To achieve the best performance of electronic circuits, the layout of components and the routing of wires are very important. In order to design a 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 it is too small, the heat dissipation will not be good, and the adjacent lines will be easily disturbed. After determining the PCB size, determine the location of special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
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. Components that are susceptible to interference cannot be too close to each other, and input and output components should be kept as far away as possible.
② There 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 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 components should be kept away from heating components.
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.
According to the functional unit of the circuit, when laying out all the components of the circuit, the following principles must be complied with:
①Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal circulation, and the direction of the signal is kept as consistent as possible.
② Take the core components of each functional circuit as the center and make layout around it. Components should be evenly, neatly and compactly drawn on the PCB, minimizing and shortening the leads and connections between components.
③ For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the circuit should arrange the components in parallel as much as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.
④The components located on the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board. The best shape for a circuit board is a rectangle. The aspect ratio is 3:2 or 4:3. When the size of the circuit board surface is greater 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 avoid being adjacent to and parallel to each other as much as possible. It is best to add a ground wire between lines to avoid feedback coupling.
② The minimum width of the printed circuit board wire is mainly determined by the adhesion strength between the wire 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 to 15 mm, the temperature will not be higher than 3 °C through a current of 2 A, so the width of the wire is 1.5 mm to meet the requirements. For integrated circuits, especially digital circuits, the wire width of 0.02-0.3 mm is usually selected. Of course, as far as possible, use wide wires, especially power and ground wires.
The minimum spacing of the conductors is mainly determined by the worst-case insulation resistance between the lines and the breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the pitch can be as small as 5-8 um.
③ The corners of printed wires are generally arc-shaped, while right angles or included angles will affect electrical performance in high-frequency circuits. In addition, try to avoid using a large area of copper foil, otherwise, when heated for a long time, it is easy to cause copper foil to expand and fall off. 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 adhesive between the copper foil and the substrate when heated.
Pad
The center hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a virtual solder joint. The outer diameter D of the pad is generally not less than d+1.2 mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be d+1.0 mm.
PCB board software editing
Post time: Mar-13-2023