The specification standard of FR-4 is set by NEMA (National Electrical Manufacturers Association). FR in NEMA classification standard stands for Flame-Retardant. FR-4 means the resin is epoxy resin, the reinforcement material is glass fiber cloth, and the flame retardant grade is UL94 V-0.
FR-4 is laminated with epoxy resin as adhesive. It is reinforced with electronic grade fiberglass cloth. One or both sides are coated with copper foil and hot pressed under high temperature and pressure. It is a composite board because it is made of different materials. It is also called copper clad laminate (CCL) in the electronics manufacturing industry.
Specifically，FR4 is mainly composed of the following two parts:
1.Glass fiber cloth:
Glass fiber cloth is woven from extremely fine glass fiber filaments, which have high strength and high modulus, providing FR4 with good mechanical properties.
As a reinforcing material, glass fiber cloth plays the role of a skeleton in FR4, enhancing the overall strength and rigidity of the material. At the same time, it also has good heat resistance and chemical corrosion resistance, allowing FR4 to maintain stable performance in harsh environments.
2. Epoxy resin adhesive:
Epoxy resin is a thermosetting plastic with excellent adhesion, electrical insulation and chemical stability.
As a matrix material, epoxy resin bonds the glass fiber cloth together to form a strong composite material. During the curing process, a strong interface bond is formed between the epoxy resin and the glass fiber cloth, giving FR4 excellent mechanical and electrical properties.
Flame retardancy: The “FR” in FR4 stands for “Flame Retardant”, which means that this material can delay combustion or prevent the spread of combustion when encountering a fire source, and has a certain self-extinguishing property.
Electrical insulation: FR4 is an excellent electrical insulator with a high insulation resistance, generally around 10¹² – 10¹⁵Ω (ohms), which can effectively isolate different signal paths in the circuit.
Mechanical properties: FR4 has high strength and high rigidity and can withstand a certain amount of external force, making FR4 less likely to deform or be damaged.
Heat resistance: FR4 has good heat resistance and can maintain stable performance at higher temperatures.
Moisture resistance: FR4 is not easy to absorb water and can maintain stable performance in a humid environment.
Dielectric properties: FR4 has stable electrical performance parameters such as dielectric constant (Dk) and dielectric loss factor (Df), which is suitable for the design of various electronic circuits.
Synergy: The combination of glass fiber cloth and epoxy resin plays a synergistic role, making FR4 show excellent performance in mechanical properties, electrical properties and chemical stability.
Cost-effectiveness: Both glass fiber cloth and epoxy resin are relatively cheap raw materials, which makes FR4 have great advantages in cost. At the same time, its excellent performance also makes FR4 widely used in the electronics industry.
Processability: FR4 has good processability and can be processed into circuit boards of various shapes and sizes through cutting, drilling, etching and other processes. This makes FR4 highly flexible and adaptable in electronic manufacturing.
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