PCB in the PCB layer stack in the control of EMI radiation in the role and design skills(2)

- May 10, 2017-

Electromagnetic shielding

    From the signal alignment, a good stratification strategy should put all the signal lines on a layer or layers, these layers next to the power layer or ground plane. For power, a good stratification strategy should be the power layer and the ground layer adjacent to the power layer and the ground layer as small as possible, which is what we talk about the "layered" strategy.

    PCB stacking

    What kind of stacking strategy helps to shield and suppress EMI? The following layered stacking scheme assumes that the supply current flows on a single layer, and that a single voltage or multiple voltage is distributed across different parts of the same layer. The situation of the multi-power layer is discussed later.

    4 layers

    There are several potential problems with 4-layer board design. First, the traditional thickness of 62mil four-layer board, even if the signal layer in the outer layer, power and ground layer in the inner layer, the power supply layer and the ground layer spacing is still too large.

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

    The first is the preferred solution, the outer layers of the PCB are strata, and the middle two are signal / power layers. The power supply on the signal layer is routed with a wide line, which makes the path impedance of the supply current low and the impedance of the signal microstrip path is low. From the EMI control point of view, this is the best existing 4-layer PCB structure. The second program of the outer layer of power and ground, the middle two layers of the signal. The solution is less important than the traditional 4-layer board, and the interlayer impedance is as bad as the traditional 4-layer board.

    If you want to control the trace impedance, the stacking solution should be carefully placed in the power supply and ground bridge below the copper island. In addition, the copper or the copper on the power source or formation should be interconnected as much as possible to ensure DC and low frequency connectivity.

    6 layers

    If the component density on the 4-layer board is relatively large, it is preferable to use a 6-layer board. However, some of the stacking scheme in the 6-layer board design is not good enough for the shielding of the electromagnetic field, which has little effect on the reduction of the transient signal of the power supply bus. Two examples are discussed below.

    The first example of the power and ground were placed on the 2nd and 5th, due to the power supply copper resistance is high, the control of common mode EMI radiation is very negative. However, from the signal impedance control point of view, this method is very correct.

    The second example places the power supply and ground on the 3rd and 4th layers, and this design solves the problem of copper cladding of the power supply. Since the electromagnetic shielding performance of the first and sixth layers is poor, the differential mode EMI increases. This design solves the differential mode EMI problem if the number of signal lines on both outer layers is the least and the trace length is very short (less than 1/20 of the highest harmonic wavelength of the signal). The suppression of the differential mode EMI is particularly good by filling the non-element and non-traces of the outer layer with copper and grounding the copper area (at every 1/20 wavelength interval). As mentioned earlier, the copper area is to be connected to the internal ground plane.

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