PCB board design method and key points analysis

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PCB board design method and key points analysis [Copy link]

Basic Principles of PCB Design

1. The design of the printed circuit board starts with determining the size of the board. The size of the printed circuit board is limited by the size of the chassis shell, so it is best to fit it into the shell. Secondly, the connection method between the printed circuit board and the external components (mainly potentiometers, sockets or other printed circuit boards) should be considered. The printed circuit board and the external components are generally connected through plastic wires or metal isolation wires. But sometimes it is also designed in the form of a socket. That is, when installing a plug-in printed circuit board in a device, a contact position that acts as a socket must be reserved. For larger components installed on printed circuit boards, metal accessories should be added to fix them to improve vibration and impact resistance.

２. Basic methods of wiring diagram design

First, you need to have a complete understanding of the specifications, dimensions, and areas of the selected components and various sockets; make reasonable and careful considerations on the location of each component, mainly from the perspective of electromagnetic field compatibility, anti-interference, short routing, less crossing, power supply, ground path and decoupling. After the location of each component is determined, it is time to connect the components and connect the relevant pins according to the circuit diagram. There are many ways to complete it. There are two methods for the design of printed circuit diagrams: computer-aided design and manual design.

The most primitive method is to arrange the layout manually. This is more troublesome and often requires repeated several times before it can be finally completed. This can also be done when there is no other drawing equipment. This method of manually arranging the layout is also very helpful for those who are just learning to design printed board drawings. Computer-aided drawing, there are now many drawing software with different functions, but in general, it is more convenient to draw and modify, and can be saved and printed. Next, determine the required size of the printed circuit board and, according to the schematic diagram, preliminarily determine the positions of each component. Then, through continuous adjustments, make the layout more reasonable. The wiring arrangement between the components in the printed circuit board is as follows: (1) No crossing circuits are allowed in the printed circuit board. For lines that may cross, the solution can be achieved by "drilling" or "winding". That is, let a lead "drill" through the gap under the feet of other resistors, capacitors, and transistors, or "go around" through one end of a lead that may cross. In special cases, if the circuit is very complicated, wire jumper is allowed to simplify the design to solve the cross circuit problem.

(2) There are two installation methods for components such as resistors, diodes, and tubular capacitors: "vertical" and "horizontal". Vertical means that the component body is installed and welded perpendicular to the circuit board, and its advantage is space saving. Horizontal means that the component body is parallel and close to the circuit board for installation and welding, and its advantage is that the mechanical strength of the component installation is better. The component hole spacing on the printed circuit board is different for these two different installation components.

(3) The grounding points of the same level circuit should be as close as possible, and the power filter capacitor of the circuit of this level should also be connected to the grounding point of this level. In particular, the grounding points of the base and emitter of the transistor of this level cannot be too far apart, otherwise the long copper foil between the two grounding points will cause interference and self-excitation. The circuit using this "one-point grounding method" is more stable and less prone to self-excitation.

(4) The main ground wire must be arranged in the order of high frequency - medium frequency - low frequency from weak current to strong current. It must not be connected randomly. It is better to have longer wires between levels, but this rule must be followed. In particular, the ground wire arrangement requirements for the frequency conversion head, regeneration head, and frequency modulation head are more stringent. If it is not arranged properly, self-excitation will occur and it will not work.

High-frequency circuits such as FM heads often use large-area surrounding ground wires to ensure good shielding effects.

(5) High-current leads (common ground wires, amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and its voltage drop, which can reduce self-excitation caused by parasitic coupling.

(6) High-impedance traces should be as short as possible, while low-impedance traces can be longer, because high-impedance traces are prone to whistling and absorbing signals, causing circuit instability. Power lines, ground lines, base traces of non-feedback components, emitter leads, etc. are all low-impedance traces. The base traces of emitter followers and the ground lines of the two channels of the recorder must be separated and each of them forms a path until they are combined at the end of the function. If the two ground lines are connected back and forth, crosstalk is very likely to occur, which reduces the separation.

The following points should be paid attention to in PCB design

1. Wiring direction: From the welding surface, the arrangement of components should be kept consistent with the schematic diagram as much as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram. Because various parameters usually need to be tested on the welding surface during the production process, this is convenient for inspection, debugging and maintenance in production (Note: under the premise of meeting the circuit performance and the requirements of the whole machine installation and panel layout).

2. The arrangement and distribution of each component should be reasonable and uniform, and strive to meet the process requirements of neatness, beauty and rigorous structure.

3. （1） Horizontal placement: When the number of circuit components is small and the size of the circuit board is large, it is generally better to place it horizontally; for resistors below 1/4W, the distance between the two pads is generally 4/10 inch; for resistors of 1/2W, the distance between the two pads is generally 5/10 inch; for diodes, the distance between the 1N400X series rectifiers is generally 3/10 inch; for 1N540X series rectifiers, the distance between the two pads is generally 4 to 5/10 inch. （２）Vertical placement: When there are many circuit components and the circuit board is not large, vertical placement is generally adopted. When vertical placement, the distance between the two pads is generally 1 to 2/10 inch.

４． （1）Potentiometer: It is used to adjust the output voltage in the voltage stabilizer. Therefore, the potentiometer should be designed so that the output voltage increases when it is adjusted clockwise and decreases when it is adjusted counterclockwise. In the adjustable constant current charger, the potentiometer is used to adjust the charging current. When designing the potentiometer, the current increases when it is adjusted clockwise. The placement of the potentiometer should meet the requirements of the overall machine structure installation and panel layout, so it should be placed on the edge of the board as much as possible with the rotating handle facing outwards. （２）IC socket: When designing a printed circuit board, when using an IC socket, you must pay special attention to whether the positioning groove on the IC socket is placed in the correct direction, and pay attention to whether the pins of each IC are correct. For example, the first pin can only be located at the lower right corner or upper left corner of the IC socket, and close to the positioning groove (from the soldering side). ５． Arrangement of input and output terminals

（１）The distance between the two associated lead terminals should not be too large, generally about 2 to 3/10 inches is more appropriate.

（２）The input and output terminals should be concentrated on 1 to 2 sides as much as possible, and should not be too scattered.

[backcolor=rgb(255, 255,6. When designing the wiring diagram, pay attention to the order of pin arrangement and the spacing between component pins should be reasonable.

7. Under the premise of ensuring the performance requirements of the circuit, the design should strive to make the wiring reasonable, use less external jumpers, and follow certain charging requirements for wiring, and strive to be intuitive, easy to install, height and maintenance. ８． When designing wiring diagrams, try to avoid turning the wiring as much as possible and make the lines simple and clear.

９． The width and spacing of the wiring lines should be moderate, and the spacing between the two pads of the capacitor should be as close as possible to the spacing between the capacitor lead pins;

１０. The design should be carried out in a certain order, such as from left to right and from top to bottom.

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