Electromagnetic Compatibility Consulting and Training Part 4. Eight-Layer Boards
An eight-layer board offers the opportunity to improve the electromagnetic compatibility (EMC) performance of a PCB. In this article, we will explore some stack-up configurations for eight-layer boards that have proven to provide excellent EMC performance.
The Purpose of Eight-Layer Boards
An eight-layer board can serve two main purposes: adding two more routing layers or improving EMC performance by adding two more planes. While both cases are seen, the majority of eight-layer board stack-ups are used to enhance EMC performance rather than increase the number of routing layers. The cost increase of an eight-layer board over a six-layer board is justified by the improved EMC performance it offers.
The Ideal Eight-Layer Stack-Up
The basic stack-up of an eight-layer board with excellent EMC performance consists of four wiring layers and four planes. The following stack-up provides a satisfying solution for all the objectives:
Mounting Pads/Low Freq. Signals
Pwr.
Gnd.
High Freq. Signals
High Freq. Signals
Improving Stack-Up with Embedded PCB Capacitance Technology
The stack-up shown in Figure 9 can be further improved by incorporating embedded PCB capacitance technology for layers 2-3 and 6-7. This technology, such as Zycon Buried Capacitance™, enhances high frequency decoupling and can reduce the need for discrete decoupling capacitors.
Alternative Eight-Layer Stack-Up Configurations
Another effective configuration for eight-layer boards involves including two outer layer ground planes, as shown in Figure 10. This arrangement shields all routing layers and offers the advantage of routing orthogonal signals adjacent to the same plane.
A modified version of this configuration can also be considered by moving the planes to the center, as depicted in Figure 11. While this provides a tightly coupled power-ground plane pair, it compromises the ability to shield traces.
Limitations of More Than Eight Layers
It is important to note that there is no significant EMC advantage to using a board with more than eight layers. If additional layers are required for signal traces, a ten-layer board should be used. However, for most applications, an eight-layer board with the appropriate stack-up configurations discussed above can provide optimal EMC performance.
By carefully considering the stack-up configuration of an eight-layer board, you can ensure improved electromagnetic compatibility and better performance for your PCB designs.
For more information and assistance with PCB stack-up and electromagnetic compatibility, contact our team at [Your Company Name].