**Foreword**
Electromagnetic compatibility (EMC) is a crucial aspect of electronic design, closely tied to the circuit's layout and performance. To ensure effective EMC, designers must focus on minimizing unwanted radiation from the product and improving its immunity to external interference. This involves managing both conduction coupling at low frequencies and radiation coupling at high frequencies by carefully controlling the paths through which signals and noise can travel. This article explores key considerations in PCB design that help reduce electromagnetic interference (EMI), ensuring better performance and compliance with industry standards.
**PCB Design Principles**
As electronic systems become more complex and operate at higher frequencies, the challenge of managing EMI becomes more critical. Proper PCB design plays a vital role in reducing these issues. Following specific design principles helps keep EMI within acceptable limits, meeting both functional and regulatory requirements while enhancing the overall reliability of the circuit.
**1. Selection of Circuit Board**
Choosing the right board size is essential in PCB design. A board that is too large can increase signal path lengths, leading to higher impedance and reduced noise immunity. On the other hand, a board that is too small may result in dense component placement, making heat dissipation and crosstalk management difficult. Therefore, the board size should be selected based on the system’s component count and layout needs.
PCBs are typically single-layer, double-layer, or multi-layer boards. The number of layers depends on the circuit’s complexity, signal density, and frequency requirements. For low to medium frequency circuits with fewer components, a single or double-layer board may suffice. However, for high-density, high-speed, or high-frequency designs, multi-layer boards are recommended. These allow for dedicated power and ground planes, reducing loop areas and differential mode radiation, thus improving EMI performance.
**2. Layout of Circuit Board Components**
Once the board size is determined, the next step is to position the components. High-performance layouts require careful planning to separate digital, analog, and power sections, as well as high and low-frequency circuits. This separation helps minimize interference between different parts of the circuit.
Key layout principles include:
- Placing heat-generating components near the board edges for better cooling.
- Grouping high-frequency components together to reduce signal path length.
- Keeping sensitive components away from noise sources like oscillators.
- Ensuring adjustable components are placed in accessible positions.
- Fixing heavy components securely to prevent mechanical stress.
- Positioning EMI filters close to their respective noise sources.
For power and ground lines, proper routing is essential to reduce EMI. Key techniques include increasing trace spacing to minimize capacitive coupling, using parallel power and ground lines, and ensuring wide traces for high current paths. Ground loops should be avoided, and multi-layer boards can use dedicated ground planes to further reduce impedance and improve shielding.
**Grounding Techniques**
Proper grounding is one of the most important aspects of EMI control. Different grounding methods—such as single-point and multi-point grounding—are used depending on the circuit’s needs. Single-point grounding is suitable for low-frequency circuits, while multi-point grounding is preferred for high-speed applications due to its lower impedance and reduced interference.
To enhance grounding effectiveness:
- Keep ground wires short and thick (minimum 3mm).
- Avoid unnecessary loops that can create potential differences.
- Use separate grounding for different signal types.
- In multi-layer boards, place power and ground planes adjacent to each other for better capacitance and shielding.
**24 Tips to Reduce Noise and Electromagnetic Interference**
(1) Use high-speed chips only where necessary.
(2) Add resistors in series to reduce control circuit edge resistance.
(3) Provide damping for relays and similar components.
(4) Choose the lowest possible clock frequency that meets system needs.
(5) Place the clock generator close to the device it powers, and ground the crystal housing.
(6) Surround the clock area with a ground ring and keep clock lines as short as possible.
(7) Position I/O drivers near the board edge, filter incoming signals, and use termination to reduce reflections.
(8) Connect unused microcontroller pins to VCC or ground, and avoid leaving them floating.
(9) Ground unused inputs and connect negative inputs to outputs.
(10) Use 45-degree angles instead of 90-degree corners to reduce high-frequency emissions.
(11) Separate high-noise areas from low-noise sections on the board.
(12) Use thick power and ground lines on single or double-layer boards, and consider multi-layer boards for better EMI reduction.
(13) Keep clock, bus, and chip select signals away from I/O lines and connectors.
(14) Keep analog voltage inputs and reference voltages far from digital signals, especially clocks.
(15) Ensure that analog and digital sections of A/D devices do not cross.
(16) Route clock lines perpendicular to I/O lines to reduce interference.
(17) Keep component and decoupling capacitor leads as short as possible.
(18) Make key signal lines thick with grounded shields, and keep high-speed lines short and straight.
(19) Avoid placing noise-sensitive lines parallel to high-current or high-speed lines.
(20) Do not route under quartz crystals or noise-sensitive components.
(21) Prevent weak signal circuits from forming loops around low-frequency circuits.
(22) If loops are unavoidable, keep their area as small as possible.
(23) Use a decoupling capacitor per IC, and add a small high-frequency bypass capacitor to each electrolytic.
(24) Replace electrolytic capacitors with tantalum or ceramic ones for better performance, and ground the casing if using tubular capacitors.
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