A trace is a thin, conductive copper wire placed on a non-conductive material to carry signals across the circuit. This trace has a certain defined width, height, and thickness depending on the application. In high-frequency applications, PCB traces may produce EMI, which may disturb the functioning of the device. Also, incorrect decoupling may lead to EMI being produced by power and ground planes. Hence, shielding PCB traces is essential to protect against electromagnetic interference (EMI) and to ensure signal integrity, especially in high-frequency and sensitive circuits. While using PCB EMI shields helps effectively mitigate noise, this post discusses various ways to shield PCB traces effectively and prevent signal distortion and losses.
Effective Ways to Shield PCB Traces to Prevent EMI
While shielding of PCB traces may not be necessary for low-frequency applications, it is recommended for medium to high-sensitivity signals, analog signals, and so on. Here are several methods to shield PCB traces effectively from EMI.
- Ground Planes: This method involves placing a continuous ground plane on one or multiple layers of the PCB. Grounding provides a low-impedance path for return currents that helps shield signal traces from EMI, improving signal integrity and reducing crosstalk.
Implementation: Place the ground plane adjacent to the signal layer, typically on a multi-layer PCB.
- Ground Pours and Copper Pours: A copper pour is basically an area filled with copper. Adding copper pour creates a solid ground plane and a low impedance path for return signals. It also works as a power plane to distribute power uniformly throughout the board. Ground pours are especially beneficial around sensitive signal traces.
Implementation: Use ground pours around sensitive traces, ensuring proper connections to the ground plane.
- Guard Traces: These are ground traces placed at both ends and parallel to sensitive signal traces, also known as guard rings. They provide a low-impedance path connected either to the ground plane or a specific guard trace, diverting unwanted signals from sensitive portions of the board. This helps block EMI between transmission lines, reduces crosstalk, and shields the signal from external interference.
Implementation: Route a guard trace parallel to the signal trace, ensuring they are closely spaced.
- Shielding Cans: These are metal enclosures placed over certain portions of the board containing sensitive components and traces, creating a physical barrier against EMI. They form a Faraday cage directly soldered onto the board to achieve circuit isolation.
Implementation: Place shielding cans over critical areas and connect them to the ground plane.
- Differential Pairs: Differential pair PCB routing carries equal and opposite signals across the board, used for high-speed and high-frequency signals. It reduces susceptibility to EMI by canceling out noise equally affecting both lines.
Implementation: Route signal pairs with equal lengths and proximity, maintaining a constant distance.
- Trace Routing: This involves laying out signal traces with careful routing to minimize EMI and prevent signal integrity issues.
Implementation: Keep high-speed traces short and away from noisy power lines, avoid right-angle bends, and maintain consistent trace lengths.
- Decoupling Capacitors: Decoupling capacitors isolate or decouple two circuits, separating AC signals from DC ones to mitigate noise. Placing capacitors close to IC power supply pins filters out high-frequency noise.
Implementation: Use capacitors with appropriate values close to the ICs.
- Ferrite Beads and Chokes: Ferrite beads suppress EMI in electronic circuits by placing them on power and signal lines to suppress high-frequency noise from ground planes, supply lines, and data signal lines.
Implementation: Select components based on frequency and current requirements.
- EMI Shielding Paint: Conductive paint applied over specific areas of a circuit board to shield EMI, providing an additional layer of protection and encapsulating sensitive components.
Implementation: Use as needed on enclosures or directly on the PCB surface.
- Controlled Impedance: Designing traces with controlled impedance for high-frequency signals helps improve signal integrity and reduce EMI. Impedance design is critical for circuit performance.
Implementation: Calculate and design trace widths and spacing to achieve desired impedance.
- Plasticized Metal Shields: Thin, lightweight thermoformed sheets or films made of plasticized metal, with polyimide on one side and a conductive metal layer on the other. These shields can be cut to size and shape to fit the board's dimensions.
Implementation: Cut and apply these shields on sensitive components to suppress EMI effectively.
Achieving immunity against EMI at board level is crucial for any device. Effective shielding of PCB traces involves a combination of grounding techniques, physical barriers, careful routing, and the use of specific components to mitigate EMI. Most of these methods ensure the reliability and performance of the electronic circuit by protecting sensitive signals from interference. While most devices now are designed to be EMC compatible, EMI shielding helps control board level internal and external noise. Plasticized EMI shields prevent unwanted signals and subsequent damage to the board and in turn the system of device. These shields must conform to the shape of the board, and hence be lightweight and flexible to ensure proper fitment and area coverage. XGR Technologies offers SnapShot® EMI shields made from a thin polyetherimide film on the interior and conductive tin on the exterior surfaces. These flexible and lightweight shields can be easily used on any type of PCBs mitigate electromagnetic inference across applications. If you need further information on SnapShotTM EMI shields and their effectiveness in shielding EMI in PCBs, you can contact the team at XGR Technologies via phone or email.
