FR1 is basically the same as FR2. FR1 has a higher TG of 130oC instead of 105oC for FR2. FR3 is also basically FR2. But instead of phenolic resin it uses an epoxy resin binder.
FR4 (FR = Flame Retardent) is a glass epoxy laminate. It is the most commonly used PCB material. FR4 uses 8 layers glass fiber material. The maximum ambient temperature is between 120o and 130oC, depending on thickness.
FR4 is the most widely used PCB base material, next is FR1 then FR2. But FR1 and FR2 are usually used for 1-layer PCBs because they are not good for plating through holes. FR3 is not recommended to building multi-layer PCBs. FR4 is the best selection. FR4 is widely used because it is good to make from one-layer to multi-layer PCBs. With only FR4, PCB companies can make all kinds of PCBs, which leaves the management and quality control much easier, and eventually it can reduce the cost!
“FR” denotes that the material complies with the standard UL94V-0.