PCBs are the foundational platform for mounting and interconnecting components in a compact space. These PCBs were able to achieve compactness by the introduction of insulation material between the conductive layers. Dominating the numbers in the industry for insulation material is FR4. It is perhaps the most common material used in PCBs, and for a very good reason: FR4 offers an ideal combination of mechanical strength, electrical insulation, flame resistance, and affordability. This article dives deep into the properties, advantages and applications of FR4 PCBs.
What is FR4 PCB?
FR4 PCB is a circuit board that is made of FR4 substrate. FR4 or Flame Retardant 4 is a grade given to a material that is composed of woven fibreglass cloth with an epoxy resin binder (the epoxy resin gives it the flame retardant properties). It was introduced to substitute phenolic paper laminates like G10, FR-2 and FR-3 that 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. It can be said that FR4 has set the Gold standard benchmark for other materials to compare with. FR4 is a robust material that can cater for a variety of applications. In fact, FR4 has a unique ability to form a category of PCB known as semi-flex PCB, where 2 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.
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 that share the manufacturing process and the basic structure of the material, which differ by various properties. FR4 is most widely differentiated by the Tg of the material, and is classified into 3 major variants –
- 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 are generally not used for general applications due to a higher cost, but is very popular 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 popular due to its favourable properties, and is sufficient for most of the 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. However, its Tg is among the lowest of most of the materials used in modern PCBs. It has sufficiently good mechanical properties that are expected from a Rigid PCB. It has ROHS and REACH compatibility, which most modern materials have. But HDI support is limited to a few high-performance FR4.
Benefits of Using FR4 PCB
FR4 PCB is popular 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 FR4 PCB board, even when considering high-performance FR4. It has the lowest cost among all materials and sets a gold standard for other materials. Even other materials’ costs are compared against the cost of FR4. In fact, high-end materials use FR4 in their inner layers to reduce the overall cost of the PCBs, such PCBs are also called hybrid PCBs.
- Availability – Almost all the PCB manufacturers offer FR4 PCBs. Although most of the manufacturers may have limited variants of FR4 that they offer, they do at least offer standard FR4. Availability is also boosted by the fact that FR4 PCB fabrication is now a set standard among PCB fabricators. Even other materials with the same manufacturing process as FR4 consider it an advantage over other materials.
- Versatile Use Cases – FR4 PCB material has so many variants that are capable of handling general applications, high-end applications, to some extent. Variants of FR4, like Isola FR406 and Shengyi S1000H, also cater to complex applications that would otherwise require other high-end materials.
- Sufficiently Good Properties – Although the properties of FR4 are not as impressive as other high-performance materials, it is important that it ticks the boxes for most of the physical properties and thermal properties that are required for the PCB 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 consist of a few ICs, and signals lower than 100MHz do not even consider materials other than FR4 (if flexural properties are not required) due to it being very affordable.
- Networking Equipment – SOHO ( Small Office/Home Office) networking equipment generally does not handle speeds beyond 1Gb/s, which can easily be hosted on a 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 that are widely used in LED displays for signage and hoarding boards.
Conclusion
FR4 is a cheap, reliable and versatile laminate to use in PCBs. However, it is worth noting that it is not suitable for high power, high heat or very high frequency applications and is not recommended for the same. Suppose you are considering using a FR4 variant for high-speed applications or HDI designs. In that case, it is highly recommended to check the datasheet of the laminate or ask your fabricator for its properties. Nonetheless, FR4 PCB is more than sufficient for most of the applications and the rest is advocated by the massive number it is being used in.
