why fr4 is used in pcb

What is FR4 PCB?

FR4 PCB is a circuit board that is made of FR4 substrate material. Generally used for rigid PCBs, FR4 has a unique ability to form a category of PCB known as semi-flex PCB. Here two major PCB parts are electrically connected by a thin FR4 that can bend, hosting just traces and not components, enabling a big PCB to fold and fit into a smaller volume without compromising performance.

What is FR4 PCB Material?

FR4 or Flame Retardant 4 is a grade given to a material that is composed of woven fiberglass cloth with an epoxy resin binder (the epoxy resin gives it the flame-retardant or self-extinguishing properties). It is a robust material that can cater to a variety of applications.

It was introduced to substitute phenolic paper laminates, like G10, FR-2, and FR-3, which had relatively poor mechanical, thermal, and electrical performance. FR4 is the go-to material for most of the designers due to its capabilities, cost, and availability.

Types of FR4 PCB Materials

As mentioned earlier, FR4 is a grade of material; hence, it has many variants and types offered by various manufacturers, like Isola, ITEQ, Shenyagi, and Panasonic. These variants share the manufacturing process and the basic structure of the material, but differ in properties. FR4 is most widely differentiated by the Tg of the material, that is, the glass transition temperature.

What does Tg Mean in FR4 PCB Material?

Tg, or the glass transition temperature, refers to the temperature up to which the substrate material remains physically stable.At or above this value, the material may start changing its mechanical state; that is, the solid structure may transform into a soft, malleable state. Even after the return of the temperature to its Tg value, the substrate material may stay distorted and not convert back into its original mechanical form.

This unwanted transformation caused by higher temperatures than the Tg value of the substrate material can cause circuit failure, component malfunction, and device errors. This damage may be irreparable and may require a complete replacement of the PCB.

Therefore, it is crucial to use PCB material as per the heat and temperature resistance requirements for your device. In case the device is expected to heat up or function in high-temperature environmental conditions, the PCB should be built accordingly.

The high Tg values of FR4 materials make them the top choice for printed circuit boards. FR4 circuit boards also have variants depending upon the Tg values and are classified into 3 major types–

• Standard Tg (130°C to 140°C) – Used for general applications and the cheapest among all materials.
• Mid Tg ( 140°C to 160°C) – Also used for general applications, but provides better results in the case of lead-free soldering.
• High Tg (170+°C) – High Tg FR4 boards are generally not used for general applications due to a higher cost, but are very extensively used among multilayer boards (4 plus layers). High-Tg FR4 generally also offers superior electrical and thermal performance. These materials are used in cost-sensitive high-performance boards. These laminates are the costliest among the three categories.

Properties of FR4 Material

FR4 PCB material is known for its favourable properties and is sufficient for most applications. It has the following properties (since FR4 has many variants, most of the properties will have a range of the highest and the lowest numeric factor available among variants) –

• Dielectric Constant (Dk) – 3.8 – 4.7
• Dissipation Factor (Df) – 0.02 – 0.03
• Volume Resistivity – Higher than 10¹³ Ω·cm
• Dielectric Strength – 20 – 50 kV/mm
• Tensile Strength – 350 – 500 MPa
• Flexural Strength – 400 – 600 MPa
• Glass Transition Temperature (Tg) – 130 – 180°C
• Thermal Conductivity – 0.3–0.4 W/m·K
• RoHS & REACH Compliance – Yes (in most cases)
• HDI compatibility – Limited to a few variants.

To give a brief gist of what these properties imply, FR4 PCB material has a relatively high Dk and Df, making it suitable for applications hosting up to 1GHz signals. While its Tg is among the lowest of most of the materials used in modern PCBs, it suits the requirements of most general applications used in everyday life. It has sufficiently good mechanical properties that are expected from a Rigid PCB. It has ROHS and REACH compatibility, but HDI support is limited to a few high-performance FR4 circuit boards.

Benefits of Using FR4 PCB Material

FR4 PCB is widely used because of its plethora of advantages when compared to other materials. Some of the major benefits of using FR4 circuit boards are –

• Cost – No PCB can beat the cost of an FR4 PCB board, even when considering high-performance FR4. It is the most economical choice among all materials and sets a gold standard for other materials. In fact, high-end products use FR4 in their inner layers to reduce their overall cost; such PCBs are also called hybrid PCBs.
• Availability – Almost all PCB manufacturers offer FR4 PCBs. Although they may offer limited variants of FR4, standard FR4 is widely available.
• Physical Strength – As explained earlier, FR4 is a composite material made of woven fiberglass and epoxy resin. This structure provides it with immense mechanical strength, which further strengthens the device body. It also stabilises the PCB’s performance under vibration and other physical stress.
• Flame Resistance and Thermal Stability – The self-extinguishing property is so significant that the PCB material is named after it – FR4, meaning flame retardant. High Tg values of the different variants of FR4 PCBs prevent degradation and ensure stability under high temperatures.
• Insulating Property – The high dielectric value (Dk) of FR4 PCB material provides it with electrical insulation. This property is quite useful in making multi-layer PCB boards as it checks electrical leakage between the layers.
• Chemical and Moisture Resistance – PCBs need to endure harsh environmental conditions. The chemical resistance and a low moisture absorption rate of FR4 boards increase their durability, further increasing their demand.
• Versatile Use Cases – FR4 PCB material has many variants that can handle general applications and some high-end applications, as well. Variants of FR4, like Isola FR406 and Shengyi S1000H, also cater to complex applications that would otherwise require other high-end materials.
• Sufficiently Suitable Properties – Although FR4 is the most basic PCB substrate material among other high-performance materials, it is notable that it ticks the boxes for most physical and thermal properties required for PCBs to function properly.

Applications of FR4 PCB

FR4 is the most common material and is used in almost all industries. But the most notable applications for FR4 PCB are –

• General Electronics – General electronics consisting of a few ICs and signal requirements lower than 100MHz do not even consider materials other than FR4 (if flexural properties are not required) due to its cost-effectiveness.
• Networking Equipment – SOHO ( Small Office/Home Office) networking equipment generally does not handle speeds beyond 1Gb/s, which can easily be hosted on an FR4 PCB.
• Consumer Electronics – Most of the consumer electronics use FR4 PCBs. Devices like smartphones and Laptops use a higher-grade FR4 for their motherboards. For handling high-frequency analog signals, modules with high-performance materials are used and connected with FR4 PCB motherboards.
• Medical Devices – FR4 PCBs are used in diagnostic equipment, monitoring devices, and low-risk portable instruments.
• Display PCBs – High Tg FR4 PCBs are widely used in LED displays for signage and hoarding boards.

FR4 vs Rogers vs Polyimide

If FR4 material is so versatile with all properties necessary for a PCB, wondering why and when other materials are used? Here is a quick comparison among popular types of PCB substrate material – FR4, Rogers, and Polyimide.

Limitations of FR4 PCB Material

Like every coin has two sides, seemingly perfect FR4 material comes with its own set of limitations. It is due to these limitations that other materials came into use for certain PCB requirements.

1. Thermal management

While the low electrical conductivity of FR4 material works in favour of PCB requirements, its low thermal or heat conductivity works against the same. Applications that need good heat dissipation usually prefer PCB substrate materials like aluminum over FR4.

At the same time, with a few design considerations, a learned designer can easily mitigate this limitation. For example, incorporating elements like heat sinks, thermal pads, and thermal vias can help dissipate the excess heat well. Additionally, thicker copper layers and hybrid designs can resolve the issue without affecting the budget a lot.

2. Signal loss

In high-frequency (HF) and radio-frequency (RF) applications, FR4 PCB boards are rarely used due to their higher signal loss tendency and dissipation factor. Rogers is used instead in such applications.

3. Environmental compliance

To achieve flame-resistant properties, standard PCB FR4 material is halogenated. These halogens may lead to environmental pollution and health hazards upon careless disposal of FR4 PCBs.

To prevent this, newer halogen-free variants of FR4 printed circuit boards are available now. They are used in eco-friendly versions of consumer electronics and certain medical devices.

So, When Should You NOT Use FR4 PCB?

If your application requires a flexible PCB, microwave or RF circuits, environmentally conscious design, or high heat dissipation, FR4 PCB substrate material might not be the best choice for you.

Although certain design considerations may help overcome these limitations. For example, a semi-flex PCB can be fabricated with FR4 material in a way that the rigid structures are connected with a small bendable band that allows mobility while ensuring cost-effectiveness.

Consult the experts to navigate through such challenges and find the right FR4 PCB solutions as per your specific application and budget requirements.

How to Choose the Right FR4 Grade for Your Application?

As explained above, FR4 material is available in various variants, each with its own characteristic properties and capabilities. Therefore, consider all of the properties well to find the right balance between performance needs and manufacturing costs.

• Consider the electrical demands of your application and check the corresponding properties of FR4 material, such as dielectric constant.
• Check if the Tg values are suitable for your application’s expected environmental conditions and operational temperatures.
• Ensure that your application does not require high heat dissipation or high-frequency signal circuits.
• Refer to the IPC regulations, NEMA standards, and FR4 design considerations to meet all safety standards and ensure the quality of the final product. Compliance is non-negotiable.
• Remember, not all applications need high-end PCB solutions, and cheap does not always mean low quality. Compare the performance requirements with costs and balance both accordingly.

Talk with us today to make the right choice for your FR4 PCB needs.

Conclusion

FR4 is a cost-effective, reliable, and versatile laminate to use in PCBs. While it is not suitable for high power, high heat, or very high frequency applications, it is more than sufficient for most applications, nonetheless. The capabilities of FR4 circuit boards are advocated by the massive number of applications in which they are being used.

Though almost all possible details regarding FR4 properties, applications, and limitations have been discussed here, it is highly recommended to check the datasheet of the laminate or ask your fabricator for its suitability for your application requirements. You may also drop your query below or get in touch with our experts for personalised assistance.

Additional Resources:

• What is Bare PCB?
• What is Polyimide PCB?
• What is Multilayer PCB?