PCB Technical - PCB layout design principles and anti-interference measures

Printed circuit board (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 anti-interference ability. Therefore, in the PCB design. The general principles of PCB design must be followed, and the requirements of anti-interference design must be met.

General principles of PCB design

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. PCB 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.

The following principles should be observed when determining the location of special components:

(1) Shorten the wiring between high-frequency components as much 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 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 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 mounted on the printed circuit board, but should be mounted 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) The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should consider the structural requirements of the whole machine. 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, not only beautiful. And easy to install and weld. Easy to mass produce.

(4) The components located at 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 principle of wiring is as follows:

(1) The wires used for the input and output terminals should try to avoid being adjacent and parallel. 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 bends of the printed conductors are generally arc-shaped, and the right angle or the included angle will affect the electrical performance in the high-frequency circuit. In addition, try to avoid using large-area copper foil, otherwise. When heated for a long time, the copper foil is prone to swell and fall off. 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. Pad

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-interference measures

The anti-jamming design of the printed circuit board has a close relationship with the specific circuit. Here, only a few common measures of PCB anti-jamming design are explained.

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, which helps to enhance the anti-noise ability.

2. Ground wire design

The principles of ground wire design are:

(1) 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 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 grounding 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 grounding wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.

3. Decoupling capacitor configuration

One of the conventional practices of PCB manufacturers for PCB design is to configure appropriate decoupling capacitors on each key part of the printed circuit 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 changes when shutting down, 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 should not be too long, especially for high-frequency bypass capacitors.

In addition, the following two points should be noted:

(1) When there are contactors, relays, buttons and other components in the printed board. When operating them all