Microstrip Impedance

Formula

Description

A microstrip is a transmission line formed by a trace on the top or bottom layer of a PCB with a ground plane beneath it, separated by the dielectric substrate. The characteristic impedance depends on the trace geometry and the substrate's dielectric constant. This empirical approximation works well for w/h ratios between 0.1 and 2.0 and is accurate to within 2% for typical PCB geometries. Controlled impedance traces are essential for high-speed digital signals (USB, PCIe, DDR, Ethernet) and RF circuits to prevent signal reflections.

Variables

• Z₀ — Characteristic impedance (Ω)
• εr — Relative dielectric constant of substrate (FR-4 ≈ 4.2-4.8)
• h — Height of dielectric between trace and ground plane (mm)
• w — Trace width (mm)
• t — Copper thickness (mm)

Practical Notes

Common impedance targets: USB 90Ω differential, PCIe 85Ω differential, DDR4 40Ω single-ended, 100BASE-TX 100Ω differential, RF 50Ω single-ended. PCB fabricators can hold impedance tolerances of ±10% for standard stackups. Always use the fab house's stackup data for accurate calculations, as εr varies with frequency and manufacturing process.