7 Tips and PCB Design Guidelines for EMI and EMC
The electromagnetic compatibility (EMC) will monitor the generation of the radiated and conducted electromagnetic interference (EMI) from electronic circuitry. Poor EMI/EMC, noise generation and poor signal transmission are found to be the key reasons for electronic circuitry failure. It is estimated that the failure rate due to electromagnetic interference (EMI) from electronic circuitry for prototype PCB is as high as 50%. A poor design is the ultimate cause of unwanted EM emission or susceptibility towards it, which is why we combined our PCB design guidelines for EMI and EMC.
More critical applications involving mil-grade, aerospace, and healthcare will witness a higher failure rate. These industries are placing stringent regulations in terms of PCB design and manufacturing processes. The telecommunication sector is one of the key sectors that is driving demand for EMI free PCBs. For example, telecommunication businesses and mobile phone manufacturers are pushing for new reforms to suppress unwanted radiations and improve connectivity. The advent of 5G will further boost the demand for EMI free PCB over the coming years.
What Are EMI and EMC in PCB Design?
Electromagnetic compatibility (EMC) is the ability of an electronic system to operate within an electromagnetic environment peacefully without generating an unintentional EMI (electromagnetic interference). Understandably, EMC is treated as a property of the electronic circuit where the circuit radiates lesser EMI to its environment. For example, we have talked about the importance of line spacing between two PCB traces and its ripple effect. In the same context, we discussed a noisy neighbor story. Now, can you imagine living in a noisy neighborhood? Similarly, EMC talks about the ability of the electronic systems to co-exist in a noisy neighborhood without generating an unintentional noise.
An electronic system that consists of printed circuit boards, integrated chips, interconnect, and I/O cables will likely emit electromagnetic interference (EMI). ICs typically act as a source and cables or PCBs act as a radiating antenna. Sometimes, the PCB also acts as a connecting medium from the source to the antenna. Thus, it is highly important to measure the electromagnetic compatibility of the electronic circuit.
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What Are the Sources of EMI?
There are two electromagnetic emission types, such as conducted and radiated EMI. Conducted electromagnetic emission will propagate through the system via a power or signal bus. While, the radiated EMI is an electromagnetic wave that propagates from the source – which can be an integrated chip or a location where electromagnetic interference is generated – the electromagnetic wave that is propagated from the edge of the PCB will be captured by a receptor. This receptor can be nearby equipment and will be affected severely by the interference.
It is common knowledge that electromagnetic radiation is emitted from the edges of PCBs. Prorogation of the EMI is best described by the example where a drop of water hits a puddle of water and creates a water ripple. The electromagnetic wave propagated as a ripple will reach the PCB edge where the two reference planes will act as a slot antenna. This will radiate noise and generate electromagnetic interference to nearby equipment.
Electromagnetic emissions may also occur from high-frequency traces that are routed in close proximity with the edge of the PCB. Similarly, electromagnetic emission can generate from power and ground planes, due to poor decoupling practices. This also results in unintentional currents, such as common-mode (CM) and differential-mode (DM) currents.
How to Reduce EMI in Your PCB?
How to design board with low or absolutely zero electromagnetic interference? Well, it isn’t impossible. All you need are world-class industry experts who will tell you the best design practices to follow for neglecting EMI/EMC problems in your design. Another option is to read the entire blog and perhaps, you will gather a set of engineering knowledge that you might not have.
The below design practices will make sure you do not create antennas, which will emit electromagnetic energy. Or these best design practices will nullify the potential signal return paths that may increase unwanted EM emissions.
In a multi-layer PCB design, it is important to route return paths for I/O pins on the layout design. The multi-layers stack-up will also play a critical role, particularly in high-power and digital applications. Signal tracing from component to the processor should be properly routed to avoid any return path, which can lead to common-mode signal generation.
The use of surface-mount devices (SMD) instead of leaded devices will further reduce EMI/EMC issues. Surface-mount devices (SMD) offer lower inductances in comparison with RF energy. Additionally, SMDs offer higher density due to closer component placements. This is particularly critical in a two-layer or four-layer circuit board. However, the rising complexity in the PCB design will create more problems associated with line spacing or trace spacing. Yet, the dense physical dimensions of SMDs will offer more effectiveness and noise-control.
Leaded components with higher inductances will generate a resonate frequency of more than 100 MHz. Due to which the adoption of a large number of through-hole components is not recommended as they generate excessive noise.
Here is a list of techniques to reduce EMI/EMC problems:
1. Line/Trace Spacing
We have discussed the importance of line spacing in a previous article. However, talking about line spacing strictly in terms of EMI/EMC from the PCB designer’s perspective is an entirely different ball game. We are well aware of the fact that the EMI (electromagnetic interference) is propagated through the edge of the board. To avoid EMI propagation, it is preferred that PCB traces should be bent at a 45-degree angle on their edges. It is also recommended to avoid microstrips and adopt striplines. Other important design practices include avoid layer changes and avoid routing for high-speed signals over the slots. Also, PCB designers should keep differential traces at close proximity.
2. Importance of Shielding:
EMI/EMC shielding protects the signal transmission from external noise and prevents information loss. The key purpose of EMI/EMC shielding is to minimize the effect of EMI and RFI on the electronic circuitry. EMI/EMC shielding is carried out by adding a metallic screen for absorbing the electromagnetic interference.
3. Controlled Impedance for Transmission Line Design
It is important to design a PCB with the right line impedance that matches the source impedances to suppress EMI/EMC. Controlled impedance also decides the signal rise and fall times. The impedance of traces also depends on the PCB materials used on the board. Read why controlled impedance matters.
4. Importance of Grounding
Reducing EMI/EMC totally depends on how effectively PCB designers apply the ground plane in their design. You must be careful while splitting ground paths. Adoption of a large, unbroken ground reference plane, and connecting it to the ground plane with the ground vias will reduce interference. Designers must provide return paths for the ground plane. Trying a zero-impedance ground is an ideal scenario in most cases.
Adding a low-pass filter for attenuation with ferrite core inductors during the high-speed signal transmission can go a long way. The decoupling capacitors are used for faster switching between IC power and ground connections, thus limiting the radio frequency emissions. This will also reduce resonance in the electronic circuit.
6. Avoid Antennas
Try to avoid antennas at any cost. Special attention must be given towards unconnected stubs and traces without return paths.
7. Separate Sensitive Components
You need to assign different PCB areas for diverse circuits in order to keep oscillator circuits away from other components. Additionally, high-speed components must be separated away from disturbing signals and from I/O connections.
These practices will attenuate and eliminate EM emissions at the source as well as designing for clean and smooth signal transmission.
How to Test for Electromagnetic Interference
The electromagnetic emissions in an electronic system are measured by implementing various modeling techniques. The computer simulation is often regarded as the fundamental approach in EMC analysis. The computer simulation is performed via an integration technique to get an accurate measurement of essential parameters.
Several steps are followed to test electromagnetic emission in an electronic system:
- The finite difference time domain modeling is implemented to measure the frequency response of the common-mode current during the high voltage applications.
- The common-mode current is evaluated by considering factors such as current-mode antenna impedance, and the distributed circuit constant.
- The electric coupling between the power plane and the ground plane will also impact the common-mode current.
Sierra Circuits measures the frequency response of electromagnetic emission from the stripline structure with the help of a high-end tool and our own proposed model. We understand the importance of keeping EMI out, thus we offer physical insights and design guidelines to keep your circuit safe and sound.
Electromagnetic compatibility of any electronic circuitry is associated with the generation, propagation, and reception of electromagnetic noise. Electromagnetic noise is not a welcomed character in a PCB design. We take intensive care to ensure signals do not interfere with each other when comes to traces, vias, and even PCBs operating in unison.
Tags: EMI, pcb design, signal integrity