Detailed analysis of PCB board design methods and key points

Basic principles and requirements for PCB board 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 external components (mainly potentiometers, sockets or other printed circuit boards) should be considered. The printed circuit board and external components are generally connected through plastic wires or metal isolation wires. But sometimes they are designed in the form of sockets. That is, when installing a plug-in printed circuit board in the equipment, a contact position that acts as a socket must be reserved. For larger components installed on the printed circuit board, metal accessories should be added to fix them to improve vibration and impact resistance.

2. Basic methods of wiring diagram design

First of all, 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, few crossovers, 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 this. There are two methods for designing 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 method can also be used 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, stored and printed.

Next, determine the required size of the printed circuit board, and preliminarily determine the positions of various components according to the schematic diagram, and then make continuous adjustments to make the layout more reasonable. The wiring arrangement between the components in the printed circuit board is as follows:

(1) Crossing circuits are not allowed in printed circuits. For lines that may cross, the problem can be solved by "drilling" or "winding". That is, let a lead "drill" through the gap under other resistors, capacitors, and transistors, or "wind" 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 problem of cross circuits.

(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 soldered perpendicular to the circuit board, and its advantage is that it saves space. Horizontal means that the component body is installed and soldered parallel to and close to the circuit board, and its advantage is that the mechanical strength of the component installation is better. For these two different installation components, the component hole spacing on the printed circuit board is different.

(3) The grounding points of the same level circuit should be as close as possible, and the power filter capacitor of the circuit should also be connected to the grounding point of the same level. In particular, the grounding points of the base and emitter of the transistor at this level should not 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 connect 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 wire, amplifier power lead, 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) The high-impedance traces should be as short as possible, and the low-impedance traces can be longer, because the high-impedance traces are prone to whistling and absorbing signals, causing circuit instability. The power line, ground line, base traces of non-feedback components, emitter leads, etc. are all low-impedance traces. The base traces of the emitter follower and the ground lines of the two channels of the recorder must be separated and each of them is connected to the end of the function. If the two ground lines are connected, crosstalk is very likely to occur, which reduces the separation. The following points should be noted 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 be neat, beautiful, and meet the rigorous structural process requirements.

3. The placement of resistors and diodes: There are two types: horizontal placement and vertical placement:

(1) Laying flat: When there are not many circuit components and the circuit board is large, it is generally better to lay it flat. 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. When diodes are laid flat, 1N400X series rectifiers are generally 3/10 inch, and 1N540X series rectifiers are generally 4 to 5/10 inch.

(2) Vertical placement: When there are many circuit components and the circuit board size is not large, vertical placement is generally adopted. When placed vertically, the distance between the two pads is generally 1 to 2/10 inch.

4. Potentiometer: IC holder placement principle

(1) Potentiometer: It is used to adjust the output voltage in the voltage regulator. 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 outward.

(2) IC socket: When designing a printed circuit board, when using an IC socket, special attention must be paid to whether the positioning groove on the IC socket is placed in the correct direction, and whether the pins of each IC are placed correctly. For example, pin 1 can only be located at the lower right corner or upper left corner of the IC socket, and close to the positioning groove (viewed from the soldering surface).

5. Arrangement of input and output terminals

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

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

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 circuit performance requirements, the design should strive to make the routing reasonable, use less external jumper wires, and route the wires according to certain charging requirements, strive to be intuitive, easy to install, height and maintenance.

8. When designing a wiring diagram, try to make the wiring as simple and clear as possible with as few turns as possible.

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 consistent as possible with the spacing of the capacitor lead pins;

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