What is the difference, advantages and disadvantages between eight-layer board and fake eight-layer board

PCBs are core components of modern electronic devices, and their design and manufacturing are critical to device performance and reliability. In PCB design, impedance control and stackup design are one of the key factors. This article will delve into PCB impedance knowledge, explore the differences between eight-layer boards and fake eight-layer boards, and analyze their respective advantages and disadvantages.

1. Basic knowledge of PCB impedance
Impedance is an electrical property in a circuit that describes the relationship between current and voltage. In PCBs, the control of impedance is crucial for high-frequency signal transmission, especially in modern communications and high-speed data transmission. Control of PCB impedance is accomplished by designing the board's hierarchy and specific material selections.

2. PCB stackup design
PCB stack-up design involves stacking multiple layers of circuit boards together to meet specific electrical and mechanical requirements. In general, a typical PCB hierarchy consists of signal, ground, and power layers. Higher layer count PCBs offer more design flexibility and better electrical performance.

3. Characteristics of eight-layer boards
An eight-layer board is a common multi-layer PCB structure. Its typical hierarchical structure includes signal layer, ground layer, power layer and plane layer. This structure provides better impedance control, signal integrity and electromagnetic compatibility. Eight-layer boards are widely used in high-performance computers, communication equipment and other complex electronic systems.
4. The concept of "fake eight-layer board"
"Fake eight-layer board" refers to the appearance of using an eight-layer PCB in the design, but in fact only a few layers are used for wiring, and the remaining layers mainly consist of power layers and ground layers. This design approach is primarily driven by cost considerations to reduce the cost of PCB manufacturing.
5. The difference between eight-layer boards and "fake eight-layer boards"
Impedance control: Eight-layer boards make it easier to achieve precise impedance control in design because there can be more signal layers and plane layers, making wiring more flexible. Since the "fake eight-layer board" has fewer layers, impedance control may be relatively difficult, especially in high-frequency applications.
Signal integrity: The multi-level structure of the eight-layer board helps reduce crosstalk and signal attenuation between signal pins and improve signal integrity. In contrast, the "fake eight-layer board" may have more signal integrity problems in high-frequency environments due to fewer signal layers.
Electromagnetic compatibility: The eight-layer board design can better control electromagnetic radiation and sensitivity and reduce electromagnetic interference. The "fake eight-layer board" may have poor electromagnetic compatibility due to the lack of isolation between the power supply and the ground layer.
Cost: "Fake eight-layer boards" are generally more competitive on manufacturing costs because fewer layers are used. However, this savings may come at a cost in circuit performance and stability.
6. Analysis of the advantages and disadvantages of eight-layer boards and "fake eight-layer boards"
(1) Advantages of eight-layer boards
Better impedance control: Eight-layer boards provide better impedance control with more signal layers and plane layers, suitable for high-performance and high-frequency applications.
Superior Signal Integrity: Multi-layer structure helps reduce signal attenuation and crosstalk, improving signal integrity.
Better electromagnetic compatibility: Adequate hierarchical structure helps control electromagnetic radiation and sensitivity and improve electromagnetic compatibility.
(2) Disadvantages of eight-layer boards
Higher manufacturing costs: Eight-layer boards have relatively high manufacturing costs due to their large number of layers. They are suitable for applications that have higher performance requirements and are willing to bear higher costs.
(3) Advantages of "fake eight-layer board"
Lower manufacturing costs: "Fake eight-layer boards" have relatively low manufacturing costs due to their fewer layers, making them suitable for cost-sensitive applications.
(4) Disadvantages of "fake eight-layer board"
Impedance control is relatively difficult: Due to the small number of layers, impedance control may be relatively difficult and is suitable for applications that do not require high impedance control.
Relatively poor signal integrity and electromagnetic compatibility: Fewer signal layers may cause problems with signal integrity and electromagnetic compatibility, and are suitable for applications that do not have high requirements in these aspects.

Conclusion: When selecting a PCB structure, design engineers need to consider performance requirements, manufacturing costs, and application scenarios. Eight-layer boards have obvious advantages in high-performance and high-frequency applications, especially in impedance control, signal integrity and electromagnetic compatibility. The "fake eight-layer board" is suitable for cost-sensitive applications, but may sacrifice performance and stability. Design engineers should weigh these factors based on specific needs to achieve the best balance of price/performance and performance.