1. SIG -GND (PWR) -PWR (GND) -sig; 2. GND -SIG (PWR) -Sig (PWR) -gnd;

　　For the above two layers of stacking design, the potential problem is the thickness of the traditional 1.6mm (62mil) board. The laminar spacing will become very large, which is not only not conducive to controlling impedance, but also inter -layer coupling and shielding; especially the distance between the power supply formation is very large, reducing the board capacitance, which is not conducive to filtering noise.

　　For the first scheme, there are usually many chips on the board. This scheme can get better SI performance, which is not very good for EMI performance. It is mainly controlled by wiring and other details. Main attention: The connected layer of the signal layer with the most dense signal layer is conducive to absorbing and suppressing radiation; increasing the area of the board and reflecting the 20H rules.

　　For the second scheme, the chip density of the board is usually low enough and there is sufficient area around the chip (the required power supply to cover the copper layer). The outer layers of this scheme PCB are all stratums, and the middle two layers are signal /power layer. The power supply on the signal layer is wiring with a wide line, which can impedance the path current of the power current, and the impedance of the signal micro -with pathway is also low. It can also shield the inner signal radiation through the outer layer. From the perspective of EMI control, this is the existing best 4 -layer PCB structure.

　　Note: The distance between the two layers of signal and the power supply mixed layer in the middle should be opened, and the direction of the wiring is vertical to avoid skewers; appropriate control board area, reflecting the 20H rules; Below the power and ground pavement. In addition, the paving copper on the power supply or the ground should be connected as much as possible to ensure the connection of DC and low -frequency.