Some advantages and disadvantages about PCB copper cladding

Copper is an important part of PCB design. Whether it is the domestic Qingyuefeng PCB design software, but also some foreign Protel, PowerPCB provides intelligent copper-clad function, then how can I apply copper, I will share some ideas with you. Share it together and hope to bring benefits to your peers.

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, which is 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 far 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 you don’t appreciate it, is it that “benefits outweigh 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 poorly 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 grounded. This is "ground." "Line" must be made with a hole on 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.

There are two basic ways to cover copper. It is a large area of ​​copper and grid copper. It is often asked whether a large area of ​​copper is good or a grid of copper. 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, large area copper is covered, and generally several slots are opened to alleviate the blistering of the copper foil. The simple grid copper is mainly shielded, and 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. So for the colleagues who use the grid, my suggestion is to choose according to the design of the board work, do not hold a thing. Therefore, the high-frequency circuit is resistant to the multi-purpose grid with high interference requirements, 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 the copper should pay attention to those problems:

1. If the PCB has more ground, there are SGND, AGND, GND, etc., depending on the position of the PCB surface, the main "ground" is used as the reference to separate the copper, and the digital ground and the analog ground are separated. Copper is not to be said, 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 in the vicinity of the crystal oscillator, the crystal oscillator in the circuit is a high-frequency emission source. The method is to surround the crystal oscillator and then ground the crystal casing separately.

4. The problem of an isolated island (dead zone), if it feels great, it would not be a big deal to define a place to add it.

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. This has a bad effect.

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. Metals inside the equipment, such as metal radiators, metal reinforcement strips, etc., must be “good grounded”.

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.

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