The function and design technique of PCB layer stacking in EMI radiation control in PCB board

- Nov 03, 2018-

PCB stack

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.

6 layer board

If the density of elements on 4 layers is high, it is better to use 6 layers.However, some lamination schemes in the 6-layer board design have not good enough shielding effect on electromagnetic field, and have little effect on reducing transient signal of power bus.Two examples are discussed below.

In the first case, the power supply and ground are placed on the second and fifth layers respectively. Due to the high impedance of the copper clad in the power supply, the control of common mode EMI radiation is very unfavorable.However, from the point of view of signal impedance control, this method is very correct.

In the second case, the power supply and ground were placed on the third and fourth layers respectively. This design solved the problem of the copper clad impedance of the power supply. Due to the poor performance of the electromagnetic shielding in the first and sixth layers, the EMI of the differential mode was increased.If the number of signal lines on the two outer layers is the least and the routing length is very short (less than 1/20 of the maximum harmonic wavelength of the signal), this design can solve the EMI problem of the differential mode.The non-component and non-routing areas on the outer layer are filled with copper and the copper clad areas are grounded (every 1/20 wave length is interval), so the EMI suppression of the differential mode is particularly good.As mentioned above, the copper-spreading area should be connected with the inner multipoint layer.

Generally, the first and sixth layers are distributed as layers, and the third and fourth layers are connected with power supply and ground.Because there are two layers of double microstrip signal line between the power supply layer and the ground layer, the EMI suppression ability is excellent.The drawback of this design is that the routing layer has only two layers.As mentioned above, if the outer layer is short and copper is laid in the non-routing area, the same stacking can be achieved with the traditional 6-layer plate.

The other 6-layer layout is signals, ground, signals, power supply, ground, signals, which can realize the environment needed for advanced signal integrity design.The signal layer is adjacent to the substratum, and the power layer is paired with the substratum.Clearly, the downside is the layer stack imbalance.

This usually causes problems for processing and manufacturing.The solution is to fill all the blank areas of the third layer with copper. If the copper density of the third layer is close to the power supply layer or the ground layer, this plate can be regarded as a structurally balanced circuit board.The copper filling area must be connected to power or ground.The distance between the connected via holes is still 1/20 of the wavelength, not necessarily everywhere, but ideally should be.

10 layer board

Due to the very thin insulation between layers, the impedance between layers of 10 or 12 boards is very low, and excellent signal integrity is fully expected as long as the layering and stacking are not problematic.It is more difficult to process and manufacture 12 layers at 62mil thickness, and there are not many manufacturers capable of processing 12 layers.

Since there is always an insulating layer between the signal layer and the loop layer, it is not optimal to allocate the middle six layers to go the signal line in the 10-layer design.In addition, it is important to keep the signal layer adjacent to the loop layer, that is, the layout of the board is signal, ground, signal, signal, power supply, ground, signal, signal, ground, signal.

This design provides a good path for the signal current and its circuit current.The appropriate wiring strategy is that layer 1 follows the X direction, layer 3 follows the Y direction, layer 4 follows the X direction, and so on.Visually, layers 1 and 3 are a pair of layers, layers 4 and 7 are a pair of layers, and layers 8 and 10 are the last pair of layers.When the routing direction needs to be changed, the signal line on the first layer should be changed from the "through hole" to the third layer.In fact, it may not always be possible, but as a design concept you should try to stick to it.

Similarly, when the signal's routing direction changes, it should be borrowed through holes from the 8th and 10th layers or from the 4th to the 7th layers.This wiring ensures that the coupling between the forward path and the loop of the signal is most tight.For example, if the signal is routed on the first layer and the loop is routed on the second layer and only on the second layer, the signal on the first layer is still in the second layer even if the signal is transferred from the "through hole" to the third layer, thus maintaining low inductance, large capacitance characteristics and good electromagnetic shielding performance.