FR4 Laminate
07 December 2002
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 "pre-preg") and copper foil, which is commonly supplied in thicknesses of "half-ounce" (approx 18 microns) or "one-ounce" (approx 35 microns). Foil is generally formed by electrodeposition ("ED Foil"), with one surface electrochemically roughened to promote adhesion. A typical 1.6 mm rigid laminate is made by bonding together eight plies of heavyweight pre-preg, with half-ounce copper foil either side, in a hot hydraulic press.
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 high layer count multilayer circuits.
Dielectric constant, typically in the range 4.4-5.2, depends on glass-resin ratio.
It decreases with increasing resin content and increasing signal frequency. At microwave frequencies, dielectric losses make FR4 unsuitable and materials such as PTFE are more appropriate.
Expansion coefficient of FR4 laminate in the xy plane is approximately 16 ppm per degree C (copper 17-18ppm, glass fibre 5-7ppm, resin 40 ppm). The glass transition temperature (Tg) of FR4, at which the resin changes from a glassy to a plastic state, ranges from 120°C to 180°C depending on resin chemistry. Above Tg, the expansion coefficient of epoxy resin in the z axis (which is not reinforced) is approx 330 ppm per degree C. Continuous operating temperature for FR4 is typically 130°C.
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