Printed circuit board (PCB) is the support of circuit components and components 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 ability of PCB design to resist interference makes a big difference. Therefore, in 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

The layout of components and wires is important for optimal performance of electronic circuits. For good design quality. PCB with low cost should follow the following general principles:

1. The layout

First of all, it is necessary to consider the PCB size is too large. When the PCB size is too large, the printed line is long, the impedance increases, the anti-noise ability decreases, and the cost increases. Too small, the heat dissipation is not good, and adjacent lines are susceptible to interference. After determining the PCB size. Then locate the 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) Shorten the connection between high-frequency components as far as possible, and try to reduce their distribution parameters and electromagnetic interference between each other. Easily disturbed components 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, so the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be as far as possible placed in places not easily accessible by hand during debugging.

(3) Components whose weight exceeds 15g. It should be braced and then welded. Those are big and heavy. The components with high calorific value should not be installed on the printed board, but on the chassis of the whole machine, and the problem of heat dissipation should be considered. Thermal elements should be kept away from heating elements.

(4) for potentiometer. Adjustable inductor coil. Variable capacitor. The layout of adjustable components such as microswitch should consider the structural requirements of the whole machine. If the machine adjustment, should be placed on the printed board above easy to adjust the place; If the machine is adjusted outside, its position should be adapted to the position of the adjusting knob on the chassis panel.

(5) The position occupied by the positioning hole and fixing bracket of the printing lever should be set aside.

According to the functional unit of the circuit. The layout of all components of the circuit shall comply with the following principles:

(1) Arrange the position of each functional circuit unit according to the circuit process, so that the layout is convenient for signal flow and the signal keeps the same direction as far as possible.

(2) To the core components of each functional circuit as the center, around it to carry out the layout. Components should be uniform. And tidy. Tightly arranged on the PCB. Minimize and shorten leads and connections between components.

(3) For circuits working at high frequencies, the distributed parameters between components should be considered. In general circuits, components should be arranged in parallel as much as possible. In this way, not only beautiful. And easy to assemble and weld.

(4) Components located at the edge of the circuit board, generally not less than 2mm from the edge of the circuit board. The best shape of a circuit board is a rectangle. The length to width ratio is 3:20 and 4:3. The size of the circuit board is greater than 200x150mm. Consideration should be given to the mechanical strength of the circuit board.

2. The wiring

The principles of wiring are as follows:

(1) Parallel wires at the input and output terminals should be avoided as far as possible. It is better to add ground wire between wires to avoid feedback coupling.

(2) The minimum width of printed wire is mainly determined by the adhesion strength between wire and insulating substrate and the current value flowing through them.

When the thickness of copper foil is 0.05mm and the width is 1~15mm. For the current through 2A, the temperature will not be higher than 3℃, so a wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, 0.02~0.3mm wire width is usually selected. Of course, use as wide a line as you can. Especially power cables and ground cables.

The minimum spacing of wires is mainly determined by the insulation resistance and breakdown voltage between wires in the worst case. For integrated circuits, especially digital circuits, as long as the process allows, the spacing can be as small as 5~8mm.

(3) Printed wire bend generally take circular arc, and right Angle or included Angle in high frequency circuit will affect the electrical performance. In addition, try to avoid using large areas of copper foil, otherwise. When heated for a long time, copper foil expands and falls off easily. When large areas of copper foil must be used, it is best to use a grid. This is conducive to the removal of copper foil and substrate bonding between the heat produced by the volatile gas.

3. The welding plate

The center hole of the pad should be slightly larger than the device lead diameter. Too large pad is easy to form virtual welding. Pad outer diameter D is generally not less than (D +1.2)mm, where D is the lead aperture. For high density digital circuits, the minimum diameter of pad is desirable (D +1.0)mm.

PCB and circuit anti-interference measures

The anti-interference design of printed circuit board is closely related to the specific circuit. Here only a few common measures of anti-interference design of PCB are described.

1. Power cable design

According to the size of the printed circuit board current, as far as possible to increase the width of the power line, reduce the resistance of the loop. At the same time. Make the power cord. The direction of the ground wire is consistent with the direction of data transmission, which helps to enhance noise resistance.

2. Lot design

The principle of ground wire design is:

(1) Digital ground is separated from analog ground. If there are both logic and linear circuits on the circuit board, keep them as separate as possible. The ground of low-frequency circuit should adopt single point parallel grounding as far as possible. When the actual wiring is difficult, part of the circuit can be connected in series and then parallel grounding. High frequency circuit should use multi-point series grounding, grounding should be short and rent, high frequency elements around as far as possible with a large area of grid foil.

(2) The grounding wire should be as thick as possible. If the grounding line is very long, the grounding potential changes with the current, so that the anti-noise performance is reduced. The grounding wire should therefore be thicker so that it can pass three times the allowable current on the printed board. If possible, the grounding cable should be larger than 2 mm to 3mm.

(3) The ground wire constitutes a closed loop. Most of the printed board composed only of digital circuit can improve the anti-noise ability of the grounding circuit.

3. Decoupling capacitor configuration

One of the common practices in PCB design is to deploy appropriate decoupling capacitors in each key part of the printed board. The general configuration principle of the decoupling capacitor is:

(1) The power input end is connected with an electrolytic capacitor of 10~ 100uF. If possible, it is better to connect 100uF or above.

(2) in principle, each IC chip should be equipped with a 0.01pF ceramic capacitor. If the printed board space is not enough, a 1~10pF capacitor can be arranged for every 4~8 chips.

(3) The anti-noise ability is weak. For devices with large power changes during shutdown, such as RAM.ROM memory devices, the decoupling capacitor should be directly connected between the power line and ground line of the chip.

(4) The capacitor lead can not be too long, especially the high-frequency bypass capacitor can not have the lead. In addition, the following two points should be noted:

(1 There is a contactor in the printed board. Relay. Large spark discharge will be generated when operating the buttons and other components, and the RC circuit shown in the attached drawing must be used to absorb the discharge current. Generally, R is 1~2K, and C is 2.2~47UF.

The input impedance of 2CMOS is very high and sensitive, so the unused end should be grounded or connected to a positive power supply.