To obtain the best performance of electronic circuits, the layout of components and wiring is very important. So what principles should be followed in order to design a good quality PCB?

1. Layout

1. 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 capability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will be poor, and the adjacent lines will be easily interfered. After determining the size of the PCB, determine the location of special components. Finally, according to the functional units of the circuit, all components of the circuit are laid out.

Observe the following guidelines when locating special components:

① Shorten the connection between high-frequency components as much as possible, and 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 apart as possible.

②There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be arranged as far as possible in places that are not easily accessible by hand during debugging.

③ Components weighing more than 15 g should be fixed with brackets and then welded. Those components that are large, heavy and generate a lot of heat should not be installed on the printed board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal elements should be kept away from heating elements.

④ For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed board where it is convenient for adjustment; if it is adjusted outside the machine, its position should be adapted to the position of the adjustment knob on the chassis panel.

2. 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 positions of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signals are kept in the same direction as possible.

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

③ For circuits that work at high frequencies, the distribution parameters between components should be considered. In general circuits, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass produce.

④ Components located at the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board. The optimal shape of the circuit board is a rectangle. The aspect ratio is 3:2 or 4:3. When the size of the circuit board surface is larger than 200 mm✖150 mm, the mechanical strength of the circuit board should be considered.

Second, wiring, the principles are as follows:

①The wires used at the input and output terminals should avoid being adjacent to each other as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling.

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

When the thickness of the copper foil is 0.05 mm and the width is 1-15 mm, the temperature will not be higher than 3 ℃ through the current of 2 A, so the wire width of 1.5 mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, whenever possible, use as wide a wire as possible, especially power and ground wires.

The minimum spacing of wires is mainly determined by the worst-case wire-to-wire insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5-8 um.

③The corner of the printed wire is generally arc-shaped, and the right angle or the included angle will affect the electrical performance in high-frequency circuits. In addition, try to avoid the use of large-area copper foil, otherwise, the copper foil will easily expand and fall off when heated for a long time. When a large area of ​​copper foil must be used, it is best to use a grid shape, which is beneficial to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

3. Pad

The pad center hole is slightly larger than the device lead diameter. If the pad is too large, it is easy to form a virtual solder. The outer diameter D of the pad 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 that the PCB design should follow the three originals.