What is FR4?


FR4 laminate is considered to be the backbone of the industry being layers of woven glass cloth impregnated with epoxy resin. It is the most widely used material providing the best compromise between cost and electrical and mechanical requirements. FR4 laminate is the usual base material from which plated-through-hole and multilayer printed circuit boards are constructed. “FR” means Flame Retardant, and Type “4” indicates woven glass reinforced epoxy resin. The laminate is constructed from glass fabric impregnated with epoxy resin (known as “prepreg”) and copper foil, which is commonly supplied in thicknesses of “half-oz” (approximately 18 microns) or “one-oz” (approximately 35 microns). FR4 laminate displays a reasonable compromise of mechanical, electrical and thermal properties. Dimensional stability is influenced by construction and resin content. Generally, heavyweight glass fabric gives a more dimensionally stable material, although it is necessary to use lightweight fabrics for