Electronic Design - Basic principles of PCB board drawing design

This article mainly introduces the basic principles and precautions of PCB design in PCB design. We know that an instrument and equipment with excellent performance, in addition to selecting high-quality components and reasonable circuits, the layout of printed circuit boards and electrical connections The correct structure of the direction PCB design is a key issue that determines whether the instrument can work reliably. For circuits with the same components and parameters, different results will be produced due to the different component layout PCB design and electrical connection directions, and the results may be very large. The difference. Therefore, it is necessary to combine the three aspects of how to correctly design the PCB layout of the printed circuit board component layout, the correct selection of the wiring direction and the process structure of the overall instrument. A reasonable process structure can eliminate the noise interference caused by improper wiring, and at the same time It is convenient for installation, debugging and maintenance in PCB production.

In the following we will discuss the above issues. Since there is no strict "definition" and "model" for a good "structure", the following discussion will only serve as an introduction and is for reference only. The structure of each instrument must be based on specific requirements (electrical performance, overall structure installation and panel layout requirements), adopt the corresponding structural PCB design scheme, and compare and repeatedly modify several feasible PCB design schemes.

PCB board power, ground bus wiring structure selection-system structure: analog circuits and digital circuits have many similarities and differences in the PCB design and wiring methods of the component layout. In the analog circuit, due to the existence of the amplifier, the very small noise voltage generated by the wiring will cause serious distortion of the output signal. In the digital circuit, the TTL noise tolerance is 0.4V ~ 0.6V, and the CMOS noise tolerance is 0.3 of Vcc. ~ 0.45 times, so the digital circuit has a strong anti-interference ability.

The reasonable choice of a good power and ground bus mode is an important guarantee for the reliable operation of the instrument. Quite a lot of interference sources are generated through the power and ground bus, and the noise interference caused by the ground wire is the largest.

One, the basic principle requirements of PCB board drawing design

1. The PCB design of the PCB board starts from determining the size of the board. The size of the PCB board is limited by the size of the chassis shell. Connector, socket or another PCB board). The PCB board and external components are generally connected by plastic wires or metal isolation wires. But sometimes PCB is designed into socket form. That is: to install a plug-in PCB board in the device, leave a contact position as a socket.

For larger components installed on the PCB board, metal accessories should be added to fix them to improve vibration and impact resistance.

2. The basic method of wiring diagram PCB design

First of all, it is necessary to have a complete understanding of the specifications, dimensions, and areas of the selected components and various sockets; reasonable and careful consideration of the location of each component, mainly from the perspective of electromagnetic field compatibility and anti-interference. Short line, less crossover, power supply, ground path and decoupling are considered. After the position of each component is determined, it is the connection of each component. Connect the relevant pins according to the circuit diagram. There are many ways to complete the PCB design of the printed circuit diagram. There are two methods of computer-aided PCB design and manual PCB design.

The most primitive is to arrange the layout by hand. This is more laborious, and it often takes several iterations to complete it. This is also possible when there is no other drawing equipment. This manual layout method is also very helpful for designers who are just learning PCB board drawings. Computer-aided drawing, now there are many kinds of drawing software with different functions, but generally speaking, drawing and modification are more convenient, and they can be saved and printed.

Next, determine the required size of the PCB board, and according to the schematic diagram, initially determine the position of each component, and then make the layout more reasonable after continuous adjustment. The wiring arrangement between the components in the PCB board is as follows:

(1) Cross circuits are not allowed in printed circuits. For lines that may cross, you can use "drilling" and "winding" to solve them. That is, let a certain lead "drill" through the gap under other resistors, capacitors, and triode pins, or "wind" from one end of a lead that may cross. In special circumstances, how the circuit is complicated, in order to simplify the PCB design It is also allowed to use wires to bridge to solve the cross circuit problem.

(2) Components such as resistors, diodes, and tubular capacitors can be installed in "vertical" and "horizontal" installation methods. The vertical type refers to the installation and welding of the component body perpendicular to the circuit board, which has the advantage of saving space. The horizontal type refers to the installation and welding of the component body in parallel and close to the circuit board, and its advantage is that the mechanical strength of the component installation is better. For these two different mounting components, the component hole spacing on the PCB board is different.

(3) The grounding point of the same level circuit should be as close as possible, and the power filter capacitor of this level circuit 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 should not be too far apart, otherwise the copper foil between the two grounding points will be too long, which will cause interference and self-excitation. Using such a "one-point grounding method" circuit will work better. Stable and not easily self-excited.

Two, the following points should be paid attention to in PCB board drawing design

1. Wiring direction: From the perspective of the welding surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction is best to be consistent with the wiring direction of the circuit diagram. Because the welding surface usually needs to be tested for various parameters during the production process, so This is convenient for inspection, debugging and maintenance during production (Note: It refers to the premise that the circuit performance and the installation and panel layout requirements of the whole machine are met).

2. The arrangement and distribution of the components should be reasonable and even, and strive to be neat, beautiful, and strict in structure.

3. Resistor and diode placement methods: there are two types: horizontal placement and vertical placement:

(1) Lay flat: When the number of circuit components is small and the size of the circuit board is large, it is generally better to lay flat; for resistors below 1/4W, the distance between the two pads is general. Take 4/10 inches, when the 1/2W resistor is placed flat, the distance between the two pads is generally 5/10 inches; when the diode is placed flat, 1N400X series rectifier tubes, generally take 3/10 inches; 1N540X series rectifier tubes, generally Take 4 to 5/10 inches.

(2) Vertical installation: When there are a large number of circuit components and the size of the circuit board is not large, the vertical installation is generally adopted, and the distance between the two pads is generally 1 to 2/10 inches in the vertical installation.

4. Potentiometer: the placement principle of IC holder

(1) Potentiometer: It is used to adjust the output voltage in the voltage regulator, so the PCB design potentiometer should be fully adjusted clockwise when the output voltage rises, and the output voltage of the counterclockwise regulator decreases when the output voltage is reduced; in the adjustable constant current charging The potentiometer in the device is used to adjust the size of the charging current. When the PCB design potentiometer should be fully adjusted clockwise, the current will increase.

The potentiometer should be placed in a position that meets the requirements of the whole machine structure installation and panel layout. Therefore, it should be placed on the edge of the board as much as possible, with the rotating handle facing outward.

(2) IC holder: When designing the PCB board drawing, when using the IC holder, pay special attention to whether the positioning slot on the IC holder is placed in the correct position, and pay attention to whether the IC pins are correct, for example, the first pin can only be used. It is located at the lower right or upper left corner of the IC socket, and is close to the positioning groove (viewed from the welding surface).