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PCB design for best EMC/EMI performance
Printed Circuit Boards (PCBs) are the main part of any electronic and electrical devices that determine their high performance and correct functionality. The quality and advantage of electronic devices are directly related to the quality and professional design of their PCBs. Any PCB consists of different parts such as power supply, ground, circuits, electronic components (ICs, resistors, capacitors, and inductors), traces, antennas, and RF modules.
Electronic devices that have the best functionality but are not compliant to EMC/EMI are not sellable in the market. Therefore, they are required to meet all electronic systems design standards and requirements beside their quality. One of the most important standards is EMC/EMI.
Based on Faraday’s law, any circuit through which electrical current passes produces an electromagnetic field around itself. The measure of the electromagnetic field is related to the rate of change in the loop area and current.
If this electromagnetic energy is excessive, then harmful effects on other components will be destructive. Therefore, in PCBs with several existing circuits and different types of electronic components, especially RF modules, the unwanted electromagnetic energy emitted with high effect on nearby components interferes with their operation in PCB and even on other electronic devices. Therefore, in some cases, it is a significant challenge to reduce electromagnetic interference (EMI) in the PCB design process.
EMC/EMI is an essential item in any electronic device. EMC (Electromagnetic Compatibility) is the electronic device’s immunity against external noise to operate correctly and efficiently. The best EMC performance PCB has three characteristics: it doesn’t interfere with other boards, has immunity against the EM emission of other PCBs, and doesn’t interfere with its operation.
Several resources generate EM emissions. Unwanted Emissions take in two states, radiated emission and conducted emission. Recognizing the EMI sources helps to provide a limited EMI solution related to that specific source.
The EMI comes from electronic components, PCB layout, and external elements. Electronics components with high voltage consumption prepare EMI problems. Impedance mismatching, poor capacitance coupling, switching circuits, and high-frequency traces are the important factors that cause EM emissions from the PCB layout. Consider a suitable design practice for reducing EMI related to each of these factors.
In general, electromagnetic compatibility (emc) is only achievable by a lot of considerations in design of PCB, such as having solid ground planes in different layers of PCB, special considerations for high-speed signals, such as impedance matching and using special elements such as filters or frequency rejection elements in PCB design.
Some practices for limited component EMI
- Electronic Components with high power produce EM emissions, so try to use low power consumption components.
- In high-frequency applications, use SMD (surface mount devices). SMD provides lower resistance and inductance and reduces RF emission.
- Grouped and separated the same type of signal component from the different types of components, for instance, analog components located nearby together and isolated from digital components.
- Using a Faraday cage for sensitive electronic components helps to protect the component from External EMI and also avoids RF emission into the environment.
Top design practices for limited PCB layout and traces EMI
- Different Types of Signal traces must be divided from each other. Signal traces are categorized as high-speed, low-speed, digital, analog, clock, video, audio, and reset signals.
- Trace clearance distance should be based on IPC standards.
- Use differential signal pairs because they produce the inverse polarity and can operate at lower voltage to help to reduce EMI.
- Make 45° turns instead of 90° bends. Sharp angles in traces cause a change in characteristic impedance value and growth in reflection.
- Shielding clock signal traces protect them from EMI.
- A long signal return path provides higher impedance and causes higher EMI. So, minimize the return path of the signal to the Ground.
- Decoupling capacitor technique in high-speed circuits helps to reduce radiated emissions.
- Matching impedance between source to load in high-speed application helps to reduce the high-frequency ringing and reflection.
PCB layer arrangement and Stack Up
Correct PCB layer arrangement and Standard can help to increase EMI from signal layers. Prevent locating conductive signal layers close to each other and use of ground plane between them.
Shielding to protect PCB against External EMI
- Shielding a part of the board or the entire board protects the PCB from external EMI.
- Use shielded cable for digital and analog signals to reduce the EM emission of these signals to the environment.
It’s logical and important to observe EMI issues in initiating steps in the design process. Finally, an electronic device based on its application category and marketplaces needs to pass the EMI testing or simply called compliance tests. According to government rules, electronic and RF devices shouldn’t disturb each other’s operation and interfere in wireless communication. FCC certificate for electronic devices expresses that the device passed all steps of the EMI standards testing process.
Arshon Technology, for its IoT and wireless products, takes all the testing processes under the supervision of an accredited laboratory such as Nemko to assure products meet their EMC/EMI obligations.