Definition

Resin-rich regions form between tightly-spaced differential traces during PCB lamination. As prepreg flows under heat and pressure, pure resin (with different dielectric properties than glass-reinforced prepreg) fills the narrow gaps between conductors. This creates a zone with lower Dk than the surrounding material, affecting differential impedance calculations. The effect becomes significant for fine-pitch differential pairs where the gap is narrow enough to exclude glass fibers. Field solvers like Polar Si9000e model this with a separate Rer (resin-rich dielectric constant) parameter, typically 0.3-0.5 lower than the bulk prepreg Dk.