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FR-4 laminate is currently the most widely used and best value-for-money core substrate for PCBs in the electronics and electrical sectors, and is the mainstream choice for the mass production of all types of printed circuit boards. As the core foundation of electronic devices, printed circuit boards (PCBs) are now ubiquitous across all industries; however, the operating conditions and usage scenarios of different devices impose strict selection criteria on PCB substrates. The industry’s mainstream PCB substrates are categorised into three main types: rigid, flexible and metal-based. These generally possess excellent insulation and heat dissipation properties, capable of meeting the requirements of the vast majority of conventional electronic devices.
Within the PCB manufacturing system, in addition to the core FR-4 rigid boards, the industry has established three mainstream material selection systems—rigid fibreglass boards, flexible polymer substrates and metal substrates—based on differences in board rigidity, flexibility and material structure, comprehensively covering all types of PCB production scenarios.
Epoxy resin-impregnated glass fibre laminates, to which FR-4 belongs, are the core substrate for rigid PCBs. Thanks to their stable performance and affordable cost, they have become the standard choice for conventional circuit boards in both industrial and consumer applications. In contrast to rigid boards, flexible PCBs are manufactured using lightweight polymeric flexible materials. The mainstream materials include PET (polyethylene terephthalate), PEN (polynaphthalate) and PI (polyimide), specifically designed for specialised flexible electronic devices that require bending or folding. Aluminium substrates, as a typical type of metal-based PCB material, are characterised by their highly efficient heat dissipation properties. They form a distinct application landscape alongside FR-4 fibreglass substrates and flexible polymer substrates, filling a gap in material selection for high-power heat-generating equipment.
Structure, Raw Material Properties and Parameter Characteristics of FR-4 PCB
1. Core Characteristics and Raw Material Composition of FR-4 Glass Fibre Boards
FR-4 is a flame-retardant glass fibre-reinforced substrate commonly used in the electronics industry. It belongs to the glass fibre-reinforced epoxy resin laminate family, and its most distinctive characteristic is self-extinguishing flame retardancy; the board will extinguish itself when exposed to an open flame, fully complying with fire safety regulations for electronic equipment.
FR-4 is a composite functional board, primarily synthesised from a blend of three raw materials: epoxy resin, functional fillers and glass fibre. This scientifically formulated composition endows the board with stable and balanced overall performance. Among these, epoxy resin serves as the core matrix material of FR-4 pcbs, undergoing thermosetting reactions to achieve polymerisation and moulding. During the PCB manufacturing process, the epoxy resin not only provides excellent electrical insulation but also acts as a bonding medium, firmly adhering the copper foil to the glass fibre-reinforced substrate. Furthermore, it possesses resistance to high and low temperatures, chemical corrosion, and water and moisture, ensuring the stable operation of FR-4 substrate circuit boards under complex operating conditions.
Glass fibre cloth serves as the core skeletal material supporting the structural strength of FR-4 boards. It is formed by melting inorganic raw materials at high temperatures, followed by cooling and solidification to create an amorphous, rigid substrate, which is then woven into shape using a warp-and-weft interlacing process. Currently, E-glass fibre cloth is the most widely used in the industry, with 106, 1080, 3313, 2116 and 7628 being the mainstream specifications. Different specifications correspond to varying board thicknesses and mechanical strengths, enabling adaptation to the production requirements of FR-4 circuit boards with different structures, such as single-layer and multi-layer boards.
2. Core Characteristics of Aluminium Substrates
Aluminium substrates are metal-based, heat-dissipating PCB substrates that differ from FR-4 fibreglass boards. They feature a three-layer composite structure comprising a top layer of conductive copper foil, a middle layer of insulating thermal barrier, and a bottom layer of aluminium substrate. Compared to conventional FR-4 fibreglass laminates, aluminium-based substrates offer significantly improved heat dissipation efficiency, rapidly conducting away heat generated during circuit operation and effectively addressing the issue of overheating and component failure. Consequently, they have become the core specialised substrate in the LED lighting sector and are widely used in various high-power lighting equipment.
Definition and Evolutionary Advantages of the FR-4 Material System within the FR Class
FR is the standard industry designation for glass fibre-reinforced epoxy resin laminates. This system comprises several sub-grades, including G-10, G-11, FR-4, FR-5 and FR-6. Among these, FR-4 has become the most widely used and cost-effective laminate type on the market today, thanks to its balanced performance and reliable flame-retardant properties.
In the evolution of these materials, G-10 was the predecessor to FR-4, a basic glass fibre laminate with good insulation properties and mechanical strength. However, it lacked flame-retardant capabilities, posing safety risks during equipment operation. By incorporating brominated flame-retardant compounds into its formulation, FR-4 achieves stable self-extinguishing flame-retardant properties, thereby completely addressing the safety shortcomings of G-10 boards. It has gradually and comprehensively replaced G-10, becoming the preferred substrate for general-purpose PCBs in both consumer and industrial applications.
Comprehensive Properties and Classification of FR-4 PCBs
FR-4 has long dominated the mainstream market for PCB substrates, primarily due to its well-balanced comprehensive properties. This board offers excellent insulation, high mechanical strength, high-temperature resistance and extremely low water absorption. It maintains stable electrical insulation and structural integrity in both dry, normal environments and humid, complex operating conditions, making it suitable for the vast majority of electrical and mechanical applications. With a glass transition temperature (Tg) range of 115°C to 200°C, it offers an exceptionally broad range of performance capabilities. Conventional standard FR-4 PCBs employ a classic structure comprising two layers of copper-clad laminate bonded to an FR-4 core.
Based on differences in heat resistance, electrical parameters and structural form, FR-4 laminates can be divided into four main categories, catering to production requirements of varying precision and operating conditions:
1. Standard FR-4: A general-purpose, basic laminate with a stable heat resistance of 140°C–150°C, suitable for the mass production of PCBs and prototype manufacturing for the vast majority of standard consumer electronic devices.
2. High Tg FR-4: With a glass transition temperature (Tg) of up to 180°C, this variant offers significantly enhanced high-temperature resistance, capable of withstanding high-temperature operations and multilayer lamination processes. It is primarily used in the production of high-end precision PCBs.
3. High CTI FR-4: Featuring a relative tracking index (CTI) exceeding 600 volts, this material offers outstanding resistance to tracking and high-voltage withstand capability, providing superior electrical safety performance. It is suitable for equipment applications with stringent insulation and protection requirements.
4. Copper-free FR-4 laminates: The surface copper plating is removed, retaining only the properties of the high-quality insulating substrate. These are primarily used to manufacture electronic auxiliary components such as equipment insulation pads and circuit board supports.
Advantages and Limitations of FR-4 Board Applications
The extremely high prevalence of FR-4 boards in the PCB industry stems primarily from their dual advantages of exceptional practicality and cost-effectiveness. With low raw material costs and a mature, stable production process, FR-4 boards enable effective control of production costs, whether for small-batch prototyping or large-scale industrial mass production, thereby meeting the production budget and capacity requirements of enterprises of all sizes.
In terms of performance, circuit boards made from FR-4 are structurally robust and resistant to water and moisture. The substrate’s inherent high insulation properties effectively isolate the copper circuits across different layers, reducing signal interference and ensuring stable signal integrity for equipment. This fully meets the requirements of the vast majority of conventional low-to-medium frequency electronic devices on the market.
However, FR-4 laminates have clear application limitations and are not a universal substrate suitable for all scenarios. In high-frequency, high-speed circuit equipment, the dielectric properties of FR-4 cannot meet the demands of high-speed transmission of high-frequency signals, necessitating the use of specialised high-frequency boards; furthermore, as FR-4 is a rigid material lacking flexibility or foldability, it cannot be used in flexible electronic devices, where flexible substrates such as PI (polyimide) must be employed.
The Industrial Value of High-End Specialised FR-4 Boards
In response to the high-specification demands of high-end industrial and precision electronics applications, the industry has developed high-performance specialised FR-4 laminates. The VENTEC series is a prime example, meeting the production standards for high-end PCBs requiring high reliability and heat resistance. The core model, VENTEC VT-47, is a high-Tg, high-end FR-4 material whose specifications match those of premium heat-resistant laminates in the industry, making it an excellent alternative for high-end substrate applications.
This high-end FR-4 laminate is suitable for the production and processing of double-layer, multi-layer precision PCBs, as well as rigid-flex boards. Thanks to its excellent resin rheological properties, it is perfectly suited to lead-free soldering processes and complies with modern environmental production standards in the electronics industry. It is currently widely used in high-end sectors such as automotive electronics, precision medical equipment, industrial automation, telecommunications equipment and core computer hardware, making it the preferred choice for PCB substrates in high-performance electronic devices.
Thanks to its multiple advantages—including flame retardancy and safety, balanced performance, outstanding value for money, and strong process adaptability—FR-4 board firmly maintains its core position as a general-purpose PCB substrate, serving as the foundational material linking everyday consumer electronics with conventional industrial equipment. Furthermore, through the iteration of sub-categories such as high Tg and high CTI variants, as well as the upgrading of high-end special-purpose FR-4 boards, this material continues to push the boundaries of performance, gradually adapting to high-end precision electronic applications.
Although FR-4 has certain limitations in high-frequency and flexible applications, when combined with complementary substrate options such as aluminium substrates and PI flexible boards, it can cover the vast majority of PCB application scenarios. This is the core reason why it has long remained the industry mainstream and remains irreplaceable. In the future, as electronic devices evolve towards greater precision, higher end-of-market positioning and environmental sustainability, the processes and performance of FR-4 laminates will continue to be optimised, further consolidating its dominant position in the PCB substrate sector.