FR1, FR2, and FR4 are classifications used to categorize different types of materials used in the construction of printed circuit boards (PCBs). These classifications are based on the flammability and electrical characteristics of the substrate materials. Here's a brief overview of each: 

1. FR1 (Flame Retardant 1):

• Material Type: FR1 is a basic flame-retardant substrate material commonly made of paper impregnated with a phenolic resin.

• Applications: FR1 is suitable for low-cost, general-purpose PCBs with relatively simple designs. It is not commonly used in high-frequency or high-performance applications.

2. FR2 (Flame Retardant 2):

• Material Type: Similar to FR1, FR2 is a flame-retardant material, but it typically uses a different resin, such as epoxy, instead of phenolic. It may also include a paper or cotton substrate.

• Applications: FR2 is used in general-purpose PCBs and may offer slightly better electrical performance than FR1. However, it is still not suitable for high-frequency or high-performance applications.

3. FR4 (Flame Retardant 4):

• Material Type: FR4 is a widely used flame-retardant substrate made of woven glass fabric impregnated with an epoxy resin. It is known for its good electrical insulation properties, mechanical strength, and flame retardancy.

• Applications: FR4 is the most common material used in the construction of PCBs. It is suitable for a wide range of applications, including high-frequency circuits and high-performance electronic devices. FR4 is widely used in various industries due to its versatility.

In summary, the key differences among FR1, FR2, and FR4 lie in the material composition and performance characteristics. FR1 and FR2 are simpler and less advanced materials, typically used in low-cost or general-purpose applications, while FR4 is a more advanced material suitable for a wide range of applications, including those with higher performance and frequency requirements. When selecting a PCB material, the choice depends on the specific requirements of the application, including factors such as cost, electrical performance, and environmental considerations.