PCB Tech - Difficulties in PCB copper design

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 also require PCB designers to fill the open area 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"?

Everyone knows 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 pour in the PCB, the copper pour 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 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 generally used to alleviate the blistering of the copper foil. The pure mesh copper coating is mainly used for shielding, and the effect of increasing the current is reduced. From the perspective of heat dissipation, the mesh 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 working frequency is available, 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 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, it is recommended that you can choose according to the working conditions of the designed circuit board, and don't cling to one thing. Therefore, high-frequency PCB circuits have high requirements for multi-purpose grids for anti-interference, and low-frequency circuits have circuits with large currents, such as commonly used complete copper.

Having said that, in order to achieve the desired effect of copper coating, what issues need to be paid attention to in copper coating:

1. If the PCB has many 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, digital ground and analog ground. It is not necessary to separate the copper pour. At the same time, before the copper pour, first thicken the corresponding power connection: 5.0V, 3.3V, etc., in this way, multiple deformed 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. Copper pour near the crystal oscillator. The crystal oscillator in the circuit is a high-frequency emission source. The method is to pour copper around the crystal oscillator, and then ground 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. You cannot rely on adding vias to eliminate the ground pins for the connection after copper plating. 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 big 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 device, 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: PCB copper, if the grounding problem is dealt with, it is definitely "pros outweigh the disadvantages", it can reduce the return area of the signal line, and reduce the electromagnetic interference of the signal to the outside.