FR4

FR-4 is a widely used glass-reinforced epoxy laminate serving as the standard substrate for printed circuit boards. It consists of woven fiberglass fabric impregnated with an epoxy resin and is formulated to be flame-retardant.

Electrical and thermal properties for FR-4 are dominated by its epoxy/glass structure. The material typically has a dielectric constant around 4.5 at low frequencies, with a dielectric loss tangent on the order of a few hundredths. It remains stable up to about 125–135°C in terms of glass transition temperature, and its service temperature is commonly limited to around 130°C. In-plane coefficients of thermal expansion are relatively low in the board plane (around 12–18 ppm/°C) but higher through the thickness (roughly 40–60 ppm/°C), reflecting its anisotropic nature.

Physical characteristics and form factors include standard thicknesses from about 0.2 to 3.2 mm, with 1.6 mm being the most common for desktop and consumer electronics. FR-4 laminates are often copper-clad, providing single- or double-sided copper foils of commonly 1 oz (35 µm) or lighter/heavier weights for circuit fabrication.

Standards and usage: FR-4 is defined as a flame-retardant glass epoxy laminate and is a prevalent grade described in IPC-4101 specifications. It is used for the vast majority of through-hole and surface-mmount printed circuit boards due to its balance of mechanical strength, electrical insulation, and cost.

Limitations and considerations: FR-4 absorbs moisture, which can affect dimensional stability and electrical properties if boards are not properly dried before soldering. While generally RoHS-compliant, the resin system may include flame retardants, and environmental handling should consider local regulations regarding recycling and disposal of epoxy/glass composites.