Guidelines for RF and Microwave PCB Design

 

Radio frequency printed circuit board (RF PCB) and microwave printed circuit board are defined as high-frequency printed circuit boards. RF PCB is operating above 100MHz, while microwave PCB is operating above 2GHz. High-frequency PCB is mainly used for applications that need special signal transmission between objects. Examples of applications are advanced automotive safety systems, cell phones, radar installations, etc.

Compared to conventional PCB, the layouts of RF PCB and microwave PCB are difficult to design. The first reason is that both of them are distributed parameter circuits, which are easy to produce both skin and coupling effects. Moreover, the interference and radiation in circuits are also hard to control in the actual PCB design. The other common issues related to receiving or transmitting radio signals are noise sensitivity, cross-interference between digital circuits and analog circuits, and tighter impedance tolerances. Therefore, balancing the advantages and disadvantages of a PCB design and minimizing interference will help RF PCB and microwave PCB to achieve their best performance.

Choosing the best substrate material for RF PCB and microwave PCB  plays a vital role in the early stage of circuit design. This contributes to the outstanding performance and reliability of final products. Certain aspects such as relative permittivity, loss tangent, thickness and environment should take into considerations when deciding the substrate material for the PCB design.

The ratio between vacuum permittivity and dielectric constant is relative permittivity. In order to meet the space and weight requirements of the board, the relative permittivity of RF and microwave PCB substrate must be high enough. Such parameters as line width, the wavelength of circuit operating frequency and approximate sizes of main components must be determined before finalizing substrate materials. This contributes to drawing the circuit board diagram, thereby establishing appropriate maximum and minimum relative permittivity. In addition, in order to keep the electrical performance within the tolerance range, the relative permittivity deviation provided by the substrate material manufacturer must be sufficiently low.

Dielectric loss is a function of loss tangent and relative permittivity. To meet the input and output power requirements while avoiding heat dissipation issues within a certain frequency range, the substrate material loss has to be sufficiently low since the dielectric loss will affect the frequency characteristic.

As for the thickness of the substrate material, it is related to trace width, mechanical performance, size stability, cost, conformability, dielectric breakdown and power handling capability. During manufacturing, high-impedance traces on thin substrate materials may require extremely low trace width. In terms of size stability, thin substrate materials have worse performance than thick substrate materials. Furthermore, thin substrate materials can also bring setbacks to manufacturers or result in increased costs. The limitation of the power handling capacity of high-frequency PCB can be alleviated by increasing the thickness of the substrate material.

The main material properties that should be considered in the selection of RF and microwave PCB substrate materials include temperature stability, humidity resistance and chemical, and anti-radiation performance. The relative permittivity may affect the electrical performance within the operating temperature range since the relative permittivity changes with temperature. The substrate material selected for high-frequency PCB should have low moisture absorption so that the electrical performance of the board will not be significantly reduced in a high-humidity environment. Also, the technology used needs to be compatible with the chemical resistance and solvent resistance of the substrate material. In space or nuclear applications, the substrate material will be affected by a large amount of ionizing radiation. The impact of ionizing radiation on the mechanical and electrical properties of the substrate needs to be estimated. At the same time, the cumulative effect should be compared with the effective operating life of the board.

 

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Reference:

https://www.epectec.com/pcb/rf-microwave-laminate-material.html

https://www.sfcircuits.com/pcb-production-capabilities/rf-pcb

https://www.pcbcart.com/article/content/rf-and-microwave-pcb-design.html