3.The design of PCB---General principles
5.General principles of PCB design
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The special components layout
First, consider the size of PCB: when the size of PCB is too large, the printing line is long, the impedance increases, the anti-noise ability decreases, and the cost increases too small, so the heat dissipation is not good, and adjacent lines are easily disturbed. After determining the PCB size, make sure the location of the special components. Finally, lay out.the entire components of the circuit according to the function unit of the circuit.
The following principles shall be observed in determining the location of special components:
A.To minimize the connection between high frequency components, try to reduce their distribution parameters and electromagnetic interference between them. Vulnerable components cannot get too close to each other, input and output elements should be kept away from each other.
B.There may be high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit. The components with high voltage should be arranged where the hand is not easy to reach when debugging.
C.The components weighing more than 15 g shall be fixed with a bracket and then welded. Those large, heavy, calorific components should not be installed on the printing board, but should be installed on the chassis of the machine box, in addition, consider heat dissipation. Heat sensitive elements should be kept away from heating elements.
D. For the distribution of adjustable elements such as potentiometer, adjustable inductor coil, variable capacitor and microdynamic switch, the structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be conveniently regulated in the printing plate. In case of external adjustment, its position should adjust to the position of the adjusting knob on the cabinet panel.
E.The position of the printed plate positioning hole and the fixed bracket shall be set aside.
The general components layout
According to the functional unit of the circuit, the overall components of the circuit should be arranged in accordance with the following principles:
A. According to the process of the circuit, the location of each functional circuit unit is arranged so that the layout can be easily circulated and the signal is as consistent as possible.
B. The core component of each functional circuit is centered, around it to make a layout. The components shall be arranged evenly, neatly and tightly in PCB, minimizing and shortening the leads and connections between the components.
C.The circuit that works at high frequency should consider the distribution parameters between components. The general circuit shall be arranged in parallel with the components as much as possible, so that it is not only beautiful, but also easy to weld and easy to mass production.
D. The component located on the edge of the circuit board is not less than 2 mm from the edge of the circuit board. The best shape of the circuit board is the rectangle. The length and width is 3:2 or 4:3. The mechanical strength of the circuit board should be considered when the circuit board size is greater than 200 mm x 150 mm.
PCB design wiring
The principles of wiring are as follows:
A. The wires in the input and output terminals should avoid the adjacent parallel, preferably with a line between the lines so as to avoid feedback coupling.
B. The minimum width of a printed plate conductor is determined by the strength of the adhesion between the conductor and the insulating substrate and the current value of the conductor. When the thickness of the copper foil is 0.5mm, width is 1~15 mm, with 2A current, temperature rise will not higher than 3 ℃. Therefore, the wire width of 1.5mm can meet the requirements. For the integrated circuit, especially the digital circuit, the width of the wire is usually 0.02 ~ 0.3mm wire. Of course, as long as you allow, use wide lines, especially power lines and ground lines. The minimum spacing of the wire is determined by the insulation resistance and breakdown voltage of the line in the worst case.
For integrated circuits, especially digital circuits, the spacing is less than 0.1 ~ 0.2mm as long as the process permits.
C. The bending of the printed wire usually takes a circular arc, while the right Angle or Angle will affect the electrical performance in the high frequency circuit. In addition, avoid large area copper foil as far as possible, otherwise, the long time heat, easy to happen copper foil expansion and shedding phenomenon. A large area of copper foil must be used, which can be used to control the volatile gas generated by heat between copper foil and substrate.
PCB design welding disc
The center hole of the welding disc is slightly larger than the diameter of the device. Soldering disk is too large to form virtual welding, outside diameter D is generally not less than (D + 1.2) mm, where D is the lead aperture. For high density digital circuit, the minimum diameter of welding disk is recommended (d + 1.0) mm.
The other more
The other more