When designing high-speed PCBs, the layer stack-up plays a crucial role in ensuring signal integrity, minimizing crosstalk, and achieving optimal electromagnetic compatibility (EMC). For a standard six-layer board with a thickness of 1.6mm, selecting the right structure can significantly impact performance. Below is an analysis of common six-layer board structures and their suitability for high-speed designs.

Standard Six-Layer Board Structure

A typical six-layer board consists of two core boards and two copper foils, laminated with PP glue (semi-cured film). This is widely used for conventional PCBs without high-speed signal requirements.

Structure Analysis

Structure 1: Top-GND-Signal_1-Signal_2-VCC-Bottom

• Features:
  • Ground (GND) and Power (VCC) layers provide effective shielding.
  • Top and Bottom layers are well-isolated from Signal_1 and Signal_2.
  • Signal_1 and Signal_2 are separated by a distance of >20mil to reduce crosstalk.
• Pros:
  • Excellent shielding between layers, reducing external interference.
  • Suitable for designs with moderate signal routing density.
• Cons:
  • Requires an eight-layer fabrication process (commonly referred to as a “fake eight-layer” board).

Structure 2: Top-Signal_1-GND-VCC-Signal_2-Bottom

• Features:
  • Ground and Power layers are fully coupled, ensuring stable power delivery.
  • Signal layers (Signal_1 and Signal_2) are closer to the Top and Bottom layers.
• Pros:
  • Suitable for lower-frequency designs.
  • Simple fabrication process.
• Cons:
  • Higher potential for crosstalk due to insufficient isolation between Signal_1 and Signal_2.
  • Limited shielding effectiveness.

Structure 3: Top-GND-Signal_1-VCC-GND-Bottom

• Features:
  • Additional GND layer between Signal_1 and VCC improves shielding.
  • Reduced number of signal layers (one fewer compared to other structures).
• Pros:
  • Superior crosstalk prevention.
  • Ideal for designs with high-speed signals requiring robust shielding.
• Cons:
  • One fewer signal layer, potentially limiting routing space.

Selecting the Right Structure

The choice of stack-up depends on the circuit’s requirements, signal speed, and interference considerations:

1. For high-speed signals:
   • If routing can be managed on a single signal layer, Structure 3 is the best option due to its superior shielding capabilities.
2. For moderate-speed signals or higher routing density:
   • Structure 1 is the most balanced, offering effective shielding while accommodating multiple signal layers.
3. For low-speed signals or cost-sensitive designs:
   • Structure 2 is simpler and more cost-effective but less effective for high-speed applications.

Practical Example

The RK3399 embedded motherboard uses Structure 1, ensuring optimal performance with high-speed interfaces. Key specifications include:

• Impedance: Single-ended 50Ω, Differential 90Ω/100Ω.
• Crosstalk prevention: 40mil spacing between Signal_1 and Signal_2 layers.

This layout effectively isolates high-speed signals and minimizes crosstalk, making it ideal for embedded applications.

Key Considerations for High-Speed PCB Design

• Signal Integrity: Use proper spacing and shielding to maintain signal quality.
• Crosstalk Mitigation: Ensure adequate layer separation and avoid unnecessary vias.
• Power Integrity: Optimize GND and VCC coupling for stable operation.
• Impedance Matching: Follow design rules for controlled impedance routing.

By carefully selecting the layer structure based on circuit requirements, engineers can achieve reliable, high-performance PCB designs.