PCB News - What is the circle of vias or metal edging around the PCB circuit board?

What is the circle of vias or metal edging around the PCB circuit board?

It is often seen that many industrial control cards or radio frequency cards on printed circuit boards will be perforated in a round and copper-colored circle, and even some radio frequency cards will be metal-plated on the map.

what is this? Today, with the increase of system speed, the timer and integrity of high-speed digital signals are not only important, but also CEM problems caused by electromagnetic interference and the integrity of the power of high-speed digital signals in the system are also very important.

The electromagnetic interference generated by the high-speed digital signal will not only cause serious interference to the system, but also reduce its interference ability, but also produce a strong electromagnetic radiation, which will lead to the seriousness of the EMC standard.

Make the product unable to pass the EMC standard certification. Peripheral radiation from multilayer PCBs is a common source of electromagnetic radiation.

When the unexpected current reaches the edge of the earth's layer and the power layer, radiation from the edge is generated.

These unpredictable flows may come from: ground and power noise caused by improper power diversion

The cylindrical magnetic field generated by the inductance holes between the layers of the printed circuit board is marginalized on the printed circuit board.

Three times the return current used to deliver high-frequency signals is too close to the edge of the printed circuit board.

There are two main sources of power noise:

1. Transitional AC current is too important in the high-speed switching state of the equipment; the other is the inductance in the current loop.

This term can be divided into the following three categories: Synchronous switching noise (SSN), sometimes referred to as noise i, can also be attributed to quality.

2. Impedance effect of non-ideal power supply; Resonance and effect In high-speed digital circuits, when the digital integrated circuit is tightened by voltage, the output of its internal gate circuit will go from top to bottom or bottom to bottom, that is, "0" and " Conversion between 1".

In the process of change, the transistors in the gate circuit will continuously activate and deactivate. At this time, the current flows from the surface connected to the input circuit or the gate circuit to the ground, resulting in an imbalance between the power and the current on the ground, resulting in a delta current change.

Switch voltage and generate noise. If there are more output buffers and simultaneous state transitions, the voltage drop is enough to cause food integrity problems. This noise is called synchronous switching noise (SSN).

The power supply noise will be transferred between the power supply layer and the laminated layer. By using the resonant cavity of these two plans to transmit the replacement noise, it will be moved to the free space at the edge of the plan, thereby preventing the certification of the product.

The above figure is a schematic diagram of simultaneous switching noise (SSN) while using a resonator to propagate replacement noise between the power plan and the quality plan. Of course, in the case of poor signal integrity, these resonators not only propagate the AC noise of the SSN, but also propagate the noise of high-speed signals.

Regarding the noise generated through the through hole, we know that the signal lines connected on the printed circuit include the microstrip line on the outer layer of the printed circuit and the line on the inner layer between the two planes, and the plating is Divided into through holes.

Tts, TRON BORON, and TROUS are connected to the signal exchange layer. The surface layer and belt line between these two plans can be properly designed through a good reference plan structure.

Control radiation. When the high-frequency signal transmission line passes through the hole to replace the layer, not only the impedance of the transmission line, but also the reference plan of the return path of the signal is included.

When the frequency of the signal is relatively low, the influence of the signal transmission hole can be ignored, but when the signal frequency reaches the radio frequency or high frequency band, the influence of the signal transmission hole on the signal can be ignored.

The hole reference plan caused the current return path to change. The TEM generated by the hole diffuses laterally between the resonators formed in the two planes, and finally draws it into the free space from the edge of the map, so that the EMI index exceeds the standard.

We know that on high-frequency and high-frequency circuit boards, there will be a radiation problem on the printed circuit board.

The three components of the CEM problem are: from EMI, coupling channels and sensitive equipment. Sensitive equipment that we cannot control, cut off the coupling channel, such as adding metal shielding, but Lao Wu didn't say anything, the rest is to find a way to eliminate the source of interference.

First, we must optimize the key signals on the printed circuit to avoid electromagnetic interference problems. Compared with the hole of the replacement layer, we can perforate the key signal hole to provide an additional return path for the key signal hole.

In order to reduce the edge of PCBs, Lao Wu has heard a rule 20 hours ago. The 20: 00 rule was first proposed by W. Michael King and written by Mark. I am on his book.

The management emphasized this and it is often regarded as an important EMI design rule. h refers to the thickness of the plane, that is, the distance of the 20H electrical plane is reduced compared with the horizon plan.

In order to reduce the effect of edge radiation, the power plan must be compared with the neighboring ground plan, but when the power plan shrinks within about 10 hours, the effect is not obvious; when the power plan is restored at 20 o'clock, it absorbs 70% % Of the marginal flow limit.

(Boundady flow); When the power plan goes from about 100 hours to the inside, it absorbs 98% of the marginal flow limit; therefore, the power layer can effectively suppress the radiation caused by the marginal effect.

Old Wu believes that the 20H rule is no longer suitable for the design of high-frequency and high-frequency circuits. The old circuit board has a large surface, and the resonant frequency of the antenna is not obvious when it is withdrawn.

At present, the radiation intensity generated from the design of the retractable power layer is very different from the size of the resonance point of the output power layer, resulting in higher radiation energy at high frequencies.

Although the frequency of 430 MHz is increased, and the frequency below 590 MHz is lower than 90 MHz, the resonance frequency is increasing due to the decrease in area, which does not help eliminate radiation in the higher frequency band.

In the future design of EMI, because the 20H nutrient layer is not useful and the map is small, the high-frequency radiation becomes more serious due to the change of the planar antenna effect. Therefore, the 20H theory no longer meets the current actual needs.

Since the 20-hour rule becomes ineffective in the design of current high-frequency and high-frequency circuits to eliminate the edge of the printed circuit, a protective structure must be used to handle the edge, thereby sending noise back to the internal space.

This will increase the voltage noise on these layers, but will reduce the radiation at the edges. The low-cost method is to drill a circular hole in the printed circuit board to form a 1/20 wavelength hole and form a hole shield on the ground to prevent the TME wavelength from being external.

For microwave cards, the wavelength is still decreasing, and because of the PCB production technology, the distance between the holes cannot be very small. At present, the distance between the 1/20 wavelength shielding holes for PCBs is not obvious to the microwave card.

At this stage, in the packaging process, the PCB and the entire metal card are surrounded to convey high-frequency messages. NO1 cannot be marked from the edge of PCBs. Of course, the use of metal packaging processes will also lead to the production costs of PCBs.

For RF high frequency cards, certain sensitive circuits and high radiation source circuits, a shielded room can be designed for soldering on a printed circuit board. The printed circuit must be designed with "through the shielding wall", that is, a hole through the ground is added near the shielding cavity wall on the printed circuit board.

This creates a relatively isolated area, similar to the PCBs below, and you can feel it.

4. The requirements for designing a cross-shielding wall are as follows: There are two or more holes The two rows are spaced apart from each other. The hole spacing in the same row is less than LAMBDA/20. The compression seal between the copper foil of the PCB and the wall of the protective chamber is prohibited.