What should be paid attention to when copper coating PCB

Copper coating is an important part of PCB design. Whether it is the domestic Qingyuefeng PCB design software, or some overseas Protel, Power PCB, it provides intelligent copper coating function. So how to apply copper well? I will share some of my ideas with you, hoping to bring benefits to peers.

The so-called copper coating refers to taking the idle space on the PCB as the reference plane, and then filling it with solid copper. These copper areas are also called copper filling. The significance of copper coating is to reduce the ground wire impedance and improve the anti-interference capability; Reduce voltage drop and improve power efficiency; Connecting with the ground wire can also reduce the loop area. In order to keep PCB from deforming as much as possible during welding, most PCB manufacturers also require PCB designers to fill the open area of PCB with copper sheet or grid shaped ground wire. If copper coating is handled improperly, it will be rewarded or lost. Is copper coating "more beneficial than harmful" or "more harmful than beneficial"?

As we all know, the distributed capacitance of the wiring on the printed circuit board will play a role in high frequency. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect will be generated, and the noise will be emitted through the wiring. If there is a poorly grounded copper coating in the PCB, the copper coating will become a tool to spread the noise. Therefore, in high frequency circuits, do not think that the ground wire is grounded somewhere, This is the "ground wire", which must be less than λ/ 20, make holes on the wiring, and "well grounded" with the ground plane of the multilayer board. If the copper coating is properly treated, it not only increases the current, but also plays a dual role in shielding interference.

PCB copper coating generally has two basic methods, namely, large-area copper coating and grid copper. It is often asked whether large-area copper coating or grid copper coating is better. It is not easy to generalize. Why? Large area copper coating has the dual functions of increasing current and shielding. However, if large area copper coating is used for wave soldering, the board may be warped or even blistered. Therefore, for a large area of copper coating, several grooves are usually opened to alleviate the blistering of copper foil. The simple grid copper coating is mainly used for shielding, and the effect of increasing current is reduced. From the perspective of heat dissipation, the grid is beneficial (it reduces the heating surface of copper) and plays a certain role in electromagnetic shielding. However, it should be pointed out that the grid is composed of lines in staggered directions. We know that for circuits, The width of the wiring has its corresponding "electrical length" for the operating frequency of the circuit board (the actual size is divided by the digital frequency corresponding to the operating frequency, see the relevant books for details) When the working frequency is not very high, perhaps the role of grid lines is not very obvious. Once the electrical length matches the working frequency, it is very bad. You will find that the circuit does not work properly at all, and there are signals everywhere that interfere with the work of the system. Therefore, for colleagues who use grid, my suggestion is to choose according to the working conditions of the designed circuit board, instead of holding on to something. Therefore, high-frequency circuit requires high anti-interference multi-purpose grid, and low-frequency circuit has high current circuit and other commonly used complete copper coating.

Having said that, in order to achieve the desired effect in copper cladding, we should pay attention to the following issues in copper cladding:

1. If there are many PCB with ground, such as SGND, AGND, GND, etc., the most important "ground" shall be used as the reference for independent copper coating according to the different positions of the PCB board, and the digital and analog copper coating shall be separated. At the same time, before copper coating, the corresponding power supply lines shall be thickened: 5.0V, 3.3V, etc. In this way, multiple deformation structures with different shapes are formed.

2. For the single point connection of different places, the method is to connect through 0 ohm resistance or magnetic bead or inductance;

3. Copper coating near the crystal oscillator. The crystal oscillator in the circuit is a high-frequency emission source. The method is to cover copper around the crystal oscillator, and then ground the shell of the crystal oscillator separately.

4. If the problem of isolated island (dead zone) is too big, it doesn't take much to define and add individual vias.

5. When PCB wiring is started, the ground wire should be treated equally. The ground wire should be well laid when wiring. It cannot be eliminated as a connected ground pin by adding vias after copper coating. This effect is very bad.

6. It is better not to have sharp corners on the board ("=180 degrees"), because from the electromagnetic point of view, this constitutes a transmitting antenna! For others, it will always have an impact on whether it is big or small. I suggest using the edge of the arc.

7. The open area of wiring in the middle layer of multilayer PCB board shall not be covered with copper. Because it is difficult for you to make this copper clad "good grounding"

8. Metal inside the equipment, such as metal radiator, metal reinforcing strip, must be "well grounded".

9. The heat dissipation metal block of the three terminal voltage regulator must be well grounded. The grounding isolation belt near the crystal oscillator must be well grounded. In a word: if the grounding problem of copper coating on PCB is solved properly, it must be "more advantages than disadvantages". It can reduce the return area of the signal line and reduce the external electromagnetic interference of the signal.