PCB Technical - Pay attention to PCB copper coating, otherwise don't copper coating!

Copper-clad is an important part of PCB design. Whether it is domestic PCB design software or some foreign Protel, PowerPCB provides intelligent copper-clad function, so how to apply copper, I will share some of my thoughts with you, I hope to bring benefits to colleagues.

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.

In order to keep the PCB from being deformed as much as possible during soldering, most PCB manufacturers also require PCB designers to fill the open areas of the PCB with copper or grid-like ground wires. If the copper coating is handled improperly, the gain will not be worth the loss. Is the copper coating "more advantages than disadvantages" or "disadvantages more than advantages"?

In the case of high frequency, 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 corresponding wavelength of the noise frequency, an antenna effect will occur, and noise will be emitted through the wiring. If there is a poorly grounded copper pour in the PCB, the copper pour becomes a noise propagation tool. Therefore, in a high-frequency circuit, do not think that the ground wire is connected to the ground. This is the "ground wire" and must be less than λ/20. Punch holes in the wiring to "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 has the dual role of shielding interference.

There are generally two basic methods for 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-clad grid 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 the copper) and plays a certain role of electromagnetic shielding. However, 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 a 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 is available, see related books for details). When the operating frequency is not very high, the side effects of the grid lines may not be obvious. Once the electrical length matches the operating frequency, it will be very bad. It is found that the circuit is not working properly at all, and signals that interfere with the operation of the system are being transmitted 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, high-frequency circuits have high requirements for multi-purpose grids for anti-interference, and low-frequency circuits, circuits with large currents, etc. are commonly used and complete copper.

In order to achieve the desired effect of copper coating, we need to pay attention to the following issues:

1. If the PCB has many grounds, such as SGND, AGND, GND, etc., according to the position of the PCB board, the main "ground" should be used as a reference to independently pour copper. The digital ground and the analog ground are separated to pour copper. At the same time, before the copper pour, first thicken the corresponding power connection: 5.0V, 3.3V, etc., in this way, multiple polygons of different shapes are formed structure.

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

3. Copper-clad 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-clad, 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. When routing the ground wire, the ground wire should be routed well. The ground pin cannot be added by adding vias. This effect is very bad.

6. It is best not to have sharp corners on the board (<=180 degrees), because from the perspective of electromagnetics, this constitutes a transmitting antenna! There will always be an impact on other places, but whether it is large or small, I recommend using the edge of the arc.

7. Do not pour copper in the open area of the middle layer of the multilayer board. Because it is difficult for you to make this copper clad "good ground"

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

9. The heat dissipation 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 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 electromagnetic interference to the outside.