1. **Introduction**
Many reliability and stability issues in electronic products arise from poor electromagnetic compatibility design. Common problems include signal distortion, excessive signal noise, unstable signals during operation, system crashes, systems vulnerable to environmental interference, and weak resistance to interference. Electromagnetic compatibility design is a highly complex discipline, encompassing everything from design principles to an understanding of electromagnetics. This article shares some practical insights, ranging from layer design to layer layout, to offer valuable guidance for electronic engineers.
2. **Layer Configuration**
1. The layers of a PCB primarily include the power layer, the ground layer, and the signal layers. The total number of layers is the sum of each individual layer. During the design process, the first task is to organize and classify all power, ground, and signal sources, and to plan the layout based on this classification. Typically, different power supplies should be allocated to separate layers, and different grounds should have their corresponding ground planes. Special signals, such as high-speed clocks or high-frequency signals, should be designed separately, with a ground plane added to shield these signals to enhance electromagnetic compatibility (EMC). When cost considerations are also important, the design should strike a balance between EMC and the system’s cost.
2. The primary consideration when designing the power layer is the type and number of power supplies. If there is only one power supply, a single power layer can be used. For high-power applications, multiple power layers may be required to supply different parts of the circuit. In cases with multiple power supplies, you can either design separate power layers or allocate different power supplies within the same layer, as long as there is no overlap between the power domains. If overlap is unavoidable, multiple power layers must be used.
3. The design of signal layers should account for the characteristics of all signals. Special signal routing and shielding must be prioritized. Typically, design software is first used to create the layout, with adjustments made based on specific details later. Both signal density and the integrity of special signals must be carefully considered during the layer design process. For sensitive signals, a dedicated ground plane layer may be used as a shielding layer when necessary.
4. Generally, it is not recommended to design single-sided or double-sided PCBs unless cost is the sole factor under consideration. While single- and double-sided PCBs are simpler to manufacture and more cost-effective, they are less suitable for high signal density and complex signal structures, such as high-speed digital circuits or mixed-signal (analog-to-digital) circuits. In single-sided designs, the lack of a dedicated ground reference layer increases the loop area, leading to higher radiation. Without effective shielding, the system’s immunity to interference is also compromised.
5. **PCB Layer Layout Design**
Once the signals and layers are determined, the layout of each layer must also be carefully planned. The layout of the inner layers of a PCB typically follows these principles:
(1) **Position the power plane adjacent to the corresponding ground plane.** The goal is to create a coupling capacitor, which works in tandem with decoupling capacitors on the PCB to reduce the impedance of the power plane while providing a broader filtering effect.
(2) **Selecting the reference plane is crucial.** While the power layer and the bottom plane can theoretically serve as the reference, the ground plane is generally preferred, as it offers better shielding than the power plane. Therefore, the ground plane is usually chosen as the reference plane.
(3) **Key signals on adjacent layers should not cross the boundary between them.** Crossing layers can lead to larger signal loops, increasing both radiation and coupling between layers.
(4) **To maintain ground plane integrity, avoid routing signal traces on the ground plane.** If signal trace density is too high, routing on the edge of the power plane may be considered.
(5) **Design a ground plane beneath critical signals**, such as high-speed, test, or high-frequency signals. This minimizes the signal loop path, reducing radiation.
(6) **Consider power supply radiation and interference.** In high-speed circuit designs, the power plane area should be smaller than the ground plane area to ensure the ground plane can shield the power supply effectively. Typically, the power plane should be offset by twice the thickness of the dielectric layer relative to the ground plane. To reduce this offset, the dielectric thickness should be minimized.
6. **General Principles for Multi-Layer PCB Layout:**
(1) The power plane should be placed near the ground plane, ideally below it.
(2) The signal layers should be adjacent to the entire metal plane.
(3) Digital and analog signals should be kept isolated. To avoid mixing, digital and analog signals should not share the same layer. If this is unavoidable, use physical separation (e.g., slots) to isolate the analog and digital areas. The same isolation principle applies to analog and digital power supplies. Since digital power supplies generate significant radiation, they must be shielded.
(4) Printed traces on the inner layers act as a planar waveguide, while traces on the outer layers form microstrip lines. Their transmission characteristics differ.
(5) Clock circuits and high-frequency circuits are major sources of interference and radiation, so they must be placed far away from sensitive areas.
(6) Stray currents and high-frequency radiation currents vary across layers and should be treated differently when designing the routing.
7. **Layer Design and Layout for Improved EMC:**
The number of layers primarily depends on the power layer, ground layer, high-frequency signals, special signals, and sensitive signals.
The layout focuses on minimizing coupling, optimizing ground and power routing, and carefully placing clocks, high-speed signals, and analog and digital signal areas to achieve superior electromagnetic compatibility.
