PCB Technical - The "disadvantages and advantages" of PCB copper

Copper coating is an important part of PCB design. Whether it is the domestic Qingyuefeng PCB design software, some foreign Protel and PowerPCB provide intelligent copper coating function, so how can we apply copper?

The so-called copper pour is to use the unused space on the PCB as a reference surface 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; reduce the voltage drop and improve the efficiency of the power supply; connecting with the ground wire can also reduce the loop area. Also for the purpose of making the PCB as not deformed as possible during soldering, most PCB manufacturers will also require PCB designers to fill the open area of the PCB with copper or grid-like ground wires. If the copper is not handled properly, it will Whether the gains or losses are rewarded or lost, is the copper coating "the advantages outweigh the disadvantages" or "the disadvantages outweigh the advantages"?

We all know that under high frequency conditions, 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 coating in the PCB, the copper coating becomes a tool for spreading noise. Therefore, in a high-frequency circuit, do not think that the ground wire is connected to the ground. This is the "ground wire". ", be sure to punch holes in the wiring with a spacing less than λ/20, and "good ground" with the ground plane of the multilayer board. If the copper coating is handled properly, the copper coating not only increases the current, but also plays a dual role of shielding interference.

There are generally two basic methods of copper coating, namely large-area copper coating and grid copper. It is often asked whether large-area copper coating is better than grid copper coating. It is not good 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 lift up and even blisters. Therefore, for large-area copper coating, several grooves are usually opened to relieve the blistering of the copper foil. The pure copper coating of the grid is mainly used for shielding, and the effect of increasing the current is reduced. From the perspective of heat dissipation, the grid is beneficial (It lowers the heating surface of the copper) and plays a role of electromagnetic shielding to a certain extent. But it should be pointed out that the grid is made up of traces in staggered directions. We know that for the circuit, the width of the trace has a corresponding "electrical length" for the operating frequency of the circuit board (the actual size is divided by the actual size). The digital frequency corresponding to the operating frequency is available, see related books for details). When the operating frequency is not very high, perhaps the role of the grid lines is not very obvious. Once the electrical length matches the operating frequency, it will be very bad. You will find that the circuit does not work properly at all, and signals that interfere with the operation of the system are being emitted everywhere. 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 high-frequency circuit has high requirements for multi-purpose grids for anti-interference, and the low-frequency circuit has a large current circuit and other commonly used complete copper.

Then in the copper coating, in order to make the copper coating achieve the expected effect, those issues need to be paid attention to in the copper coating:

1. If the PCB has more grounds, such as SGND, AGND, GND, etc., according to the position of the PCB board, the main "ground" is used as a reference to independently pour copper, and the digital ground and analog ground are separated. There is not much to say about copper coating. At the same time, before copper coating, first thicken the corresponding power connections: 5.0V, 3.3V, etc., in this way, multiple deformable structures with different shapes are formed.

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

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

4. The island (dead zone) problem, if you think it is too big, it won't cost much to define a ground via and add it in.

5. At the beginning of the wiring, the ground wire should be treated the same, and the ground wire should be routed well when routing the wire. You cannot rely on the copper pour to eliminate the ground pin for the connection by adding vias. This effect is very bad.

6. It is best not to have sharp corners (<=180 degrees) on the board, because from the perspective of electromagnetics, this constitutes a transmitting antenna! For other things, it is only large or small. It is recommended to use the edge line of the arc.

7. Do not apply copper in the open area of the middle layer of the multilayer board. Because it is difficult to make this copper "good grounding"

8. The metal inside the device, such as metal radiators, metal reinforcement strips, etc., must be "good grounding".

9. The heat-dissipating metal block of the three-terminal regulator must be well grounded. The ground isolation strip near the crystal oscillator must be well grounded. In short: if the grounding problem of the copper coating on the PCB is dealt with, it is definitely "pros outweigh the disadvantages". It can reduce the return area of the signal line and reduce the signal's external PCB electromagnetic interference.