PCB Blog - Design of Electromagnetic Compatibility Based on PCB board

1. Introduction to PCB
PCB, the Chinese name is printed circuit boards,is an important electronic component, a support body for electronic components, and a provider of electrical connections for electronic components. Because it is made using electronic printing, it is called a "printed" circuit board. The inventor of the printed circuit board was the Austrian Paul Eisler, who in 1936 used the printed circuit board in a radio unit. In 1943, Americans used the technology extensively in military radios. In 1948, the United States officially recognized the invention for commercial use. Printed circuit board technology has only been widely adopted since the mid-1950s. Before the advent of printed circuit boards, the interconnection between electronic components was achieved by direct connection of wires. Now, circuit panels exist only as effective experimental tools; printed circuit boards have come to dominate the electronics industry. Classification according to the number of circuit layers: divided into single-sided, double-sided and multi-layer boards. Common multi-layer boards are generally 4-layer boards or 6-layer boards, and complex multi-layer boards can reach more than ten layers.

1. There are three main division types of PCB board:
1) Single panel
A single-sided board is a basic PCB with parts concentrated on one side and wires on the other side. Because the wires only appear on one side, this type of PCB is called single-sided. Because single boards have many strict constraints on designing circuits, only early circuits used these types of boards.
2) Double panel
The double-sided circuit board has wiring on both sides, but to use the wires on both sides, there must be a proper circuit connection between the two sides. Such "bridges" between circuits are called vias. Vias are small holes on a PCB, filled or painted with metal, that can be connected to wires on both sides. Because double-sided boards have twice the area of single-sided boards, and because wiring can be interleaved, it is more suitable for more complex circuits than single-sided boards.
3) Multilayer board
In order to increase the area that can be wired, multilayer boards use more single or double-sided wiring boards. A printed circuit board with a double-sided inner layer, two single-sided outer layers, or two double-sided inner layers and two single-sided outer layers, alternated together by a positioning system and insulating bonding materials, and conductive patterns. Printed circuit boards that are interconnected according to design requirements become four-layer and six-layer printed circuit boards, also known as multi-layer printed circuit boards. The number of layers of the board represents several independent wiring layers, usually the number of layers is even, and includes the two outer layers. Most motherboards are 4 to 8-layer structures, but technically, nearly 100-layer PCB boards can be achieved. Most large supercomputers use fairly multi-layer motherboards, but because such computers can be replaced by clusters of many ordinary computers, ultra-multi-layer boards have gradually fallen out of use. Because the layers in the PCB are tightly combined, it is generally not easy to see the actual number, but if you look closely at the motherboard, you can still see it.

According to the soft and hard classification, it is divided into ordinary circuit boards and flexible circuit boards. PCB is a working platform for circuit components in electronic equipment. It provides electrical connections between circuit components. Its performance is directly related to the quality of electronic equipment. With the rapid development of microelectronics technology and the improvement of circuit integration, the density of components on the PCB board is getting higher and higher, and the system working speed is getting faster and faster, which makes the PCB electromagnetic compatibility design more and more important, becoming a circuit The key to the stable and normal operation of the system.

2. Common EMI in PCB

There are two ways to solve the electromagnetic compatibility problem in PCB design: active reduction and passive compensation. For this reason, the interference source and propagation path of electromagnetic interference must be analyzed. Electromagnetic interference that usually exists in PCB design includes: conducted interference, crosstalk interference and radiation interference.
2.1 Conducted interference
Conducted interference mainly affects other circuits through wire coupling and common mode impedance coupling. For example, if noise enters a system through a power supply circuit, all circuits that use the power supply will be affected. Figure 1 shows that the noise is coupled through the common mode impedance. Circuit 1 and circuit 2 use a common wire to obtain the power supply voltage and ground return. If the voltage of circuit 1 suddenly needs to rise, then the voltage of circuit 2 must be due to the common power supply and The impedance between the two circuits decreases.
2.2 Crosstalk Interference
Crosstalk is when one signal line interferes with another adjacent signal path. It usually occurs on adjacent circuits and conductors, and is characterized by the mutual capacitance and mutual inductance of circuits and conductors. For example, a strip line on the PCB carries a low-level signal, and when the length of the parallel wiring exceeds 10cm, crosstalk interference will occur. Since the crosstalk can be caused by the mutual capacitance of the electric field and the mutual inductance of the magnetic field, when considering the crosstalk problem on the PCB stripline, the main problem is to determine which of the electric field and the magnetic field coupling is the main factor.
2.3 Radiated Interference
Radiated interference is the interference introduced by the radiation of space electromagnetic waves. The radiation interference in the PCB is mainly the common mode current radiation interference between cables and internal traces. The problem of field-to-line coupling occurs when electromagnetic waves are radiated onto a transmission line. Distributed small voltage sources along the line can be decomposed into common-mode and differential-mode components. Common mode current refers to the current on the two wires with a small amplitude difference but the same phase, while the differential mode current refers to the current on the two wires with the same amplitude and opposite phase.

3. Electromagnetic compatibility design of PCB
With the increasing density of electronic components and circuits on the PCB board, in order to improve the reliability and stability of the system, corresponding measures must be taken to make the design of the PCB board meet the requirements of electromagnetic compatibility and improve the anti-interference performance of the system.

3.1 Selection of PCB board
In PCB board design, crosstalk occurs between signals on adjacent transmission lines due to the mutual coupling of electromagnetic fields. Therefore, when designing PCB electromagnetic compatibility, first consider the size of the PCB. The size of the PCB is too large, the printed line is too long, and the impedance It will inevitably increase, the anti-noise ability will decrease, and the cost will also increase; if the PCB size is too small, crosstalk is prone to occur between adjacent transmission lines, and the heat dissipation performance is not good. The number of layers of the PCB board is determined according to the comprehensive factors of the power supply, the type of ground, the density of the signal lines, the signal frequency, the number of signals required for special wiring, the surrounding elements, and the cost and price. To meet the strict requirements of EMC and consider the manufacturing cost, appropriately increasing the ground plane is one of the methods of PCB EMC design. For the power supply layer, the internal electrical layer division can generally meet the needs of multiple power supplies, but if multiple power supplies are required and are interleaved, two or more layers of power supply planes must be considered. For the signal layer, in addition to considering the routing density of signal lines, from the EMC point of view, it is also necessary to consider the shielding or isolation of key signals, so as to determine whether to increase the number of corresponding layers.

3.2 PCB layout design
The layout of the PCB should generally follow the following principles:
(1) Try to shorten the connection between high-frequency components and reduce their distribution parameters and mutual electromagnetic interference. Components that are easily disturbed should not be too close together, and the input and output should be kept as far away as possible.
(2) There may be a high voltage between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge.
(3) The device with large heat generation should leave space for the heat sink, or even install it on the bottom plate of the whole machine to facilitate heat dissipation. Thermal elements should be kept away from heating elements.
(4) Arrange the position of each functional unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal can keep the same direction as possible.
(5) Take the component of each functional module as the center, and arrange around it to minimize and shorten the lead and connection length between components.
(6) Comprehensively consider the distribution parameters among the components. Arrange the components in parallel as much as possible, which is not only conducive to enhancing the anti-interference ability, but also has a beautiful appearance and is easy to mass produce.

3.3 Layout design of components
Compared with discrete components, integrated circuit components have the advantages of good sealing, fewer solder joints and low failure rate, and should be preferred. At the same time, choosing a device with a slower signal slope can reduce the high-frequency components generated by the signal. The full use of SMD components can shorten the length of the connection, reduce the impedance, and improve the electromagnetic compatibility. When arranging components, firstly group them in a certain way, put them together in the same group, and arrange incompatible components separately to ensure that the components do not interfere with each other in space. In addition, heavy components should be fixed with brackets.

3.4 Wiring design of PCB board
The general principle of PCB layout design is to first lay the clock and sensitive signal lines, and then lay out the high-speed signal lines, and the unimportant signal lines. When wiring, under the premise of the general principle, the following details need to be considered:
(1) In the multi-layer board wiring, a "well"-shaped network structure is used between adjacent layers;
(2) Reduce wire bending and avoid sudden change in wire width. In order to prevent characteristic impedance changes, the corners of the signal lines should be designed into arcs or connected with 45-degree broken lines;
(3) The distance between the outer conductors or components of the PCB board is not less than 2mm from the edge of the printed board, which not only prevents the characteristic impedance from changing, but also facilitates PCB clamping;
(4) For devices that must lay a large area of copper foil, they should be grid-shaped and connected to the ground through vias;
(5) Short and thin wires can effectively suppress interference, but too small wire width will increase wire resistance. The width of the wire depends on the current passing through the wire. Generally speaking, for copper with a thickness of 0.05mm and a width of 1mm The allowable current load of the foil is 1A. For low-power digital integrated circuits, the line width of 0.2-0.5mm can be selected. In the same PCB, the width of ground and power lines should be larger than that of signal lines.

3.5 Power line design of PCB board
(1) According to the size of the PCB current of the printed board, try to thicken the width of the power line and the ground line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. .
(2) Try to use SMD components, shorten the length of the pins, reduce the area of the decoupling capacitor power supply loop, and reduce the influence of the distributed inductance of the components.
(3) Add a power filter to the front end of the power transformer to suppress common mode noise and differential mode noise, and isolate the interference of external and internal impulse noise.
(4) Filter capacitors and decoupling capacitors should be added to the power supply lines of the printed circuit board. Add a large-capacity electrolytic capacitor to the power inlet of the board for low-frequency filtering, and then connect a small-capacity ceramic capacitor in parallel for high-frequency filtering.
(5) Do not overlap the analog power supply and the digital power supply to avoid coupling capacitance and mutual interference.

3.6 Ground wire design of PCB board
(1) In order to reduce the ground loop interference, it is necessary to find a way to eliminate the formation of the loop current. Specifically, an isolation transformer, optocoupler isolation, etc. can be used to cut off the formation of the ground loop current, or a balance circuit can be used to eliminate the loop current.
(2) In order to eliminate the coupling of the common impedance, the impedance of the common ground wire should be reduced, the wire should be thickened or copper should be laid on the ground wire; on the other hand, mutual interference can be avoided by appropriate grounding methods, such as single-point grounding in parallel, series Hybrid single-point grounding completely eliminates common impedance.
(3) In order to eliminate the interference of digital devices to analog devices, digital ground and analog ground should be separated, and analog ground and digital ground should be set separately. High-frequency circuits are mostly grounded in series. The ground wire should be short and thick, and the surrounding of high-frequency components should be shielded with a grid-like large area of copper.

3.7 Layout of crystal oscillator circuit on PCB board
The frequency of the crystal oscillator circuit is high, which makes it an important source of interference in the system. Regarding the layout of the crystal oscillator circuit, there are the following considerations:
(1) The crystal oscillator circuit is as close as possible to the integrated block, and all the printed lines connecting the input/output terminals of the crystal oscillator are as short as possible to reduce the influence of noise interference and distributed capacitance on the crystal oscillator.
(2) The ground wire of the crystal oscillator capacitor should be connected to the device using the wide and short printed wire as possible; the digital ground pins close to the crystal oscillator should minimize the number of vias.
(3) The crystal case is grounded.

3.8 Electrostatic protection design of PCB board
Electrostatic discharge is characterized by high potential, low charge, high current and short time. The electrostatic protection of PCB design can be considered from the following aspects:
(1) Try to choose components with high anti-static level, and sensitive components with poor anti-static ability should be kept away from electrostatic discharge sources. Tests have shown that the breakdown distance of each kilovolt of electrostatic voltage is about 1mm, so if the components are kept at a distance of 16mm from the electrostatic discharge source, the electrostatic voltage of about 16KV can be resisted;
(2) Ensure that the signal return has a short path, and selectively add filter capacitors and decoupling capacitors to improve the electrostatic discharge immunity of the signal line;
(3) Use protection devices such as voltage transient suppression diodes to protect the circuit;
(4) Relevant personnel must wear an electrostatic wristband when touching the PCB to avoid electrostatic accumulation damage caused by the movement of human charge.

4. Conclusion
PCB electromagnetic compatibility design is to reduce external electromagnetic radiation and improve the ability to resist electromagnetic interference. Reasonable layout and wiring are the key to the design. The various methods and techniques described in this article are beneficial to improve the EMC characteristics of high-speed PCBs. Of course, these are only part of the EMC design. Reflected noise, radiated emission noise, and other process technology issues are usually considered. Interference. In the actual design, reasonable anti-electromagnetic interference measures should be adopted according to the design target requirements and design conditions, and comprehensive considerations should be made to design the printed circuit boards with good EMC performance.