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Suggestions for multilayer PCB design

2020-05-31

Multilayer PCB design suggestions and examples (4, 6, 8, 10, and 12-layer PCB boards) illustrate the PCB design requirements: A. The surface is a complete ground plane (shield); B. There are no adjacent parallel wiring layers; C . All signal layers are as close as possible to the ground plane; D. The key signals are adjacent to the ground layer and do not cross the partition.


 

4-layer PCB board

 

Option 1: There is a ground plane under the component surface, and the key signals are preferentially distributed on the TOP layer. As for the layer thickness setting, there are the following suggestions: 1: Meet the impedance control 2: The core board (GND to POWER) should not be too thick to reduce the ground. The distributed impedance of the plane; to ensure the decoupling effect of the power plane.

 

Option 2: Defect 1: The power and ground are too far apart, the impedance of the power plane is too large 2: The power and ground plane are extremely incomplete due to the influence of component pads, etc. 3: The signal impedance is discontinuous due to the incomplete reference plane

 

Option 3: Similar to option 1, applicable to the case where the main device is placed in the BOTTOM layout or the key signals are routed at the bottom layer.

 

6 layer PCB board

 

Option 1: A signal layer is reduced, and an additional electrical layer is added. Although the layer available for wiring is reduced, this PCB design solution solves the defects common to Option 1 and Option 2. Advantages: 1: Power layer and ground layer are tightly coupled. 2: Each signal layer is directly adjacent to the inner electrical layer and is effectively isolated from other signal layers, making crosstalk less likely. 3: Siganl_2 (Inner_2) is adjacent to the two inner electrical layers GND (Inner_1) and POWER (Inner_3), which can be used to transmit high-speed signals. The two inner electric layers can effectively shield the outside interference to the Siganl_2 (Inner_2) layer and Siganl_2 (Inner_2) to the outside.

 

Option 2: 4 signal layers and 2 internal power / ground layers are used, with more signal layers, which is beneficial to the wiring work between. Defects: 1: The power layer and ground layer are far apart and not fully coupled. 2: The signal layers Siganl_2 (Inner_2) and Siganl_3 (Inner_3) are directly adjacent to each other. The signal isolation is not good and crosstalk is likely to occur.

 

Regarding 8, 10, 12 layer PCB board.

 

Personal summary:

1. The key signal layer of PCB design should be adjacent to ground, and GND should be adjacent to power to reduce the impedance of the power plane.

 

2. Do not adjoin between signal layers, increase the isolation between signals to avoid crosstalk.

 

3. The signal layer should be as close as possible to the ground plane, and do not run in parallel between adjacent layers.

 

4. For the transmission line, the microstrip line model is used for the top and bottom layers, and the stripline model is used for the internal signal layer. It is best to use software for the signal layers on both sides of the 6-layer / 10-layer / 14-layer / 18-layer substrate.

 

5. If there are other power supplies, give priority to the thick line on the signal layer, try not to divide the electrical ground layer. The high-speed line is best to go to the inner layer, the top and bottom layers are easily affected by external temperature, humidity, and air, and are not easy to stabilize. 


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