PCB Tech - PCB design principles and anti-interference measures

Printed circuit board (PCB circuit board) 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 circuit board design has a great influence on the anti-interference ability. Therefore, in the PCB circuit design. The general principles of PCB circuit design must be followed, and the requirements of EMC/EMI design must be met.

The general principle of PCB circuit board design, in order to obtain the best performance of the electronic circuit, the layout of the components and the direction of the wires are very important. In order to design PCB circuit boards with good quality and low cost, the following general principles should be followed:

1. Layout

First, consider the size of the PCB (generally determined by the shape of the product). When the PCB size is too large, the printed lines will be longer, 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 the adjacent lines will be easily interfered; after determining the PCB size, determine the location of special components ; Finally, according to the functional unit of the circuit, layout all the components of the circuit.

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 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, 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 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 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 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 PCB 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 ~ 1.5mm. With a current of 2A, the temperature will not be higher than 3°C, therefore. The wire width is 1.5mm to 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 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-interference design of the printed PCB is closely related to the specific circuit. The anti-interference design of the PCB is based on the current of the printed circuit board, and the width of the power line should be increased as much as possible 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.