RF PCB Design — Controlled Impedance Traces
At RF frequencies, PCB traces behave as transmission lines. Their characteristic impedance must be controlled to prevent reflections, signal integrity problems, and radiation. The four standard controlled-impedance structures are microstrip, stripline, coplanar waveguide (CPW), and differential pairs. Each has different shielding characteristics, loss properties, and PCB layer requirements.
Comparison of PCB Transmission Line Types
| Type | Layers needed | Shielding | Via needed for termination? | Best for |
|---|---|---|---|---|
| Microstrip | 2 (trace + ground) | Partial (one side open) | No | General RF up to ~10 GHz, antennas |
| Stripline | 3+ (trace between 2 grounds) | Full TEM, no radiation | Yes (for shunt elements) | High isolation, sensitive circuits, digital backplanes |
| CPW (coplanar waveguide) | 1 (trace + ground coplanar) | Partial to full (GCPW) | No — shunt connections on same layer | MMIC, mmWave, no-via shunt connections |
| Differential pair | 2 | Partial (common-mode rejects noise) | No | USB, HDMI, LVDS, balanced RF |
Coplanar Waveguide (CPW)
CPW places the signal trace between two coplanar ground planes. No ground plane is needed below the substrate, making it ideal for flip-chip and MMIC die attachment where ground connections must be made on the surface. The characteristic impedance depends on the trace width w and gap s to the ground planes:
where K(k) is the complete elliptic integral of the first kind. Grounded CPW (GCPW) adds a bottom ground plane, suppressing the parasitic microstrip mode and is preferred for multilayer PCBs.
Differential Pair Impedance
Edge-coupled microstrip differential pairs carry balanced signals. The key parameter is the differential impedance \(Z_{diff} = 2 Z_{odd}\), where \(Z_{odd}\) is the odd-mode impedance. Standard targets: 100 Ω differential for USB/HDMI/LVDS, 90 Ω for PCIe.
where s is the edge-to-edge spacing and h is the substrate height. The even-mode impedance \(Z_{even} \approx 2Z_0/(1 + 0.347e^{-2.655 s/h})\) determines common-mode behaviour. Tight spacing reduces Z_diff; looser spacing increases it toward 2×Z₀_single.
Stripline
Stripline is a flat conductor centred symmetrically between two ground planes. It is a TEM mode structure with no dispersion and no radiation — unlike microstrip whose fields extend into air above the trace. The impedance:
where b is the ground-plane separation and \(w_{eff}\) accounts for trace thickness. Because the medium is uniform, \(v_p = c/\sqrt{\varepsilon_r}\) with no effective permittivity correction.
Via Parasitics
A PCB via connecting signal layers has inductance and capacitance:
where h is the via height (mm), d the drill diameter (mm), and D the antipad diameter (mm). At 1 GHz, a standard FR4 via (0.3 mm drill, 1.6 mm deep) has about 0.5 nH inductance and 0.15 pF capacitance — enough to cause significant reflections at microwave frequencies. Via stubs (unused portions below the signal layer) are particularly problematic and are removed by backdrilling.
Bond Wire Inductance
Gold or aluminium bond wires connecting a die to a package or PCB have parasitic inductance (Grover formula):
where ℓ and d are in mm. A typical 1.5 mm gold bond wire (d = 25 µm) has about 1.2 nH inductance, giving 7.5 Ω reactance at 1 GHz — often requiring resonating capacitors in MMIC matching networks.
Common Material Properties
| Substrate | εᵣ | tan δ | Typical use |
|---|---|---|---|
| FR4 | 4.2–4.6 | 0.018–0.025 | General PCB, < 3 GHz |
| Rogers 4003C | 3.55 | 0.0027 | RF PCB, antennas, up to 30 GHz |
| Rogers 4350B | 3.66 | 0.0037 | Low-cost RF alternative |
| PTFE (Teflon) | 2.1 | 0.001 | High-frequency, low-loss |
| Alumina (Al₂O₃) | 9.8 | 0.0002 | MMICs, hybrid circuits |
| GaAs | 12.9 | 0.0006 | MMIC substrates |