NodeLoop

Microstrip Impedance Calculator

Estimate characteristic impedance (Z₀) for microstrip PCB traces based on key stackup parameters.

Input Parameters

mm
mm

Characteristic Impedance (Z₀)

-- Ω

Cross-Section View

Microstrip Cross Section Visualization Visual representation of trace width (W), dielectric height (H), and copper thickness (T). εr = 4.3 Ground Plane

🔬 Formula & Theory

The characteristic impedance (Z₀) of a microstrip transmission line is primarily determined by its physical dimensions (trace width W, dielectric height H, copper thickness T) and the dielectric constant (εr) of the substrate material.

This calculator uses approximations common in the industry. One set of widely used formulas is from Wheeler, or more refined versions by Hammerstad and Jensen. These involve calculating an effective dielectric constant (εeff) which accounts for the fact that some of the electromagnetic field lines exist in the air above the trace and some in the dielectric.

For W/H ≤ 1 (narrow traces):

Z₀ ≈ (60 / √εeff) * ln(8H/W' + W'/4H)

For W/H > 1 (wide traces):

Z₀ ≈ (120π / √εeff) / (W'/H + 1.393 + 0.667 * ln(W'/H + 1.444))

Where W' is an effective trace width that accounts for the copper thickness T:

W' = W + (T/π) * (1 + ln(2H/T)) (approximation)

And the effective dielectric constant εeff is approximately:

εeff ≈ (εr + 1)/2 + ((εr - 1)/2) * (1 / √(1 + 12H/W))

Note: These are simplified representations. The actual implementation in the calculator might use more detailed or slightly different variations of these formulas for improved accuracy across a wider range of W/H ratios and to account for thickness. Always verify with a more advanced field solver for critical designs.

FAQ - Controlled Impedance

Which Er should I use for FR-4 in impedance calculations?
Use the laminate's Dk at the operating frequency from the fab stackup. If unknown, start with Er~3.9-4.2 at 1-5 GHz and validate with a field solver or TDR.
Microstrip vs coplanar microstrip: when to prefer CPW?
CPW with ground improves isolation and reduces radiation. Prefer it for dense RF, edge routing, or when a continuous reference plane under the trace is interrupted.
How does solder mask affect impedance?
Solder mask increases effective Er and lowers Z0 a few ohms for microstrips. Model it as a dielectric layer or use 'mask over' presets from the fabricator.
How to dimension a 90 Ω differential pair?
Choose topology (edge-coupled microstrip or stripline), set target Er and copper, then iterate width and spacing to hit 90 Ω diff while meeting manufacturability rules.
When is stripline preferable to microstrip?
Stripline reduces radiation and crosstalk at the cost of higher loss and complexity. Use it for high-speed cores or when external layers are congested.
What tolerance can I expect on 50 Ω traces?
±5-10% is typical depending on copper thickness, etch/laminate tolerances and mask. Ask your fab for a controlled-impedance stackup and target width.

Related resources