PCB Tech - PCB copper clad from the antenna perspective

The so-called copper pour is to use the unused space as a reference surface and then fill it with solid copper. These copper areas are also called copper pour. 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 board as undistorted as possible during soldering, most people will also require people to fill the open area of the PCB with copper or grid-like ground wires. If the copper is not handled properly, it will not be rewarded or lost. Whether copper coating is "the advantage outweighs the disadvantages" or "the disadvantages outweigh the advantages"

The following measurement results are obtained using the EMSCAN electromagnetic interference scanning system. EMSCAN enables us to see the distribution of electromagnetic fields in real time. It has 1218 near-field probes and uses electronic switching technology to scan the electromagnetic field generated by the PCB at high speed. It is the only electromagnetic field near-field scanning system in the world that adopts array antenna and electronic scanning technology, and it is also the only system that can obtain complete electromagnetic field information of the measured object.

Let's look at an actual case. On a multi-layer PCB, the engineer puts a circle of copper around the PCB, as shown in Figure 1. In this copper coating process, the engineer only placed a few vias at the beginning of the copper skin, and connected the copper skin to the ground layer. There were no vias in other places.

Frequency 22.894Mhz The electromagnetic field generated by poorly grounded PCB copper

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 the noise will be emitted through the wiring.

From the results of the actual measurement above, there is a 22.894MHz interference source on the PCB, and the laid copper sheet is very sensitive to this signal, and the signal is received as a "receiving antenna". At the same time, the copper sheet is also used as a "transmitting antenna". "Antenna" emits strong electromagnetic interference signals to the outside.

The relationship between frequency and wavelength is f = C/λ.

In the formula, f is the frequency, the unit is Hz, λ is the wavelength, the unit is m, and C is the speed of light, which is equal to 3*108 m/s. For the signal of 22.894MHz, its wavelength λ is: 3*108/22.894M=13 meters. λ/20 is 65cm.

The copper of this PCB is too long, exceeding 65cm, which leads to the antenna effect.

Currently, in our PCBs, chips with a rising edge less than 1ns are commonly used. Assuming that the rising edge of the chip is 1ns, the frequency of electromagnetic interference generated by it will be as high as fknee = 0.5/Tr = 500MHz. For a 500MHz signal, its wavelength is 60cm, λ/20=3cm. In other words, a 3cm-long wiring on the PCB may form an "antenna".

Therefore, in a high-frequency circuit, don't think that if you connect the ground somewhere to the ground, this is the "ground". Be sure to punch holes in the wiring with a pitch less than λ/20 to "good ground" with the ground plane of the multilayer board.

For general digital circuits, at a distance of 1cm to 2cm, perforate the "ground filling" of the component surface or the soldering surface to achieve a good grounding with the ground plane, so as to ensure that the "ground filling" will not produce "bad" effects. .

Therefore, the following extensions are carried out:

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

Ø Regardless of how many power sources there are on a PCB, it is recommended to use power splitting technology and only use one power layer. Because the power supply is the same as the ground, it is also the "reference plane". The "good grounding" between the power supply and the ground is achieved through a large number of filter capacitors. Where there is no filter capacitor, there is no "grounding".

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

Ø The heat dissipation metal block of the three-terminal regulator must be well grounded.

Ø The ground isolation strip near the crystal oscillator must be well grounded.

Conclusion: PCB copper, if the grounding problem is dealt with, it must be "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.