Microstrip Impedance

Microstrip Impedance

Impedance control is critical for high-speed PCB design. When a signal's wavelength approaches the trace length, impedance mismatches cause reflections that corrupt data.

A microstrip is a trace on an outer layer with a ground plane below it. Its characteristic impedance Z₀ is determined by the trace geometry and dielectric constant.

The IPC-2141 formula:

Z₀ = (87 / √(εr + 1.41)) × ln(5.98H / (0.8W + T))

Where:

• εr = substrate dielectric constant
• H = height of dielectric (distance to ground plane), in the same units as W and T
• W = trace width
• T = trace thickness

Common target impedances:

• 50 Ω — single-ended signals (RF, USB, Ethernet)
• 90 Ω differential — USB, PCIe
• 100 Ω differential — Ethernet, HDMI

Your Task

Implement microstripImpedance(er, H, W, T) where H, W, T are in mm, returning Z₀ in ohms.