Electromagnetic Compatibility Consulting and Training Part 2
In the world of electronics, PCB stack-up is a crucial factor in ensuring proper functionality and electromagnetic compatibility (EMC). In this article, we will explore the different configurations and considerations for four-layer PCB stack-up.
Four-Layer Boards
The most common configuration for a four-layer board consists of four uniformly spaced layers with internal power and ground planes. Typically, the external trace layers have orthogonal routing directions. While this configuration is an improvement over a two-layer board, it has some less-than-ideal characteristics.
In terms of objectives, this stack-up only satisfies the first objective mentioned in Part 1. With equally spaced layers, there is a large separation between the signal layer and the current return plane, as well as between the power and ground planes. To address these deficiencies, we must decide which aspect is more important.
Traditional construction techniques for four-layer boards do not provide sufficient inter-plane capacitance between adjacent power and ground planes for adequate decoupling. Therefore, it is recommended to opt for tight coupling between the signal and current return planes. This simple improvement can greatly enhance the EMC performance of the board.
Advantages of Tight Coupling
By spacing the signal layers as close to the planes as possible (<0.010") and using a large core (>0.040") between the power and ground planes, several advantages can be achieved. Firstly, smaller signal loop areas result in reduced differential mode radiation. Secondly, tight coupling between the signal trace and the ground plane reduces plane impedance, which in turn decreases common-mode radiation. Lastly, close trace to plane coupling minimizes crosstalk between traces. This method is often overlooked but highly effective in reducing radiation on a four-layer PCB.
By implementing this stack-up, we have met objectives (1) and (2) as outlined in Part 1.
Other Four-Layer Board Stack-Up Possibilities
While the aforementioned stack-up is commonly used, there are alternative configurations worth considering. One option is to reverse the signal layers and the plane layers, providing shielding to the signal traces on the inner layers. However, this may lead to challenges such as ground plane fragmentation and difficulty in board rework.
A modified stack-up, shown in Figure 3b, addresses some of the issues by using ground planes on the outer layers and routing power as a grid on the signal layers. This configuration offers lower ground impedance, reduced common-mode cable radiation, and the ability to enclose all signal traces in a Faraday cage. From an EMC perspective, this is an excellent alternative stack-up for a four-layer PCB.
Another less commonly used but still effective configuration replaces the power plane with a ground plane and routes power as a trace on the signal layers. This stack-up overcomes the rework challenge mentioned earlier and maintains a low ground impedance. However, it does not provide any shielding.
As you can see, there are more options available for four-layer board stack-up than initially thought. It is possible to meet four out of five objectives with a four-layer PCB by choosing the appropriate configuration.
In conclusion, selecting the right stack-up configuration is essential for achieving optimal performance and EMC in a PCB. Each configuration comes with its own advantages and disadvantages, so it is important to consider the specific requirements of your project. By understanding the principles and possibilities of PCB stack-up, you can ensure the success of your electronic designs.
Disclaimer: The article is based on the research and expertise of Henry W. Ott and Henry Ott Consultants. Please consult with professionals in the field for specific recommendations and guidance.
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