Copper laying is an important part of PCB design. Whether it is domestic PCB design software or some foreign Protel, PowerPCB provides intelligent copper laying function. So how to lay copper well, I will share some of my ideas with you. Hope to bring benefits to colleagues.
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 copper coating 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. Also for the purpose of keeping the PCB from deforming as much as possible during soldering, most PCB manufacturers will also require PCB designers to fill the open area of the PCB with copper or grid-shaped ground wires. If the copper is not handled properly, it will Gain does not reward loss, is copper cladding "the advantage outweighs the disadvantages" or "the disadvantages outweigh the advantages"?
We all know that in the case of high frequency, 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 poorly grounded copper in the PCB, the copper will become a tool for spreading noise. Therefore, in high-frequency circuits, don't think that connecting a certain part of the ground to the ground is a "ground fault". Line", must be less than λ/20 spacing, punch holes in the wiring, and "good grounding" with the ground plane of the multilayer board. If the copper coating is handled properly, the copper coating can not only increase the current, but also play a dual role of shielding interference.
There are generally two basic methods of copper cladding, namely, large-area copper cladding and grid copper. People often ask whether it is better to coat large areas of copper or grid copper, and it is not easy to generalize. why? A large area of copper coating has the dual functions of increasing the current and shielding. However, if a large area of copper coating is used for wave soldering, the board may warp or even bubble. Therefore, for large-area copper coating, several slots are generally opened to alleviate copper foil blistering. The pure grid copper coating is mainly for shielding, and the effect of increasing the current is reduced. From the perspective of heat dissipation, the grid is good. (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 a circuit, the width of the trace 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 working frequency can be obtained, see related books for details), when the working frequency is not very high, perhaps the role of the grid line is not very obvious, once the electrical length matches the working frequency, it is very bad, you You will find that the circuit is not working properly at all, and signals are being emitted everywhere that interfere with the working of the system.
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, the multi-purpose grid with high anti-interference requirements for high-frequency circuits, and the circuits with large currents for low-frequency circuits are commonly used for complete copper laying. Having said so much, then we need to pay attention to those issues in copper cladding in order to make copper cladding achieve our expected effect:
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 to independently pour copper, digital ground and analog ground according to the position of the PCB board. It is not much to say that copper is deposited separately. At the same time, before the copper is poured, the corresponding power supply wiring is first thickened: 5.0V, 3.3V, etc. In this way, multiple deformable structures of different shapes are formed.
2. For the single-point connection of different grounds, the method is to connect through a 0 ohm resistor or a magnetic bead or an inductance;
3. Pour copper near the crystal oscillator. The crystal oscillator in the circuit is a high-frequency emission source. The method is to coat copper around the crystal oscillator, and then ground the crystal oscillator shell 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, and the ground wires should be routed well when wiring. You can't rely on adding vias after pouring copper to eliminate the connected ground pins. This effect is very bad.
6. It is best not to have sharp corners on the board (<=180 degrees), because from an electromagnetic point of view, this constitutes a transmitting antenna! For other things that always have an effect whether it's big or small, I recommend using the edge of the arc.
7. Do not apply copper to the open area of the wiring in the middle layer of the multilayer board. Because it is difficult for you to make this copper "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 on the PCB is dealt with properly, it must 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.
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