Key Technologies for RF PCB Route: Microstrip/Stripline Wiring, Shielding Design, and Parasitic Parameter Control

Radio Frequency (RF) PCBs (typically operating at frequencies ≥300 MHz up to 6 GHz or higher) are critical components in wireless systems such as 5G base stations, Wi-Fi routers, and satellite communication devices. Unlike low-frequency PCBs, RF signals are highly sensitive to wiring geometry, electromagnetic interference (EMI), and parasitic effects—even minor route design flaws (e.g., inconsistent trace width, poor shielding) can cause signal attenuation, phase distortion, or EMI leakage, directly degrading system performance.

This article focuses on three core technologies of RF PCB route design: microstrip/stripline wiring (the foundation of RF signal transmission), shielding design (to suppress EMI), and parasitic parameter control (to maintain signal integrity). It details technical principles, design standards, and practical optimization methods to help engineers achieve reliable RF signal transmission.

I. Microstrip/Stripline Wiring: The Core of RF Signal Transmission

RF signals rely on specialized transmission lines (microstrip or stripline) to propagate with minimal loss and distortion. The choice between microstrip and stripline depends on frequency requirements, EMI sensitivity, and PCB layer constraints—each has unique structural characteristics and design rules.

1. Microstrip Wiring: Suitable for Low-to-Medium RF Frequencies

Microstrip is a surface-mounted transmission line, consisting of a conductive trace on the PCB’s top/bottom layer and a continuous ground plane on the adjacent inner layer. It is widely used in RF systems operating at 300 MHz–3 GHz (e.g., Wi-Fi 6 modules) due to its easy fabrication and low cost.

(1) Structural Characteristics and Advantages

Structure: A single-conductor trace (width W) on the outer layer, separated by a dielectric substrate (thickness H, relative permittivity εr) from the ground plane below.

Key Advantages:

Easy access for testing (e.g., using a network analyzer probe to measure signal parameters directly on the trace).

Lower fabrication complexity compared to stripline (no need for inner-layer routing or blind vias).

(2) Critical Design Parameters

The performance of microstrip wiring (impedance, loss, phase velocity) depends on three core parameters—impedance matching, trace width/space, and substrate selection—which must be precisely controlled.

Parameter
Design Requirements
Technical Rationale

Characteristic Impedance (Z₀)
Must match the source/destination impedance (typically 50Ω for RF systems) with a tolerance of ±5%.
Impedance mismatch causes signal reflection (measured by Voltage Standing Wave Ratio, VSWR—VSWR ≤1.5 is acceptable for most RF applications).

Trace Width (W)
Calculated based on εr, H, and Z₀. For example: εr=4.4 (FR-4 substrate), H=1.6mm, Z₀=50Ω → W≈3.2mm (for 1oz copper).
Incorrect width leads to Z₀ deviation. Too narrow → Z₀ increases (signal reflection); too wide → Z₀ decreases (excessive loss).

Trace Spacing (S)
Minimum spacing between adjacent microstrip traces ≥2×W (or ≥0.5mm, whichever is larger).
Prevents crosstalk (electromagnetic coupling between traces). At 2.4 GHz, spacing<2w can="" cause="" crosstalk="" attenuation="">-20dB, distorting signals.

Substrate Selection
Use low-loss substrates (e.g., Rogers 4350B, εr=3.48, loss tangent tanδ=0.0037) for frequencies >2 GHz; FR-4 (tanδ=0.02) is acceptable for<1 GHz.
High tanδ increases dielectric loss (signal energy is absorbed by the substrate), especially at high frequencies.

(3) Routing Best Practices

Minimize Trace Bends: Avoid right-angle bends (90°) as they create impedance discontinuities and radiate EMI. Use 45° bends or curved bends (radius ≥3×W) to maintain Z₀ consistency.

Reduce Trace Length: Keep microstrip traces as short as possible (preferably<100mm for="" frequencies="">3 GHz) to minimize conductor loss (due to skin effect) and dielectric loss.

Ground Plane Continuity: Ensure the ground plane under the microstrip has no gaps or cuts—even a small break (e.g., a 1mm-wide slot) can increase Z₀ by 10% or more, causing reflection.

2. Stripline Wiring: Ideal for High-Frequency and EMI-Sensitive Applications

Stripline is an inner-layer transmission line, with a conductive trace sandwiched between two parallel ground planes. It is preferred for high-frequency RF systems (3 GHz–6 GHz, e.g., 5G millimeter-wave modules) or environments with strict EMI requirements (e.g., aerospace devices).

(1) Structural Characteristics and Advantages

Structure: A single-conductor trace (width W) in the inner layer, separated by two dielectric substrates (thickness H1 and H2, typically H1=H2 for symmetry) from the top and bottom ground planes.

Key Advantages:

Excellent EMI shielding: The two ground planes enclose the trace, preventing RF signal radiation and external interference.

Stable impedance: Less affected by external environmental factors (e.g., humidity, nearby components) compared to microstrip.

(2) Critical Design Parameters

Stripline design shares similarities with microstrip but requires additional attention to layer symmetry and ground plane alignment.

Characteristic Impedance (Z₀): For symmetric stripline (H1=H2=H), Z₀ is calculated using εr, H, and W. Example: εr=3.48 (Rogers 4350B), H=0.8mm, Z₀=50Ω → W≈1.8mm (1oz copper).

Layer Symmetry: Ensure H1 and H2 differ by ≤10%—asymmetry causes the trace to "couple" more strongly with one ground plane, leading to impedance deviation and increased loss.

Trace Placement: Avoid routing stripline near PCB edges (keep ≥5mm distance) or through areas with dense vias—vias can create gaps in the ground plane, breaking the enclosed shielding effect.

(3) Routing Best Practices

Blind/Buried Vias for Layer Transitions: If the RF signal must switch from a stripline (inner layer) to a microstrip (outer layer), use blind vias (connecting inner layer to outer layer without passing through all layers) instead of through-hole vias. Through-hole vias create unnecessary connections to other ground planes, introducing parasitic inductance.

Consistent Substrate Thickness: Use PCBs with uniform dielectric thickness (tolerance ≤±0.1mm) to maintain H1 and H2 stability—thickness variations can cause Z₀ fluctuations along the trace.