How to choose a suitable stacking scheme for PCB design with more than 4 layers?

Nowadays, PCB designs with more than 4 layers are commonly used in high-speed and complex circuit design. How to choose a suitable stack? This article analyzes the commonly used PCB stacks.

Nowadays, PCB designs with more than 4 layers are commonly used in high-speed and complex circuit design. How to choose a suitable stack? This article analyzes the commonly used PCB stacks.
 

How to choose a suitable stacking scheme for PCB design with more than 4 layers?

1. Stacking scheme 1: TOP, GND2, PWR3, BOTTOM

This solution is the current mainstream 4-layer selection solution in the industry. There is a perfect ground plane under the main device plane (TOP), which is the optimal wiring layer. When the layer thickness is set, the thickness of the core board between the ground plane layer and the power plane layer should not be too thick to reduce the distributed impedance of the power supply and the ground plane and ensure the filtering effect of the plane capacitor.

2. Stacking scheme two: TOP, PWR2, GND3, BOTTOM

If the main component surface is designed on the BOTTOM layer or the key signal line is on the BOTTOM layer, the third layer must be arranged on a complete ground plane. When the layer thickness is set, the thickness of the core board between the ground plane layer and the power plane layer should not be too thick.

3. Stacking scheme three: GND1, S2, S3, GND4/PWR4

This kind of scheme is usually applied to the design of interface filter board and backplane. Since the entire board has no power plane, GND and PGND are arranged on the first and fourth layers, respectively. The surface layer (TOP layer) only allows a small number of short wires to be run. Similarly, we lay copper on the S02 and S03 wiring layers to ensure the reference plane of the surface wiring and control the stacking symmetry.

Six-layer laminate design scheme

1. Stacking scheme 1: TOP, GND2, S3, PWR4, GND5, BOTTOM. This scheme is the mainstream 6-layer selection scheme in the industry, with 3 wiring layers and 3 reference planes. The thickness of the core board between the 4th and 5th layers should not be too thick in order to obtain a lower transmission line impedance. Low impedance characteristics can improve the decoupling effect of the power supply.

The third layer is the optimal wiring layer. High-risk lines such as clock lines must be laid on this layer to ensure signal integrity and resist EMI energy. The bottom layer is the next best wiring layer. The top layer is the routable layer.

2. Stacking plan two: TOP, GND2, S3, S4, PWR5, BOTTOM When there are too many wires on the circuit board and the three wiring layers cannot be arranged, this stacking plan can be used. This kind of scheme has 4 wiring layers and two reference planes, but there are two signal layers sandwiched between the power plane and the ground plane, and there is no power decoupling effect between the power plane and the ground plane.

Since the third layer is close to the ground plane, it is the best wiring layer, and high-risk lines such as clocks should be arranged. The first layer, the fourth layer, and the sixth layer are layers that can be routed.

3. Stacking scheme three: TOP, S2, GND3, PWR4, S5, BOTTOM. This scheme also has 4 wiring layers and two reference planes. The power plane/ground plane of this structure adopts a structure with a small spacing, which can provide lower power supply impedance and better power decoupling.

The top and bottom layers are poor wiring layers. The second layer close to the ground plane is the best wiring layer and can be used to route high-risk signal lines such as clocks. Under the condition of ensuring the RF co-current path, the fifth layer can also be used as the wiring layer for other high-risk signal lines. The first and second layers, the fifth and the sixth layers should use cross wiring.

Eight-layer laminate design scheme

1. Stacking scheme 1: TOP, GND2, S3, GND4, PWR5, S6, GND7, BOTTOM This scheme is the main selection layer setting scheme of the current eight-layer PCB in the industry, with 4 wiring layers and 4 reference planes. The signal integrity and EMC characteristics of this laminated structure are the best, and the best power decoupling effect can be obtained.

The top and bottom layers are EMI routing layers. The adjacent layers of the 3rd and 6th layers are reference planes and are the best wiring layers. The two adjacent layers of layer 3 are ground planes, so they are the best routing layers. The thickness of the core board between the 4th and 5th layer should not be too thick in order to obtain a lower transmission line impedance, which can improve the decoupling effect of the power supply.

2. Stacking scheme two: TOP, GND2, S3, PWR4, GND5, S6, PWR7, BOTTOM Compared with scheme 1, this scheme is suitable for the situation where there are many types of power supplies and one power plane cannot handle it. The third layer is the optimal wiring layer. The main power supply should be arranged on the 4th floor, which can be adjacent to the main ground.

The power plane on the 7th layer is a split power supply. In order to improve the decoupling effect of the power supply, the ground copper should be used on the bottom layer. In order to balance the PCB and reduce the warpage, the top layer also needs to be covered with copper.

3. Stacking scheme three: TOP, S2, GND3, S4, S5, PWR6, S7, BOTTOM This scheme has 6 wiring layers and two reference planes. The power decoupling characteristics of this laminated structure are very poor, and the EMI suppression effect is also very poor. The top and bottom layers are wiring layers with poor EMI characteristics. Layers 2 and 4 next to the ground plane are the best wiring layers for clock lines, and cross wiring should be used.

Layers 5 and 7 next to the power plane are acceptable wiring layers. This solution is usually used for 8-layer backplane designs with fewer SMD components. Because there are only sockets on the surface, a large area of ​​copper can be laid on the surface.

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