Using various methods and techniques to improve the electromagnetic compatibility of PCB design

PCB is the abbreviation of printed circuit board (Printed Circuit Board). Usually, a conductive pattern made of printed circuits, printed components or a combination of the two on an insulating material according to a predetermined design is called a printed circuit. The conductive pattern that provides electrical connections between components on an insulating substrate is called a printed circuit. In this way, the finished board of a printed circuit or printed circuit is called a printed circuit board, also known as a printed board or printed circuit board. PCB is indispensable for almost all electronic devices we can see, from electronic watches, calculators, general computers to computers, communication electronic equipment, aviation, aerospace, and military weapon systems. As long as there are electronic components such as integrated circuits, PCB is used for electrical interconnection between them. Its performance is directly related to the quality of electronic equipment. With the rapid development of electronic technology, electronic products are becoming more and more high-speed, high-sensitivity, and high-density. This trend has led to the seriousness of electromagnetic compatibility (EMC) and electromagnetic interference problems in PCB circuit board design . Electromagnetic compatibility design has become a technical problem that needs to be solved urgently in PCB design.

1 Electromagnetic compatibility

Electromagnetic compatibility (EMC) is an emerging comprehensive discipline that mainly studies electromagnetic interference and anti-interference issues. Electromagnetic compatibility means that electronic equipment or systems do not reduce performance indicators due to electromagnetic interference under the specified electromagnetic environment level, and the electromagnetic radiation they generate themselves is not greater than the specified limit level, does not affect the normal operation of other systems, and achieves the purpose of non-interference and reliable operation between equipment and equipment, and between systems. Electromagnetic interference (EMI) is caused by the electromagnetic interference source transferring energy to sensitive systems through coupling paths. It includes three basic forms: conduction by wires and common ground wires, radiation through space, or near-field coupling. Practice has proved that even if the circuit schematic is designed correctly, improper printed circuit board design will have an adverse effect on the reliability of electronic equipment, so ensuring the electromagnetic compatibility of printed circuit boards is the key to the entire system design.

1.1 Electromagnetic Interference (EMI)

When an EMI problem occurs, three elements are needed to describe it: interference source, propagation path, and receiver.

Therefore, if we want to reduce electromagnetic interference, we must think of ways to deal with these three elements. Below we mainly discuss the wiring technology of printed circuit boards.

2. Printed circuit board wiring technology

Good printed circuit board (PCB) layout is a very important factor in electromagnetic compatibility.

2.1 Basic characteristics of PCB

The structure of a PCB is a series of lamination, routing and prepreg treatments on a vertical stack. In a multi-layer PCB, designers will lay out the signal lines on the outermost layer for the convenience of debugging.

The wiring on the PCB has impedance, capacitance and inductance characteristics.

Impedance: The impedance of a trace is determined by the weight of the copper and the cross-sectional area. For example, 1 ounce of copper has an impedance of 0.49 mΩ/unit area. Capacitance: The capacitance of a trace is determined by the insulator (EoEr), the range of current flow (A), and the spacing between traces (h). This is expressed as C = EoErA/h, where Eo is the dielectric constant of free space (8.854 pF/m) and Er is the relative dielectric constant of the PCB substrate (4.7 in FR4 laminate).

Inductance: The inductance of a trace is evenly distributed throughout the trace and is approximately 1 nH/m.

For 1 ounce copper wire, in the case of 0.25 mm (10 mil) thick FR4 laminate, a 0.5 mm (20 mil) wide, 20 mm (800 mil) long wire located above the ground plane can produce an impedance of 9.8 m∧, an inductance of 20 nH, and a coupling capacitance of 1.66 pF to the ground. Compared with the parasitic effects of components, these values ​​are negligible, but the sum of all wiring may exceed the parasitic effects. Therefore, the designer must take this into account. General guidelines for PCB wiring:

(1) Increase the spacing between traces to reduce capacitive coupling crosstalk;

(2) Lay out power and ground wires in parallel to optimize PCB capacitance;

(3) Place sensitive high-frequency wires away from high-noise power lines;

(4) Widen the power line and ground line to reduce the impedance of the power line and ground line. 2.2 Segmentation

Segmentation refers to the use of physical separation to reduce coupling between different types of lines, especially through power and ground lines.

An example of using segmentation technology to separate four different types of circuits. On the ground plane, non-metallic trenches are used to isolate the four ground planes. L and C act as filters for each part of the board to reduce coupling between power planes of different circuits. High-speed digital circuits require placement at the power input due to their higher instantaneous power requirements. Interface circuits may require electrostatic discharge (ESD) and transient suppression devices or circuits. For L and C, it is better to use L and C of different values ​​rather than using a large L and C, because it can provide different filtering characteristics for different circuits.

2.3 Decoupling between local power supply and IC

Local decoupling can reduce noise propagation along the power mains. The large-capacity bypass capacitor connected between the power input port and the PCB acts as a low-frequency pulsation filter and as a potential reservoir to meet sudden power requirements. In addition, there should be decoupling capacitors between the power supply and ground of each IC. These decoupling capacitors should be as close to the pins as possible. This will help filter out the switching noise of the IC.

2.4 Grounding Technology

Grounding technology is applied to both multi-layer PCBs and single-layer PCBs. The goal of grounding technology is to minimize ground impedance, thereby reducing the potential of the ground loop from the circuit back to the power supply.

(1) Ground wire of single-layer PCB

In a single-layer (single-sided) PCB, the width of the ground line should be as wide as possible and at least 1.5 mm (60 mil). Since star wiring cannot be achieved on a single-layer PCB, changes in the width of the jumper and ground line should be kept to a minimum, otherwise it will cause changes in line impedance and inductance.

(2) Ground wire of double-layer PCB

In double-layer (double-sided) PCB , ground grid/dot matrix wiring is preferred for digital circuits. This wiring method can reduce ground impedance, ground loops and signal loops. Like in single-layer PCB, the width of ground and power lines should be at least 1.5mm. Another layout is to put the ground layer on one side and the signal and power lines on the other side. In this arrangement, the ground loop and impedance will be further reduced, and the decoupling capacitor can be placed as close as possible to the IC power line and the ground layer.

(3) Protection ring

A guard ring is a grounding technique that isolates noisy environments (such as RF currents) outside the ring because no current flows through the guard ring during normal operation.

(4) PCB capacitors

On multi-layer boards, PCB capacitance is generated by the thin insulating layer separating the power plane and the ground. On single-layer boards, the parallel routing of power and ground lines will also lead to this capacitance effect. One advantage of PCB capacitance is that it has a very high frequency response and low series inductance evenly distributed over the entire surface or line. It is equivalent to a decoupling capacitor evenly distributed over the entire board. No single discrete component has this feature.

(5) High-speed circuits and low-speed circuits

When laying out high-speed circuits, they should be placed closer to the ground plane, while low-speed circuits should be placed closer to the power plane.

(6) Ground copper filling

In some analog circuits, unused board areas are covered by a large ground plane to provide shielding and increase decoupling. However, if this copper area is left floating (i.e. it is not connected to ground), it may act as an antenna and cause EMC problems.

(7) Ground plane and power plane in multi-layer PCB

In a multi-layer PCB , it is recommended to place the power plane and the ground plane as close as possible in adjacent layers to generate a large PCB capacitance across the board. The fastest critical signal should be placed close to the ground plane, and non-critical signals should be placed close to the power plane.

(8) Power requirements

When a circuit requires more than one power supply, grounding is used to separate each power supply. However, multi-point grounding is not possible in a single-layer PCB. One solution is to separate the power and ground wires from one power supply from the other power and ground wires. This also helps to avoid noise coupling between power supplies.

3 Conclusion

The various methods and techniques introduced in this article are helpful to improve the EMC characteristics of PCB . Of course, these are only part of EMC design. Usually, we also need to consider the interference caused by reflection noise, radiation emission noise, and other process technology problems. In actual design, we should adopt reasonable anti-electromagnetic interference measures according to the design target requirements and design conditions to design a PCB with good EMC performance .