PCB Tech - What are PCB principles and anti-interference measures

Design principles and anti-interference measures of printed circuit boards Printed circuit boards (PCBS) are the supporting parts of circuit components and devices in electronic products. It provides electrical connections between circuit components and equipment. With the rapid development of power technology, the density of PGB continues to increase. PCB design has a great influence on the anti-interference ability. Therefore, when performing PCB design.

The general principles of PCB design must be followed, and the requirements of anti-interference design must be met.

General principles of PCB design In order to obtain the best performance of electronic circuits, the structure of the components and the layout of the wires are very important. In order to design a PCB with good quality and low cost.

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 circuit will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase. If the PCB size is too small, the heat dissipation will not be good, and adjacent circuits will be easily interfered. After determining the PCB size. Then determine the location of special components.

Finally, according to the functional unit of the circuit, arrange all the components of the circuit.

The following principles should be followed when determining the location of special components: (1) Shorten the connections between high-frequency components as much as possible, and minimize their distribution parameters and mutual electromagnetic interference.

Components that are susceptible to interference should not be too close to each other, and input and output components should be as far away as possible. (2) There may be a high potential difference between some components or conductors, and the distance between them should be increased to avoid accidental short circuits.

High-voltage components should be arranged as far as possible in places that are not easy to reach during manual debugging. (3) Parts weighing more than 15g should be fixed with brackets and then welded. The heat of those large and heavy components should not be installed on the printed circuit board, but should be installed on the bottom of the machine, and heat dissipation issues should be considered.

The heating element should be far away from the heating element. (4) The arrangement of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should take into account the structural requirements of the whole machine.

In the case of adjustment inside the machine, it should be placed on the printed board for easy adjustment of the position. In the case of adjustment outside the machine, its position should correspond to the position of the adjustment knob on the chassis panel.

(5) The position occupied by the positioning hole of the print wrench and the fixing bracket should be put aside. According to the functional unit of the circuit.

When arranging all the components of the circuit, the following principles must be met:

(1) Arrange the position of each functional circuit unit according to the flow of the circuit, make the layout easy for signal flow, and keep the signal in the same direction as possible. (2) Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB.

Minimize and shorten the leads and connections between components. (3) Circuits operating at high frequencies should consider the allocation parameters between components. The general circuit should connect components in parallel as much as possible. In this way, it is not only beautiful. Easy to install and solder.

Easy to mass produce. (4) The component is located on the edge of the circuit board, and it is generally not less than 2mm from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 to 4:3. When the surface size of the circuit board is larger than 200x150mm. The mechanical strength of the circuit board should be considered.

2. Wiring

The wiring principle is as follows: (1) The wires used at the input and output terminals should try to avoid adjacent parallel connections.

It is best to add wires between the ground wires to avoid feedback coupling. (2) The minimum width of the printed line mainly depends on the bond strength between the line and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1~15mm. With 2A current, the temperature will not be higher than 3°C, so. The line width is 1.5mm to meet the requirements. For integrated circuits, especially digital circuits, a line width of 0.02~0.3mm is usually selected. Of course, as long as you allow, still use as wide a line as possible. Especially the power line and the ground line. In the worst case, the minimum spacing of wires is mainly determined by the insulation resistance and breakdown voltage between the wires.

For integrated circuits, especially digital circuits, as long as the process permits, the spacing can be as small as 5~8mm. (3) The printed lines at the corners are usually arc-shaped, and the right angle or angle in the high-frequency circuit will affect the electrical performance. In addition, try to avoid using large area copper foil, otherwise. When heated for a long time, the copper foil is likely to swell and fall off. When a large area of copper foil must be used, it is best to use a grating shape.

This facilitates the removal of volatile gases from the heat of the adhesive between the copper foil and the substrate.

3. The center hole of the soldering board pad is slightly larger than the diameter of the device lead. If the weld plate is too large, it is easy to form a false weld. The outer diameter of the pad d is usually not less than (d + 1.2) mm, where D is the guide hole.

For high-density digital circuits, the minimum diameter of the pad is ideal (d + 1.0) mm.

PCB and circuit anti-interference measures

The anti-interference design of the printed circuit board is closely related to the specific circuit, and here is just an explanation of several commonly used measures in the PCB anti-interference design.

1. The power line design should increase the width of the power line as much as possible to reduce the loop resistance according to the current of the printed circuit board.

At the same time, the direction of the power line, the ground line and the data transmission are the same, which helps to enhance the anti-noise ability.

2. Ground wire design

The principle of ground line design is: (1) The digital ground is separated from the analog place. If there are logic circuits and linear circuits on the circuit board, they should be kept as separate as possible. Low-frequency circuits should try to use single points and be grounded. If the actual wiring is difficult, part of them can be connected and then grounded.

The high-frequency circuit should be grounded in series with multiple points, the ground wire should be short and rented, and the high-frequency components should be filmed with a large area of grating as much as possible. (2) The ground wire should be as thick as possible. If the ground wire uses a very shaped wire, the ground potential changes with the current, resulting in a decrease in 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 grounding wire should be 2~3mm or more. (3) The grounding wire forms a closed loop.

Most of the printed circuit boards and their grounding circuits composed of only digital circuits can improve their anti-noise ability by forming a ring.

3. Decoupling capacitor configuration

The general practice of PCB design is to configure appropriate decoupling capacitors on each key part of the printed circuit board.

The general configuration principles of decoupling capacitors are: (1) An electrolytic capacitor spanning 10~100uf at the power input end. If possible, it is better to exceed 100uF.

(2) In principle, a 0.01pF ceramic capacitor should be arranged for each IC chip. If the gap of the printed circuit board is insufficient, a 1~10pf capacitor can be arranged for each 4~8 chips.

(3) For devices with weak anti-noise capability and large power changes when shutting down, such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power line and the chip's ground line.

(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.

In addition, pay attention to the following two points: (1) When there are contactors, relays, buttons and other components on the printed circuit board. When operating them, a large spark discharge is generated, and the discharge current must be absorbed by using the RC circuit shown in the figure. Generally, R needs 1~2k, and c needs 2.2~47uf. (2) CMOS has high input impedance and is acceptable to be able to sense, so it is grounded or inserted at the end of use.