PCB wiring design methods and requirements

In this article, we will discuss the PCB cabling design methods and requirements. The layout design of components and the direction of electrical connections may produce different results, and the results may vary greatly, therefore, the structure of printed circuit board components should be correctly designed and the correct PCB design should be selected.

I. Design of layout structure of printed line components

An instrument with excellent performance, in addition to selecting a proper circuit for high-quality components, the layout of printed circuit board components and the correct Structure Design of Electrical connections are a key issue that determines whether the instrument can work reliably, for the circuit of the same component and parameter, the layout design of the component and the direction of the electrical connection may produce different results, and the results may be very different. Therefore, we must combine three aspects: how to correctly design the layout of printed circuit board components, how to correctly select the PCB wiring direction, and the overall process structure of the instrument, and a reasonable process structure, it can eliminate noise interference caused by improper PCB wiring design and facilitate installation during production. Debugging and repair.

As there is no strict "Definition" or "Mode" in the excellent "structure", we will discuss the above issues for reference only. The structure of each instrument must comply with specific requirements (electrical performance. The system adopts the corresponding structure design scheme, and compares and repeatedly modifies several feasible design schemes. Printed Board power supply. Local Bus cabling structure selection-System Structure: analog and digital circuits have many similarities and differences in the design of component layout diagrams and PCB Cabling Design methods. In a analog circuit, the extremely low noise voltage generated by the PCB wiring design may cause serious distortion of the output signal. In a digital circuit, the TTL noise tolerance is-4 V ~ 0.6 V. The CMOS noise margin is 0.3 ~ Of The Vcc ~ 0.45 times, so digital circuits have strong anti-interference capabilities. Reasonable Selection of good power supply and ground bus modes is an important guarantee for the reliable operation of the instrument. A considerable number of sources of interference are generated by power supply and ground bus, among which the ground line causes the most noise interference.

II. Basic principles and requirements of printed circuit board diagram design

1. the design of the printed circuit board starts from determining the size of the Board. The size of the printed circuit board is limited by the size of the chassis so that it can be placed in the shell. Secondly, the printed circuit board and external components (mainly potentiometer) should be considered. Socket or other printed circuit board) connection mode. Printed circuit boards and external components are generally connected offline through plastic wires or Metal insulation. But sometimes it is designed as a socket. That is, to install a plug-in printed circuit board in the device, leave the contact position to act as the plug port. For the larger components installed on the printed circuit board, the metal accessories should be fixed to improve vibration resistance. Impact Resistance.

2. Basic PCB layout diagram design method: First, you need to configure the selected components and various sockets. Size. Have a complete understanding of the area, and make reasonable arrangements for the location of each part. Careful consideration is mainly from the electromagnetic field compatibility. Anti-interference angle, short cabling, less crossover, power supply, ground path and decoupling. After the positions of the components are determined, the components are connected to the relevant pins according to the circuit diagram. There are various methods to complete the process. The design of the printed circuit diagram includes two methods: computer-aided design and manual design. The original layout is arranged manually. This is a waste of time. It usually takes several times to complete the process. This is also possible if there are no other drawing devices, this manual layout method is also very helpful for the designers who just learned the layout chart. Computer-aided plotting now has a variety of plotting software with different functions, but in general, plotting. The modification is convenient, and can store disks for storage and Sakura. Then, determine the size of the printed circuit board, and preliminarily determine the positions of each component according to the principle diagram, then, the layout is adjusted to make the layout more reasonable.

The wiring of each component in a printed circuit board is arranged as follows:

(1) crossover circuits are not allowed in printed circuits. You can "drill" the lines that may be crossed ". There are two solutions. That is, let a lead resistance from another. Capacitor. The gap at the foot of the transistor is "drilled" in the past, or "wound" at one end of a lead that may be crossed. In special circumstances, how is the circuit very complicated, in order to simplify the design, it also allows the use of wire cross-connection to solve the cross circuit problem.

(2) resistance. Diode. Tubular capacitors and other components have two installation methods: vertical and horizontal. Vertical refers to the installation of component bodies perpendicular to the circuit board. The advantage of welding is space saving. Horizontal refers to the parallel installation of components closely related to the circuit board and welding. Its advantage is that the mechanical strength of component installation is good. For these two different installation elements, the component hole distance on the printed circuit board is different.

(3) The grounding point of the same level-1 circuit should be as close as possible, and the power filter capacitor of the level-1 circuit should also be connected to the level-1 grounding point. Especially the base pole of the current transistor. The grounding point of the emission pole cannot be too far away. Otherwise, the copper foil between the two grounding points is too long, which may cause interference and self-excitation. Using such a "one-point grounding" circuit, the operation is relatively stable, not self-excited.

(4) The total ground line must be arranged strictly in the order of high-frequency, medium-frequency, and low-frequency levels from weak current to strong current. It cannot be switched over or over again. The level and level are more likely to be connected to the Long Point of the line, this provision must also be observed. In particular, the frequency conversion head. Regeneration header. The ground line of the FM head has stricter requirements. If there is any improper arrangement, it will generate self-excitation and thus cannot work. Frequent frequency circuits of the FM class usually use a large area enclosed ground wire to ensure good shielding effect.

(5) Strong current leads (public ground wires, power amplifier leads, etc.) should be as wide as possible to reduce the PCB wiring resistance and voltage drop, and reduce the self-excitation caused by parasitic coupling.

(6) The Strip with high impedance should be as short as possible, and the strip with low impedance can be longer, because the strip with high impedance is easy to generate flute and absorb signals, resulting in unstable circuit. Power cord. Ground. Baseline without feedback components. The transmitting pole leads are the baseline of Low-impedance cabling and follow-up devices. The ground wires of the two sound channels of the recorder must be separated, each of which is independent of each other until the end of the efficacy is combined. For example, two ground wires are connected to each other, which can easily produce crosstalk and reduce the degree of separation.

3. Pay attention to the following points in the design of the Printed Panel chart

1. PCB wiring direction: From the Perspective of the welding surface, the arrangement orientation of components should be as consistent as possible with the schematic diagram. The PCB wiring direction should be the same as the wiring direction of the circuit diagram, because various parameters of the welding surface need to be checked during the production process, it is easy to inspect, debug and maintain the welding surface (note: it must meet the requirements of circuit performance, device installation, and panel layout ).

2. Various components should be arranged in a reasonable and even manner, and strive for neat, beautiful, and well-structured process requirements.

3. resistor and diode placement: divided into two types: horizontal and vertical:

(1) flat release: when the number of circuit components is small and the size of the circuit board is large, the flat release is usually better. When the resistance below 1/4 W is flat, the distance between two pad is generally 4/10 inch, when the resistance of 1/2 W is flat, the gap between the two pad is generally 5/10 inch; when the diode is flat, the 1N400X series of the entire tube, generally 3/10 inch; n540X series FCL, generally 4 ~ 5/10 inch.

(2) vertical layout: when the number of circuit components is large and the size of the circuit board is small, vertical layout is generally used. When vertical layout is used, the gap between the two pad is generally 1 ~ 2/10 inch.

4. potentiometer: IC seat Placement Principle

(1) potentiometer: used to adjust the output voltage in the voltage regulator. Therefore, when the design potentiometer is full, the output voltage increases clockwise and the output voltage decreases at the anti-clockwise regulator; in the Adjustable constant current charger, the potentiometer is used to adjust the charge current fold size. When the design potentiometer should be filled with a clockwise adjustment, the current increases. The potentiometer shall be filled with the structural installation of the entire machine and the layout of the Panel. Therefore, it shall be placed at the edge of the Board as much as possible, and the rotating handle shall be placed outward.

(2) IC seat: when designing a Printed Panel chart, when using the IC seat, be sure to pay special attention to whether the orientation of the positioning slot on the IC seat is correct, and pay attention to whether the foot positions of each IC are correct, for example, the 1st foot can only be located in the lower-right or upper-left corner of the IC seat, and is close to the positioning slot (from the welding surface ).

5. cabling

(1) do not have too much distance between the associated two leads, generally 2 ~ About 3/10 inch is suitable.

(2) Try to concentrate the inbound and outbound ends on one to two sides. Do not be too discrete.

6. When designing a PCB layout, pay attention to the pin arrangement order, and ensure proper pin spacing.

7. While ensuring the circuit performance requirements, the design should strive for reasonable cabling, less use of external cross-line, and follow certain charge requirements to walk the line, and strive to be intuitive, easy to install, high and repair.

8. When designing the PCB wiring design drawing, the line should be bent as little as possible, and the lines should be simple and clear.

9. when designing PCB wiring, the width and line spacing should be moderate. The gap between the two pad of the capacitor should be as close as possible to the gap between the capacitive Wire Guide; 10. the design should be carried out in a certain order, for example, from left to right and from top to bottom. Source: Sina Blog of the mouse)

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