The meaning of PCB copper and the design difficulties

The meaning of PCB copper and the design difficulties

 

The so-called copper coating is to use the unused space on the PCB as the reference surface, and then fill it with solid copper. These copper areas are also called copper filling. The significance of copper clad is to reduce the ground line impedance, improve the anti-interference ability; reduce the voltage drop, improve the power supply efficiency; connect with the ground wire, and also reduce the loop area. Also for the purpose of making the PCB soldered as much as possible, most PCB manufacturers will also require the PCB designer to fill the copper or grid-like ground wire in the open area of ​​the PCB. If the copper is not handled properly, it will If it is not worth the loss, is the copper-clad ‘better than the disadvantages’ or ‘does more harm than good’?

 

Everyone knows that at high frequencies, the distributed capacitance of the wiring on the printed circuit board will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect will occur and the noise will be emitted through the wiring. If there is a badly grounded copper in the PCB, copper is a tool for propagating noise. Therefore, in high-frequency circuits, don't think that somewhere on the ground is connected to the ground. This is the ground. Be sure to make a hole in the wiring at a pitch less than λ/20, and ‘good grounding’ with the ground plane of the multilayer board. If the copper coating is properly treated, the copper cladding not only has an increased current, but also plays a dual role of shielding interference.

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There are two basic ways to cover copper. It is a large area of ​​copper and grid copper. It is often asked that large areas of copper are good or grid copper is good. It is not easy to generalize. why? Large-area copper-clad, with the dual function of increasing current and shielding, but a large area of ​​copper, if the wave soldering, the board may be tilted up, and even foaming. Therefore, a large area of ​​copper, generally will open a few slots to ease the blistering of copper foil, the simple grid copper is mainly shielding, the effect of increasing the current is reduced, from the perspective of heat dissipation, the grid is beneficial (It reduces the heating surface of copper) and plays a role in electromagnetic shielding. However, it should be pointed out that the grid is composed of traces in the staggered direction. We know that for the circuit, the width of the trace has its corresponding ‘electrical length’ for the operating frequency of the board ( the actual size is divided). The digital frequency corresponding to the working frequency is available, specifically related books. When the working frequency is not very high, perhaps the effect of the grid line is not very obvious. Once the electrical length matches the working frequency, it is very bad, you It will be found that the circuit is not working at all, and signals that interfere with the operation of the system are being transmitted everywhere. Therefore, for colleagues who use the grid, it is recommended to choose according to the design of the board work, do not hold a thing. Therefore, the high-frequency circuit is resistant to interference with a multi-purpose grid, and the low-frequency circuit has a large current circuit and the like is generally used for copper plating.

 

Having said that, then in the copper cladding, in order to make the copper meet our expected results, then what issues should be paid attention to in the copper cladding:

 

1. If the PCB has more ground, there are SGND, AGND, GND, etc., according to the difference of PCB board position, the most important ‘ground’as the reference reference to separate copper, digital ground and analog ground Separate copper to cover, and at the same time, before the copper coating, first increase the corresponding power connection: 5.0V, 3.3V, etc., thus forming a plurality of different shapes of multi-deformation structure.

 

2. For single-point connections to different locations, the method is to connect through 0 ohm resistors or magnetic beads or inductors.

 

3. The copper near the crystal oscillator, the crystal oscillator in the circuit is a high-frequency emission source, the method is to surround the crystal copper, and then the crystal casing is grounded separately.

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4. The problem of isolated islands (dead zones), if you feel great, it will not cost much to define a hole in the hole.

 

5. When starting the wiring, the ground wire should be treated equally. When the wire is routed, the ground wire should be taken well. It is not possible to rely on the copper to add the via hole to eliminate the ground pin. The effect is very bad.

 

6. It is best not to have sharp corners on the board (‘180 degrees’), because from the electromagnetic point of view, this constitutes a transmitting antenna! For other things that always have an effect, it's just big or small. I recommend using the edge of the arc.

 

7. The wiring of the middle layer of the multi-layer board is not covered with copper. Because it is very difficult for you to make this copper ‘good grounding’.

 

8. The metal inside the equipment, such as metal radiators, metal reinforcement strips, etc., must achieve ‘good grounding’.

 

9. The heat-dissipating metal block of the three-terminal regulator must be well grounded. The grounding isolation strip near the crystal must be well grounded.

 

In short: the copper on the PCB, if the grounding problem is dealt with, it must be ‘profits outweigh the disadvantages’, it can reduce the return area of ​​the signal line, and reduce the external electromagnetic interference of the signal.