In order to get the best performance of electronic circuit, the layout of components and wires is very important. In order to design PCB with good quality and low cost, the following general principles should be followed:

layout

First, consider the size of the PCB. If 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 also increase; if the PCB size is too small, the heat dissipation will not be good, and the adjacent lines will be easily disturbed. After the PCB size is determined, the position of special components is determined. Finally, according to the functional units of the circuit, all the components of the circuit are arranged.

When determining the location of special components, the following principles should be observed:

① Shorten the connection between high frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. The components that are easy to be disturbed should not be too close to each other, and the input and output components should be as far away as possible.

② There may be high potential difference between some components or wires, so the distance between them should be increased to avoid accidental short circuit caused by discharge. The components with high voltage should be arranged in the place that is not easy to reach when debugging.

③ Components weighing more than 15 g should be fixed with bracket and then welded. Those large, heavy and calorific components should not be installed on the printed circuit board, but on the chassis of the whole machine, and the problem of heat dissipation should be considered. The thermal element should be far away from the heating element.

④ For the layout of potentiometer, adjustable inductance coil, variable capacitor, micro switch and other adjustable elements, the structural requirements of the whole machine should be considered. In case of internal adjustment, it should be placed on the top of the printed circuit board for easy adjustment; in case of external adjustment, its position should match the position of the adjustment knob on the chassis panel.

According to the functional units of the circuit, the layout of all components of the circuit should comply with the following principles:

① The position of each functional circuit unit is arranged according to the circuit flow, so that the layout is convenient for signal flow, and the signal keeps the same direction as far as possible.

② The core component of each functional circuit is taken as the center, and the layout is carried out around it. Components should be evenly, neatly and compactly pulled out on the PCB to minimize and shorten the lead and connection between components.

③ The distribution parameters between components should be considered when the circuit works at high frequency. In general, components should be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.

④ The distance between the components located at the edge of the circuit board and the edge of the circuit board is generally not less than 2 mm. The best shape of circuit board is rectangle. The aspect ratio is 3:2 or 4:3. The size of circuit board is larger than 200 mm ✖ 150 mm, the mechanical strength of the circuit board should be considered.

wiring

The principles are as follows:

① The wires used at the input and output terminals should be parallel to each other as far as possible. It is better to add ground wire between wires to avoid feedback coupling.

② The minimum width of PCB wires is mainly determined by the adhesion strength between wires and insulating substrate and the current flowing through them.

When the thickness of copper foil is 0.05 mm and the width is 1-15 mm, the temperature will not be higher than 3 ℃ under the current of 2 A, so the wire width of 1.5 mm can meet the requirements. For integrated circuits, especially digital circuits, the wire width of 0.02-0.3 mm is usually selected. Of course, as long as allowed, or as far as possible with wide wire, especially the power line and ground wire.

The minimum conductor spacing is mainly determined by the insulation resistance and breakdown voltage under the worst condition. For integrated circuits, especially for digital circuits, the spacing can be as small as 5-8 um as long as the process permits.

③ The corner of printed wire is generally circular arc, and the right angle or included angle will affect the electrical performance in high frequency circuit. In addition, avoid using large area copper foil as far as possible, otherwise, when heated for a long time, the copper foil is easy to expand and fall off. When it is necessary to use large area copper foil, it is better to use grid shape, which is helpful to eliminate the volatile gas generated by the heating of adhesive between copper foil and substrate.

Pad

The center hole of pad is slightly larger than the diameter of device lead. If the pad is too large, it is easy to form false soldering. Generally, the pad outer diameter D is not less than D + 1.2 mm, where D is the lead hole diameter. For high density digital circuits, the minimum pad diameter can be D + 1.0 mm.