PCB Tech - Use PCB layered stacking to control EMI radiation

There are many ways to solve EMI problems. Modern EMI suppression methods include: using EMI suppression coatings, selecting appropriate EMI suppression parts, and EMI simulation design. Starting from the most basic PCB layout, this article discusses the role and design techniques 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 IC output voltage jump faster. However, the problem does not end here. Due to the limited frequency response of the capacitor, the capacitor cannot generate the harmonic power required to drive the IC output cleanly in the full frequency band. In addition, the transient voltage formed on the power bus will form a voltage drop across the inductance of the decoupling path, and these transient voltages are the main common mode EMI interference sources.

For the IC on the circuit board, the power layer around the IC can be regarded as an excellent high-frequency capacitor, which can collect the part of the energy leaked by the discrete capacitor that provides high-frequency energy for clean output. In addition, the inductance of a good power layer should be small, so the transient signal synthesized by the inductance is also small, thereby reducing common mode EMI. Of course, the connection between the power layer and the IC power pin must be as short as possible, because the rising edge of the digital signal is getting faster and faster, and it is best to connect it directly to the pad where the IC power pin is located. This needs to be discussed separately. In order to control common-mode EMI, the power plane must help decoupling and have a sufficiently low inductance. This power plane must be a well-designed pair of power planes. Someone may ask, how good is good? The answer to the question depends on the layering of the power supply, the materials between the layers, and the operating frequency (that is, a function of the rise time of the IC). Generally, the spacing of the power layer is 6mil, and the interlayer is FR4 material, the equivalent capacitance of the power layer per square inch is about 75pF.

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

What stacking strategy for PCB stacking helps shield and suppress EMI? The following layered stacking scheme assumes that the power supply current flows on a single layer, and the single voltage or multiple voltages are distributed in different parts of the same layer. The case of multiple power layers will be discussed later.

4-layer board

There are several potential problems with the 4-layer board design. First of all, the traditional four-layer board with a thickness of 62 mils, even if the signal layer is on the outer layer, and the power and ground layers are on the inner layer, the distance between the power layer and the ground layer is still too large.

If the cost requirement is first, you can consider the following two traditional 4-layer board alternatives. These two solutions can improve the performance of EMI suppression, but they are only suitable for applications where the component density on the board is low enough and there is enough area around the components (place the required power copper layer). The first is the preferred solution. The outer layers of the PCB are all ground layers, and the middle two layers are signal/power layers. The power supply on the signal layer is routed with a wide line, which can make the path impedance of the power supply current low, and the impedance of the signal microstrip path is also low. From the perspective of EMI control, this is the best 4-layer PCB structure available. In the second scheme, the outer layer 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 impedance 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 arrange the traces under the power and ground copper islands. In addition, the copper islands on the power supply or ground layer should be interconnected as much as possible to ensure DC and low-frequency connectivity.

6-layer board

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

In the first example, the power supply and ground are placed on the 2nd and 5th layers respectively. Due to the high copper impedance of the power supply, it is very unfavorable to control the common mode EMI radiation. However, from the point of view of signal impedance control, this method is very correct. In the second example, the power supply and ground are placed on the 3rd and 4th layers respectively. This design solves the problem of power supply copper impedance. Due to the poor electromagnetic shielding performance of the 1st and 6th layers, the differential mode EMI is increased. If the number of signal lines on the two outer layers is the least, and the trace length is very short (shorter than 1/20 of the wavelength of the highest harmonic of the signal), this design can solve the differential mode EMI problem. Fill the area with no components and no traces on the outer layer with copper and ground the copper-clad area (every 1/20 wavelength as an interval), which is particularly good at suppressing differential mode EMI. As mentioned earlier, it is necessary to connect the copper area with the internal ground plane at multiple points. The general high-performance 6-layer board design generally disposes the first and sixth layers as ground layers, and the third and fourth layers are used for power and ground. Since there are two double microstrip signal line layers in the middle between the power layer and the ground layer, the EMI suppression capability is excellent. The disadvantage of this design is that there are only two routing layers. As mentioned earlier, if the outer traces are short and copper is laid in the traceless area, the same stacking can also be achieved with a traditional 6-layer board. Another 6-layer board layout is signal, ground, signal, power, ground, signal, which can realize 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 disadvantage is the unbalanced stacking of layers. This usually brings trouble to manufacturing. The solution to the problem is to fill all the blank areas of the third layer with copper. After the copper is filled, if the copper density of the third layer is close to the power layer or ground layer, this board can not be strictly counted as a structurally balanced circuit board . The copper-filled area must be connected to power or ground. The distance between the connection vias is still 1/20 wavelength, and it may not be necessary to connect everywhere, but it should be connected under ideal circumstances.

10-layer board

Since the insulating isolation layer between the multilayer boards is very thin, the impedance between the 10 or 12 layers of the circuit board is very low. As long as there is no problem with the layering and stacking, excellent signal integrity can be expected. It is more difficult to manufacture 12-layer boards with a thickness of 62mil, and there are not many manufacturers that can process 12-layer boards.

Multi-power layer design

If the two power layers of the same voltage source need to output large currents, the circuit board should be laid out into two sets of power layers and ground layers. In this case, an insulating layer is placed between each pair of power and ground layers. In this way, we obtain the two pairs of power bus bars with equal impedances that divide the current we expect. If the stacking of the power layers causes the impedance to be unequal, the shunt will not be uniform, the transient voltage will be much larger, and the EMI will increase sharply.

If there are multiple power supply voltages with different values on the circuit board, multiple power supply layers are required accordingly. Remember to create their own paired power supply and ground layers for different power supplies. In the above two cases, when determining the position of the paired power layer and ground layer on the circuit board, keep in mind the manufacturer's requirements for the balanced structure.

The thickness, via process and the number of layers of the circuit board in the circuit board design are not the key to solving the problem. Excellent layered stacking is to ensure the bypass and decoupling of the power bus, and minimize the transient voltage on the power layer or ground layer. And the key to shielding the electromagnetic field of the signal and power supply. Ideally, there should be an insulating isolation layer between the signal routing layer and the return ground layer, and the paired layer spacing (or more than one pair) should be as small as possible. Based on these basic concepts and principles, a circuit board that can always meet the design requirements can be designed.