FR is a material category for fiberglass reinforced epoxy laminates. FR-4 material is a reinforced epoxy laminate made of epoxy resin and flame-retardant fiberglass fabric. It is a self-extinguishing material due to the addition of bromine. The category designations for fiberglass laminates include G-10, G-11, FR-4, FR-5, and FR-6. Among them, FR-4 is the most widely used grade today. G-10 is the predecessor of FR-4 and lacks the flame retardancy of FR-4. Therefore, in most cases, FR-4 replaces G-10 and is an ideal choice for manufacturing PCB.

FR-4 material is a common and widely used substrate material with excellent electrical insulation properties, mechanical strength and high temperature resistance. It absorbs little water and FR-4 is most commonly used as an electrical insulator with some mechanical strength.

This material is known for maintaining its high mechanical values and electrical insulation properties in dry or wet conditions. These attributes, along with good manufacturing properties, make this grade practical in a variety of electrical and mechanical applications.

Its glass transition temperature (TG) is roughly 115°C to 200°C. The standard FR-4 material for PCB manufacturing will be between two thin layers of copper-plated FR-4.

FR-4 is affordable, making it a suitable choice for small batch or prototype PCB production. Its affordability helps consumers reduce the cost of PCB manufacturing. FR-4 is the most widely used material in PCB manufacturing. Circuit boards made of FR-4 are strong, waterproof, and provide good insulation between copper layers to minimize interference and promote good signal integrity.