Why do PCB boards have metal edges? Do the role of metallization edges?

2019-10-31 22:09:33 Juding Circuit Technology

I believe that people in the industry often see a lot of industrial PC board or RF pcb board, the PCB board will be surrounded by a circle of vias and copper strips, and even some RF board will be metalized on the side of the board. On the other hand, is the purpose of doing this to make people think that there is content? Is the engineer loading a big X?

As the system speed of each product increases, not only the timing of high-speed digital signals, but also the integrity of the signal is particularly prominent. At the same time, the electromagnetic interference caused by the system from the high-speed digital signals and the EMC problem caused by the power integrity are also prominent. . The electromagnetic interference generated by high-speed digital signals not only causes serious mutual interference inside the system, but also reduces the anti-interference ability of the system. At the same time, it also generates strong electromagnetic radiation to the outer space, which causes the electromagnetic radiation emission of the system to seriously exceed the EMC. The standard makes the product not certified by the EMC standard. The radiation on the side of the multi-layer pcb board is a relatively common source of electromagnetic radiation.

When the unintended current reaches the edge of the ground plane and the power plane, edge radiation is generated. These are unintended currents that can come from grounding and power supply noise caused by inadequate power supply bypassing. The cylindrical radiating magnetic field generated by the inductive vias, the radiation between the layers of the board, will eventually gather at the edge of the board. The stripline return current carrying the high frequency signal is too close to the edge of the board.

The root cause of power supply noise is usually two aspects:

One: In the high-speed switching state of the device, the transient alternating current is too large.

Second: the inductance present on the current loop.

From the aspect of performance, they can be divided into three categories:

Synchronous switching noise (SSN), sometimes referred to as ΔI noise, can also be attributed to the Groundbounce phenomenon.

Non-ideal source impedance effects.

Resonance and edge effects.

In a high-speed digital circuit, when the digital integrated circuit is powered up, its internal gate output will undergo a state transition from high to low or low to high, that is, a transition between "0" and "1". During the change process, the transistors in the gate circuit will be turned on and off continuously. At this time, there will be current flowing from the connected power supply to the start circuit, or from the gate circuit to the ground plane, so that the current on the power plane or ground plane. An imbalance is generated, resulting in a transiently varying current ΔI.

This current flows through the return path, and the existing inductance causes a drop in the AC voltage, which in turn causes noise. If there are more output buffers for state transitions at the same time, this voltage drop will be large enough to cause power supply incompleteness. This noise is called Simultaneous Switch Noise (SSN).

The power supply AC noise will be between the power supply layer and the ground layer. The resonant cavity mode of these two planes will be used to conduct AC noise. When it is transmitted to the edge of the plane, it will radiate into the free space, which will lead to the product EMI standard test cannot be achieved. The standard for EMI certification.

The AC noise is propagated by the resonant cavity between the power plane and the ground plane. Of course, such a resonator not only propagates the AC noise of the SSN, but may propagate the noise of the high-speed signal if the signal integrity is not completed.

I know the noise generated by the vias. The signal lines interconnected on the pcb board include the microstrip line on the outer layer of the pcb board, and the strip line between the two layers in the inner layer, and the signal layer is connected to form a connection. The via hole is subdivided into a through hole, a blind hole, and a buried hole. The microstrip line in the surface layer and the strip line between the two planes pass well, and the reference plane laminated structure design can well control the radiation.

The via hole penetrates a plurality of layers in the vertical direction. When the high-frequency signal transmission line is changed through the via hole, not only the impedance of the transmission line changes, but also the reference plane of the signal return path changes. When the frequency of the signal is relatively At lower times, the effect of the via on the signal transmission can be completely neglected, but when the signal frequency rises to the RF or microwave frequency band, the TEM of the via will be generated due to the change of the current return path due to the change of the reference plane of the via. The wave will propagate laterally between the resonant cavities formed by the two planes, and finally radiate into the free space through the edge of the pcb board. In severe cases, the EMI index will exceed the standard.

Now we know that for high-frequency high-speed pcb boards, edge radiation problems will occur on the board side of the pcb board. How to protect it?

The three elements that cause EMC problems are: electromagnetic interference sources, coupling paths, sensitive equipment, sensitive equipment we may not be able to control, cut off the coupling path, such as adding a metal shielding equipment shell, etc., then how to find a way to get rid of the interference source.

First of all, we must optimize the signal traces of the critical circuits on the PCB to avoid EMI problems. More than the vias of the layers can be grounded through the vias of the key signals to provide additional signal vias. Return path. For the reduction of PCB edge radiation, a 20H rule has been said before. The 20H rule was first proposed by W. Michael King in 1980, and was described by Mark. I. Montrose in his book, and is regarded by management and often listed. For important EMI design rules, where H refers to the thickness of the board, that is, the power plane is 20H away from the ground plane.

In order to reduce the effect of edge radiation, the power plane should be retracted from the adjacent ground plane, and the effect is not obvious when the power plane is shrunk by about 10H. When the power plane is shrunk by 20H, it absorbs 70% of the marginal flux boundary; when the power plane is shrunk by 100H, it can absorb 98% of the marginal flux boundary; thus, the indented power layer can effectively suppress the marginal effect. Radiation.

We believe that the 20H rule is no longer suitable for the current high-frequency high-speed PCB design. The previous printed circuit board area is large, and the resonating frequency of the planar antenna caused by the shrinkage is less obvious. Now due to the small PCB surface, the internal power supply is reduced. The radiant intensity of the layer design varies significantly with the resonance point of the size of the different retracting power supply layers, resulting in higher radiant energy at high frequencies.

The use of 20H inward does not completely solve the radiation drop, although the frequency below 430MHz is improved, while 40H improves the frequency below 590MHz, but the resonance frequency becomes higher due to the area reduction, and the radiation in the higher frequency band for the resonance frequency Suppression is not helpful.

The future design of EMI, because the internal power supply layer 20H will become helpless, and the smaller the board, the higher the frequency of radiation will become more serious due to the change of the planar antenna effect, so the theory of 20H is not in line with the current reality. Needs.

Since the 20H rule has become ineffective for the current high-frequency high-speed PCB design, it is necessary to use a shielding structure to process the edge to eliminate the interference from the PCB side radiation, thereby reflecting the noise back to the inner layer. In space, this increases the voltage noise on these layers, but the edge radiation is also reduced. The low-cost implementation method is to make a grounding via hole at the 1/20 wavelength hole spacing around the PCB board to form a grounding via shield to prevent the TME wave from radiating externally.

For the microwave circuit pcb board, the wavelength is further reduced, and due to the current production process of the pcb board, the spacing between the holes and the holes cannot be made small, and the spacing of the 1/20 wavelength is shielded around the pcb board. The way of via hole is not obvious to the microwave plate. At this time, it is necessary to adopt the process of metallizing the edge of the pcb board, and the edge of the whole pcb board is surrounded by metal, so that the microwave signal cannot be obtained from the pcb board. While external radiation, the use of pcb board edge metallization edging process will also lead to a lot of manufacturing costs of pcb board.

For some sensitive circuits of RF microwave pcb board, and circuits with strong radiation source, we can design a soldering shield cavity on the pcb board. The pcb should be designed with “via-hole shielding wall”, which is on the pcb board and shielded. The area where the cavity wall is in close contact with the grounded via. This creates a relatively isolated area, similar to the pcb board below.

Everyone feels the design requirements of the via shield wall:

*Two or more rows of vias

*Two rows of vias are staggered from each other

*The pitch of the vias in the same row is less than λ/20

* The grounded PCB copper foil and the shield cavity wall are not bonded with solder resist

Well, the above sharing is provided by: (Juding Circuit Technology) pcb circuit board board factory, I believe everyone should understand: What is the role of metallized edge, why do PCB boards need to be metallized?

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