Dk, the dielectric constant or relative permittivity, describes how strongly a laminate stores electric field energy compared with vacuum. It sets signal speed, roughly the speed of light divided by the square root of Dk, and is a key input to trace impedance. FR-4 runs about 3.8 to 4.6 and shifts with frequency, resin content and glass style, while RF laminates hold values such as 3.0 or 3.5 tightly. Its partner Df, the dissipation factor or loss tangent, sets how much signal turns into heat.
Fab says the effective Dk of that 1080 prepreg is closer to 3.7, rerun the impedance calc before we lock the stackup.
Also heard as
- dielectric constant
- Er
- relative permittivity
Related terms
Impedance control
Designing and building traces to a target impedance, such as 50 or 100 ohms, within a set tolerance.
Rogers
Shop shorthand for high-frequency PTFE or ceramic-filled laminate, from the name of a major RF materials maker.
FR-4
Flame-retardant glass-epoxy laminate grade that is the default base material for rigid circuit boards.
Stackup
Ordered arrangement of copper layers, cores and prepregs that makes up a multilayer board, with each thickness.