# Analyze the role of PCB layered stack design in suppressing EMI

There are many ways to solve EMI problems. Modern EMI suppression methods include: EMI suppression coatings, selection of suitable EMI suppression components, and EMI simulation design. This article starts from the most basic PCB layout and discusses the role and design skills of PCB layered stacking in controlling EMI radiation.

Power bus

Properly placing a capacitor of appropriate capacity near the power supply pin of the IC can make the output voltage of the IC jump faster. However, the problem is not here. Due to the finite frequency response of the capacitor, this makes it impossible for the capacitor to generate the harmonic power required to cleanly drive the IC output over the full frequency band. In addition, the transient voltage developed on the power busbar creates a voltage drop across the inductor of the decoupling path, which is the primary source of common-mode EMI interference. How should we solve these problems?

As far as the ICs on our boards are concerned, the power plane around the IC can be thought of as an excellent high-frequency capacitor that collects the energy that is leaked by discrete capacitors that provide high-frequency energy for clean outputs. In addition, the excellent power supply layer has a small inductance, so that the transient signal synthesized by the inductor is also.

Of course, the connection from the power plane to the IC power supply pin must be as short as possible because the rising edge of the digital signal is getting faster and faster, preferably directly to the pad where the IC power supply pin is located, as discussed further.

To control common-mode EMI, the power plane must be decoupled and have a low enough inductance. This power plane must be a well-designed pair of power planes. Someone may ask, to what extent is it good? The answer to the question depends on the layering of the power supply, the material between the layers, and the operating frequency (ie, the function of the rise time of the IC). Typically, the power supply is layered at 6 mils and the interlayer is FR4, and the equivalent capacitance per square inch of power plane is approximately 75 pF. Obviously, the smaller the layer spacing, the larger the capacitance.

There are not many devices with a rise time of 100 to 300 ps, ​​but according to the current development speed of ICs, devices with a rise time of 100 to 300 ps will occupy a high proportion. For circuits with rise times of 100 to 300 ps, ​​the 3 mil layer spacing will no longer be suitable for most applications. At that time, it was necessary to use a layering technique with a layer spacing of less than 1 mil and to replace the FR4 dielectric material with a material having a high dielectric constant. Ceramics and ceramics now meet the design requirements of 100 to 300 ps rise time circuits.

Although new materials and methods may be used in the future, for today's common 1 to 3 ns rise time circuits, 3 to 6 mil layer spacing and FR4 dielectric materials, it is usually sufficient to handle high-end harmonics and make the transient signal low enough, that is to say Common mode EMI can be reduced very low. The PCB layered stack design example presented here will assume a layer spacing of 3 to 6 mils.

Electromagnetic shielding

From the perspective of signal routing, a good stratification strategy should be to place all signal traces in one or several layers, which are next to the power or ground plane. For the power supply, a good stratification strategy should be that the power plane is adjacent to the ground plane, and the distance between the power plane and the ground plane is as small as possible. This is what we call the "layering" strategy.

PCB stacking

What kind of stacking strategy helps shield and suppress EMI? The following layered stacking scheme assumes that the supply current flows over a single layer, with single or multiple voltages distributed across different parts of the same layer. The case of multiple power planes is discussed later.

4-layer board

There are several potential problems with 4-layer board design. First, the conventional four-layer board with a thickness of 62 mils, even if the signal layer is on the outer layer, the power supply and the ground layer are in the inner layer, the distance between the power supply layer.

If the cost requirement is first, consider the following two alternatives to traditional 4-layer boards. Both of these solutions improve EMI suppression performance, but only for applications where the on-board component density is low enough and there is sufficient area around the component to place the required copper layer on the power supply.

The first is the preferred solution. The outer layers of the PCB are all 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 available. The outer layer of the second scheme takes the power and the ground, and the middle two layers take the signal. Compared with the traditional 4-layer board, the improvement is smaller, and the interlayer resistance is as poor as the traditional 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, the copper or copper islands on the ground should be interconnected as much as possible to ensure DC and low frequency connectivity.

6-layer board

If the density of the components on the 4-layer board is relatively large, it is preferable to use a 6-layer board. However, some of the stacking schemes in the 6-layer board design do not have a good shielding effect on the electromagnetic field, and have little effect on the reduction of the power bus bus transient signal. Two examples are discussed below.

In the first case, the power supply and the ground are placed on the 2nd and 5th layers respectively. Since the copper resistance of the power supply is high, it is very disadvantageous for controlling the common mode EMI radiation. However, from the point of view of impedance control of the signal, this method is very correct.

In the second example, the power supply and the ground are placed on the 3rd and 4th layers respectively. This design solves the problem of the copper-clad impedance of the power supply. Due to the poor electromagnetic shielding performance of the first layer and the sixth layer, the differential mode EMI is increased. This design solves the differential mode EMI problem if the number of signal lines on the two outer layers is the least and the trace length is short (less than 1/20 of the highest harmonic wavelength of the signal). The suppression of differential mode EMI is particularly good by copper-filling the no-component and trace-free areas on the outer layer and grounding the copper-clad area (interval every 1/20 wavelength). As mentioned earlier, the copper area is connected to the internal ground plane at multiple points.

The general-purpose high-performance 6-layer board design generally lays the first and sixth layers as the ground layer, and the third and fourth layers take the power and ground. Since there are two layers of the centered double microstrip signal line layer between the power supply layer and the ground layer, the EMI suppression capability is excellent. The disadvantage of this design is that the trace layer has only two layers. As mentioned earlier, if the outer traces are short and copper is laid in the unlined area, the same stack can be achieved with a conventional 6-layer board.

Another 6-layer board layout is signal, ground, signal, power, ground, and signal, which enables the environment required for advanced signal integrity design. The signal layer is adjacent to the ground layer, and the power layer and the ground layer are paired. Obviously, the downside is that the stacking of the layers is unbalanced.

This usually causes troubles in manufacturing. The solution to the problem is to fill all the blank areas of the third layer with copper. If the copper layer density of the third layer is close to the power layer or the ground layer after copper filling, the board can be regarded as a structurally balanced circuit board. . The copper filled area must be connected to power or ground. The distance between the connecting vias is still 1/20 wavelength, and it is not necessary to connect everywhere, but ideally it should be connected.

10-layer board

Since the insulating isolation layer between the multilayer boards is very thin, the impedance between the 10 or 12 layer circuit board layers and the layers is very low, and excellent signal integrity is expected as long as there is no problem in delamination and stacking. It is difficult to machine a 12-layer board at a thickness of 62 mils, and there are not many manufacturers that

Since the signal layer and the loop layer are always separated by an insulating layer, the scheme of distributing the middle 6 layers in the 10-layer board design to walk the signal line is not optimal. In addition, it is important to have the signal layer adjacent to the loop layer, that is, the board layout is signal, ground, signal, signal, power, ground, signal, signal, ground, and signal.

This design provides a good path for signal current and its loop current. The proper routing strategy is that the first layer is routed along the X direction, the third layer is routed along the Y direction, the fourth layer is routed along the X direction, and so on. Intuitively looking at the traces, the first layer 1 and the third layer are a pair of layered combinations, the fourth layer and the seventh layer are a pair of layered combinations, and the eighth layer and the tenth layer are the last pair of layered combinations. When it is necessary to change the direction of the trace, the signal line on the first layer should be changed by the "via" to the third layer. In fact, you may not always be able to do this, but as a design concept you should try to comply.

Similarly, when the direction of the signal is changed, it should be through the vias from the 8th and 10th layers or from the 4th to the 7th. This routing ensures that the coupling between the forward path and the loop of the signal is tightest. For example, if the signal is routed on the first layer and the loop is on the second layer and only on the second layer, then the signal on the first layer is transferred to the third layer even by "via". The loop is still on the second layer, thus maintaining low inductance, large capacitance characteristics and good electromagnetic shielding performance.

What if the actual route is not the case? For example, the signal line on the first layer passes through the via hole to the 10th layer. At this time, the loop signal has to find the ground plane from the 9th layer, and the loop current needs to find the nearest ground via (such as the grounding pin of the component such as resistor or capacitor). . If there is such a via in the vicinity, it is really lucky. If no such via is available, the inductance will increase, the capacitance will decrease, and EMI will increase.

When the signal line must leave the current pair of wiring layers through the via to other wiring layers, the ground via should be placed near the via, so that the loop signal can be smoothly returned to the proper ground plane. For the layer 4 and layer 7 combination, the signal loop will return from the power or ground plane (ie, layer 5 or layer 6) because the capacitive coupling between the power plane and the ground plane is good and the signal is easily transmitted.

Multi-power layer design

If the two power planes of the same voltage source need to output a large current, the board should be laid into two sets of power and ground planes. In this case, an insulating layer is placed between each pair of the power supply layer and the ground layer. This gives us two equal pairs of equal power supply busbars that we want to divide the current. If the stack of power planes causes unequal impedances, the shunt is not uniform, the transient voltage will be much larger, and EMI will increase dramatically.

If there are multiple supply voltages with different values ​​on the board, multiple power planes are required accordingly. It is important to remember to create separate pairs of power and ground planes for different power supplies. In both cases, when determining the location of the paired power and ground planes on the board, remember the manufacturer's requirements for the balanced structure.

to sum up

Given that most engineers design boards that are 62 mils thick and have no blind holes or buried holes, the discussion of board delamination and stacking is limited. For boards with too much thickness difference, the layering scheme recommended in this paper may not be ideal. In addition, the processing method of the circuit board with blind holes or buried holes is different, and the layering method of this paper is not applicable.

The thickness, via process, and number of layers in the board design are not the key to solving the problem. Excellent layered stacking ensures bypass and decoupling of the power bus and minimizes transient voltages on the power or ground plane. The key to shielding the signal and the electromagnetic field of the power supply. Ideally, there should be an insulating isolation between the signal trace layer and its return ground plane, and the matching layer spacing (or more than one pair) should be as small as possible. Based on these basic concepts and principles, it is possible to design a board that always meets the design requirements. Now, IC's rise time is already short and will be shorter, and the techniques discussed in this article are essential to address EMI shielding issues.