PCB Design Principles – knowledge base

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

  • PCB size
  • Location of special components
  • COmponents layout

First, the PCB size needs to be taken into consideration. The PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability is reduced, and the cost also increases; if the PCB size is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.

Location of special components

The following principles should be observed when determining the location of special components:

  1. Shorten the wiring between high-frequency components as much as possible, 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 away as possible.
  2. There may be a high potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuits caused by the discharge. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands 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 circuit 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 far away from heating components.
  4. The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should consider the structural requirements of the whole machine. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should match the position of the adjustment knob on the chassis panel.

Components layout

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 position of each functional circuit unit according to the circuit flow so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.
  2. Take the core component of each functional circuit as the center and layout around it. The components should be pulled uniformly, neatly, and compactly on the PCB to minimize and shorten the leads and connections between the components.
  3. For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit 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. The 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 of the circuit board is rectangular. The aspect ratio is 3:2 or 4:3. When the size of the circuit board is larger than 200 mm*150 mm, the mechanical strength of the circuit board should be considered.

Layout

The principles are as follows:

  1. The wires used for the input and output terminals should try to avoid being parallel adjacent to each other. It is best to add ground wires between wires to avoid feedback coupling.
  2. The minimum width of the conductors of the printed circuit board is mainly determined by the adhesion strength between the conductors 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 a 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, as long as possible, use as wide a line as possible, especially the power line and the ground line.
    The minimum spacing of wires is mainly determined by the worst-case insulation resistance and breakdown voltage between the wires. For integrated circuits, especially digital circuits, as long as the process permits, the pitch can be as small as 5 to 8 um.
  3. The bends of the printed conductors are generally arc-shaped, and the right angle or angle will affect the electrical performance in the high-frequency circuit. 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 copper foil is required, it is best to use a grid shape, which helps to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

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 false 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.

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