PCB is the support for circuit components and devices in electronic products. It provides electrical connections between circuit elements and devices. With the rapid development of electrical technology, the density of PGB is getting higher and higher. The quality of PCB design has a great influence on the ability to resist interference. Therefore, in the PCB design. The general principles of PCB design must be followed and the requirements of anti-interference design should be met.
The general principle of PCB design is that the layout of components and the layout of wires are very important to obtain the best performance of electronic circuits. In order to design the PCB with good quality and low cost. The following general principles should be followed:
1. Layout
First, consider the size of the PCB. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise capability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will be poor, and the adjacent lines will be easily interfered. After determining the PCB size. Then determine the location of special components. Finally, according to the functional units of the circuit, all components of the circuit are laid out.
Observe the following guidelines when locating special components:
1.1. Shorten the connection 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 should not be too close to each other, and input and output components should be kept as far apart as possible.
1.2. 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 arranged as far as possible in places that are not easily accessible by hand during debugging.
1.3. Components weighing more than 15g 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 board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal elements should be kept away from heating elements.
1.4. 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 inside 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 be adapted to the position of the adjustment knob on the chassis panel.
1.5. The position occupied by the positioning hole of the printed pulley 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 should be followed:
a. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the direction of the signal is kept as consistent as possible.
b. Take the core components of each functional circuit as the center and make layout around it. Components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten leads and connections between components.
c. For circuits that work at high frequencies, the distribution parameters between components should be considered. In general circuits, the components should be arranged in parallel as much as possible. In this way, not only beautiful. And easy to install and weld. Easy to mass produce.
d. Components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The optimal shape of the circuit board is a rectangle. The aspect ratio is 3:2 into 4:3. When the size of the circuit board is larger than 200x150mm. Consideration should be given to the mechanical strength experienced by the circuit board.
2. Wiring
2.1. The wires used for the input and output terminals should avoid the adjacent parallel as far as possible. It is best to add a ground wire between the wires to avoid feedback coupling.
2.2. The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating base plate and the value of the current flowing through them. When the copper foil thickness is 0.05mm and the width is 1 ~ 15mm. Through the current of 2A, the temperature will not be higher than 3 ℃, so. Wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, use as wide a line as possible whenever possible. Especially the power and ground wires. The minimum spacing of wires is mainly determined by the worst-case wire-to-wire insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5~8mm.
2.3. The corners of printed wires are generally arc-shaped, and right angles or included angles will affect electrical performance in high-frequency circuits. In addition, try to avoid using a large area of copper foil, otherwise. When heated for a long time, the copper foil is prone to expand and fall off. When a large area of copper foil must be used, it is best to use a grid. This is beneficial to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.
3. Pad
The pad center hole is slightly larger than the device lead diameter. If the pad is too large, it is easy to 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 is desirable (d+1.0) mm.
PCB and circuit anti-interference measures
The anti-jamming design of the printed circuit board is closely related to the specific circuit. Here, only a few common measures for the anti-jamming design of the PCB are explained.
3.1. 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 the ground line consistent with the direction of data transmission, which will help to enhance the anti-noise capability.
3.2. Ground wire design
The principles of ground wire design are:
a. Separate the digital ground from the analog ground. If there are 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 be grounded in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the large-area grid-shaped ground foil should be used around the high-frequency components as much as possible.
b. The ground wire should be as thick as possible. If the ground wire is very slender, the ground potential will change with the change of the current, which will reduce 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 more than 2~3mm.
c. The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of the grounding circuits are arranged in a loop, which can improve the anti-noise ability.
4. Decoupling capacitor configuration
One of the common practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board.
The general configuration principles of decoupling capacitors are:
4.1. Connect an electrolytic capacitor of 10 to 100uf across the power input. If possible, it is better to connect with more than 100uF.
4.2. In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. If the printed board space is not enough, a 1 ~ 10pF capacitor can be arranged every 4~8 chips.
4.3. For devices with weak anti-noise ability and large power changes when turned off, 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.
4.4. Capacitor leads should not be too long, especially high-frequency bypass capacitors should not have leads.
In addition, the following two points should be noted:
a. When there are contactors, relays, buttons and other components on the printed board. When operating them, a large spark discharge occurs, and the RC circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47UF.
b. The input impedance of CMOS is very high, and it is susceptible to induction, so the unused terminal should be grounded or connected to a positive power supply during use.
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