Copper pouring is an important part of PCB design. Whether it is domestic PCB design software or some foreign Protel, PowerPCB provides intelligent copper pouring function, so how to lay copper well?
The so-called copper filling is to use the idle space on the PCB as a reference plane, and then fill it with solid copper. These copper areas are also called copper filling. The significance of pouring copper is to reduce the impedance of the ground wire and improve the anti-interference ability; to reduce the voltage drop and improve the efficiency of the power supply; to connect to the ground wire can also reduce the loop area.
In order to keep the PCB from deforming as much as possible during soldering, most PCB manufacturers will also require the PCB designer to fill the open area of the PCB with copper or grid-shaped ground wires. If the copper cladding is not handled properly, the gain will outweigh the loss. Is the copper cladding "the advantages outweigh the disadvantages" or "the disadvantages outweigh the advantages"?
Everyone knows that at high frequencies, the distributed capacitance of the wiring on the printed circuit board will play a role. When the length is greater than 1/20 of the wavelength corresponding to the noise frequency, the antenna effect will occur, and the noise will be emitted through the wiring. If there is a poorly grounded copper pour in the PCB, the copper pour becomes a vehicle for noise transmission. Therefore, in high-frequency circuits, do not think that a certain part of the ground wire is connected to the ground. This is the "ground wire", and the spacing must be less than λ/20. Drill holes in the wiring to "good ground" with the ground plane of the multilayer board. If the copper cladding is handled properly, the copper cladding not only has the dual function of increasing the current but also shielding interference.
There are generally two basic methods of copper pouring, namely large-area copper pouring and mesh copper plating. People often ask whether large-area copper pouring or grid copper pouring is better, and it is not easy to generalize. why? A large area of copper clad has the dual functions of increasing current and shielding, but if a large area of copper clad is subjected to wave soldering, the board may warp or even bubble. Therefore, for large-area copper cladding, several slots are generally opened to alleviate copper foil blistering. Pure grid copper is mainly shielding, 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 certain role in electromagnetic shielding. But it should be pointed out that the grid is composed of traces in staggered directions. We know that for the circuit, the width of the trace has its corresponding "electrical length" for the operating frequency of the circuit board (the actual size divided by The digital frequency corresponding to the working frequency is available, see related books for details), when the working frequency is not very high, perhaps the side effect of the grid line is not obvious, once the electrical length matches the working frequency, it is very bad, you will It was found that the circuit was not working properly at all, and signals were being emitted everywhere that interfered with the system's operation. So for colleagues who use grids, my suggestion is to choose according to the working conditions of the designed circuit board, and don't cling to one thing. Therefore, multi-purpose grids with high anti-interference requirements for high-frequency circuits, low-frequency circuits, circuits with large currents, etc. are commonly used for complete copper laying.
Having said so much, then we need to pay attention to the following issues in order to make the copper pour achieve our expected effect in the copper pour:
1. If the PCB has many grounds, such as SGND, AGND, GND, etc., it is necessary to use the most important "ground" as a reference for independent copper pouring according to the position of the PCB board. It’s not much to say that the digital ground and the analog ground are separated to pour copper. At the same time, before pouring copper, first thicken the corresponding power supply wiring: 5.0V, 3.3V, etc. In this way, multiple polygons of different shapes are formed. structure.
2. For the single-point connection of different grounds, the method is to connect through 0 ohm resistance, magnetic beads or inductance;
3. Pour copper near the crystal oscillator. The crystal oscillator in the circuit is a high-frequency emission source. The method is to surround the crystal oscillator with copper, and then ground the shell of the crystal oscillator separately.
4. The island (dead zone) problem, if you think it is too big, it will not take much to define a ground via and add it.
5. At the beginning of wiring, the ground wires should be treated equally. When wiring, the ground wire should be routed well. It is not possible to increase the ground pin by adding vias. This effect is very bad.
6. It is best not to have sharp corners on the board
7. Do not cover copper in the open area of the wiring in the middle layer of the multilayer board. Because it is difficult for you to make this copper pour "good ground"
8. The metal inside the equipment, such as metal radiators, metal reinforcement strips, etc., must be "well grounded".
9. The heat dissipation metal block of the three-terminal voltage regulator must be well grounded. The ground isolation zone near the crystal oscillator must be well grounded. In short: if the grounding problem of the copper pour on the PCB is handled well, it will definitely be "better than harm". It can reduce the return area of the signal line and reduce the electromagnetic interference of the signal to the outside.