General principles of PCB design
In order to get the best performance of the electronic circuit, the layout of the components and the wiring of the wires are very important. In order to design a good quality, low cost PCB. The following general principles should be followed:
First, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is also increased; if the size is too small, the heat dissipation is not good, and adjacent lines are susceptible to interference. After determining the PCB size. Then determine the location of the special components. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
Observe the following principles when determining the location of a particular component:
(1) Try to shorten the wiring 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 placed too close together, 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 circuit caused by discharge. Components with high voltage should be placed as far as possible in the hands of the hand when debugging.
(3) Components weighing more than 15g shall be fixed by brackets and then welded. Those components that are large, heavy, and have a lot of heat should not be mounted on the printed board, but should be installed on the chassis of the whole machine, and heat dissipation should be considered. The thermal element should be kept away from the heating element.
(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, microswitches, etc., 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 to facilitate adjustment; if it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.
(5) The position occupied by the printing plate positioning hole and the fixing bracket should be left.
According to the functional unit of the circuit. When laying out all the components of a circuit, the following principles must be met:
(1) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(2) Center around the core components of each functional circuit and arrange it around it. Components should be arranged evenly, neatly and compactly on the PCB. Minimize and shorten leads and connections between components.
(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. This way, not only beautiful. And it is easy to load and weld. Easy to mass produce.
(4) Components located at the edge of the board are generally not less than 2 mm from the edge of the board. The optimal shape of the board is rectangular. The aspect ratio is 3:2 to 4:3. When the board surface size is greater than 200x150mm. The mechanical strength of the board should be considered.
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should be avoided as far as possible. It is best to add the ground wire between the wires to avoid feedback.
(2) The minimum width of the printed photoconductive wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the copper foil thickness is 0.05mm and the width is 1~15mm. With a current of 2A, the temperature will not be higher than 3 ° C, therefore. A wire width of 1.5 mm is sufficient. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as long as it is allowed, use wide lines as much as possible. Especially the power and ground lines. The minimum spacing of the wires is primarily determined by the worst case interline insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, the pitch can be as small as 5 to 8 mm as long as the process allows.
(3) The curved corner of the printed conductor generally takes a circular arc shape, and the right angle or angle affects the electrical performance in the high frequency circuit. In addition, try to avoid using large areas of copper foil, otherwise. When heated for a long time, copper foil is prone to expansion and shedding. When large areas of copper foil must be used, it is best to use a grid. This is advantageous in eliminating volatile gases generated by the heat of the adhesive between the copper foil and the substrate.
The center hole of the pad is slightly larger than the diameter of the device lead. The pad is too large to form a solder joint. The pad outer diameter D is generally not less than (d + 1.2) mm, where d is the lead aperture. For high-density digital circuits, the minimum pad diameter can be (d + 1.0) mm.
PCB and circuit anti-interference measures
The anti-jamming design of printed circuit boards is closely related to the specific circuit. Here, only some common measures of PCB anti-interference design are explained.
1. Power cord design
According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, the direction of the power line and the ground line are consistent with the direction of data transmission, which helps to enhance the anti-noise capability.
2. Ground design
The principle of ground design is:
(1) The digital ground is separated from the simulated ground. If there are both logic and linear circuits on the board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel with a single point. If the actual wiring is difficult, it can be partially connected and then grounded in parallel. The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and rented, and the grid-like large-area foil should be used as much as possible around the high-frequency components.
(2) The grounding wire should be as thick as possible. If the grounding wire uses a very thin line, the ground potential changes with the change of the current, which reduces the noise immunity. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.
(3) The grounding wire constitutes a closed loop. In a printed circuit board composed only of digital circuits, the grounding circuit is mostly formed into a ring circuit to improve the anti-noise capability.
3. Untwisting capacitor configuration
One of the usual practices in PCB design is to configure appropriate decoupling capacitors at various critical points in the printed board.
The general configuration principle for the untwisting capacitor is:
(1) The power input terminal is connected to an electrolytic capacitor of 10~100uf. If possible, it is better to pick up 100uF or more.
(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If there is not enough space in the printed board, a 1~10pF capacitor can be placed every 4~8 chips.
(3) For devices with weak anti-noise capability and large power supply changes during shutdown, such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power cable and the ground of the chip.
(4) The capacitor leads should not be too long, especially the high-frequency bypass capacitors must not have leads.
In addition, you should also pay attention to the following two points:
(1) When there are contacts, relays, buttons and other components in the printed board. A large spark discharge is generated when operating them, and the RC circuit shown in the drawing must be used to absorb the discharge current. Generally, R takes 1~2K, and C takes 2.2~47UF.
(2) The input impedance of CMOS is very high and it is susceptible to induction. Therefore, it is necessary to ground or connect the power supply to the unused terminal during use.