Microstrip Impedance
Calculator
Calculate the physical trace width needed to achieve a target impedance on a PCB, given substrate thickness, dielectric constant, and copper thickness. Essential for anyone designing RF PCBs.
Substrate & Trace Parameters
Common PCB Substrate Properties
| Material | εr | Loss Tangent | Max Frequency | Cost |
|---|---|---|---|---|
| FR-4 (standard) | 4.2 – 4.7 | 0.020 | ~6 GHz | Low |
| Rogers RO4003C | 3.55 | 0.0027 | 40+ GHz | Medium |
| Rogers RO3003 | 3.00 | 0.0013 | 40+ GHz | Medium |
| Rogers RT/duroid 5880 | 2.20 | 0.0009 | 77+ GHz | High |
| Taconic TLY-5 | 2.20 | 0.0009 | 77+ GHz | High |
| Rogers RO3010 | 10.2 | 0.0022 | 40+ GHz | High |
| Alumina (Al₂O₃) | 9.8 | 0.0001 | 100+ GHz | Very High |
How Microstrip Impedance Works
A microstrip transmission line consists of a signal trace on one side of a dielectric substrate with a ground plane on the opposite side. The characteristic impedance is determined by the trace width, substrate height, dielectric constant, and to a lesser extent, the trace thickness and frequency.
Unlike stripline (where the trace is sandwiched between two ground planes), microstrip fields extend partially into the air above the trace. This creates a non-homogeneous dielectric medium, which is why the "effective dielectric constant" (εeff) is used: it is always between 1 (air) and εr (substrate).
Key Design Relationships
- Wider trace = lower impedance. To decrease impedance, increase trace width.
- Thicker substrate = higher impedance for the same trace width. More height means less capacitance to ground.
- Higher εr = lower impedance for the same geometry. More dielectric concentration increases capacitance.
- Higher εr = narrower traces needed for 50 Ω. This can be advantageous for miniaturization but increases manufacturing tolerance sensitivity.
Practical Design Tips
- Always verify your calculated trace width with your PCB fabricator's capabilities. Minimum trace widths typically range from 3 to 5 mils (75 to 125 μm).
- For frequencies above 6 GHz, switch from FR-4 to a low-loss laminate (Rogers, Taconic) to avoid excessive dielectric losses.
- Ground vias should be placed within λ/20 of the trace edge to prevent parallel-plate mode excitation.
- Maintain consistent trace width through transitions, bends, and connector launches to minimize reflections.
- At mmWave frequencies, surface roughness of the copper affects loss. Specify low-profile or rolled copper for best performance.
Frequently Asked Questions
How wide should a 50-ohm microstrip trace be?
It depends on substrate parameters. On FR-4 (εr = 4.4) with 20 mil (0.508 mm) substrate, a 50-ohm trace is about 0.92 mm (36 mils) wide. On Rogers RO4003C (εr = 3.55), it is about 1.10 mm (43 mils). Use this calculator with your exact substrate specs for precise results.
What is the dielectric constant of FR-4?
FR-4 has a nominal εr of 4.2 to 4.7, commonly specified as 4.4 at 1 GHz. This value decreases slightly with frequency. For precision RF work above 6 GHz, dedicated RF laminates with tightly controlled εr are recommended.
What is effective dielectric constant?
εeff accounts for the fact that microstrip fields exist partly in the substrate and partly in air above the trace. It is always between 1 and εr. It determines the actual propagation velocity and wavelength on the line, and is needed for calculating physical lengths of quarter-wave transformers and stubs.
Can I use FR-4 at mmWave frequencies?
FR-4 is generally not suitable above 6-10 GHz due to its high loss tangent (0.02) and poorly controlled dielectric constant. At mmWave frequencies (28 GHz+), use materials like Rogers RT/duroid 5880 (loss tangent 0.0009) or alumina for best performance.