PCB News - Talking about the general principles of PCB design

Printed circuit board (PCB) is the support of circuit components and devices in electronic products. It provides circuit components and devices
Electrical connection between. With the rapid development of electrical technology, the density of PCB is getting higher and higher. The quality of PCB design has a great influence on the anti-interference ability. Therefore, in the PCB design. The general principles of PCB design must be followed, and the requirements of anti-interference design must be met. To get the best performance of the electronic circuit, the layout of the components and the layout of the wires are very important. In order to design PCB with good quality and low cost. The following general principles should be followed:

1. Layout
First, consider the PCB size. When the PCB size is too large, the printed lines 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 lines will be easily disturbed. After determining the PCB size. Then determine the location of special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.

2. Shorten the wiring between high-frequency components as much as possible, try to reduce 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 kept as far away as possible.

3. There may be a high potential difference between some components or wires, so the distance between them should be increased to avoid discharging accidents.
External short circuit. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands during debugging.

4. Components weighing more than 15g should be fixed with brackets and then welded. Those that are big, heavy, and generate a lot of heat
The components should not be mounted on the printed circuit board, but should be mounted on the chassis bottom plate of the complete machine, and the heat dissipation problem should be considered. Thermal element
Keep away from heating elements.

5. For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc., the configuration of the whole machine should be considered.
Institutional requirements. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should be the same as
The position of the adjusting knob on the panel of the chassis is suitable.

6. The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved.
According to the functional unit of the circuit. When laying out all the components of the circuit, the following principles must be met:
1. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, 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. Components should be uniform and neat
Arranged neatly and compactly on the PCB board. Minimize and shorten the leads and connections between components.
3. For circuits operating at high frequencies, the distributed parameters between components should be considered. The general circuit should be used as much as possible
The components are arranged in parallel. In this way, not only beautiful. And easy to install and weld. Easy to mass produce.
4. The components located on 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
It is a rectangle. The aspect ratio is 3:2 to 4:3. When the size of the circuit board is larger than 200x150mm. The mechanical strength of the circuit board should be considered.

7. wiring
The principle of wiring is as follows:
1. The wires used for the input and output terminals should try to avoid being parallel adjacent to each other. It is best to add ground wires between wires to avoid feedback coupling.

2. The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them.
Certainly. When the thickness of the copper foil is 0.05mm and the width is 1 ~ 1.5mm. With a current of 2A, the temperature will not be higher than 3°C, therefore. A wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, as long as possible, use as wide a line as possible. Especially the power cord and ground wire. The minimum spacing of wires is mainly determined by the worst-case 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.

4. Pay attention to the following issues in the wiring of the printed circuit board: dedicated zero-volt line, the wiring width of the power line is ≥1mm; the power line
As close as possible to the ground wire, the power supply and ground on the entire printed board should be distributed in a "well" shape so that the distributed wire current can reach
To balance; a zero-volt line must be provided for analog circuits; to reduce crosstalk between lines, if necessary, increase the number of printed lines
Care about the distance; insert some zero-volt wires as isolation between the lines; the plugs of the printed circuit should also be arranged with more zero-volt wires as
Isolation between lines; pay special attention to the size of the wire loop in the current flow; if possible, enter the control line (on the printed board)
Connect R-C decoupling at the mouth to eliminate the interference factors that may appear in the transmission; the line width on the printed arc should not change suddenly, and the wire should not suddenly corner (≥90 degrees).
5. Pad
The pad is larger than the lead diameter of the device. However, if the pad is too large, it is easy to form a false solder. The outer diameter of the pad D is generally not less than
(d+1.2)mm, where d is the lead diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm.

The above is an introduction to the general principles of PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.