How 4-layer boards use layered stacking to control EMI emissions
There are several potential issues with the 4-layer board design. First, the conventional four-layer board is 62 mils thick, even though the signal layer is on the outer layer, the power and ground planes are in the inner layer, and the distance between the power plane and the ground plane is still too large.
If you first consider the cost requirements, consider the following two alternatives to traditional 4-layer boards. Both solutions improve EMI suppression, but only for applications where the on-board component density is low enough and there is enough area around the component to place the desired copper layer on the power supply. The first is the preferred solution. The outer layers of the PCB are layers, and the middle two layers are signal/power layers. The power supply on the signal layer is routed with a wide line, which allows the path impedance of the supply current to be low and the impedance of the signal microstrip path to be low. From the perspective of EMI control, this is the best 4-layer PCB structure. The outer layer of the second solution uses power and ground, and the middle two layers use signals. Compared with the traditional 4-layer board, the improvement is smaller, and the interlayer resistance is as bad as the conventional 4-layer board. If you want to control the trace impedance, the above stacking scheme must be very careful to place the traces under the power and ground copper islands. In addition, copper or copper islands on the ground or on the ground should be interconnected as much as possible to ensure DC and low frequency connections.