PCB Cover Copper Technique

PCB cover copper is to use the spare space on the PCB as the datum, and then fill it with solid copper, which is also called copper filling. The significance of cover copper is to reduce the impedance of ground wire, improve anti-jamming ability. Reduce the pressure drop, improve the efficiency of power supply. And connect with ground wire, also reduce the area of loop. Most PCB manufacturers require PCB designers to fill open areas with copper plate or grid-like ground wires. If they don't handle it properly, it will be "more bad than good " instead of "more good than bad".

We all know that in high frequency, the distributed capacitance of the wiring on the PCB will work, when the length is greater than 1/20 of the corresponding wavelength of the noise frequency, there will be antenna effect, the noise will be emitted through the wiring. If there is a poor grounding copper cover on the PCB, it would become a tool to spread noise, so in high frequency circuit, do not think that the ground wire is connected to the ground somewhere, this is "ground wire ", be sure to make vias in the wiring at a distance less than λ/20, "ground well" with the multilayer plate. Cover copper properly not only increases the current, but also acts as a double interference shield.

Generally, there are two basic ways to cover copper, large copper plate and grid copper, it is often asked, which one is better? Large copper plate or grid copper? Not good to take in the lump. Why? Large copper plate has dual effect of increasing current and shielding interference, but with large copper plate, the board may be raised or even bubbling during over-wave soldering. Therefore, if take a large copper plate, generally will open a few slots to relieve copper foil blistering. Simply grid copper mainly acts as a shield, the effect of increasing the current is reduced, from the view of heat dissipation, the grid has the advantage (it reduces the heating surface of copper) to play a certain role of electromagnetic shielding. But it should be pointed out that the grid is made up of interlaced lines, and we know, the width of the line has a corresponding "electrical length" for the operating frequency of the circuit board (the "electrical length" can be obtained by dividing the actual size by the corresponding digital frequency of the working frequency, see the relevant books), when the operating frequency is not very high, perhaps the advantage of the grid line is not very obvious, once the electrical length and operating frequency match, it is very bad, you will find that the circuit is not working properly at all, transmitting signals interfere the system work everywhere. So for guys using the grid, my advice is to choose base on the work situation of PCB design, do not always hold one thing. Generally, high frequency circuit and with high resistance to interference requirement, choose grid copper. Low frequency circuit and with high current, choose large copper plate.

In order to achieve the desired effect of cover copper, we should pay attention to below points.

1.If there is more PCB plane, SGND, AGND, GND, and so on, according to the position of the PCB plane, we should cover copper independently with the most important ground as reference, and the copper is applied digitally and analogically separately. Before the copper cover, the corresponding power lines should be first thickened: 5.0 v、3.3v and so on. Then there will be multi-deformation structures in different shapes

2.For single-point connections on different locations, the method is to connect with a 0-o resistor or bead or inductor

3.Copper cover near the crystal oscillator, the crystal oscillator in the circuit is a high-frequency emission source, the practice is to surround the crystal oscillator with copper cover, and then the shell of the crystal oscillator is then grounded separately.

4.The island (Dead Zone) problem, if you feel it’s big, you can define a hole in the ground, it’s easy to achieve.

5.When wiring, the ground wire should be treated equally, the ground wire should be WHERE set well, do not rely on adding holes to eliminate the connection of the ground pin after copper covered, the effect won’t be good.

6.It’s better to not have sharp corner on the board (≤180 degrees), because from the angle of electromagnetism, this constitutes a transmitting antenna! For others that will always have an impact, only big or small, so the advice is using the arc along the edge.

7.Regarding the open area in the middle layer of multi-layer PCB, do not cover copper. Because it's hard for you to make this copper cover "well grounded"

8.For the metal inside the equipment, such as metal radiator, metal reinforcement strip and so on, must achieve "good grounding ".

9.The heat dissipation metal block of the three-terminal regulator must be well grounded. The grounding isolation zone near the crystal oscillator must be well grounded. All in all: for copper cover on the PCB, if the grounding problem is handled well, it must be "advantages over disadvantages ", it can reduce the return area of the signal line, reduce the external electromagnetic interference of the signal.