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PCB Stack-Up Design: Layer Principles | Gold Phoenix PCB

Time: 2018-03-24 04:03:05

PCB Stack-Up Design

This Gold Phoenix PCB Knowledge Center article explains PCB stack-up design principles, EMC considerations, and standard layer configurations.

What Is Stack-Up?

Stack-up refers to the arrangement of copper layers and insulating layers that make up a PCB prior to board layout design. While a layer stack-up allows more circuitry on a single board through various PCB board layers, the structure of PCB stack-up design confers many other advantages:

A PCB layer stack can help minimize a circuit’s vulnerability to external noise as well as minimize radiation and reduce impedance and crosstalk concerns on high-speed PCB layouts.
A good layer PCB stack-up can also help balance the need for low-cost, efficient manufacturing methods with concerns about signal integrity issues.
The right PCB layer stack can enhance the Electromagnetic Compatibility (EMC) of a design.

For multilayer PCBs, general layers include ground plane (GND plane), power plane (PWR plane), and inner signal layers.

Why Stack-Up?

The development of modern electronics has been increasingly pushing PCBs toward demands such as miniaturization, light weight, high speed, better functionality and reliability, and longer lifetime, which has resulted in the popularity of multilayer PCBs. Combined with a type of semi-solid adhesive called “prepreg,” two or more single and/or double-sided PCBs are stacked together to generate multilayer PCBs through reliable predefined mutual connections. There are three or more conductive layers in one multilayer PCB, with two layers outside and one layer synthesized in the insulation board. With the increase in PCB complexities and densities, issues such as noise, stray capacitance, and crosstalk can occur when the layer arrangement has an inefficient design.

Planning an optimal multilayer stack-up is one of the most important elements in determining the EMC performance of a product. A well-designed layer stack-up can minimize radiation and prevent the circuit from being affected by external noise sources. Well-stacked PCB substrates can also reduce signal crosstalk and impedance mismatch issues. However, an inferior stack-up may increase EMI (Electromagnetic Interference) radiation, because reflections and ringing in the system as a result of impedance mismatch can dramatically lower a product’s performance and reliability.

PCB Stack-Up Principles

After the number of circuit board layers is determined, the next step is to reasonably arrange the placement order of each layer. Two main factors should be considered:

1. The distribution of special signal layers

2. The distribution of power layer and ground layer

The more layers a circuit board has, the more varieties of arrangement are possible, making it more difficult to choose the best combination. The general principles are as follows:

1. The signal layer should be next to an internal power layer (internal power/ground layer), shielded by the copper film of the internal power layer.

2. The internal power layer should be integrated with the ground layer tightly. The thickness of the dielectric between the internal power layer and ground layer should be minimized to improve the power supply capacitance between the layers and increase the resonant frequency. If the potential difference between the internal power layer and ground layer is not significant, a smaller insulation thickness can be used, such as 5 mil (0.127 mm).

3. Two signal layers should not be placed directly adjacent to each other. Crosstalk is easily introduced between adjacent signal layers, which can lead to circuit failure. Placing a ground layer between two signal layers can efficiently avoid crosstalk.

4. Multiple grounded internal power layers can effectively reduce ground impedance. For example, when signal layer A and signal layer B each use a ground plane, common-mode interference can be effectively reduced.

5. The layer structure should be symmetrical.

Standard Stack-Up Configurations

4-layer standard stack-up:

Top — 1 oz.
PP 7628 (0.17 mm)
L2 — 1 oz.
Core 1.2 mm
L3 — 1 oz.
PP 7628 (0.17 mm)
Bottom — 1 oz.
Total thickness: 1.61 mm ±10%

6-layer standard stack-up:

Top — 1 oz.
PP 2116 (0.10 mm)
L2 — 1 oz.
Core 0.6 mm
L3 — 1 oz.
PP 2116 (0.10 mm)
L4 — 1 oz.
Core 0.6 mm
L5 — 1 oz.
PP 2116 (0.10 mm)
Bottom — 1 oz.
Total thickness: 1.6 mm ±10%

8-layer standard stack-up:

Top — 1 oz.
PP 1080×2 (0.14 mm)
L2 — 1 oz.
Core 0.35 mm (including copper thickness)
L3 — 1 oz.
PP 2116 (0.10 mm)
L4 — 1 oz.
Core 0.35 mm (including copper thickness)
L5 — 1 oz.
PP 2116 (0.10 mm)
L6 — 1 oz.
Core 0.35 mm (including copper thickness)
L7 — 1 oz.
PP 1080×2 (0.14 mm)
Bottom — 1 oz.
Total thickness: 1.6 mm ±10%

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