PCB Design Considerations for High-Performance Filtering in mmWave Applications
RF circuits for applications in the mmWave range (30 to 300GHz) require high-performance filtering to meet the high-data, high-speed functionality that operating at these higher frequencies promises. However, filters for devices operating in the mmWave range will not function optimally if your printed circuit board (PCB) is not configured appropriately. For this reason, RF design engineers need to make a number of critical PCB design decisions that range from selecting the right materials to developing a board configuration that will limit common issues such as spurious-wave-mode propagation, conductor and radiation losses, unwanted resonance, and dispersion.
To address these complex issues, when starting a new circuit design for a mmWave application, Knowles recommends carefully evaluating the following design factors for your PCB assembly:
◆ Board Material Thickness and Dielectric Constant (Dk): mmWave applications require thinner boards to eliminate unwanted resonance and minimize wave-propagation issues, but the selected material's thickness and Dk will also dictate the signal-conductor width.
◆ PCB Land Pattern Optimization: PCB land patterns need to be optimized for different board materials and thicknesses to maintain the best voltage standing-wave ratio (VSWR).
◆ Etching Tolerances: Any imprecise variation in trace width or gaps in etching will have a greater effect on the system's impedance, which will in turn degrade the VSWR.
◆ Shunt Capacitance: The RF layer's thickness and the Dk of the board material can increase or decrease the shunt capacitance that an RF filter could encounter when fully mounted, so it's important to optimize the board layout around and under the RF components to achieve optimal performance.
◆ Plating Finish: When selecting a final plating finish, consider is the insertion loss that the finish will add to your system.
◆ Ground-via Spacing: This is important to consider at mmWave frequencies because higher-order modes can easily resonate between vias.
◆ Surface Roughness: Rougher surfaces essentially slow down a wave as it passes through the board, which will impact conductor loss much more at mmWave frequencies.
◆ Solder Pattern and Reflow Profile: We recommend having a final solder standoff thickness of 2 to 3 mil once assembled, but if this is not possible, on-chip solder dams using dry film solder-mask material may be placed on the top of the circuit.
Before embarking on designing a mmWave assembly, it's a good idea to first discuss any potential board material or configuration changes, and the effects of making these changes, with your component manufacturer.
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