The most commonly used epoxy-fiberglass material standard for printed circuit boards. The “FR” refers to flame retardant.

This is the standard glass epoxy substrate with a glass transition temperature of 135 deg C. Its mechanical strength and in particular flexural strength and impact resistance are greatly superior to those of phenolic and epoxy paper laminates. FR stands for flame resistant.

FR-4, an abbreviation for Flame Resistant 4, is a type of material used for making a printed circuit board (PCB). It describes the board itself with no copper covering. The FR-4 used in PCBs is typically UV stabilized with a tetrafunctional resin system.