In three minutes, you will learn about the technical measures for PCB radiation suppression and shielding. Save it first
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1. What is EMC?
EMC (Electro Magnetic Compatibility) refers to the ability of a device or system to generate electromagnetic energy that neither interferes with other electronic devices nor is interfered by the electromagnetic energy of other electronic devices in a specific electromagnetic environment.
For EMC, the corresponding standards include Europe's CE certification, the United States' FCC certification and China's 3C certification. If electronic products want to be legally sold in the corresponding regions, they must obtain certification that complies with the corresponding local regulations.
2. EMC = EMI + EMS
EMI (Electromagnetic Interface) refers to electromagnetic interference, which specifically refers to the electromagnetic interference energy that is detrimental to other electronic systems or devices generated during the normal operation of electronic components on electronic equipment and must not exceed a certain limit.
EMS (Electromagnetic Susceptionbility) refers to the degree to which an electronic system or device can resist electromagnetic interference from other devices or components.
To put it simply, the difference is that EMC is a comprehensive attack and defense capability, EMI is the attack power, and EMS is the defense power. When conducting EMC tests on electronic equipment, the relevant standards stipulate the maximum value of EMI and the minimum value of EMS, which is like limiting the "attack power" to a lower level and requiring the "defense power" to a higher level. This is also easy to understand, just like when we raise a guard dog, you don't want it to run out and bite people, but you require it to be able to withstand the enemy's attack.
3. EMC design method
EMC design methods can be divided into the following four points: grounding, filtering, isolation and shielding. Grounding can effectively guide noise into the ground plane, conducted interference takes filtering and isolation measures, radiated interference takes grounding and shielding measures, filtering can filter out invalid signal frequency bands, isolation can block the conducted interference of noise, and shielding can effectively block the radiated interference of noise.
When conducting an EMC feasibility analysis of a product, the first thing to do is to determine the area where the product is positioned, so that corresponding tests can be performed according to its standards. For EMC feasibility analysis, you can focus on three aspects:
protecting disturbed groups;
Cut off the transmission path of interfering noise;
Isolate the source of interference.
3.1. Reduce the antenna effect of PCB traces and copper foil
The principles of EMC application are the transmission line effect principle and the antenna principle, which can be divided into near-field radiation analysis and far-field radiation analysis. When routing high-speed PCBs, the stub routing of key signals should be reduced as much as possible, which is beneficial to PCB routing and also helps to reduce the antenna effect of PCB routing. In order to reduce the stubs generated by high-speed signal vias, one method is to use back drilling technology (Back Drill), as shown in the following figure:
Back drilling technology
So some people may ask, what is back drilling?
Back drilling is actually a special PCB processing method for controlled depth drilling. In the production of multi-layer boards, such as 12-layer boards, we need to connect the 1st layer to the 9th layer. Usually we drill a through hole (one-time drilling) and then add copper. In this way, the 1st layer is directly connected to the 12th layer. In fact, we only need to connect the 1st layer to the 9th layer. Since there is no line connecting the 10th to the 12th layer, it is like a pillar. This pillar affects the signal path and causes signal integrity problems in communication signals. Therefore, this extra pillar (called stub in the industry) is drilled out from the back (secondary drilling). So it is called back drilling, but it is generally not so clean, because the subsequent process will electrolyze a little copper, and the drill tip itself is also sharp.
In high-speed circuit design, generally no wiring is done on the TOP and BOTTOM layers. In order to effectively shield the inner layer signals of the PCB and facilitate the lamination process of the PCB board, a large area of copper is usually applied on the surface. When applying copper on the surface, ground holes must be fully drilled, and isolated copper foil is strictly prohibited. This should be paid special attention to when inspecting the PCB! For the copper foil of each part of the PCB, it is recommended to trim it into a 45° angle or fillet, as shown in the figure below. When the surface copper foil is not sufficiently grounded, the isolated copper foil will show an antenna effect. At the same time, designing the copper foil of the copper foil to a 45° angle or fillet is conducive to reducing the antenna effect of the copper foil, thereby reducing the external radiation of the PCB. Similarly, try to avoid right-angle routing when routing the PCB.
3.2 Faraday electromagnetic shielding cage
Background
A "Faraday cage" is a cage made of metal or a good conductor. It is named after the founder of electromagnetism, the famous British physicist Michael Faraday. It is a device used to demonstrate equipotential, electrostatic shielding and high-voltage live work. In 1836, Faraday once risked being electrocuted and conducted a world-famous experiment - the Faraday cage experiment. Faraday locked himself in a metal cage, and when a strong electrostatic discharge occurred outside the cage, nothing happened.
Faraday Cage
The "Faraday cage" is composed of a cage body, a high-voltage power supply, a voltage display and a control part. The cage body is connected to the ground. The high-voltage power supply transmits 100,000 volts of DC high voltage to the discharge rod through a current-limiting resistor. When the tip of the discharge conductor is 10 centimeters away from the cage body, a discharge spark appears. According to the electrostatic equilibrium condition of the grounded conductor, the cage body is an equipotential body with zero potential difference and zero electric field inside. It can be inferred that for a conductor in an electrostatic equilibrium state, the charge is distributed on the outer surface close to the discharge conductor.
If the conductor is hollow, when it reaches electrostatic equilibrium, there will be no electric field inside. In this way, the outer shell of the conductor will "protect" its interior from the influence of the external electric field. This phenomenon is called electrostatic shielding. Of course, the demonstration of the Faraday cage also shows that the Faraday cage with a grounded outer shell can effectively isolate the electromagnetic interference inside and outside the cage, thereby playing the role of electrostatic shielding.
Based on the Faraday cage and electrostatic shielding principle:
The protective clothing of high-voltage live operators is made of metal wire. When they come into contact with high-voltage lines, an equipotential is formed, so that no current passes through the operator's body, which plays a good protective role.
A car is not afraid of lightning strikes, not because it is a car, but because it is a Faraday cage.
Large buildings are sometimes designed as Faraday cages, such as the Beijing Bird's Nest Stadium during the 2008 Olympic Games. Its metal frame ensures that even if it is directly struck by lightning, the Faraday cage effect can still protect the safety of tens of thousands of spectators in the venue.
Similarly, at the edge of the PCB or the edge of each functional module circuit of the PCB, a ground hole connected to the GND plane in the layer is drilled every 1/10 wavelength to build a Faraday electromagnetic shielding cage for the PCB or each functional module circuit on the PCB. The shielding cage can play a certain shielding role. When routing on each layer of the PCB, in order to maximize the effect of the Faraday electromagnetic shielding cage, the wiring cannot be routed outside the shielding cage. The application of the Faraday electromagnetic shielding cage in the shielding of mobile phone motherboards is shown in the figure below:
3.3. Clarify EMC test indicators
EMI testing mainly includes conducted spurious emissions (CE), radiated spurious emissions (RE) and harmonic interference, while EMS testing includes conducted spurious emissions (CS), radiated spurious emissions (RS), electrostatic discharge (ESD), surge and voltage drop test (DIP).
Well, this is the end of the article about the technical measures for PCB radiation suppression and shielding. If you have any suggestions, please leave a message to correct me!
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