75Ω Microstrip Calculator
Controlled Impedance | FR-4 (Er=4.5) | IPC-2141
Quick answer
On FR-4 (εr 4.5) with 0.2 mm dielectric height and 1 oz copper, an isolated microstrip line needs about 0.130 mm (5.1 mil) width for 75 Ω, by the IPC-2141-style single-ended approximation used here.
How sensitive is that result?
| Nominal (w = 0.130 mm, εr = 4.5) | 75.0 Ω |
|---|---|
| Width +10 % | 72.5 Ω |
| Width −10 % | 77.7 Ω |
| εr = 4.2 (same width) | 77.3 Ω |
| εr = 4.8 (same width) | 72.9 Ω |
Assumptions
- Single-ended isolated line; copper 0.035 mm; dielectric height 0.2 mm; εr 4.5.
- The sensitivity rows re-run the same production function with one input changed.
Limits
- This is not a differential-pair result and ignores solder mask and etch trapezoid.
- Confirm width against your fabricator’s stackup and impedance coupon.
Next step
- Change the stackup in the single-ended calculator — height, εr and copper change the result more than rounding does.
- Differential pairs — this page is single-ended only; numeric differential results are switched off, the page lists what to bring to a field solver.
For 75Ω microstrip on FR-4 with 0.2mm dielectric height:
Assumptions: εr = 4.5, copper thickness = 0.035mm, dielectric height = 0.2mm. Hammerstad–Jensen style approximation. These are approximations; fabricators' field-solver results govern the final stackup.
Impedance Parameters
MOD: IMP_CAL_V1Differential impedance results are temporarily unavailable. The previous differential model was not validated against a field solver or a published source, so it has been switched off until it is. Single-ended Z0 below is for an isolated line and is not a differential result.
Embedded microstrip results are temporarily unavailable while the model is researched and validated.
Width Design
For 75Ω microstrip
Single-ended Z0 (isolated line): 75.0 Ω
Impedance Control Tips
To Increase Impedance
- • Reduce trace width
- • Increase dielectric height
- • Review laminate dielectric constant
To Decrease Impedance
- • Increase trace width
- • Reduce dielectric height
- • Review laminate dielectric constant
Understanding 75Ω Microstrip Impedance
75Ω impedance is standard for video transmission and broadcast equipment, matching coaxial cable impedance. To achieve 75Ω characteristic impedance using a microstrip trace on standard FR-4 material (Er=4.5) with 0.2mm dielectric height, you need a trace width of approximately 0.130mm (5.1 mil). This is an approximation based on IPC-2141-style and Hammerstad equations; confirm the final stackup with your fabricator's impedance report.
Key Parameters for 75Ω Design
- Target Impedance: 75Ω (characteristic impedance Z₀)
- Trace Type: Microstrip (surface layer with air above)
- Required Width: 0.130mm (5.1 mil)
- Dielectric Constant: 4.5 (FR-4 standard)
- Dielectric Height: 0.2mm (layer spacing)
- Copper Thickness: 35µm (1oz copper)
- Effective Er: 3.148
- Propagation Delay: 5.918 ps/mm
Microstrip vs Stripline for 75Ω
Microstrip traces are routed on the outer layers of the PCB with air above the trace. This results in a lower effective dielectric constant (3.15 vs 4.5 for stripline), requiring a different trace width to achieve 75Ω. Microstrip is preferred for high-frequency designs due to lower propagation delay and easier access for probing.
Common 75Ω Applications
75Ω impedance is required by the following standards and interfaces:
75Ω Standard Applications
IPC-2141 Impedance Formula
For microstrip, the characteristic impedance is calculated using the Hammerstad & Jensen formula, which accounts for the effective dielectric constant created by the air-dielectric interface. The formula considers trace width (W), dielectric height (H), copper thickness (T), and material properties to approximate impedance; accuracy depends on the stackup, so confirm the final value with your fabricator's field-solver result.
Frequently Asked Questions
How tight should my 75Ω trace width tolerance be?
The tolerance comes from your interface specification and your fabricator's process capability. As an illustration, a ±10% tolerance on a 75Ω target would be 67.5Ω to 82.5Ω. Specify controlled impedance to your PCB manufacturer and ask what width tolerance they hold.
Does copper thickness affect 75Ω impedance?
Yes, thicker copper slightly lowers impedance due to increased effective trace width. For precision designs, always include copper thickness in your impedance calculations. Our calculator uses35µm (1oz) as the standard.
What if my PCB uses different dielectric than FR-4?
High-speed designs often use low-loss materials like Rogers 4350B (Er=3.48) or Megtron 6 (Er=3.4). Lower Er requires wider traces for the same impedance. Use our full calculator above to input your specific material properties.
Related Impedance Calculations
Complete Your PCB Design
After calculating 75Ω microstrip trace width, use our other free tools to complete your high-speed PCB design. Calculate trace current capacity and via sizing for your power delivery network.