Introduction to Printed Circuit Board (PCB) Shielding

One of the problems that come to the PCB designers is electromagnetic problems in the layout. Engineers who design and develop those printed circuit boards must always keep an eye on electromagnetic interference and compatibility to meet the system requirements. Unfortunately, even minor design flaws have the ability to raise an electromagnetic challenge. Due to decreasing number of board designers and the increment of consumer demands for faster speeds, these problems are now more prevalent than ever.

Two significant challenges for this issue are Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC).

Electromagnetic Interference (EMI) in PCB

EMI is a type of electromagnetic energy which causes a disturbance or interference in signals of electronic devices from environmental sources such as electrostatic coupling, induction, or radiation. Thus, this interference can cause the circuit or the system to perform poorly or cease functioning before the end of its life cycle. Even further, the electronic circuit's signaling channels and data commutation paths may also get distorted by generating noises, which increases the error rate or data loss. However, these EMI distortions are all around us; hence, strategic approaches are used to reduce their effect on the circuits.

Although when it comes to the design of PCBs, high-frequency signal transmission always needs EMI shielding to prevent signal and data loss. For that, a carbon paste, silver paste, or EMI shielding film is added to the surface of the PCB during the PCB manufacturing process, which can be commonly observed in flex and rigid-flex boards. Thus, as a standard practice, a stainless-steel EMI shielding cover must permanently be installed over specified locations during PCB construction.

Sources for EMI

Conducted emission and radiated emission are two different forms of electromagnetic emission. Power input lines and cables are the entry points for conducted emission into the system, while the electromagnetic waves from power and communication cables, switching equipment, and electrostatic discharges cause the radiated emission. It spreads via the air from electronic devices and traces to interfere with other electronic systems. Examples include the interference of computers and mobile devices with airplane systems. By adding line filters close to the power input or close to the connections, conducted interference can be reduced. A ferrite core/ring is another successful technique for decreasing conducted interference. Since the ferrite core uses high-frequency current dissipation a ferrite ceramic is one of the best options to create high-frequency noise-reduction devices.

What is PCB shielding?

The mechanical technique used to eliminate or reduce the effects of EMI in an electronic system by using magnetic, conductive, or both types of materials is known as PCB shielding. Since those mechanical shields are conductive, closed containers linked to the PCB's ground significantly reduce the size of loop antennas and reflect or absorb some of the electromagnetic energy.

How does EMC PCB shielding work

The capability of an electronic system to operate correctly in an electromagnetic environment without inducing electromagnetic interference in neighboring devices or systems is known as electromagnetic compatibility (EMC). This means that the relevant system will function as intended within the specified parameters and safety measures if it is electromagnetically compatible. Thus, several standards govern commercial electronic products, and it is a must to meet the prescribed requirements of EMC. It is crucial to test EMI during the PCB design stage. Later in the production process, EMI might be expensive to control. The selection of components, circuit design, and PCB layout design are the three essential factors to consider while designing an EMC-friendly PCB.

Functioning of EMC

Effective EMC shielding's primary goal is to shield delicate electronics from radio frequency interference (RFI) or electromagnetic interference (EMI). Absorbing the electromagnetic interference that broadcast via the air using a metallic barrier may lead to the accomplishment of this. The shielding effect is based on the same idea as a Faraday cage: transmitting or the metallic screen encircles sensitive electronics. The screen absorbs the sent impulses, generating a current inside its frame. A ground connection, or virtual ground plane, absorbs this current. Hence, you have to main the shielded signal free of electromagnetic interference, increasing the efficiency of the shielding by absorbing these signals before they get to the critical circuit parts.

PCB shielding types

When it comes to PCB shielding currently there, two significant types exist today. Which are,

• PCB RF Shielding - By reducing the interaction of electromagnetic fields, electrostatic charges, and radio waves, RF shielding prevents radio frequencies and electromagnetic radiation. The Faraday cages include metal shielding cans, Elastomer electromagnetic shielding materials, Ferrite beads, rods and plates, High conductivity mesh materials, and separated/star ground replacements that prevent electrostatic fields may identify in these shields.
• Arduino Shielding - The shield pins on the EMI PCB shield from Arduino allow the user to put it on top of an Arduino PCB. The circuit's functioning will therefore improve. However, paying close attention to any reserved pins is more necessary. Because A sensor cannot serve as both the input and output on another shield if it uses the I2C or SPI bus. Hence, Arduino allows one to select a variety of assets and securities for shielding. Furthermore, Arduino shields are pre-built circuit boards that attach to additional Arduino boards to improve board interoperability. Thus, this includes wireless, ethernet, GSM, and proto shields.

Compared to other varieties, an Arduino shield is more manageable and affordable. It also has a library containing example, editable code that enables the users to create and put together a project.

Features of PCB shielding

PCB-level shielding offers numerous features and benefits to the users, as follows.

1. Smaller footprints
2. Multiple cavity patterns
3. Low profile configurations
4. Fence and cover as two-piece security
5. Through hole or surface mounts
6. Unlimited design possibilities
7. I/O holes
8. Connector cutouts
9. Ventilation holes
10. Insulation padding featuring ESD protection
11. Secure locking mechanisms to protect against shock and vibration
12. Removable covers to easily access components
13. Extended shielding with RF absorbers

Potential problems of unshielded electronics

Electronics and circuits may significantly face problems due to interference, crosstalk, power supply disturbances, and ground bounce, which can result in performance problems. Also, the threat posed by electromagnetic interference (EMI) is more severe since it can destroy an electronic device's functionality. Additionally, it might have disastrous and expensive ramifications. Especially the medical device failure brought on by EMIs, for instance, might have serious repercussions and results.

The following issues may develop by uncontrolled EMI,

1. Disruption of communications
2. Interference with wireless devices
3. Corruption of data from sensors
4. Electronic component malfunctions
5. Software errors or failures

Seven tips for designing EMI shielding in your PCB layouts

The following design principles will ensure that you don't develop antennas that generate electromagnetic energy since it is challenging to design a board with low or no electromagnetic interference. These recommended design principles will decrease the length and surface area of any potential signal return pathways that can lengthen undesired Electromagnetic emissions. Especially in high-power and digital applications, the multi-layer stack-up will be crucial. To eliminate any return path that can result in the creation of common-mode signals, signal traces from the component to the processor should be correctly routed.

Replacing the Leaded devices with the Surface Mount Devices (SMD) may decrease EMI/EMC problems further. Compared to RF energy, surface mount devices (SMD) have lower inductances. SMDs also provide increased density as a result of closer component locations. This is especially important for circuit boards with two or four layers. However, the increasing complexity of the PCB design will exacerbate already existing issues with trace or line spacing. The SMDS's thick physical dimensions will provide better noise control.

1. Trace spacing and layout

Since the traces which conduct current from components to receivers in PCBs can form an antenna when it is at a bend or cross. Therefore, there are several steps that you can use to reduce the effect.

• Trace separation

• Avoid the right angle and adopt a 45turn

• Route differential traces closely
• Use vias like a pro
• Avoid stubs in sensitive and high-frequency traces
• Use guard and shunt paths for clock lines

2. Ground planes

When designing PCBs, a ground with a low inductance value is an essential component for reducing EMC issues.  Increasing the ground area of a PCB will lead to minimizing system ground inductance, and lowering EM emission and crosstalk. When connecting the signals to the ground, there are several options as follows.

• Use entire ground plane and ground grids
• Avoid long return paths
• Use Faraday’s cage/guard ring to isolate the noisy environment
• Place high-speed circuits closer to the ground and low-speed circuits closer to the power plane
• Always ground the copper-fill areas
• Check for multi-power requirements
• Be careful with split apertures

3. Shielding

To stop EMI in the system, shielding is a mechanical approach that uses conductive, magnetic or both types of materials. A mechanical shield is a closed conductive container linked to the ground, which efficiently minimizes the size of loop antennas by absorbing and reflecting some of their radiation. Depending on the need, it can be utilized to cover the entire system or only a portion of it. Information loss is avoided, and the signal transmission is shielded from outside noise by EMI/EMC.

4. Arrangement of PCB layers

The layer configuration of a PCB affects its EMC performance as well. You have to utilize one entire layer as a ground plane when two or more layers are on a board. The layer underneath the ground layer on a four-layer board needs to serve as the power plane. For the four-layer board, signal 1, ground, power, and signal 2 are the preferable layer stacks. As far as practical, the impedance-matched traces should be on signal 1.

5. Segregate sensitive components

PCB components must be organized into groups based on the types of signals they operate on, such as analog, digital, power supply, low-speed, and high-speed signals, etc., to create an EMC-friendly design. Each component group's signal tracks should remain in the designated area. A filter should be used every time a signal needs to go from one subsystem to another.

6. Decoupling capacitor

High-frequency current switches made by ICs during operation cause switching noise in the power rails and IC-connected traces. If it's unable to reduce such noises, they will cause radiated emissions and hence EMI. Placing the decoupling capacitors next to the IC power pins is one way to decrease noise on the power rails. Additionally, directly grounding the capacitors to the ground planes.  Using of power planes instead of power traces will reduce the noise in the power.

7. Controlled impedance for transmission line design

The impedance matching between the source and destination becomes crucial when a circuit works at high speed. High-frequency ringing and signal reflection will result from improper impedance matching and control. Ringing and reflection will create extra RF energy, radiating, or coupling to other circuit components and cause EMI issues. Strategies for signal termination help in minimizing these negative impacts. Termination can quiet down the signals' rapidly rising and falling edges and reduce signal reflection and ringing using controlled impedance techniques. The PCB materials used on the board have an impact on the impedance of the traces as well.

Conclusion

Placing different electronic components in a predetermined order may lead to building an electronic circuit. If the arrangement is improper, it might lead to several EMI/EMC problems. The PCB design may influence any component's EMC performance and EMI output. It would help if you considered each component's EMI/EMC impact while designing a PCB. Achieving High EMC performance will take place only with excellent PCB design procedures—where a designer must either get rid of the interference source or shield the circuit from its damaging effects. Ultimately, the objective is to keep the circuit board functioning as designed for superior EMC.