As electronic devices become smaller and signal speeds increase, multilayer PCBs have become increasingly important. Among these, the 8-layer PCB stackup offers a balance between complexity and performance for many advanced applications. This article explores the reasons for using an 8-layer PCB, its typical structure, and best practices for design.

Why Use a Standard 8 Layer PCB Stackup?

1. Space Efficiency: With at least 4 layers for signal traces, more space is available on outer layers for components.
2. Improved Signal Integrity: Power and ground planes between signal layers reduce crosstalk.
3. Better EMI Control: Multiple layers allow for better electromagnetic interference (EMI) management.
4. Thermal Management: Increased layers provide more options for heat distribution.

Typical Structure of an 8 Layer PCB Stackup

• Read more about: 4 Layer PCB Stackup

• Read more about: 6 Layer PCB Stackup

A standard 8-layer PCB stackup usually consists of the following layers:

1. Top Layer (Signal)
2. Prepreg
3. Ground Plane
4. Core
5. Inner Layer 1 (Signal)
6. Prepreg
7. Power Plane
8. Core
9. Ground Plane
10. Prepreg
11. Inner Layer 2 (Signal)
12. Core
13. Power Plane
14. Prepreg
15. Bottom Layer (Signal)

This arrangement separates signal layers with power or ground planes to reduce EMI susceptibility and emission.

Design Techniques and Challenges

1. Routing Direction

• Route signal traces on adjacent layers perpendicularly to minimize crosstalk.
• Maintain different routing directions on subsequent layers, even when separated by power or ground planes.

2. Return Path

• Visualize and optimize the return path for high-speed signals, even on inner layers.
• Ensure short return paths to minimize interference with other components.

3. Ground Plane Management

• Avoid split ground planes to prevent impedance discontinuities.
• Ensure low-impedance connections from outer layer components to inner ground planes via vias.

4. Via Usage

• Consider using buried or blind vias to increase routing space.
• Verify manufacturer capabilities for producing complex via structures.

5. Material Selection

• Choose appropriate prepreg and core materials.
• Consider how material properties affect impedance control for transmission line design.

6. Manufacturer Capabilities

• Ensure your PCB manufacturer can produce the specified 8-layer stackup.
• Discuss any special requirements, such as impedance control or via structures, with your manufacturer.

8 Layer PCB Stackup reference for different thickness design

Conclusion

While an 8-layer PCB stackup offers numerous advantages, it also presents unique challenges. Adhering to best practices and using appropriate design tools are crucial for success. Remember that an 8-layer stackup isn’t a universal solution to all EMI problems – proper design techniques remain essential.

Designers should leverage advanced PCB design software, such as OrCAD PCB Designer, which offers features like Cross Section Editor for layer stackup control, strong DRC tools, and reliable analysis and simulation capabilities. These tools can significantly simplify the complex task of designing an 8-layer PCB.

If you need Stackup suggestion for your 8 layer PCB design, Welcome to send email to sales@raypcb.com .