Copper coating is an important part of PCB design. Whether it is the domestic Qingyuefeng PCB design software or some foreign Pro and PowerPCB, they all provide intelligent copper coating function. So, how to apply copper well? I will share some of my own ideas with everyone, hoping to bring benefits to my peers.
The so-called copper coating refers to using the idle space on the PCB as a reference plane, and then filling it with solid copper. These copper areas are also known as copper filling. The significance of copper coating is to reduce the impedance of the ground wire and improve its anti-interference ability; Reduce voltage drop and improve power efficiency; Connecting to the ground wire can also reduce the loop area. For the purpose of minimizing deformation during PCB welding, most PCB manufacturers also require PCB designers to fill the open areas of the PCB with copper sheets or grid like ground wires. If copper coating is not handled properly, it will be rewarded or lost. Is copper coating “more beneficial than harmful” or “more harmful than beneficial”?
Everyone knows that in high-frequency situations, 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 outward through the wiring. If there is a poorly grounded copper coating in the PCB, the copper coating will become a tool for spreading noise. Therefore, in high-frequency circuits, it is important not to assume that a certain part of the ground wire is grounded, This is the ‘ground wire’, which must be less than λ/ At a spacing of 20, holes are drilled in the wiring to ensure a “good grounding” with the ground plane of the multi-layer board. If the copper coating is properly treated, it not only increases the current but also plays a dual role in shielding interference.
There are generally two basic methods of copper coating, namely large-scale copper coating and grid copper coating. It is often asked whether large-scale copper coating is better or grid copper coating is better, which is not easy to generalize. Why? Large area copper coating has the dual functions of increasing current and shielding, but if the copper coating is applied on a large area, the board may curl up and even bubble during wave soldering. Therefore, when covering a large area with copper, several slots are usually opened to alleviate blistering on the copper foil. Simple grid copper coating mainly serves as a shielding effect, and the effect of increasing current is reduced. From the perspective of heat dissipation, the grid has advantages (it reduces the heating surface of copper) and also plays a certain role in electromagnetic shielding.
However, it should be pointed out that the grid is composed of staggered directional wiring, and we know that for circuits, The width of wiring has its corresponding “Electrical length” for the operating frequency of the circuit board (the actual size can be obtained by dividing 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 can not work properly at all, and signals that interfere with the system’s work are being transmitted everywhere. So for colleagues who use grids, my suggestion is to choose based on the working conditions of the designed circuit board and not cling to one thing. Therefore, high-frequency circuits with high anti-interference requirements for multi-purpose grids, low-frequency circuits with high current circuits, and other commonly used complete copper laying.
Having said so much, in order to achieve the expected effect of copper coating, what issues should we pay attention to in terms of copper coating
If the PCB has a large amount of ground, such as SGND, AGND, GND, etc., it is necessary to use the main “ground” of the PCB as a reference for independent copper coating based on the different positions of the PCB surface. It is not necessary to separate the digital ground and analog ground for copper coating. At the same time, before copper coating, the corresponding power lines should be thickened: 5.0V, 3.3V, etc. This will form multiple deformation structures with different shapes.
- For single point connections to different locations, the method is to connect through a 0 ohm resistor, magnetic bead, or inductor;
- The copper coating near the crystal oscillator is used as a high-frequency emission source in the circuit. The method is to cover the crystal oscillator with copper around it, and then ground the outer shell of the crystal oscillator separately.
- If the problem of isolated islands (dead zones) is significant, then defining a ground through hole and adding it doesn’t take much effort.
- When starting wiring, the ground wire should be treated equally. When wiring, the ground wire should be properly routed, and it cannot be eliminated by adding through holes after copper coating, as this effect is not good.
- On the board, there should be no sharp angle (“=180 degrees”) on the Zui, because from the electromagnetism point of view, it 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.
- Do not cover copper in the open area of the wiring in the middle layer of the multi-layer board. Because it’s difficult for you to make this copper clad “well grounded”
- The metal inside the equipment, such as metal radiators, metal reinforcement strips, etc., must be well grounded.
- The heat dissipation metal block of the three terminal voltage regulator must be well grounded. The grounding isolation strip near the crystal oscillator must be well grounded. In short, if the grounding issue on the PCB is properly addressed, the benefits outweigh the drawbacks. It can reduce the return area of the signal line and reduce the electromagnetic interference of the signal to the outside.
