Definition

Prepreg (pre-impregnated) is fiberglass cloth impregnated with partially cured epoxy resin. B-stage means the resin is solid at room temperature but will flow and complete curing under heat and pressure during lamination, bonding copper layers and cores together. Prepreg is specified by glass style (1080, 2116, 7628) and resin content, which determine cured thickness and dielectric properties. Multiple prepreg sheets can be stacked to achieve target dielectric thickness. Prepreg Dk and thickness are critical for impedance control, so always confirm your fabricator's available prepreg options when designing controlled-impedance stackups.