PCB cabling procedure

PCB cabling procedure Reprinted from: http://blog.21ic.com/user1/6960/archives/2011/83333.html

I. Circuit Board Design Procedure Generally, the most basic process of designing a circuit board can be divided into three steps. (1). Circuit Principle The schematic design of the circuit diagram is mainly the schematic design of protel099. System (Advanced schematic) to draw a circuit schematic. In this process, make full use of the various schematic drawing tools and various edits provided by protel99. Function To achieve our goal, that is, to get a correct and exquisite circuit schematic. (2). Generate Network Table network table is a schematic circuit design (Sch) and printed circuit board design (PCB ). It is the soul of the circuit board. The network table can be obtained from the circuit schematic diagram or from the printed circuit board. (3 ). the Design of printed circuit board the design of printed circuit board is mainly for another important part of protel99 PCB. In this process, we use the powerful functions provided by protel99 to achieve the layout design of the circuit board, complete difficult tasks. Ii. Easy to draw Circuit Diagram 2.1 schematic design process schematic design can be completed according to the following process. (1) After the design drawing size is Protel 99/schematic, you must first design the part drawing and design the drawing size. The drawing size depends on the size and complexity of the circuit diagram, Set The proper drawing size is the first step to design the schematic. (2) configure the Protel 99/schematic design environment and Protel 99/schematic design environment, including setting the lattice size and type, and the cursor type. Most parameters can also use the system default value. (3) According to the needs of the circuit diagram, the rotating parts are extracted from the parts warehouse and placed on the drawings. Encapsulation For definition and setting. (4) A schematic wiring uses various tools provided by Protel 99/schematic to connect the components on the drawing with wires and symbols of electrical significance to form a complete schematic. (5) Adjust the line and further adjust and modify the preliminary drawn circuit diagram to make the schematic diagram more beautiful. (6) Reports Output Various reports are generated using various report tools provided by Protel 99/schematic. The most important report is the network table, which prepares for subsequent circuit board design. (7) File The last step to save and print the output is to save the file and print the output. Single Chip Microcomputer The design principles of the control panel are as follows: (1) In terms of the layout of components, we should try to keep related components close to each other, such as the clock generator, crystal oscillator, and CPU clock. Input Terminals are easy to generate noise, so they should be placed closer. For devices that are easy to generate noise Current Circuit, Large Current Circuit Switch Circuit, etc., should try to keep it away from the microcontroller logical control circuit and storage circuit (ROM, Ram), if possible, you can make these circuits into another circuit board, this is conducive to anti-interference, improve the circuit work availability. (2) Try to use key components, such as ROM and ram. Chip Install decoupling nextCapacitor . In fact, printed circuit board cabling, pin connections, and wiring may all contain a large inductance effect. A large inductance may cause a severe switch noise Spike when the VCC goes online. The only way to prevent the VCC from switching noise spikes is Power Supply Place a 0.1uf Electronics Decoupling capacitor. If a surface mount component is used on the circuit board, you can use the slice capacitor to directly pin the component and fix it on the VCC pin. It is best to use a porcelain capacitor because the capacitor has a low electrostatic loss (ESL) and High-Frequency Impedance. In addition, the temperature and Time Medium stability is also very good. Do not use the TA capacitor whenever possible, because it has a high impedance at a high frequency. Pay attention to the following points when placing the decoupling capacitor: The power input of the printed circuit board is connected to an electrolytic capacitor of about UF. If the volume permits, it is better to Increase the capacity. In principle, a 0.01uf porcelain chip capacitor should be placed next to each integrated circuit chip. If the gap between the circuit board is too small to be placed, one or more chips can be placed every 10 ~ 10. When the anti-interference capability is weak and the current changes greatly during shutdown, and storage components such as Ram and Rom, the decoupling capacitor should be connected between the power cord (VCC) and the ground wire. The lead of the capacitor should not be too long. In particular, the high-frequency bypass capacitor cannot contain the lead. (3) In the single-chip microcomputer control system, there are many types of ground wires, such as systematic, shielded, logical, Simulation Whether the layout of the ground is reasonable determines the anti-interference capability of the circuit board. When designing ground and ground points, we should consider the following issues: logically and analog ground should be separately cabled, cannot be used together, and their respective Ground Wires should be connected to the corresponding power ground wires respectively. During the design, the simulated ground should be as bold as possible, and the grounding area of the lead end should be increased as much as possible. Generally, input and output SimulationSignal , It is best to isolate from single-chip microcomputer circuit through optical coupling. When designing a printed circuit, the ground line should form a closed loop to improve the anti-interference ability of the circuit. The ground line should be as rough as possible. If the ground line is very fine, the ground line Resistance It will be large, resulting in changes in the grounding potential with the current, resulting in signal level instability, leading to the decline of the circuit's anti-interference ability. Make sure that the width of the main ground wire is at least 2 ~ More than 3mm, the grounding wire on the element pin should be around 5mm. Pay attention to the selection of grounding points. When the signal frequency on the circuit board is lower than 1 MHz, the electromagnetic induction between the wiring and components has little effect, while the circulation formed by the grounding circuit has a great impact on the interference, so that it does not form a loop. When the signal frequency on the circuit board is higher than 10 MHz, the ground impedance becomes very large due to the obvious inductance effect of the wiring, and the circulation formed by the grounding circuit is no longer the main problem. Therefore, multi-point grounding should be adopted to minimize the ground impedance. The layout of the power cord should not only strip the width of the line as much as possible according to the current size, but also make the direction of the power cord and ground line and Data At the end of the cabling, the baseline of the circuit board is fully covered with a ground wire. These methods help to enhance the anti-interference capability of the circuit. The data line width should be as wide as possible to reduce the impedance. The data line must be at least 0.3mm (12mil) in width ~ 0.5mm (18mil ~ 20 mil) is more ideal. Because a circuit board through the hole will bring about 10pf capacitor effect, this for high-frequency circuit, will introduce too much interference, so when wiring, should try to reduce the number of through hole. Furthermore, excessive passing holes may reduce the mechanical strength of the circuit board.