8-Layer PCB Stackup

The 8-layer PCB laminated structure usually includes signal layer, power layer and ground layer, the specific arrangement and design principles are as follows

Signal layer: It usually includes the top layer (TOP), the bottom layer (Bottom) and the signal layer in the middle (e.g. Signal2, Signal3, etc.). The signal layer is mainly used for wiring and transmitting electrical signals.

Power Layer: It usually includes one or more power layers (e.g. Power1, Power2, etc.), which are used to provide a stable power supply. The power supply layer is adjacent to the ground layer to better realize the coupling between the power supply and the ground, and to reduce the impedance between the power plane and the ground plane.

Ground layer: includes one or more ground layers (e.g. Ground1, Ground2, etc.), which are mainly used to provide a stable ground reference plane and reduce electromagnetic interference. The ground plane is adjacent to the power plane to provide better signal integrity.

Design Principles and Common Arrangements

The layer adjacent to the main chip is the ground plane: it provides a stable reference plane for the main chip and reduces interference.
All signal layers are adjacent to the ground plane as much as possible: provides better signal integrity.
Avoid two signal layers directly adjacent to each other as much as possible: reduce signal interference.
Main power supply is adjacent to its corresponding ground plane as much as possible: to reduce the impedance between the power plane and the ground plane.
Symmetrical structure design: Dielectric layer thickness and type, copper foil thickness, and graphic distribution type should be symmetrical to minimize the impact of asymmetry.

Common design examples and tool usage

Common stacked layer design: such as TOP-Gnd-Signal-Power-Gnd-Signal-Gnd-Bottom, etc. This design can provide better signal integrity and electromagnetic compatibility.
Using Huaqiu DFM tool: This tool helps to calculate impedance, select proper line width and spacing, and ensure the accuracy of the design.

8-Layer PCB Stackup Design Analysis

Option 1: Six Signal Layer Design (Not Recommended)

Structure Characteristics:

1. Top Layer: Signal 1 (Component side/Microstrip routing layer)
2. Inner Layer: Signal 2 (X-direction microstrip, premium routing layer)
3. Inner Layer: Ground (Ground plane)
4. Inner Layer: Signal 3 (Y-direction stripline, premium routing layer)
5. Inner Layer: Signal 4 (Stripline routing layer)
6. Inner Layer: Power (Power plane)
7. Inner Layer: Signal 5 (Microstrip routing layer)
8. Bottom Layer: Signal 6 (Microstrip routing layer)

Drawback Analysis:

• Poor electromagnetic absorption
• High power impedance
• Incomplete signal return paths
• Inferior EMI performance

Option 2: Four Signal Layer Design (Recommended)

Improved Features:

1. Top Layer: Signal 1 (Component side/Microstrip, premium routing layer)
2. Inner Layer: Ground (Low-impedance ground plane, excellent EM absorption)
3. Inner Layer: Signal 2 (Stripline, premium routing layer)
4. Inner Layer: Power (Power plane forming capacitive coupling with adjacent ground)
5. Inner Layer: Ground (Ground plane)
6. Inner Layer: Signal 3 (Stripline, premium routing layer)
7. Inner Layer: Power (Power plane)
8. Bottom Layer: Signal 4 (Microstrip, premium routing layer)

Advantages:
✓ Dedicated reference plane for each signal layer
✓ Precise impedance control (±10%)
✓ Reduced crosstalk (orthogonal routing between adjacent layers)
✓ 40% improvement in power integrity

Option 3: Optimal Four Signal Layer Design (Highly Recommended)

Golden Rule Structure:

1. Top Layer: Signal 1 (Component side/Microstrip)
2. Inner Layer: Ground (Solid ground plane)
3. Inner Layer: Signal 2 (Stripline)
4. Inner Layer: Power (Power plane)
5. Inner Layer: Ground (Core ground plane)
6. Inner Layer: Signal 3 (Stripline)
7. Inner Layer: Ground (Shielding ground plane)
8. Bottom Layer: Signal 4 (Microstrip)

Outstanding Performance:
★ Five ground planes provide perfect EM shielding
★ <3mil power-ground spacing for optimal decoupling
★ Symmetrical layer distribution prevents warpage
★ Supports 20Gbps high-speed signaling

Design Recommendations:

1. Route critical signals on S2/S3 stripline layers first
2. Implement split power plane design
3. Limit top/bottom layer traces to <5mm length
4. Maintain orthogonal routing between adjacent signal layers

Stackup Thickness Reference

Note: All designs should incorporate blind/buried vias for optimal routing space utilization.