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PCB Surface Microstrip Characteristic Impedance Calculator

A microstrip is an RF and high-speed digital transmission line fabricated on a printed circuit board (PCB) outer layer, separated from a ground plane by a dielectric substrate.

Width of the top copper signal trace (0.30 mm ≈ 11.8 mils).

Distance between trace and reference ground plane (0.18 mm ≈ 7 mils).

Substrate dielectric constant (standard FR-4 is ~4.2 - 4.5, Rogers 4350B is ~3.66).

1 oz copper = 0.035 mm (1.37 mils), 0.5 oz = 0.018 mm.

Calculated Result
54.43 Ω

Characteristic Impedance (Z₀)

Characteristic Impedance (Z₀)

54.43 Ω

Effective Permittivity (ε_eff)

3.29

Propagation Delay

6.05 ps/mm

Phase Velocity (v_p)

165,189 km/s

W/H Trace Ratio

1.67

Calculation Breakdown

  1. Trace Aspect Ratiou = W / H = 0.3 mm / 0.18 mm = 1.67
  2. Effective Dielectric Constantε_eff = (4.4 + 1)/2 + [(4.4 - 1)/2] × [1 / √(1 + 12/1.67)] = 3.29
  3. Hammerstad Characteristic ImpedanceZ₀ = 54.43 Ω (6.05 ps/mm signal delay)

Microstrip Transmission Characteristics

Interactive visualization based on your current inputs

Value
0.013263952Impedance (Ω)Effective PermittivityDelay (ps/mm)W/H RatioParameterValue

What Is the PCB Surface Microstrip Characteristic Impedance Calculator?

A surface microstrip is a conductor track separated from a continuous reference ground plane by an insulating dielectric layer on a printed circuit board.

How Does the PCB Surface Microstrip Characteristic Impedance Calculator Work?

The ratio of trace width to dielectric height (W/H) and substrate permittivity determine line capacitance and inductance per unit length, yielding Z₀ = √(L/C).

PCB Surface Microstrip Characteristic Impedance Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

Z_0 = \frac{120 \pi}{\sqrt{\varepsilon_{\text{eff}}} \left( \frac{W}{H} + 1.393 + 0.667 \ln\left( \frac{W}{H} + 1.444 \right) \right)}, \quad t_{\text{pd}} = \frac{\sqrt{\varepsilon_{\text{eff}}}}{c}

Hammerstad-Jensen equations calculate PCB surface microstrip characteristic impedance Z0 and effective dielectric constant from trace width and substrate thickness.

How to Use the PCB Surface Microstrip Characteristic Impedance Calculator

  1. Enter PCB trace width and substrate prepreg thickness.
  2. Specify dielectric constant (e.g. 4.4 for FR-4) and copper thickness.
  3. Adjust trace width until target impedance (e.g. 50 Ω) is reached.

Step-by-Step Example Calculation

Standard FR-4 Outer Layer 50-Ohm Trace

Input Values:

traceWidthMm:0.30
dielectricHeightMm:0.18
dielectricConstantEr:4.4
copperThicknessMm:0.035
Worked Steps: Trace exhibits Z₀ = 51.5 Ω characteristic impedance with ε_eff = 3.25 and 6.01 ps/mm signal propagation delay.

Understanding Your Result

Characteristic Impedance (Z₀): High-frequency AC resistance of the transmission line.

Effective Dielectric Constant (ε_eff): Blended substrate-air dielectric value.

Propagation Delay: Time required for an electrical pulse to travel 1 millimeter.

Factors That Affect the Result

  • Wider traces lower impedance; thicker dielectric heights increase impedance.

When Should You Use This Calculator?

  • Designing high-speed DDR memory buses, USB 3.0 lines, Ethernet routing, and RF antenna feedlines.

Assumptions & Limitations

  • Assumes an isolated trace with no nearby parallel traces causing coplanar cross-coupling.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Hammerstad & Jensen closed-form RF microstrip formulation (IPC-2141).

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