Questioni che richiedono attenzione nel processo di progettazione dei PCBPubblicato il 16/07/2020
Per ottenere le migliori prestazioni dei circuiti elettronici, la disposizione dei componenti e la disposizione dei fili sono molto importanti. Per progettare PCB di buona qualità e basso costo. Il processo di progettazione dei PCB dovrebbe seguire i seguenti principi generali:
First, the PCB size should be considered. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost also increases. If it is too small, the heat dissipation is not good, and the adjacent lines are easily interfered. After determining the PCB size. Then determine the location of 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 special components:
(1) Keep the wiring between high-frequency components as short 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 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 in places that are not easily accessible by hand during commissioning.
(3) Components weighing more than 15g should be fixed with brackets and then soldered. Those large, heavy and heat-generating components should not be mounted on the printed board. It should be installed on the chassis bottom of the whole machine, and heat dissipation should be considered. The thermal element should be far away from the heating element.
(4) For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches. The structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be placed on the printed 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.
(5) The position occupied by the positioning hole of the printed wrench and the fixing bracket should be reserved.
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 flow of the circuit. Make the layout easy for signal flow and keep the signal in the same direction as possible.
(2) Centering on the core component of each functional circuit, lay out around it. Components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten the leads and connections between components.
(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. The general circuit should arrange the components in parallel as much as possible. In this way, not only beautiful. Moreover, it is easy to install and weld, and easy to mass produce.
(4) Components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 into 4:3. When the circuit board surface size is greater than 200x150mm. The mechanical strength of the circuit board should be considered.
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should avoid adjacent parallel as much as possible. It is best to add a ground between the lines to avoid feedback coupling.
(2) The minimum width of the printed camera wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them.
(3) The bends of the printed conductors are generally rounded, and the right angle or the included angle will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large area copper foil. Otherwise, when heated for a long time, the copper foil will swell and fall off easily. When it is necessary to use a large area of copper foil, it is best to use a grid. This helps to eliminate the volatile gas generated by the heating 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. If the pad is too large, it may form a virtual solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum diameter of the pad can be (d + 1.0) mm.
PCB and circuit anti-interference measures
The anti-interference design of the printed circuit board has a close relationship with the specific circuit. Here are some explanations on several common measures of PCB anti-interference design:
1. Power cord design
According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission. This helps to enhance the ability to resist noise.
2. Lot design
The principles of ground design are:
- The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should use single-point parallel grounding as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. High-frequency circuits should be grounded at multiple points in series, and the ground wire should be short and leased. Use grid-like large-area foil as much as possible around the high-frequency components.
- The ground wire should be as thick as possible. If the grounding wire uses a very strong line, the grounding potential changes with the change of current, which reduces the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2~3mm.
- The ground wire constitutes a closed loop. The printed circuit board composed only of digital circuits, the ground circuit is arranged in a group of loops can mostly improve the ability to resist noise.
Decoupling capacitor configuration
One of the common practices in PCBs design is to configure appropriate decoupling capacitors at each key part of the printed board. The general configuration principles of decoupling capacitors are:
(1) Connect 10~100uf electrolytic capacitor across the power input. If possible, it is better to connect more than 100uF.
(2) In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. If there is not enough space on the printed board, a capacitor of 1~10pF can be arranged every 4~8 chips.
(3) For devices with weak noise immunity and large power changes during the shutdown. Such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power line and the ground line of the chip.
(4) The lead wire of the capacitor should not be too long, especially the high-frequency bypass capacitor must not have a lead wire. In addition, the following two points should be noted:
- When there are contactors, relays, buttons and other components in the printed board. When operating them, a large spark discharge will occur, and an RC circuit must be used to absorb the discharge current. Generally R takes 1~2K, C takes 2.2~47UF.
- The input impedance of 2CMOS is very high, and it is susceptible to induction. Therefore, when using it, the unused end should be grounded or connected to a positive power supply.
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