How much do you know about PCB design principles?
To obtain the best performance of electronic circuits, component layout and wire layout are very important. In order to design good quality, low cost PCB. The following general principles should be followed:
The layout
First, consider the PCB size. PCB size is too large, printing line is long, impedance increases, anti-noise ability decreases, the cost also increases; Too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After the PCB size is determined, the location of special components is determined. Finally, according to the functional unit of the circuit, the layout of all components of the circuit.
The following principles should be observed when determining the location of special components:
(1) Shorten the connection between high-frequency components as far as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. Easily disturbed components should not be too close to each other, input and output components should be as far away as possible.
② There may be a higher potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuit from discharge. Components with high voltage should be arranged as far as possible in places that are not easily touched by the hand during debugging.
(3) Components weighing more than 15 g should be fixed with a support, and then welded. Those large and heavy, heat components, should not be installed on the printed board, but should be installed on the chassis bottom, and should consider the problem of heat dissipation. The heat sensitive element should be far away from the heating element.
④ The layout of adjustable components such as potentiometer, adjustable inductor coil, variable capacitor, micro switch should consider the structural requirements of the whole machine. If the machine adjustment, should be placed above the printed board for easy to adjust the place; If the outside of the machine adjustment, its position to adjust the knob on the chassis panel position.
According to the functional unit of the circuit, the layout of all components of the circuit should comply with the following principles:
(1) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout is convenient for signal flow, and make the signal as consistent as possible.
② The core components of each functional circuit as the center, around it to carry out the layout. Components should be evenly, neatly and compactly enfolded on the PCB to minimize and shorten the leads and connections between components.
③ The circuit working at high frequency should consider the distribution parameters between the components. General circuits should be arranged as parallel as possible. In this way, not only beautiful, but also easy to assemble welding, easy to mass production.
(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 best shape for a circuit board is a rectangle. The length to width ratio is 3:2 or 4:3. ✖ circuit board face size greater than 200 mm, 150 mm, the circuit board by mechanical strength should be considered.
Wiring
Its principles are as follows:
① The wires used at the input and output ends should be avoided as far as possible. It is better to add ground wire to avoid feedback coupling.
② The minimum width of the printed 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 copper foil is 0.05mm 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, usually choose 0.02 ~ 0.3mm wire width. Of course, whenever you can, use wide wires, especially power cables and ground cables.
The minimum distance between wires is mainly determined by the insulation resistance and breakdown voltage between wires in the worst case. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5 ~ 8 um.
(3) The bend of the printed wire is generally a circular arc, and the right Angle or included Angle will affect the electrical performance in the high-frequency circuit. In addition, try to avoid using a large area of copper foil, otherwise, when heated for a long time, it is easy to occur copper foil expansion and shedding phenomenon. When a large area of copper foil must be used, it is best to use a grid, which is conducive to the exclusion of volatile gases generated by the heat of the adhesive between the copper foil and the substrate.
Pad
The center hole of the pad should be slightly larger than the diameter of the device lead. Too large pad is easy to form virtual welding. The outer diameter D of the pad is generally not less than d+ 1.2mm, where d is the lead aperture. For high density digital circuit, the minimum diameter of the pad is d+ 1.0mm.
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