In the EMC design considerations of the PCB, the first is the layer configuration; the number of layers of a single board consists of the power supply, the number of ground layers, and the number of signal layers; in the EMC design of the product, in addition to the selection of components and circuit design In addition, good PCB design is also a very important factor.

The key to PCB's EMC design is to minimize the reflow area and allow the return path to flow in the direction we design. The design of the layer is the basis of the PCB. How can we do a good job of designing the PCB layer to optimize the EMC effect of the PCB?

A, PCB layer design ideas:

The core of the PCB stackup EMC planning and design is to properly plan the signal return path and minimize the backflow area of ​​the signal from the mirrored layer of the board, so that the magnetic flux can be cancelled or minimized.

1. Board Mirror Layer

The mirror layer is a complete layer of copper plane (power layer, ground plane) adjacent to the signal layer inside the PCB. Mainly have the following effects:

(1) Reducing back-flow noise: The mirror layer can provide a low-impedance path for signal layer reflow, especially when large current flows in the power distribution system, the role of the mirror layer is even more pronounced.

(2) Reduce EMI: The presence of the mirror layer reduces the area of ​​the closed loop formed by the signal and reflow and reduces EMI;

(3) Reducing crosstalk: It helps to control crosstalk between signal traces in high-speed digital circuits. By changing the height of the signal traces from the mirror layer, crosstalk between signal lines can be controlled. The smaller the height, the smaller the crosstalk.

(4) Impedance control to prevent signal reflection.

2, the choice of mirror layer

(1) The power supply and the ground plane can all be used as reference planes and have a certain shielding effect on the internal wiring;

(2) Relatively speaking, the power plane has a high characteristic impedance, there is a large potential difference with the reference level, and the high frequency interference on the power plane is relatively large;

(3) From the shielding point of view, the ground plane is generally treated as a ground, and as a reference level reference point, its shielding effect is far better than the power plane;

(4) When selecting the reference plane, the plane should be preferred and the power plane should be selected.

Second, the magnetic flux cancellation principle:

According to Maxwell's equations, the discrete electrical or electrical currents, all electrical and magnetic interactions between them, are transmitted through the intermediate region between them, whether the intermediate region is a vacuum or a solid matter. In the PCB, the magnetic flux always propagates in the transmission line. If the RF return path is parallel to its corresponding signal path, the magnetic flux in the return path and the magnetic flux in the signal path are opposite to each other. At this time, they overlap each other. You get the effect of flux cancellation.

Third, the essence of magnetic flux cancellation is the control of the signal return path. The specific diagrams are as follows:

Fourth, how to use the right-hand rule to explain the flux cancellation effect when the signal layer is adjacent to the stratum, as explained below:

(1) When a current flows on the wire, a magnetic field is generated around the wire. The direction of the magnetic field is determined by the right-hand rule.

(2) When there are two wires that are close to each other and are parallel, as shown in the figure below, the current of one of the conductors flows outward and the current of the other conductor flows inward. If the current flowing through the two wires is a signal current, respectively And its return current, then the two currents are equal in magnitude and direction, so their magnetic fields are also equal in magnitude, and the directions are reversed, so they cancel each other out.

Five, six-layer board design example

1, for the six-layer board, give priority to program 3;

analysis:

(1) Since the signal layer is adjacent to the return reference plane, the ground planes adjacent to S1, S2, and S3 have the best magnetic flux cancellation effect, and the wiring layer S2 is preferred, followed by S3, S1.

(2) The power plane is adjacent to the GND plane. The distance between the planes is very small, and there is an optimal flux cancellation effect and a low power plane impedance.

(3) When the main power source and its corresponding ground cloth are in layers 4 and 5, when the layer thickness is set, the spacing between S2-P is increased, and the space between P-G2 is reduced (correspondingly reducing the gap between G1-S2 layers) Pitch) to reduce the impedance of the power plane and reduce the impact of the power supply on S2.

2. When the cost requirement is high, Option 1 can be adopted;

analysis:

(1) In this structure, since the signal layer is adjacent to the return reference plane, S1 and S2 are adjacent to the ground plane and have the best magnetic flux cancellation effect;

(2) Power plane due to passing S3 and S2 to GND plane, poor flux cancellation effect and high power plane impedance;

(3) The wiring layers S1 and S2 are preferred, followed by S3 and S4.

3, for six-layer board, alternative 4

analysis:

For applications where local and small signal requirements are high, Option 4 is more suitable than Option 3, which provides an excellent wiring layer S2.

4, the worst EMC effect, program 2

analysis:

In this structure, S1 and S2 are adjacent, S3 and S4 are adjacent, and S3 and S4 are not adjacent to the ground plane, and the magnetic flux cancellation effect is poor.

Sixth, summary

PCB layer design specific principles:

(1) The complete ground plane (shielding) is below the component surface and the welding surface;

(2) Try to avoid direct neighboring of the two signal layers;

(3) All signal layers are as close as possible to the ground plane;

(4) High-frequency, high-speed, clock and other key signal wiring layers should have an adjacent ground plane.

Hope that through the brief introduction of this article, can help everyone to deepen the understanding of the PCB layer design, do a good job PCB design in advance, once and for all.