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Essential Tips for Microwave PCB Design

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 Microwave PCBs face more unusual challenges than standard PCBs, which means fabrication and designing involve complexities that a lot of dedication, precision, and good knowledge of designers and fabricators can mitigate. Most of these challenges, like optimal component placement and routing, are dealt with at the designing phase. As this guide progresses, it covers almost every aspect of designing and fabricating microwave PCBs.

What is Microwave PCB ?

Microwave PCBs, as the name suggests, can handle higher-frequency signals while keeping signal integrity intact along with minimal signal loss and minimum crosstalk between different traces. It is mainly used to process signals within the microwave frequency range of electromagnetic waves between 300MHz and 300GHz. It is made of high-frequency dielectric material with low loss and high signal transmission speed.
Microwave PCB 1

Tips For Microwave PCB Design

Microwave circuits are widely used in radar systems, satellite communications, microwave amplifiers, measurement equipment, medical imaging equipment, and other fields where high-frequency signals need to be processed. The design of microwave circuits is one of the core of microwave engineering, of which PCB design is a very critical part. Next, we’ll give you a few important tips on microwave design.

Microwave PCB Requirements

Firstly, before proceeding with the Microwave PCB design, we need to understand its design requirements.

  • Frequency Range: Knowing the adequate frequency range for the required microwave application is important, as this helps to choose the best in terms of substrate materials, components, and design techniques.
  • Environmental Conditions: It is important to know under which environmental conditions the required PCB will be exposed. Knowing such environmental conditions such as humidity, and extreme high and extreme low temperatures has a great impact on selecting components, material choice, and robustness.

Material Selection

Selecting appropriate materials for microwave PCBs is crucial for durability and reliability. So, what points must be kept in mind while choosing material for PCB are jotted down? The points that ELE usually takes into consideration when selecting materials are as follows:
  • Thermal Stability: It’s good practice to choose a thermally stable material with low thermal expansion constant. This helps the PCB to maintain its dimensions and performance under a wide range of environmental temperature variations.
  • Dielectric Constant: To ensure controlled impedance matching and proper signal propagation, it is important to choose PCB material wisely in terms of dielectric constant. Material with constant dielectric constant eventually helps to obtain high performance and PCB stability over a wide range of operational frequencies.
  • Dielectric Loss: Choosing a low-loss dielectric constant is important to have minimal signal attenuation. Such material requires special handling techniques but offers good operational performance at the microwave frequency range.

Impedance Control

Impedance control is the process of optimizing electrical characteristics and signal integrity during PCB design and manufacturing to ensure that the design meets specific impedance requirements. Impedance control is important in high-speed digital and analog circuits because it directly affects the speed, quality, and stability of signal transmission.

  • Trace: An important principle of signal integrity is that regulative impedance is to be used during the PCB routing. To achieve optimum intended impedance, it is recommended to get extremely accurate measurements of the trace length, distance between the conductors, and dielectric constant. To improve the design approaches, impedance calculators and simulation tools must be used.
  • Differential Pairs: The differential pairs must only be equal in length and should have a fixed distance from each other because this helps manage impedance and noise in high-speed signals. It is possible to have pre-emphasis via differential signaling to reduce the probability of signal degradation.
  • Ground Planes: To avoid the changes in the impedance level and to provide the return path for the high-frequency signals, there should be a continuous ground plane. In addition, ground planes also minimize the crosstalk of signals as well as electromagnetic interference, EMI

Signal Loss Minimization and Signal Integrity

At present, the electronic system design signal frequency is generally higher than 100MHz, high-frequency high-speed printed circuit boards are faced with the challenge of signal transmission integrity, the signal is prone to reflections, crosstalk, and other problems in the transmission process, and the higher the frequency, the faster the transmission rate, the more serious the signal loss. The development of high-frequency and high-speed PCB pain points is how to reduce signal loss in the transmission process, to ensure signal integrity.

  • Routing: Reduce trace lengths to help minimize phase delay and signal loss to the board. It is important not to have sharp bends or curves, and as much as possible aim for a single-line track. During bending, ensure that bends have gradual slopes to prevent reflections of the signal.
  • Surface Smoothness: Smoothen the surface of the trace since at higher frequencies, a rough surface might increase signal degradation. Ensure that the material and the chosen fabrication method result in smooth surfaces on the traces.

Component Placement

Component placement while designing the PCB layout is important to consider. Following are some points that should be kept in mind while designing PCB layout.
  • Placement of High-Frequency Components: To avoid long traces and attenuation, route high-frequency parts as close as possible to the circuits that they are interfacing with. To assemble signal routes for balance, keep the layout as symmetrical as possible.
  • Shielding: Shielding strategies should be effectively implemented to reduce EMI and avoid the exposure of sensitive components. Shielding could be done by proper ground plane designs, metal shields, and correct stacking of the PCBs.
  • Decoupling and Bypassing: To attenuate noise and ensure a clean source of power for critical junctions, include bypass and decoupling capacitors around the power terminals of high-frequency devices. It would be equally important to ensure that the routes from capacitors to ground have low inductance.

Thermal Management

High-frequency circuits generate a lot of heat since resistance is proportional to frequency for most materials. Don’t forget the use of thermal vias, heat sinks, and appropriate material to assist in the dissipation of heat. Consider the heat conductivity of the material being used for the PCB substrate.
To identify hot areas and enhance the flow and cooling management, thermal simulations should be performed. It is worth noting that thermal design determines long-term performance dependability.

Testing of Microwave PCB

  • Prototyping: Prototyping is the process of creating a series of mockups of the design to enhance its design optimization. It is good practice to develop prototyping, to check the viability of the concept, and to identify all issues that can be potentially met during the design process
  • RF Testing: To measure the signal integrity, the impedance control, as well as, the general efficiency of the circuit, perform RF testing. To perform these tests, employ spectrum analyzers, vector network analyzers (VNA), and time-domain reflectometers (TDR).
  • Environmental Testing: Environmental testing should be performed to determine whether the PCB is robust enough for real-life conditions. Some of these tests could be vibrations, humidity as well as temperature cycling tests. This testing is important to detect potential flaws that end up risking the reliability and durability of the desired PCB.

Conclusion

In microwave PCBs, there must be consideration of thermal management, signal integrity, impedance control and the material to be used. It is thus crucial to adhere to the above important rules and use powerful design and simulation tools to design efficient and reliable microwave printed circuit boards for applications that would require meeting high specifications and high-frequency performance. Constant feedback with senior manufacturers as well as integrating interactive prototyping eliminates the risks and provides microwave electrical devices with enhanced durability.

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About the Author
Irene Shi
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