How do I determine the correct microstrip trace width for 50 ohm impedance on FR4 at a given frequency?
Microstrip Design on FR4
The characteristic impedance of a microstrip line depends on the ratio of trace width (W) to dielectric height (h), the dielectric constant (εr), and the trace thickness (t). For W/h ratios typical of 50 Ω (1.5-2.0), the impedance decreases with wider traces and increases with thinner traces. The effective dielectric constant is lower than the bulk εr because part of the field propagates in air above the trace.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Does copper thickness matter?
Yes. Standard 1 oz copper (35 μm) slightly lowers the impedance compared to 0.5 oz (17 μm) for the same trace width. The effect is about 1-3 Ω for typical 50 Ω lines. Thicker copper (2 oz) has a larger effect and must be accounted for in the impedance calculation.
What about inner layers?
Inner layer traces (stripline) are surrounded by dielectric on both sides, giving a higher effective εr and requiring narrower traces for 50 Ω. On the same 0.031-inch FR4, stripline requires approximately 12 mil trace width for 50 Ω, compared to 59 mil for microstrip.
Can I use FR4 above 6 GHz?
With difficulty. Above 6 GHz, FR4 dielectric loss exceeds 1 dB/inch, εr dispersion causes phase errors, and the εr tolerance makes impedance control unreliable. For serious work above 6 GHz, use Rogers 4003C (εr=3.55, tan δ=0.0027), Isola I-Tera (εr=3.45, tan δ=0.0031), or similar low-loss laminates.