PCB Technical - Printed PCB design principles and anti-interference

Printed PCB is the support of 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, when doing PCB design. The general principles of PCB design must be followed, and the requirements of anti-interference design must be met.

The general principle of PCB design is 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:

1. Layout First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. After determining the PCB size. Then determine the location of special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.

(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 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 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 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 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) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, micro switches, 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 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.

(5) The position occupied by the positioning hole of the printed board and the fixed 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 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 lay out around it. The components should be arranged evenly, neatly and compactly on the PCB. Minimize and shorten the leads and connections between 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. And it is easy to install and weld. Easy to mass produce.

4) The 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 to 4:3. When the size of the circuit board is larger than 200x150mm. The mechanical strength of the circuit board should be considered.

2. Wiring The principles of wiring 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 printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the thickness of the copper foil 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 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, as long as possible, use as wide a line as possible. Especially the power cord and ground wire. 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 spacing can be as small as 5-8mm.

(3) The corners of printed conductors are generally arc-shaped, and right angles or angles will affect electrical performance in high-frequency circuits. 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 of copper foil must be used, it is best to use a grid shape. This helps to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

3. 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. PCB and circuit anti-jamming measures The anti-jamming design of printed circuit boards is closely related to the specific circuit. Here, only a few common measures of PCB anti-jamming design are explained.

(1) The design of the power cord is based on the current of the printed circuit board, try to increase the width of the power cord 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 helps to enhance the anti-noise ability.

(2) The principle of the ground design of the lot design is:

1) The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the PCB 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 much 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 grid-like large-area ground foil should be used around the high-frequency component as much as possible.

2) The ground wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the change of the 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 grounding wire should be 2~3mm or more.

3) The ground wire forms a closed loop. The printed circuit board PCB composed of only digital circuits, the grounding circuit of which is arranged into a group of loops can mostly improve the anti-noise ability.

(3) One of the conventional methods of PCB design for decoupling capacitor configuration is to configure appropriate decoupling capacitors on each key part of the printed board. The general configuration principles of decoupling capacitors are:

1) Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.

2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the gap of the printed board is not enough, a 1-10pF capacitor can be arranged for every 4~8 chips.

3) For devices with weak anti-noise ability and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

4) Capacitor leads cannot be too long, especially high-frequency PCB bypass capacitors. In addition, you should also pay attention to the following two points:

when contacting the printed circuit board PCB contactors, relays, buttons and other components. When operating them, large spark discharges will be generated, 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.

The input impedance of CMOS is very high and it is susceptible to induction, so when in use, the unused terminal must be grounded or connected to a positive power supply.