From the perspective of signal routing, a good layering strategy should be to place all signal routing in one or several layers, which are adjacent to the power supply layer or the ground layer.For power supply, a good stratification strategy should be that the power layer is adjacent to the ground layer and the distance between the ground layer and the ground layer is as small as possible. This is what we call the "stratification" strategy.
What stack strategies help shield and suppress EMI?The following layered stacking scheme assumes that the power supply current flows over a single layer and that single or multiple voltages are distributed across different parts of the same layer.The case of multiple power layers is discussed later.
4 layer board
There are several potential problems in the design of four layers.First, the traditional four-layer plate with a thickness of 62mil is too wide. Even if the signal layer is in the outer layer, the gap between the power supply layer and the ground layer is still too large.
If cost requirements are Paramount, consider the following two alternatives to the traditional four-tier board.Both of these solutions can improve EMI suppression performance, but only for applications where the density of the components on the board is low enough and there is enough area around the components (to place the required copper coating of the power supply).
The first is the preferred scheme. The outer layer of PCB is the formation, and the middle two layers are the signal/power layer.The power supply on the signal layer USES a wide wire, which makes the path impedance of the power supply current low and that of the signal microstrip path low.From the perspective of EMI control, this is the best 4-layer PCB structure available.In the second scheme, the outer layer goes to the power source and the ground, and the middle two layers go to the signal.Compared with the traditional 4-layer plate, the improvement is smaller, and the inter-layer impedance is as bad as the traditional 4-layer plate.
If you want to control the routing impedance, the above stacking plan should be very careful to place the routing under the power supply and the grounding copper island.In addition, the copper-clad islands on the power supply or formation should be interlinked as much as possible to ensure the DC and low frequency connectivity.