PCB Manufacturing

RF & Microwave PCB: Materials, Design and Manufacturing Considerations

RF & Microwave PCB

The world of RF PCB (radio frequency PCB) and microwave PCB board design is a specialized and rapidly growing field within the wider electronics industry. The unique challenges and requirements of these high-frequency applications require a thorough understanding of materials, design principles and manufacturing techniques that are necessary to achieve optimal performance.

What are RF & Microwave PCBs?

High-frequency circuit PCB which operates over 10 MHz to 1 GHz falls under the category of RF PCBs. The circuitry designs that employ over 1 GHz frequency is Microwave PCB type. The major difference between RF and Microwave PCB is the difference in operating frequency. All microwave applications come under the RF category but not all RF comes under the microwave PCB category. These types of PCBs tend to get reflections, crosstalk, ringing, and other EMI problems. The design is carefully done with the same trace width all over the PCB not just to match the impedance but also the signal integrity.

Key Parameters in RF and Microwave PCB Material

Opting for the right material for RF and microwave PCB application is most critical and it is difficult to choose them in the manufacturing process.

  • Dielectric Constant: Dielectric constant material is nothing but the material’s capability to store electric field. The direction of the electric field varies with thickness and axis. It will cause a drastic shift under high-frequency conditions. The materials are tested under various qualifications and these materials are selected for the targeted materials. The frequency and test condition range is calculated before selecting the substrate material value. Relative permittivity is nothing but the ratio between dielectric constant and vacuum permittivity, Dk and Df materials are chosen based on the material performance from lowest to higher frequencies with the impact of dipole movements as shown in the graph below.

Dielectric Constant

  • Coefficient of Thermal Expansion (CTE): The coefficient of thermal expansion defines the change in mechanical property of an object with the change in temperatures. Thermal stability, drill types, and assembly process changes concerning CTE of the Printed circuit board and its functional reliability. The change in shape at different ranges of temperature impacts more. PFTE is the most common material used for RF and microwave applications The assembly change and poor CTE can cause breakage during assembly and also during operations. Lower CTE is preferred for RF and microwave applications.
  • Loss Tangent: Dynamic dielectric constant is one of the challenges in RF applications just because of the higher frequency of operation where this problem does not occur in a lower range of frequency. Signal losses occur if the frequency is higher and gets degraded with heat. Multi-layer complex RF PCB board and microwave PCB board designs need to have more precise considerations on lossy tangents. Materials used in the PCB board and microwave PCB board should be able to handle all the loss in power rails of the Microwave and RF PCB. Power loss in filter circuitry also has to be maintained with sharp characteristics.
  • Spacing: Applications like RF PCB boards and microwave PCB boards need to have more constraints with spacing as crosstalk is bound to happen for higher frequency and skin effects. Intra-board cross-talk affects the performance of the RF PCB boards and microwave PCB boards. The resistivity of the traces increases and leads to more heat dissipation in high-frequency circuits. The minimum distance depends completely on the applications and manufacturers. The trace width, trace length, and return path define the signal integrity issues.
  • Moisture absorption: The operational environment is one more major factor for the performance. For applications operating in air-conditioned or controlled moisture-absorbing environments then the concern need not be an environment. If the board is subjected to work in rigorous rain or exterior temperatures then it might affect the criticality of the PCB.
  • Anti-Radiation: The substrate materials will undergo ionizing radiation when RF PCB boards and microwave PCB boards are used in a nuclear-based environment. Mechanical and electrical performance of these is affected if radiation stress is applied on the board, this reduces the lifespan of the RF PCB boards and microwave PCB boards.

Main Types of RF & Microwave PCB Materials

Laminate materials can play a major impact on the performance of RF or Microwave PCBs. FR4 has a higher dissipation factor which will lead to insertion loss and interference in signal frequencies. The Dk and Df values can vary with a 10 percent offset from real manufacturing value resulting in impedance variation. Dielectric loss is common with the PCB which occurs when electron bounce occurs while traversing through the conductor. This can be avoided by using substrates with a low dissipation factor of 0.001 with materials like Poly Tetra Fluro Ethylene (PTFE).

Bonding materials like ceramic-filled PTFE, LCP, and FEP are used. The remelting temperature of the bonding material is also considered. If the application of a PCB is going to have intense thermal conditions, then the bond might come off. FEP and LCP come under lower lamination temperatures and remelt temperatures. If the RF PCB is going to undergo any high-level temperatures, then PTFE materials are used for high reflow process and laminations with higher CTE.

Here are the base materials used in RF and Microwave PCBs.

Poly Tetra Fluro Ethylene (PTFE)

RF materials are generally made up of PTFE, Ceramics, Hydrocarbons, and forms of silica. PTFE with a combination of micro glass fibre is always opted for high-quality materials. Ceramic filled with PTFE is the combination of cheaper cost and increased manufacturing. PTFE is more expensive when compared to FR4 or other options, Hydrocarbon is added to the PTFE to reduce or refrain from moisture absorption.

Ceramic Material

Ceramic materials are always used together with PTFE and woven glass to be reinforced hybrid materials. These kinds of materials have excellent loss and well-controlled dielectric constants (Dk) which is very suitable for RF and microwave PCBs.

Reinforced FR4

Reinforced FR4 is the standard material type that is used for PCB manufacturing in many applications. This is the cheapest option for manufacturing RF and Microwave PCBs. Though it can be used for lesser frequency RF applications the loss tangent is worse in FR4 for RF applications, the dissipation factor for FR4 is around 0.02 which is higher.

Major Considerations in Microwave and RF PCB Design

Characteristic impedance, attenuation coefficient, and signal transmission speed are the major characteristics that have to be focussed on while designing high-frequency RF and Microwave applications. The attenuation coefficient determines the signal loss with relative permittivity which determines the speed of signal transmission and characteristic impedance. The Strip line and the microstrip are the most widely used routing styles for Microwave PCB applications, yet the are other techniques like bipolar pulse or groove type which depend on the soft base material. These are evaluated by various factors.

  • Strip Line: A Strip line is a type of routing that has two parallel reference ground planes on either side. Characteristics impedance is calculated for two types of narrow and wide signal lines. For the narrow signal lines, the characteristic impedance is calculated by the below formula:

Characteristic Impedance Calculation

  • Asymmetric Strip Line: An asymmetric strip line is when a strip line does not align symmetrically with the reference ground or power plane, the calculation part changes concerning change in area. The offset of the deviation should be ideally 1/3rd of the centre to keep the signal parameters stable. High-frequency RF and microwave coupling occurs with parallel lines or traces crossing each other.
  • Microstrip Line: A microstrip line is usually placed on the outer layers of the PCB with one reference ground line parallel to it. The strip line must be designed for Microwave PCBs to achieve less distortion and minimal loss. The lines that carry RF or Microwave frequencies must be routed with short traces; the dimension of the line should be 1/20 of the wavelength of the signal. If the trace lengths are longer than 1/16th of the wavelength then impedance control should be maintained. The microstrip structures are shown in the below image.

Microstrip Structures

  • Ground Considerations: Ground planes are poured to relax the return path and isolate the high-frequency signal from interfering with other circuit performance. This prevents cross-talk in the line maintaining signal reflections to zero.

Kay Factors in Microwave & RF PCBs Fabrication

Standard PCBs have a simple and neat manufacturing process but Microwave and RF PCB manufacturing processes have a lot of challenges. However, the advancement in technologies and manufacturing processes ensures the shortcomings of the RF and Microwave PCBs are rectified.

  • Precision and Accuracy in Production: The major manufacturing difficulty is maintaining precision and accuracy as Microwave PCBs demand high performance with high-frequency applications. Advanced manufacturing techniques and special circuitry testing will be one of the reliable testing methods to ensure fabrication quality. The frequency spectrum should be consistent in the overall area of the RF and Microwave PCB.
  • Impedance Control: Precise impedance control techniques are incorporated into the manufacturing process which ensures the reliability of RF and microwave PCB. The entire microwave circuit board RF microwave PCB should be manufactured to not have signal reflections and to maintain signal integrity. Ensuring performance, signal loss, and controlled impedance is essential for microwave and RF printed circuit board applications.
  • Material Handling: Special materials are required to ensure the quality of the substrate and the materials tend to degrade over time. This quality needs to be ensured and special material storages need to be deployed for Microwave PCB fabrication which operates over 1GHz. So the RF PCB suppliers need to be careful.
  • Assembly Process: A high-precision assembly process ensures the solderability and precise placement of components. So reliable RF PCB manufacturers and microwave PCB manufacturers are needed. This also minimizes the stray capacitance and inductance which will impact the circuit performance in High-frequency operation.

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