What is the effect of copper weight and plating thickness on microstrip impedance?
Copper Thickness Effects
The Hammerstad-Jensen model accounts for conductor thickness by calculating an effective trace width: Weff = W + (t/π) × [1 + ln(2h/t)] for microstrip in air. This effective width is wider than the physical width because the fields fringe around the conductor edges, and thicker conductors create more fringing.
| 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 weight affect loss?
Thicker copper has lower DC resistance but the same high-frequency surface resistance (determined by skin depth). The benefit of thicker copper for loss reduction is minimal above a few hundred MHz. Thicker copper does improve thermal performance by spreading heat more effectively.
What about differential pairs?
Copper thickness affects the coupling between differential traces by changing the effective trace width and gap. For tightly coupled pairs (S ≈ W), 1 Ω change in single-ended impedance translates to approximately 2 Ω change in differential impedance. Include thickness in all differential impedance calculations.
When can I ignore conductor thickness?
For traces wider than 10× the conductor thickness (W > 10t), the thickness effect on impedance is less than 1% and can usually be ignored. For a 1 oz copper (35 μm) trace, this means ignoring thickness for traces wider than 350 μm (14 mil). For narrow traces (< 5 mil), thickness always matters.