To get the best performance from electronic circuits, the layout of components and the layout of wires are important. In order to design good quality, low cost PCB. The following general principles should be followed:
layout
First, consider the PCB size. The PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise capability is reduced, and the cost is also increased. After the PCB size is determined, the location of the special components is determined. Finally, according to the functional units of the circuit, all components of the circuit are laid out.
Observe the following principles 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. Vulnerable components should not be placed 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 placed as hard as possible during the debugging.
③ Components weighing more than 15 g should be fixed with brackets and then soldered. Those large, heavy, and heat-generating components should not be installed on printed boards. Instead, they should be installed on the chassis base of the entire machine, and heat dissipation should be considered. Keep the thermal element away from the heating element.
④ For the layout of adjustable components such as potentiometers, adjustable inductors, 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 a printed board where it is easy to adjust. If it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.
According to the functional units of the circuit, the layout of all components of the circuit must meet the following principles:
① Arrange the positions of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signals in the same direction as possible.
② Take the core components of each functional circuit as the center and make a layout around it. The components should be pulled evenly, 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 distribution parameters between components must be considered. Generally, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to mount and weld, and easy to mass produce.
④ The 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 optimal 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.
wiring
The principles are as follows:
① The wires used for input and output should be avoided as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling.
② The minimum width of printed circuit wires is mainly determined by the adhesion strength between the wires and the insulating substrate and the value of the current flowing through them.
When the thickness of the copper foil is 0.05 mm and the width is 1 to 15 mm, the current will not exceed 3 ° C through a current of 2 A. Therefore, the width of the wire is 1.5 mm. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually chosen. Of course, as long as you can, use wide wires, especially power and ground wires.
The minimum spacing of the wires is mainly determined by the worst-case insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the pitch can be as small as 5 to 8 mm.
③ The bend of the printed conductor is generally circular, 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 swell and fall off when heated for a long time. When a large area copper foil must be used, it is best to use a grid shape, which is conducive to excluding the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.
Pad
The pad center hole is slightly larger than the device lead diameter. Pads that are too large are prone to false soldering. The pad outer diameter D is generally not less than d + 1.2 mm, where d is the lead hole diameter. For high-density digital circuits, the minimum pad diameter can be d + 1.0 mm.