Considerations for High Frequency PCB Design
High-frequency PCB design requires a completely different set of knowledge compared to simple low-speed analog design or digital circuits design. Ignoring some of these practices may make the design be highly susceptible to EMI or cause signal loss. We here list some considerations for you when design a high frequency PCB.
Impedance matching is a key factor in high-frequency PCB design. As the frequency increases, the tolerance becomes lesser. Therefore, the designer needs to ensure that the impedance matches at the specified value.
Unlike low-speed or frequency designs, material selection is critical for a high-frequency PCB design. FR-4 here is not the best choice, because it can not achieve consistent dielectric constant of the PCB which is required by high-frequency design. This can lead to unacceptable signal losses in a high speed design.
Instead, the ideal material for high-speed designs should have a much lower loss dissipation factor than the FR-4. In high-frequency design, even the slightest moisture of the PCB can affect the circuit itself. It is important to use a low moisture absorption material like PTFE.
You may have chosen a suitable substrate for high-frequency design. But if you do not pay attention to the thickness of the solder mask, your design will still suffer from signal loss. Solder mask has a high moisture absorption rate, and wet solder mask is not suitable for high-frequency transmission.
Return loss for a high-frequency design must be kept to the minimum. Since high-frequency signals use the least-inductive path, there should be no discontinuity in the ground plane below the signal traces themselves. Otherwise, high-frequency signals will use other non-ideal return paths, such as the power supply plane, and may cause ringing or reflections.