Why is the number of layers of multilayer PCB an even number?

Multi-layer PCBs are a common part of the PCB production process. Have you ever noticed that when referring to multi-layer boards, they usually come in even numbers, such as 2 layers, 4 layers, 6 layers, 8 layers, and so on? It's rare to hear about odd-layer PCBs. Why is that?

In reality, the preference for even-layer multi-layer PCBs isn't as mysterious as it may seem. To begin with, multi-layer PCBs are engineering marvels designed to handle complex circuits. They are multi-layer versions of PCBs, with multiple layers stacked together to accommodate more signal paths and power planes. This allows electronic devices to become more compact and more powerful.

The popularity of even-layer PCBs is partly due to simplicity and symmetry. Each internal layer in a multi-layer board has a corresponding symmetric layer. This symmetry helps in balancing the layout between signal and power layers. In PCB design, we need to consider not only signal transmission but also factors like power and ground. The design of even-layer PCBs makes it easier to achieve symmetrical arrangements, reducing electromagnetic interference, and enhancing performance and reliability.

Furthermore, the distribution of currents is another important consideration. Even-layer PCBs allow for an even distribution of currents between adjacent inner layers, which helps reduce resistance, differences in current density, and heat-related issues. This is particularly important for applications that require high current or high-frequency transmission.

Moreover, manufacturing multi-layer boards involves stacking these layers, usually in pairs. This stacking method is more manageable and controllable, ensuring the quality and manufacturability of the PCB.

Although even-layer PCBs are more common, odd-layer PCBs are still used for specific applications. However, their design and manufacturing are more complex and often require more sophisticated layer management. The choice of the number of layers in a multi-layer PCB typically depends on the project's requirements, complexity, and budget. Understanding why most people prefer even-layer PCBs can help us make better use of them in electronic devices.