there is a solution that is being employed to mitigate the risks posed by FR4’s usage.

High-temperature laminates are added to the FR4 core to help reduce the heat exposure on the core. This way, we attain an apt solution where the cost is lesser, durability is higher while ensuring the performance is not affected as much due to high temperatures.


Rigid PCBs are also used in the telecommunication industry for power generators, power distribution junction boxes, transmission sensors, electronic computer units (ECU), and more. Furthermore, PCBs are also used for LED display lights, filtering systems, and frequency amplifiers.  

Military And Defense

In military and defense systems, there is numerous sophisticated equipment that employs the usage of rigid PCBs. These need to be low-cost and highly durable, and hence FR4 is best suited for these.

Power systems, transmission sensors, automation networks, cross targeting systems are some of the use cases where PCBs are employed.

These cover numerous industries where FR4 Rigid PCBs are being employed. So it is pretty versatile and vast when it comes to the use cases for PCBs.


Overall, FR4 is one of the best materials for PCBs, especially for rigid PCBs. Its usage across industries and use cases stands testament to this fact.


This is because the investment to results ratio for FR4 is incredibly high compared to numerous alternatives out there in the market. Its incredible mechanical strength makes it durable and hence gives it longevity. The intricacies of the manufacturing process achieve the necessary strength.

Furthermore, FR4 provides other benefits such as low cost, making the expenditure to quality ratio at an all-time high. As organizations and businesses, people are always looking to maximize the output with minimum input. Hence, cost-effectiveness becomes one of the most significant reasons companies of various industries use the FR4 material.

Besides these, the other advantages FR4 offers are its capability to self-extinguish when burnt and nearly zero water absorption. This makes the material usable in various environmental conditions. It also ensures that the PCB won’t be damaged by humid conditions and wouldn’t burn.

All these characteristics and traits make FR4 the perfect material to be used in PCBs. It works even better for rigid PCBs because they need to be strong and perform in a myriad of conditions.


FR4 has a few disadvantages as well. It can’t be used in scenarios where high temperatures are due to its low Tg. Furthermore, it can’t be used in equipment with high frequencies such as RF and microwaves.

Having said this, the pros far outweigh the cons. Moreover, some of the challenges can be solved by employing specific manufacturing techniques and certain materials.


With the pros and cons mentioned above, it is easy to determine why FR4 is the go-to choice of most businesses and organizations across industries.

There are several alternatives to FR4, but they don’t have the same quality as FR4. Furthermore, even if they solve particular challenges faced with FR4, they lack other aspects.

Technology is advancing rapidly, and the physical infrastructure is evolving to support it. With these, there might be newer challenges even for materials such as FR4. It would be interesting to see how it evolves then.

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