Electronic equipment is assembled from components, components, connections and parts. Only through a reasonable layout and proper arrangement of its position can it be beneficial to ensure the realization of technical indicators and make it work stably and reliably. In the circuit unit of electronic equipment, the location arrangement of components is called component layout; the arrangement of component locations in electronic equipment and the location arrangement of components and mechanical parts are collectively called layout. The connection and direction arrangement of various wires between components and components is called wiring. Layout and wiring directly affect the performance and assembly process of electronic devices. This chapter focuses on layout and routing and electronic assembly processes.
Component layout principles
1. Layout principle of components
The layout of components in electronic equipment and components should follow the following principles.
Component layout should ensure the realization of electrical performance indicators
Electrical performance generally refers to: frequency characteristics, signal distortion, gain, working stability, phase shift, noise level, efficiency and other related indicators, which vary with different circuits. The layout of components has a great influence on the electrical performance. For example, when the low-frequency circuit is at high gain, improper layout will produce parasitic feedback, which will distort the output signal or cause unstable operation; for example, improper layout in high-frequency devices will increase the distribution parameters (distributed capacitance). , wiring inductance, ground impedance, etc.), which will change the circuit parameters and bring about adverse consequences; while for digital circuits, improper layout will cause waveform distortion and have adverse effects. If you pay attention to the inductive influence of electric field and magnetic field and reduce the electric and magnetic induction to a minimum during the layout of components, the occurrence of the above-mentioned undesirable phenomena can be reduced, otherwise shielding and isolation measures should be taken.
component installation
The location, installation and placement direction of components, as well as the distance between components, directly affect the length of the connection and the laying path. The length of the wire and the rationality of the wiring also affect its distribution parameters and electromagnetic induction, which will ultimately affect the circuit performance. Therefore, the layout of components should take into account the wiring and take care of each other.
Component layout
The layout of components should be considered to make the installation structure compact, the weight distribution is balanced, and the arrangement is orderly, which is beneficial to the structural design.
At present, electronic equipment is developing towards miniaturization and miniaturization, which requires compact structure and increased assembly density. Therefore, in the layout of components, careful consideration and ingenious arrangements should be made, and under the condition of taking into account all requirements, strive to increase the assembly density to reduce the size of the whole machine. In addition, the weight balance of the components should also be considered in the layout, and strive to reduce the center of gravity; at the same time, the components should be arranged in an orderly and hierarchical manner, which is easy to find and maintain. All of these should be beneficial to the structural design and facilitate assembly and commissioning.
Component layout should be conducive to heat dissipation and shock and vibration resistance
High temperature has a greater impact on most components, especially semiconductor devices, and has a greater impact on temperature-sensitive components. The layout should be conducive to heat dissipation, and the layout should be strictly in accordance with the requirements on thermal design in Chapter 2. Some components have poor resistance to shock and vibration, or shock and vibration have a greater impact on their performance. Full attention should be paid to anti-vibration and vibration resistance issues during layout.
2. Arrangement method and requirements for component layout
It is better to arrange in a straight line according to the order of the circuit diagram
As shown in Figure 3.1, according to the order of all levels of circuits in the circuit diagram, it is common and better to arrange all levels of circuits in a straight line. The circuits at all levels are arranged in the order of the horizontal axis centered on the tubes (G1, G2, G3 in the figure represent transistors or integrated circuit blocks); Specifically: the components between the output of the front-stage tube and the input of the rear-stage tube are arranged in the area between the two tubes (area 1-2 and 2-3 in the figure), and the circuit of this stage is Other components are arranged in the longitudinal axis area on both sides of the tube (areas 1, 2, and 3 in the figure).
The advantages of arranging circuit components in a straight line are:
â‘ The distance between the input stage and the output stage of the circuit is far away, which reduces the parasitic feedback (parasitic coupling) between the input and the output.
â‘¡ The ground current of circuits at all levels mainly flows within the scope of this level, which reduces the interference of ground current between levels.
â‘¢ It is convenient for shielding and isolation of circuits at all levels. It must be pointed out that when laying out in a straight line, there should be enough distance between the circuits at all levels, so that the front and rear circuits can be well connected, and the direction of the pins should be paid attention to to make the connection the shortest. For the integrated circuit block, the components connected to it should be arranged near the corresponding leads of the integrated circuit block, and the distance should be slightly closer.
When there are both high-potential components and low-potential components in the circuit, the high-potential components are arranged on the horizontal axis, and the low-potential components are arranged on the vertical axis, which can avoid ground current channeling and reduce the impact of high-potential components on low-potential components. interference.
When the circuit is limited by the installation space and cannot be arranged in a straight line, a square (L-shaped) or two-row parallel arrangement can be used, as shown in Figure 3.2. At this time, two bottom plates should be used, and the bottom plates are still arranged in a straight line. The two bottom plates are isolated from each other and only connected at one point. In Figure 3.2(a), G1 and G2 are one bottom plate, G3 and G4 are another bottom plate, and the two bottom plates are connected by wires between G2 and G3; in Figure 3.2 (b), G1, G2, G3 and G4, G5, G6, each use a base plate, and the two base plates are connected by wires between G3 and G4. The sawtooth (W-shaped) layout shown in Figure 3.2(c) is not desirable, because although this layout occupies a small area, it has large ground current interference and large parasitic coupling, which is unfavorable to the circuit operation and generally cannot be used.
Although the circuit unit using the printed circuit board has less impact on the ground current than the circuit unit using the metal base, the layout should also be arranged in a straight line, so that the input and output are far away, the parasitic feedback is small, and the printed wires of the circuits at all levels are The shortest, which can weaken the coupling interference.
Pay attention to the interaction between circuits, components, and wires at all levels
Appropriate distances should be left between the circuits at all levels, and should be arranged reasonably according to the size of the components. Pay attention to the connection between the output of the previous stage and the input of the next stage, and try to connect small components directly between the circuits. Large components can be pulled out of the circuit to be installed separately and wired into the circuit.
Iron core devices, thermal components, and high-voltage components with magnetic fields should be placed correctly, preferably away from other components to avoid interference between components.
For high-frequency circuits, in order to reduce the influence of distributed parameters, it is best not to arrange similar components in parallel, and their leads should not be parallel, but they can be arranged in a staggered manner (such as one standing upright and the other lying down).
When arranging components, pay attention to their grounding methods and grounding points
If a metal base is used to install components, it is best to lay several thick copper wires on the bottom surface as ground wires. The ground wires should be soldered to the center of the base after hot dip tinning (note that each thick copper wire must be firmly welded to the base). When the grounding components are to be grounded, the shortest path should be selected and welded on the thick copper ground wire. If large components are installed on other metal components, ground wires should be laid separately, and metal components cannot be used as ground wires.
When installing components on metal bases and metal components, there should be enough installation space for easy assembly and disassembly.
If a printed circuit board is used to install components, each grounding component should be arranged near the ground wire, and one-point grounding and nearest grounding can be used according to the situation.
The special requirements of circuit components should be met during component layout
For thermal components and components with high heat generation, attention should be paid to their thermal interference during layout, and thermal isolation or heat dissipation measures can be taken; for circuits and components that need to be shielded, there should be room for shielding structures during layout.
For push-pull circuits, bridge circuits or other circuits that require symmetrical electrical properties, attention should be paid to structural symmetry when arranging components, that is, to achieve symmetrical positions of components and symmetrical connections, so that the distribution parameters of the circuit are as consistent as possible.
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