Copper clad as an important link in PCB design, whether domestic PCB design software or foreign Protel,altium designer provide intelligent copper clad function, so how to finish copper, the following are some ideas to share with you,RenhotecIC hope to bring benefits to my peers.

　　Copper cladding is the use of idle space on the PCB as a datum and then filled with solid copper, which is also called copper filling. The significance of copper clad is to reduce the impedance of ground wire, improve the anti-interference ability, reduce the voltage drop, improve the efficiency of power supply, and reduce the loop area when connected with ground wire. Most PCB manufacturers will also require PCB designers to fill PCB open areas with copper or grid-shaped ground wires. If copper is not handled properly, it will not be rewarded. Is copper clad "more good than bad" or "more bad than good "?


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

　　Looking at a measured case, on a multi-layer PCB, the engineer applied a circle of copper around the PCB, as shown in figure 1. In this treatment, the engineer placed only a few holes in the beginning of the copper skin, connecting the copper skin to the formation, and no holes were made elsewhere.

　　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 1/20 of the corresponding wavelength of the noise frequency, the antenna effect will be produced, and the noise will be emitted outward through the wiring.

　　Judging from the actual measurement above, there is a 22.894 MHz interference source on the PCB, and the copper skin is sensitive to the signal, which is received as a "receiving antenna ". At the same time, the copper skin acts as a" transmitting antenna "to transmit a strong electromagnetic interference signal to the outside.

　　As we know, the relationship between frequency and wavelength is f=C/λ.

　　The f is frequency, the unit is Hz,λ wavelength, the unit is m,C speed of light, equal to 3×108 m / s

　　The wavelength λ of the signal MHz 22.894 is :3×108/22.894/13 m. λ/20 cm.65

　　The copper clad in this PCB is too long, exceeding 65 cm, resulting in antenna effect.

　　currently, in our PCB, chips with rising edge less than 1 ns are generally used. Suppose the rise edge of the chip is 1 ns, and the frequency of electromagnetic interference will be as high as fknee =0.5/ Tr fknee =500 MHz..1 The wavelength is 60 cm,λ/20=3 cm. for 500 signals That is to say, the 3 cm long wiring on the PCB may form a "antenna ".

　　Therefore, in the high-frequency circuit, do not think that the ground wire somewhere connected to the ground, this is the "ground wire ". Be sure to make holes in the wiring with spacing less than λ/20 and "well grounded" with the ground plane of the multilayer board ".

　　To the general digital circuit, according to the distance between 1 cm and 2 cm, the "ground filling" of the element surface or welding surface is perforated to realize the good grounding with the ground plane, so as to ensure that the "ground filling" will not have the effect of "harm ".

　　Thus, we extend as follows:

　　Do not apply copper to the open area of the middle layer of the multilayer board. Because it's hard to get this copper coating "well grounded"

　　A PCB, no matter how many kinds of power supply, it is recommended to use power segmentation technology, and only one power layer. Because the power supply is the same as the ground, it is also the "reference plane ", and the" good grounding "of the power supply and the ground is realized by a large number of filter capacitors. Where there is no filter capacitor, there is no" grounding ".

　　The metal inside the equipment, such as metal radiator, metal reinforcement bar and so on, must realize "good grounding ".


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

　　The grounding isolation zone near the crystal oscillator must be well grounded.

　　Conclusion: the copper coating on the PCB, if the grounding problem is handled well, must be "the advantage is greater than the disadvantage ", it can reduce the reflux area of the signal line and reduce the electromagnetic interference of the signal.