The critical circuits in a PCB can be guarded against EMI using EMI shields.
The shielding effectiveness of an EMI shield depends on the shield material used. The material choice is determined by the type of circuit and its frequency of operation.
A component shield encases a component that is either susceptible to or radiates EMI.
EMI shields absorb EMI
Every PCB is built to handle electromagnetic fields, as the successful operation of most circuits requires some electromagnetic field interactions. To ensure this, PCBs need to be designed so that the electromagnetic field is not interfering with the operation of devices or circuits close by and the PCB itself is not influenced by the external electromagnetic field.
To create a PCB with high electromagnetic compatibility (EMC), PCB designers should follow several guidelines for layout and design to mitigate electromagnetic interference (EMI). EMI shielding in PCBs is one solution that can increase the EMC of a circuit board.
Reduce EMI With These PCB Design Guidelines
Both conducted and radiated EMI in a PCB can come from a variety of sources, including:
- Switching devices
- Switching pulses
- High-speed digital signals
- Clock signals
Obstructions in current paths, especially in high-frequency PCBs, generates EMI and threatens the EMC of a board. At high-frequency operation, metal projections (such as heat sinks) behave as antennas radiating electromagnetic emissions. Similar to the causes mentioned above, ‘n’ number of factors influence the generation of crosstalk, noise, distortion, and EMI in PCBs.
Since EMI can stem from a variety of places, multiple areas on a PCB need to be considered to mitigate the negative effects of EMI. Here are some modifications to consider to increase the EMC of a PCB design:
- Minimize trace length
- Avoid routing over split planes
- Minimize the inclusion of vias
- Close routing of differential pairs
- Isolate high-speed traces
- Separate analog from digital circuits
- Separate power from control circuits
- When using ICs on a board, include decoupling capacitors near the VCC pin
- Guard critical circuits using EMI shields
Let’s take a closer look at this last suggestion—using EMI shielding in PCBs.
EMI Shielding in PCBs
EMI shielding is often used in PCBs used in the medical, communication, and military fields. In RF or high-frequency circuits, EMI shields are often used in the input, output, and amplifier stages, as these sections are more vulnerable to EMI.
In EMI shielding, critical circuits are covered using metal shields that protect from the radiation and absorption of EMI. EMI shields isolate the encapsulated circuit or components from the rest of the board without altering its electrical connection with other components in the PCB. Designers connect the EMI shield to the ground to transfer the interferences or noises to the ground plane.
The EMI shield absorbs the internally or externally-generated EMI. This absorption creates a current flow in the metal used for shielding. Since the EMI shields are connected to the ground plane, the currents generated from EMI leave the critical circuit without interfering with its sensitive signals. The effectiveness of any EMI shield depends on the shield material used, and the material choice is determined by the type of circuit and its frequency of operation.
Types of EMI Shielding
There are a few types of EMI shielding used in PCBs:
Component shielding: A component shield encases a component that is either susceptible to or radiates EMI.
Board shielding: A section of a board or complete board can be encapsulated inside EMI shields to prevent the detrimental effects of EMI from reaching other circuits.
Cable shielding: In high-frequency circuits, the presence of parasitic reactance aggravates the effect of EMI in cables carrying analog or digital signals. Cable shielding is an excellent solution for minimizing the EMI in cables.
EMI shielding in PCBs is a great method of protection against the ill effects of radiated and absorbed EMI. A PCB design with EMI shields can increase the reliability and signal integrity of circuits.
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