FR-4 Flame-Retardant Glass Epoxy — Sheet

FR4 sheet is the flat-panel form of NEMA FR-4 flame-retardant glass-epoxy laminate — a thermoset composite of woven fiberglass cloth impregnated with brominated epoxy resin, laminated under heat and pressure into rigid flat stock. FR-4 is the most widely used glass-epoxy grade in electrical and electronic industries, balancing high mechanical strength, reliable dielectric insulation, low moisture absorption, and a certified UL 94 V-0 flame rating. It is the standard substrate material for printed circuit boards and a preferred insulating structural material across industrial OEM applications.

FR4 sheet provides a tensile strength of 65,000 psi, flexural strength of 60,000 psi, and compressive strength of 38,000 psi. Rockwell M115 hardness and a shear strength of 21,500 psi reflect the material's structural rigidity. Water absorption over 24 hours is just 0.10%. The UL 94 V-0 flammability rating and maximum continuous operating temperature of 280°F (140°C) make FR4 sheet suitable for UL-listed enclosures, switchgear panels, and fire-rated electrical equipment.

Standards & compliance. FR4 sheet conforms to NEMA LI-1 Grade FR-4 and is tested per ASTM D709. The UL 94 V-0 rating is the defining compliance attribute of FR-4 versus G-10. Ready Plastics stocks FR4 sheet in standard thicknesses; custom-cut pieces are available on request.

Sister grades. G-10 glass-epoxy sheet is mechanically comparable but carries a 94HB flame rating — appropriate when flame retardancy is not required. G-11 glass-epoxy sheet and its flame-retardant counterpart FR-5 are rated to 355°F (180°C), offering higher thermal performance for elevated-temperature service. For extreme heat up to 425°F (220°C), see G-7 glass-silicone sheet.