Design Guidelines for RF PCB Layout and Manufacturing

RF PCBs work on some fundamental rules that involve prior knowledge of digital and analog design circuits. Comparing directly to analog and digital is not directly possible because here the frequency range is in the GHz range. The frequency range makes the PCB so different and we have look for almost everything:

• Impedance mismatches
• Trace lengths
• Grounding
• Shielding

If not done so, may cause the full failure of the system. Or if designed not considering all the requirements it will lead to performance degradation, signal loss, EMI (electromagnetic interference), and increased power consumption. We will see what type materials can be used, how the traces are routed in microwave RF PCB and most importantly the rules for signal integrity and impedance. Careful layout is required to prevent signal degradation and crosstalk between sections. Before starting an RF layout, consider the following parameters:

7 Key RF PCB Layout Considerations

1. Material Selection

RF PCBs require a stable substrate now. What do you mean by stable? It only means a stable Dielectric constant (Dk) and lower loss tangent (Df). Common materials which are used in RF are Rogers RO4000, RO3000, Taconic RF laminates and PTFE. Lower dielectric constant means there will be small propagation delay. On the other hand lower loss tangents minimize the energy loss.

2. Controlled Impedance

For the trace we can use  microstrip, stripline, or coplanar waveguide structures which will be discussed below. Other than that we have to maintain consistent trace width and height from the reference ground plane. By implying that, characteristic impedance (typically 50Ω or 75Ω) can be set. If the line does not match to the network there will be reflections and losses. For more we can use curved corners traces instead of sharp 90° bends. After designing the circuit and PCB, simulation is very important. There are many online tools like impedance calculators & simulation tools to determine the appropriate trace dimensions and working. See our detailed article on impedance control.

3. Trace Design and Routing

Due to high frequency we need minimum parasitic and for that, Keep RF traces as short and direct as possible. Avoid sharp corners and use 45° bends or curved traces to prevent impedance discontinuities. Avoid via transitions in RF paths, use back-drilled or blind vias. Avoid running RF and non-RF traces in parallel to prevent coupling. Place test points outside the traces to preserve impedance continuity. Keep digital and RF sections isolated; maintain at least 20 mm separation, otherwise we have 20H rules, H is the distance from L1 to L2 of PCB, it can be prepreg between them if 4 layer PCB is considered.

4. Using RF Transmission Lines

1. Microstrip: A signal trace routed on an outer layer with a solid ground plane directly underneath.

2. Stripline: Signal traces routed on inner layers with ground planes above and below for balanced performance.

3. Coplanar Waveguide (Grounded): A center conductor flanked by ground traces with via fences for excellent isolation.

5. Grounding and Return Paths

A solid ground strategy is essential to prevent noise and EMI. Use a continuous ground plane directly beneath RF traces. Avoid ground plane interruptions that break return paths. The usual procedure is to route the RF on the top layer and ground just below it. Although vias are not recommended but where signal spreading is more we can stitch the PCB with vias which help to reduce parasitic capacitance. Ensure digital grounds are separated from RF grounds to reduce noise coupling. They should join only at one point in the circuit with some special considerations.

6. Power Supply Decoupling

Clean power is crucial for RF circuitry. Use multiple capacitors with varying values to filter a wide frequency range. Placing decoupling capacitors close to the power pins of RF ICs always helps to reduce noise. Route power traces separately and isolate them with filters if necessary. You can see a PMIC utilizing all these outside the chip.

7. Shielding and Isolation

EMI can degrade RF performance if not managed properly. Use grounded shielding cans to isolate RF sections. Separate RF, digital, and analog sections on the PCB. Implement guard traces (grounded) around sensitive RF traces to reduce coupling. Minimize crosstalk by spacing traces appropriately.

Conclusion:

RF PCB design starts with careful substrate selection as we do in step 1. Then the layer stack planning and controlled impedance routing is done. RF circuits are more susceptible to layout induced losses because of crosstalking and parasitic effects of the material. Following the given guidelines helps you build RF boards. Which are capable of handling high frequencies with minimal loss and interference.  As frequencies increase into GHz ranges we can not just skip the simulation and testing.