What is the role of PCB's metal edges?
Many industrial control boards or radio frequency boards will be traveled and copper bands around the PCB, and even some RF boards will be metallic bonding around. What is the purpose of this?
Today, with the improvement of the system rate, it is not only the timing and signal integrity of high -speed digital signals. At the same time, the EMC problem caused by electromagnetic interference and power integrity caused by high -speed digital signals in the system is also very prominent. The electromagnetic interference generated by high -speed digital signals will not only cause severe mutual disturbance inside the system, reduce the anti -interference ability of the system, but also produce strong electromagnetic radiation in the external space, causing the system's electromagnetic radiation to emit more than the EMC standard, making the product make the product Can not pass EMC standard certification. Multi -layer PCB plate radiation is a relatively common source of electromagnetic radiation.
When the non -expected current reaches the edge of the ground layer and the power layer, edge radiation occurs. These non -expected currents may be derived from: 1. The ground and power noise generated by the inadequate power by the power supply; 2. The cylindrical radiation magnetic field generated by the perceptual over -perforated, it radiates between the various layers of the circuit board, and eventually on the circuit on the circuit The edge of the board will be closed; 3. The load -loading back -shaped back current loading current is too close to the edge of the circuit board.
The root cause of the noise of power supply is two aspects: one is that in the state of high -speed switching of the device, the transition current of the transient is too large; the other is the inductance existing in the current loop. From the perspective of expression, it can be divided into three categories: 1. Synchronous switching noise (SSN), sometimes called ΔI noise, and the phenomenon of ground bomb can also be attributed to this type; 2. Non -ideal power impedance impact; 3. Residents and edges effect.
In high -speed digital circuits, when the digital integrated circuit is power -on, the internal door circuit output will occur from high to low or from low to high state conversion, that is, the conversion between "0 ″ and" 1 ″. In the process of changing, the transistor in the door circuit will continue to be turned on and cut off. At this time, there will be a current flowing from the power supply to the door circuit, or flowing from the door circuit into the ground plane, so that the current plane or the current on the ground plane Election is generated to produce a current change current △ i. This current will form an AC voltage drop when the inductance exists on the flowing path, which will cause noise. If there are many output buffers with a state conversion at the same time, this voltage drop is large enough, which causes the power integrity problem. This noise is called synchronous switching noise SSN.
The noise of power AC will use the two plane resonant mode between the two plane resonance mode between the power layer and the strata. It will radiate to the free space when it is transmitted to the plane edge, which will cause the product EMI to be certified.
For the noise generated by perforated, the signal line of the PCB upper connection includes the micro -band line of the outer layer of the PCB and the inner layer on the two planes and the electroplating pores that are connected to the signal conversion. Pole, blind holes, buried holes), the micro -band line on the surface layer and the straps on the two planes can be well controlled by the design of the plane layer stack structure.
The excessive pores run through multiple layers in the vertical direction. When the high -frequency signal transmission line passes through the perforated conversion layer, not only the impedance of the transmission line changes, the reference plane of the signal return path also changes. When low, the effect of Tongkou on signal transmission can be ignored, but when the signal frequency rises to the radio frequency or microwave frequency band, the current changes in the current back path due to the reference plane change of the perforated is In the horizontal transmission between the resonant cavity formed by the two planes, the EMI indicator exceeded the standard through the edge of the PCB out of the edge of the PCB.
After understanding the high -frequency and high -speed PCB, the edge radiation problem will be generated on the edge of the PCB. How to protect it?
Three elements of EMC problems are: electromagnetic interference source, coupling pathway, sensitive equipment
The sensitive device cannot be controlled. Cutting the coupling path, such as adding a metal shielding device shell, etc., you need to solve the problem of interference sources.
First of all, we must optimize the key signal on the PCB to avoid the EMI problem with ourselves. More than the excessive perforated, we can use the holes around the excess of the key signal to provide an additional return path for the perforated excess of the key signal Essence
Reduce PCB edge radiation, there is a 20H rule. The 20H rules were first proposed by W.Michael King in 1980. It is often listed as an important EMI design rule. A 20H distance in the plane.
In order to reduce the effect of marginal radiation, the plane of the power supply should be contracted from the adjacent plane, and the effect of shrinking about 10h in the power supply plane is not obvious; when the power plane shrinks 20 hours, the absorption of 70%of the marginal flux boundary (Flux Boundary); When the power plane is 100h, it can absorb 98%of the marginal general -volume boundary; therefore, the internal shrinkage source layer can effectively suppress the radiation caused by the marginal effect.
The 20H rules are no longer suitable for the current high -frequency and high -speed PCB design. The previous printing circuit board has a large area and the resonance frequency of the flat antenna caused by internal shrinkage is not obvious. Nowadays The radiation intensity changes with the size of the size of the source layer of different internal shrinkage, resulting in high radiation energy at high frequencies.
The use of internal shrinkage 20h cannot completely solve the decline in radiation. Although the frequency below 430MHz has improved, and 40H has improved the frequency below 590MHz, but due to the reduction of the area of the resonance, the radiation frequency of the resonance frequency is higher. Inhibition is no help.
The future design of EMI will have no help because the 20H internal reduction of the power layer will not help, and the smaller the board, the higher the frequency of radiation will be more serious due to changing the floor antenna effect.
Since the 20H rules have become invalid for the current high -frequency and high -speed PCB design, it is necessary to use the shielding structure to deal with the edge of the shielding structure for killing PCB board, so as to reflect the noise back to the inner space. This will increase the voltage noise on these layers, but the edge radiation is reduced.
The low -cost implementation method is to hit a circle of ground with a 1/20 wavelength hole spacing around the PCB to form a grounded shield to prevent TME waves from radiation.
For the microwave circuit board, its wavelength has become smaller, and because the PCB production process is now, the distance between the hole and the hole cannot be done very small. At this time The role of microwave boards is not obvious. At this time, you need to use the PCB version of the metal -based edging process to surround the entire board with metal, so that the microwave signal cannot be radiated from the PCB board. The edging process will also lead to a lot of increase in PCB production and manufacturing costs.
For radio frequency microwave boards, some sensitive circuits, and circuits with strong radiation sources can be designed with a shielding cavity welded on PCB. PCB must be added with the "cross -shielding wall" during design, which is on the PCB and the shielding cavity wall on the wall of the shielding cavity. The close part is added to the ground. In this way, a relatively isolated area is formed.
