EMI occurs when electromagnetic sources transfer energy to sensitive systems through coupling paths, manifesting as conduction through wires and shared grounds, space radiation, or near-field coupling. Experience demonstrates that improper PCB design, despite correct circuit schematics, significantly compromises electronic equipment reliability. Therefore, ensuring PCB EMC is critical for overall system design integrity. This article focuses on EMC technology and its application in multilayer PCB design.
1. The PCB board serves as the foundation for electronic product circuit components and devices, facilitating electrical connections among them. It is an essential component in various electronic devices. Today, the prevalence of large-scale and very large-scale integrated circuits in electronic equipment has significantly increased the component density on printed circuit boards (PCBs), alongside the faster signal transmission speeds. Consequently, electromagnetic compatibility (EMC) issues have become increasingly prominent.
2. PCBs are categorized into single-sided (single-layer boards), double-sided (double-layer boards), and multi-layer boards. Single-sided and double-sided boards are typically used for low- to medium-density circuits and those with lower integration levels. In contrast, multi-layer boards support high-density wiring and highly integrated circuits. From an EMC perspective, single-sided and double-sided circuits are inadequate for high-speed circuits. The development of multi-layer wiring circuits has addressed these challenges, leading to their widespread adoption.
3. **Characteristics of Multi-layer Wiring**: PCBs consist of organic and inorganic dielectric materials arranged in multiple layers. These layers are interconnected using vias, which are filled or plated with metal to facilitate electrical signal transmission between layers. The key advantages of multi-layer wiring include:
– Incorporation of dedicated power and ground layers within the board structure. The power layer serves as a noise loop to minimize interference and offers a path for system-wide signal currents, reducing common impedance coupling.
– Specialized ground layers for signal lines ensure stable impedance and facilitate impedance matching, thereby reducing waveform distortion from reflections. Additionally, these layers increase distributed capacitance between signal and ground lines, minimizing crosstalk.
4. **PCB Laminate Design**:
– **Wiring Rules**: High-speed and high-performance systems often utilize multi-layer PCBs where dedicated layers serve as DC power or ground reference planes. These solid planes provide low impedance return paths crucial for EMC compliance. Signal layers are strategically placed between these reference planes and can be symmetric or asymmetric striplines. For instance, a 12-layer board might be structured as T – P – S – P – S – P – S – P – S – S – P – B, where T and B denote the top and bottom layers, respectively.
– **Routing Considerations**: When routing high-speed signals, it’s crucial to minimize signal loop areas to reduce differential and common-mode radiation. Signal traces should be routed to ensure the return current flows efficiently between reference planes, maintaining signal integrity and minimizing electromagnetic emissions.
5. **Ground Wire Layout**: At high frequencies, any metal wire exhibits impedance due to its resistance and inductance characteristics. Thus, grounding strategies—whether single-point or multi-point—must form low-impedance loops to the actual ground or rack. It’s essential to minimize the length of ground paths to prevent them from becoming antennas that radiate high-frequency currents.
6. **Power Cord Arrangement**: Multi-layer boards typically employ a power supply layer-ground structure for powering components. This structure’s low characteristic impedance obviates the need for high-frequency decoupling capacitors adjacent to each integrated circuit. Instead, decoupling capacitors can be strategically placed anywhere on the board.
7. **Conclusion**: In the design of multi-layer PCBs, it’s imperative to group components to minimize interference, carefully route high-speed circuits to avoid coupling interference, and reduce power supply loop areas to mitigate magnetic coupling. Isolation of incompatible signal lines and minimizing signal loop areas further helps in reducing radiation and improving EMC performance.
These edits aim to clarify technical concepts and improve readability while maintaining the original content’s technical depth.
