In the EMC design of PCB, the first concern is the layer setting; The layers of the board are composed of power supply, ground layer and signal layer. In EMC design of products, besides the selection of components and circuit design, good PCB design is also a very important factor.

The key to the EMC design of PCB is to minimize the backflow area and make the backflow path flow in the direction we designed. The layer design is the basis of PCB, how to do a good job of PCB layer design to make the EMC effect of PCB optimal?

Design ideas of PCB layer:

The core of PCB laminated EMC planning and design is to reasonably plan signal backflow path to minimize the backflow area of signal from the board mirror layer, so as to eliminate or minimize magnetic flux.

1. Board mirroring layer

The mirror layer is a complete layer of copper-coated plane layer (power supply layer, grounding layer) adjacent to the signal layer inside the PCB. The main functions are as follows:

(1) Reduce backflow noise: the mirror layer can provide a low impedance path for the signal layer backflow, especially when there is a large current flow in the power distribution system, the role of the mirror layer is more obvious.

(2) EMI reduction: the existence of the mirror layer reduces the area of the closed loop formed by the signal and reflux and reduces EMI;

(3) reduce crosstalk: help to control the crosstalk problem between signal lines in high-speed digital circuit, change the height of the signal line from the mirror layer, you can control the crosstalk between signal lines, the smaller the height, the smaller the crosstalk;

(4) Impedance control to prevent signal reflection.

Selection of mirror layer

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

(2) Relatively speaking, the power plane has a high characteristic impedance, and 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 perspective of shielding, the ground plane is generally grounded and used as the reference point of the reference level, and its shielding effect is far better than that of the power plane;

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

Magnetic flux cancellation principle:

According to Maxwell’s equations, all electrical and magnetic action between separate charged bodies or currents is transmitted through the intermediate region between them, whether it be a vacuum or solid matter. In a PCB, the flux is always propagated in the transmission line. If the rf backflow path is parallel to the corresponding signal path, the flux on the backflow path is in the opposite direction to that on the signal path, then they are superimposed on each other, and the effect of flux cancellation is obtained.

The essence of flux cancellation is the control of signal backflow path, as shown in the following diagram:

How to use the right hand rule to explain the magnetic flux cancellation effect when the signal layer is adjacent to the stratum is explained as follows:

(1) When a current flows through the wire, a magnetic field will be generated around the wire, and the direction of the magnetic field is determined by the right hand rule.

(2) when there are two close to each other and parallel to the wire, as shown in the figure below, one of the conductors of electricity to drain out, the other a conductor of electricity to flow, if the electric current flows through the wire are current and its return current signal, then the two opposite direction of current is equal, so their magnetic field are equal, but the direction is opposite,So they cancel each other out.

Six layer board design example

1. For six-layer plates, scheme 3 is preferred;

Analysis:

(1) As the signal layer is adjacent to the reflow reference plane, and S1, S2 and S3 are adjacent to the ground plane, the best magnetic flux cancellation effect is achieved. Therefore, S2 is the preferred routing layer, followed by S3 and S1.

(2) The power plane is adjacent to the GND plane, the distance between the planes is very small, and it has the best magnetic flux cancellation effect and low power plane impedance.

(3) The main power supply and its corresponding floor cloth are located at layer 4 and 5. When layer thickness is set, the spacing between S2-P should be increased and the spacing between P-G2 should be reduced (the spacing between layer G1-S2 should be correspondingly reduced), so as to reduce the impedance of the power plane and the influence of the power supply on S2.

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

Analysis:

(1) Because the signal layer is adjacent to the reflow reference plane and S1 and S2 are adjacent to the ground plane, this structure has the best magnetic flux cancellation effect;

(2) Due to the poor magnetic flux cancellation effect and high power plane impedance from the power plane to the GND plane through S3 and S2;

(3) Preferred wiring layer S1 and S2, followed by S3 and S4.

3. For six-layer plates, option 4

Analysis:

Scheme 4 is more suitable than Scheme 3 for local, small number of signal requirements, which can provide an excellent wiring layer S2.

4. Worst EMC effect, Scheme,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, so the magnetic flux cancellation effect is poor.

Conclusion

Specific principles of PCB layer design:

(1) There is a complete ground plane (shield) below the component surface and welding surface;

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

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

(4) Wiring layer of high frequency, high speed, clock and other key signals should have an adjacent ground plane.