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General principles to be observed in PCB design.


       Circuit board (PCB) is the support component of circuit components and devices in electronic products. It provides electrical connections between circuit elements and devices. With the rapid development of technology, the density of PGB is higher and higher. The design of PCB has a great influence on its ability to resist interference. Therefore, in the design of PCB. The general principles of PCB design must be adhered to and the requirements of anti-interference design should be met.

General principles of PCB design:

It is important for electronic circuits to get the best performance, the cloth of the components and the layout of the wires. In order to design good quality, low cost PCB. The following principles should be followed:

1. The layout

(1) reduce the connection between high frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between them. Vulnerable components cannot get too close to each other, and input and output components should be kept away from each other.

(2) there may be a high potential difference between some components or wires, and the distance between them should be increased so as to avoid an unexpected short circuit. The components with high voltage should be arranged as far as possible when the hand is not easy to reach.

(3) components weighing more than 15g shall be fixed with brackets and then welded. Those large, heavy and calorific components should not be installed on the printed board, but should be installed on the chassis of the machine, and the heat dissipation should be considered. The thermal element shall be removed from the heating element.

(4) the layout of adjustable elements such as potentiometers, adjustable inductors, variable capacitors, and micro switches should be considered as the structural requirements of the whole machine. If the adjustment in the machine, should be placed in the printing plate to facilitate the adjustment of the place; In case of external adjustment, the position should be adjusted to the position of the adjusting knob on the cabinet panel.

(5) the positioning holes and the position of fixed brackets shall be set aside.

According to the functional unit of the circuit. For the layout of all components of the circuit, the following principles should be met:

(1) arrange the location of each functional circuit unit according to the process of the circuit, so as to facilitate the distribution of signals and keep the signals as consistent as possible.

(2) center around the core components of each functional circuit and arrange the layout around it. The components shall be arranged in a uniform, neat and compact manner on the PCB. Minimizing and shortening leads and connections between components.

(3) the circuit that works at high frequency should consider the distribution parameters between components. General circuits should be arranged in parallel as far as possible. In this way, it is not only beautiful. And it's easy to weld. Easy to mass production.

(4) the component on the edge of the circuit board is not less than 2mm from the edge of the circuit board. The best shape of the circuit board is the rectangle. The ratio of length to width is 3:2 to 4:3. The circuit board size is greater than 200x150mm. The mechanical strength of the circuit board shall be considered.

2. The wiring

(1) the wires used in the input and output terminals should be avoided in parallel. It is better to add line between lines to avoid feedback coupling.

(2) the minimum width of the printed conductor is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them.

(3) the curve of the printed wire is generally rounded, while the right Angle or Angle will affect the electrical performance in the high frequency circuit. In addition, avoid large area copper foil, otherwise. When heated for a long time, copper foil is prone to expand and fall off. A large area of copper foil must be used for the best use of a raster. This facilitates the removal of volatile gases produced by the heat between copper foil and substrate.

3. The bonding pad

1. Power cord design.

According to the printed circuit board current size, as far as possible to rent the power line width, reduce the loop resistance. At the same time, the direction of the power line, ground wire and the direction of data transmission are consistent, which will help to enhance the anti-noise ability.

2. Location design

The ground line design principle is;

(1) the digital ground is separated from the simulation. If there are logic circuits and linear circuits on the circuit board, they should be separated as far as possible. The low frequency circuit should be connected with single point in parallel, and the actual wiring can be connected in series and connected in parallel. The high frequency circuit should adopt multi-point series grounding, the ground wire should be short and rent, and the high frequency element should be surrounded by a grid large area.

(2) the ground wire should be as thick as possible. If the ground wire is used in a very tight line, the ground potential changes with the current, and the anti-noise performance is reduced. The grounding line should therefore be thickened to allow it to pass three times the allowable current on the printed board. If possible, the grounding line should be above 2~3mm.

(3) the ground wire forms a closed loop. Only the printed plate made up of digital circuit, the ground circuit of the ground circuit can improve the anti-noise ability.

3. Decoupling capacitor configuration.

One of the conventional methods of PCB design is to configure the appropriate fallback capacitance in each key part of the printed board. decoupled

General configuration principles are:

(1) electrolytic capacitor with 10~100uf on the input end of the power supply. If possible, connect 100uF or above.

(2) in principle, each IC chip should be equipped with a chip capacitor of 0.01pF. If there is not enough space in the printed plate, a 1-10pf capacitor can be arranged for every 4~8 chips.

(3) for devices with weak anti-noise capability and power supply changes when the power is switched off, such as RAM and ROM storage devices, it should be directly connected to the back coupling capacitance between the power line and ground wire of the chip.

(4) capacitance leads cannot be too long, especially for high frequency bypass capacitors. In addition, the following two points should be noted:

(1) there are contactors, relays, buttons and other components in the printed board. When operating them, large spark discharge will occur. The RC circuit shown in the attached diagram must be used to absorb the discharge current. In general, R is 1~2K, and C is 2.2~47UF. (2CMOS input impedance is very high and sensitive, so it is not necessary to earthing or connect to a positive power when used.


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