intermediate PCB Design

Controlled Impedance

Use the impedance calculator to design 50 Ω microstrip and stripline traces. Understand how trace geometry and dielectric constant affect characteristic impedance.

εr (dielectric constant)

W — trace widthmm

H — substrate heightmm

T — trace thicknessµm

50.2 Ω

εr,eff = 3.33

Z₀ (Ω) vs trace width — operating point ●

Formula reference

Wheeler (1977) — Microstrip Z₀:

Wide trace (W/H ≥ 1): Z₀ = 120π / (√ε_eff · (W/H + 1.393 + 0.667·ln(W/H + 1.444)))

Narrow trace (W/H < 1): Z₀ = (60/√ε_eff) · ln(8H/W + W/(4H))

The calculator shows the microstrip formula. Start by setting εr = 4.4 (standard FR4), H = 1.6 mm (standard board thickness), T = 35 µm (1 oz copper). Adjust W until Z₀ ≈ 50 Ω.

Show hint

For 1.6 mm FR4, a 50 Ω microstrip trace is approximately W = 3.0 mm.
Notice that increasing W lowers Z₀. This is because a wider trace has more capacitance per unit length to the reference plane, which lowers the characteristic impedance Z₀ = √(L/C).

Show hint

Double the trace width → lower Z₀ by roughly 25–30%. The relationship is logarithmic.
Increase εr to 10 (ceramic substrate). The Z₀ drops significantly. Higher permittivity increases trace capacitance, reducing impedance. Rogers and ceramic substrates are used at RF to achieve compact trace geometries.

Show hint

εr,eff is less than εr because part of the field is in air above the trace. Stripline fully embeds the trace, so εr,eff ≈ εr.
Switch to 'stripline' mode. A stripline trace is sandwiched between two reference planes. Its impedance is lower than microstrip for the same W because the field is entirely in the dielectric (no air).

Show hint

Striplines have lower EMI and crosstalk than microstrips since they're shielded on both sides.
The mini Z₀ vs W chart shows how impedance varies across the width range. The orange dot is your operating point. Use this to quickly choose trace width for common targets: 50 Ω (RF, single-ended), 75 Ω (video), 100 Ω differential.

Show hint

In KiCad, use the Interactive Router > Differential Pair and set the impedance in the constraints manager.