The printed circuit board (PCB) is the support for circuit components and devices in electronic products. It provides the electrical connection between the circuit components and devices. With the rapid development of electrical technology, the density of the PGB is getting higher and higher, the PCB design has a great impact on the anti-interference ability. Therefore, in the PCB design. Must comply with the general principles of PCB design, and should be consistent with the requirements of anti-interference design.
The general principles of PCB design
To get the best performance of the electronic circuit, the layout of components and the layout of the wire is very important. In order to design a good quality, low cost PCB. should follow the following general principles.
First, consider the size of the PCB size. PCB size is too large when the printed lines are long, increased impedance, noise immunity decreases, the cost also increases; too small, it is not good heat dissipation, and neighboring lines are vulnerable to interference. After determining the size of the PCB. Then determine the location of special components. Finally, according to the functional unit of the circuit, the layout of all components of the circuit.
In determining the location of special components to comply with the following principles.
1. shorten the connection between high-frequency components as much as possible, trying to reduce their distribution parameters and mutual electromagnetic interference. Components susceptible to interference can not be too close to each other, input and output components should be as far away as possible.
2. Some components or wires may have a high potential difference between them, should increase the distance between them, so as not to discharge lead to an accidental short circuit. Components with high voltage should be laid out as far as possible in places that are not easily accessible by hand when debugging.
3. Components weighing more than 15g should be fixed with brackets and then welded. Those large and heavy, heat-generating components, should not be installed on the printed board, but should be installed in the chassis of the whole chassis, and should consider the problem of heat dissipation. Thermal components should be far from the heat-generating components.
4. For potentiometers, adjustable inductor coils, variable capacitors, micro switches and other adjustable components layout should consider the structural requirements of the entire machine. If the machine is adjusted, it should be placed on the printed board to facilitate the adjustment of the place; if the machine is adjusted outside, its location should be adapted to the location of the adjustment knob on the chassis panel.
5. should leave the printing trigger positioning holes and fixed bracket occupied by the location.
According to the functional unit of the circuit. The layout of all components of the circuit should conform to the following principles.
1. Arrange the location of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal in the same direction as far as possible.
2. Take the core component of each functional circuit as the center, and layout around it. 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, to consider the distribution parameters between components. General circuit should be as far as possible so that the components are arranged in parallel. In this way, not only beautiful. And easy to assemble and solder. Easy to mass production.
4. Components located on the edge of the board, from the edge of the board is generally not less than 2mm. the best shape of the board is rectangular. Length to width ratio of 3:2 to 4:3. board surface size greater than 200x150mm. Should consider the mechanical strength of the board.
The principles of wiring are as follows.
1. Input and output terminals with the wire should try to avoid adjacent parallel. It is best to add interline ground to avoid feedback coupling.
2. The minimum width of the printed registers is mainly determined by the adhesion strength between the wire and the insulated base trigger and the current value flowing through them. When the thickness of copper foil is 0.05mm, the width of 1 ~ 15mm. The temperature will not be higher than 3℃ for a current of 2A, therefore. The conductor width of 1.5mm can meet the requirement. For integrated circuits, especially digital circuits, usually choose 0.02 ~ 0.3mm wire width. Of course, as long as possible, or as long as possible with a wide line. Especially the power and ground lines. The minimum spacing of the wire is mainly determined by the worst-case insulation resistance and breakdown voltage between the lines. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5 to 8 mm.
3. Printed wire corners are generally taken rounded, while right angles or pinch angles in the high-frequency circuit will affect the electrical performance. In addition, try to avoid using a large area of copper foil, otherwise. When heated for a long time, the copper foil is prone to expansion and shedding phenomenon. Must use a large area of copper foil, it is best to use the grid shape. This will help to exclude the volatile gas generated by the heat of the adhesive between the copper foil and the substrate.
The center hole of the pad should be slightly larger than the device lead diameter. The pad is too large and easy to form a false solder. Solder pad outside diameter D is generally not less than (d + 1.2) mm, where d is the lead aperture. For high-density digital circuits, the minimum diameter of the pad can be (d + 1.0) mm.
