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PCB Circuit Board Design Principles and Anti-interference Measures

Time : 2024-01-09 Hits : 6

Printed circuit board (PCB) is the support of circuit components and devices in electronic products, it provides electrical connections between circuit components and devices. With the rapid development of electricity in technology, the density of PGB is getting higher and higher. PCB circuit board design is good or bad on the anti-jamming ability has a great impact, therefore, in the PCB circuit design, must comply with the general principles of PCB circuit design, and should be in line with the requirements of EMC / EMI design.

General principles of PCB circuit board design, in order to obtain the best performance of electronic circuits, the layout of components and the direction of the wire is very important. In order to design good quality and low cost PCB circuit boards, the following general principles should be followed:

A. Layout

First of all, consider the size of the PCB size (generally determined by the shape of the product). PCB size is too large, the printed line is long, the impedance increases, the noise immunity decreases, and the cost increases; too small, it is not good for heat dissipation, and the neighboring lines are susceptible to interference; in the determination of the After determining the PCB size, and then determine the location of special components; Finally, according to the functional unit of the circuit, the layout of all components of the circuit.

In determining the location of special components to comply with the following principles:

a, Shorten the connection line between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components susceptible to interference can not be too close to each other, input and output components should be as far away as possible.

b, Some components or wires may have a high potential difference between them, the distance between them should be increased, so as to avoid the discharge of accidental short circuit. Components with high voltage should be arranged in a place that is not easily accessible by hand during commissioning.

c, Weight more than 15g components, should be fixed with a bracket, and then welded. Those large and heavy, heat generating components, should not be mounted on the printed circuit board, but should be mounted on the chassis of the machine on the bottom plate, and should consider the heat dissipation problem. Thermal components should be kept away from heat generating components.

d, For potentiometers, adjustable inductor coil, variable capacitors, microswitches and other adjustable components of the layout should take into account the structural requirements of the machine. If it is adjusted in the machine, it should be placed on the printed board in a convenient place for adjustment; if it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.

e, Should leave the printed wrench positioning holes and fixed bracket occupied by the position.

PCB-13

According to the functional unit of the circuit, the layout of all components of the circuit should comply with the following principles:

a, Arrange the position of each functional circuit unit according to the flow of the circuit so that the layout facilitates signal circulation and keeps the signals in the same direction as far as possible.

b, Take the core component of each functional circuit as the center and layout around it. Components should be evenly, neatly, compactly arranged on the PCB, minimize and shorten the lead and connection between the components.

c, In the high-frequency operation of the circuit, to consider the distribution parameters between the components. General circuits should be as far as possible so that the components are arranged in parallel. In this way, not only beautiful, but also easy to load and solder, easy to mass production.

d, Components located at the edge of the circuit board, from the edge of the circuit board is generally not less than 2mm. the best shape of the circuit board for the rectangle. Aspect ratio of 3:2 to 4:3. circuit board surface size greater than 200x150mm, should consider the mechanical strength of the circuit board.

B. Wiring

The principles of wiring are as follows:

a, Input and output wires should try to avoid parallel neighbor. It is best to add a line between the ground to avoid feedback coupling.

b, The minimum width of the printed PCB wire mainly by the wire and insulation between the adhesion strength of the base wrench and the value of the current flowing through them. When the copper foil thickness of 0.05mm, width of 1 ~ 1.5mm, through the 2A current, the temperature will not be higher than 3 ℃, therefore, the wire width of 1.5mm can be Meet the requirements. For integrated circuits, especially digital circuits, usually choose 0.02 ~ 0.3mm wire width. Of course, as long as it is allowed, or as wide as possible, especially the power line and ground. 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 allows, can make the pitch as small as 5 ~ 8mm.

c, Printed wire corners generally take the arc shape, and right angles or corners in the high-frequency circuit will affect the electrical performance. In addition, try to avoid the use of large-area copper foil, otherwise, when subjected to heat for a long time, prone to copper foil expansion and fall off phenomenon. Must use a large area of copper foil, it is best to use the grid, which is conducive to the elimination of copper foil and the substrate between the adhesive heat generated by volatile gases.

PCB-14

C. Solder Pads

The center hole of the pad should be slightly larger than the diameter of the device leads.The pads are too large to form a false solder. Pad outside 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 the pad can be taken (d + 1.0) mm.

PCB and circuit anti-interference measures printed circuit board interference-free design and specific circuit has a close relationship, here only on the PCB interference-free design of several commonly used measures to do some explain.

a, Power line design

According to the size of the printed circuit board current, try to rent the width of the power line to reduce the loop resistance. At the same time, so that the power line, the direction of the ground line and the direction of the data transfer is consistent, which helps to enhance the ability to resist noise.

b, Ground designThe principle of ground design is:

 a) Separate digital ground and analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible.Low-frequency circuit ground should try to use a single point of parallel connection, the actual wiring difficulties can be part of the series connection and then parallel connection.High-frequency circuits should use multi-point series ground, the ground should be short and rent, high-frequency components around the grid-like large-area ground foil as far as possible.

 b) The ground wire should be as thick as possible.If the ground wire with a very stringy line, the ground potential changes with the current, so that the noise resistance performance is reduced. Therefore, the ground wire should be thickened so that it can pass three times the permissible current on the printed circuit board. If possible, the ground wire should be more than 2~3mm.

 c) The ground wire forms a closed loop. Printed circuit boards consisting of digital circuits only, the grounding circuitry of the cloth into a group of loops can mostly improve the noise immunity.

c, The configuration of the uncoupling capacitor one of the conventional practices in PCB circuit board design is to configure appropriate decoupling capacitors in various critical areas of the printed circuit board.Retro-coupled capacitors are generally configured as follows:

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

 b) In principle, each IC chip should be arranged with a 100pF ceramic capacitor, such as the printed circuit board gap is not enough, every 4 ~ 8 chips can be arranged with a 1 ~ 10pF but the capacitance.

 c) For devices with weak noise immunity and large power supply changes during shutdown, such as RAM and ROM memory devices, the decoupling capacitors should be directly connected between the power supply line and the ground line of the chip.d) Capacitor leads should not be too long, especially high-frequency bypass capacitors should not have leads.

d, In addition, the following two points should be noted:

 a) When there are contactors, relays, buttons and other components in the printed circuit board, the operation of them will produce a large spark discharge, the RC circuit shown in the attached figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, C is 2.2 ~ 47UF.

 b) The input impedance of CMOS is very high and susceptible to inductance, so the unused end should be grounded or connected to a positive power supply when in use.


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