FR4 is a code of the fire-resistant material grade, which means that it is a kind of material specification that the resin material must be able to self-extinguish after combustion state,it is not a kind of material name,but a kind of material grade.So it has many types for FR4 grade materials of the current general circuit board used,but most of them are made of so-called four-function (Tera-Function) epoxy resin with filler and composite materials made of glass fiber.

It is a kind of refractory material that epoxy resin and glass fiber after high temperature and high pressure,also is a kind of major material that the production of PCB. Heat resistance can reach to 300 degrees, the operating frequency can be several GHz, the dielectric constant is about 4.3.

As shown in below pictures and they all are FR4.