Anti-interference design rules in PCB design process

PCB is the support for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the anti-interference ability. Therefore, when designing PCB, the general principles of PCB design must be followed and the requirements of anti-interference design must be met.

General principles of PCB design To achieve the best performance of electronic circuits, the layout of components and wires is very important. In order to design high-quality and low-cost PCBs, the following general principles should be followed:

1.Layout First, consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance increases,

the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of special components. Finally, layout all components of the circuit according to the functional units of the circuit. When determining the location of special components, follow the following principles:

(1) Shorten the connection between high-frequency components as much as possible,

and try to reduce their distributed parameters and mutual electromagnetic interference. Components susceptible to interference should not be placed too close to each other, and input and output components should be kept as far apart as possible.

(2) Some components or wires may have a high potential difference,

so the distance between them should be increased to prevent discharge from causing an accidental short circuit. Components with high voltage should be placed as far as possible in places that are not easily touched by hands during debugging.

(3) Components weighing more than 15g should be fixed with a bracket and then soldered.

Components that are large, heavy, and generate a lot of heat should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and heat dissipation should be considered. Thermistors should be kept away from heating components.

(4) The layout of adjustable components such as potentiometers, adjustable inductors,

variable capacitors, and micro switches should take into account the structural requirements of the whole machine. If they are adjusted inside the machine, they should be placed in a convenient place on the printed circuit board for adjustment; if they are adjusted outside the machine, their position should be consistent with the position of the adjustment knob on the chassis panel.

(5) Space should be reserved for the positioning holes of the printed circuit board and the space occupied by the fixing bracket.

According to the functional unit of the circuit. When laying out all the components of the circuit, the following principles should be followed:

1) Arrange the positions of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal flow and the signal keeps the same direction as much as possible.

2) Take the core component of each functional circuit as the center and lay it out around it. The components should be arranged evenly, neatly and compactly on the PCB. Try to reduce and shorten the leads and connections between the components.

3) For circuits working at high frequencies, the distribution parameters between the components should be considered. Generally, the components should be arranged in parallel as much as possible in the circuit. This is not only beautiful, but also easy to install and solder. It is easy to mass produce.

4) Components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is a rectangle. The aspect ratio is 3:2 to 4:3. When the surface size of the circuit board is greater than 200x150mm, the mechanical strength of the circuit board should be considered.

2.Wiring The wiring principles are as follows;

(1) The wires used for the input and output terminals should be avoided to be adjacent and parallel as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling.
(2) The minimum width of the printed conductor is mainly determined by the adhesion strength between the conductor and the insulating substrate and the current flowing through them. When the copper foil thickness is 0.05mm and the width is 1~15mm. When a current of 2A passes through, the temperature will not be higher than 3℃, so a conductor width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a conductor width of 0.02~0.3mm is usually selected. Of course, as long as it is allowed, wide wires should be used as much as possible. Especially for power lines and ground lines. The minimum spacing of the conductors is mainly determined by the insulation resistance and breakdown voltage between the wires in the worst case. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5~8mm.
(3) The bends of printed conductors are generally arc-shaped, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large-area copper foil, otherwise, when heated for a long time, the copper foil is prone to expansion and shedding. When large-area copper foil must be used, it is best to use a grid shape. This helps to remove the volatile gas generated by the heat of the adhesive between the copper foil and the substrate.

3.The center hole of the pad should be slightly larger than the diameter of the device lead.

A pad that is too large is prone to cold soldering. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm. PCB and circuit anti-interference measures The anti-interference design of printed circuit boards is closely related to the specific circuit. Here we only explain several common measures for PCB anti-interference design.

(1) Power line design According to the current size of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.
(2) Ground design The principles of ground line design are:

1) Separate digital ground from analog ground.

If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of low-frequency circuits should be connected in parallel at a single point as much as possible. If the actual wiring is difficult, some circuits can be connected in series and then connected in parallel. High-frequency circuits should be connected in series at multiple points. The ground wire should be short and loose, and a large grid-shaped ground foil should be used around high-frequency components as much as possible.

2) The ground wire should be as thick as possible.

If the ground wire is made of very thin wire, the ground potential will change with the change of current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed circuit board. If possible, the ground wire should be more than 2~3mm.

3) The ground wire forms a closed loop.

For printed circuits composed only of digital circuits, the ground circuit is mostly arranged in a loop to improve the anti-noise ability.

(3) Decoupling capacitor configuration One of the common practices in PCB design is to configure appropriate decoupling capacitors at various key parts of the printed circuit board. The general configuration principle of decoupling capacitors is:

1) Connect a 10~100uf electrolytic capacitor across the power input terminal.

If possible, it is better to connect 100uF or more.
2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the printed circuit board does not have enough space, a 1~10pF capacitor can be arranged for every 4~8 chips.

3) For devices with weak noise resistance and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

4) The capacitor lead should not be too long, especially the high-frequency bypass capacitor should not have leads. In addition, the following two points should be noted:

When there are contactors, relays, buttons and other components on the printed circuit board. When operating them, large spark discharges will be generated, and the RC circuit shown in the attached figure must be used to absorb the discharge current. Generally, R is 1~2K and C is 2.2~47UF