Design Recommendations for Stack-up Structure in High-Speed PCB Design

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In high-speed PCB design, the stack-up structure directly determines signal integrity, electromagnetic compatibility, and manufacturing yield. A poorly designed stack-up introduces impedance discontinuities, crosstalk, and warpage—issues that cannot be fixed by layout alone. Based on over 16 years of manufacturing experience, the following design recommendations provide practical, process-aware guidelines for achieving reliable, high-performance rigid, flex, and rigid-flex PCBs.


1. Stack-up Method
The recommended PCB stack-up method is Foil Stacking.

2. Material Type Control
Minimize the use of different types and models of prepreg (PP) sheets and cores within the same stack-up. The number of PP sheets per dielectric layer shall not exceed three.

3. Dielectric Thickness Control
The thickness of the PP dielectric between two adjacent layers shall not exceed 21 mil.
Note: Thick PP dielectrics are difficult to process and typically require the addition of a core, which increases the total layer count and adds manufacturing cost.

4. Copper Foil Thickness Selection

• Outer layers (Top/Bottom): 0.5 oz copper foil (recommended)
• Inner layers: 1.0 oz copper foil (recommended)

Note: Copper thickness should be determined based on current carrying requirements and trace width.

• Power boards typically use 2–3 oz copper.
• Standard signal boards typically use 1 oz copper.
• For fine-pitch routing, 1/3 oz copper may be used to improve yield.
• Cores with different copper thicknesses on opposite sides shall be avoided for inner layers.

5. Symmetrical Stack-up Design
The distribution of routing layers and plane layers shall be symmetrical about the centerline of the stack-up. This includes:

• Layer count
• Distance from centerline
• Copper thickness of routing layers
• Dielectric thickness and prepreg type
• Pattern distribution (copper planes vs. routing layers)

Note: Symmetrical design is essential to prevent warpage and ensure manufacturing reliability. Insulation layer thickness, prepreg type, copper thickness, and pattern type should all be balanced about the centerline.

6. Design Margin for Signal Integrity
Sufficient margin shall be reserved for trace width and dielectric thickness to prevent signal integrity (SI) issues and other reliability concerns caused by insufficient design tolerance.

Layer Type Definition
PCB stack-ups consist of three fundamental layer types:

• Signal Layers: Dedicated for routing signal traces.
• Plane Layers: Include power planes and ground planes, collectively referred to as plane layers.
• Hybrid Layers: In some designs, routing is performed on power or ground planes, or power/ground nets are routed on signal layers. For such cases, these layers are uniformly classified as signal layers for design and manufacturing purposes.

 

Application Across PCB Variants
The above stack-up design recommendations apply to the following PCB technologies, with additional considerations as noted:
Rigid PCB
Traditional rigid circuit boards that provide structural strength and mechanical stability. The stack-up guidelines outlined above are directly applicable to rigid PCB designs.
Flex PCB
Flexible circuit boards manufactured from materials such as polyimide, enabling bending and folding applications. When designing flex PCBs, the following additional factors must be considered alongside the core principles:

• Flex layer stack-up symmetry
• Adhesive material selection
• Bending radius and mechanical cycle life

Rigid-Flex PCB
A hybrid construction integrating rigid and flexible circuit sections into a single interconnected structure. Stack-up symmetry must be maintained across the entire assembly, with particular attention to:

• Transition zone design between rigid and flex sections
• Mechanical reliability under dynamic flexing
• Signal integrity continuity across the interface

With over 16 years of industry-leading expertise, Kingsheng PCBA specializes in high-precision PCBA design and manufacturing. Our deep experience in stack-up optimization, material selection, and process control ensures that every project meets the highest standards of signal integrity, reliability, and manufacturability. Whether rigid, flex, or rigid-flex boards, we deliver professional solutions tailored to your high-speed design requirements.
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