PCB wiring principles

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In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are all done for it. In the entire PCB, the design process of wiring is the most demanding, the most skillful, and the most labor-intensive. PCB wiring includes single-sided wiring, double-sided wiring, and multi-layer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, you can use interactive wiring to pre-wire the lines with stricter requirements. The edge lines of the input and output ends should avoid adjacent parallel lines to avoid reflection interference. If necessary, ground wire isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parallel wiring is prone to parasitic coupling. The routing rate of automatic wiring depends on a good layout. The wiring rules can be pre-set, including the number of bends in the routing, the number of vias, the number of steps, etc. Generally, exploratory wiring is performed first to quickly connect the short lines, and then maze wiring is performed. The wiring path of the connection to be routed is optimized globally. It can disconnect the routed lines as needed. And try to re-route to improve the overall effect. The through hole is not suitable for the current high-density PCB design, which wastes a lot of precious wiring channels. To solve this contradiction, blind and buried hole technologies have emerged. They not only complete the role of the through hole, but also save a lot of wiring channels to make the wiring process more convenient, smoother and more perfect. The design process of the PCB board is a complex and simple process. To master it well, the majority of electronic engineering designers need to experience it by themselves to get the true meaning. 1. Processing of power supply and ground wire: Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsiderate consideration of the power supply and ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power supply and ground wire should be taken seriously, and the noise interference generated by the power supply and ground wire should be reduced to a minimum to ensure the quality of the product. For every engineer engaged in the design of electronic products, they understand the cause of the noise between the ground wire and the power line. Now only the reduction type suppression of noise is described: It is well known that decoupling capacitors are added between the power supply and ground wires. Try to widen the width of the power supply and ground wires. It is best that the ground wire is wider than the power supply wire. The relationship between them is: ground wire > power supply wire > signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power supply wire is 1.2~2.5 mm. For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way). Use a large area of copper layer as a ground wire, and connect all unused areas on the printed circuit board to the ground as a ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each. 2. Common ground processing of digital circuits and analog circuits: Now many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog circuits. Therefore, when wiring, it is necessary to consider the mutual interference between them, especially the noise interference on the ground wire. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world, so the problem of digital and analog common ground must be handled inside the PCB. In fact, the digital ground and analog ground are separated inside the board. They are not connected to each other, but only at the interface where the PCB is connected to the outside world (such as plugs, etc.). There is a short circuit between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design. 3. Signal lines are laid on the power (ground) layer: When wiring a multi-layer printed circuit board, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the power (ground) layer. First, you should consider using the power layer, and then the ground layer. Because it is best to retain the integrity of the ground layer. 4. Treatment of connecting legs in large-area conductors: In large-area grounding (electricity), the legs of common components are connected to it. The treatment of connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but there are some bad hidden dangers for the welding and assembly of components, such as: ① Welding requires a high-power heater. ② It is easy to cause cold solder joints. Therefore, taking into account both electrical performance and process requirements, a cross-shaped pad is made, which is called heat shield, commonly known as thermal pad. In this way, the possibility of cold solder joints caused by excessive heat dissipation in the cross section during welding can be greatly reduced. The treatment of the connecting (ground) layer legs of multi-layer boards is the same. 5. The role of the network system in wiring: In many CAD systems, wiring is determined by the network system. The grid is too dense, and although the number of pathways has increased, the step is too small, and the amount of data in the drawing field is too large, which will inevitably have higher requirements for the storage space of the equipment, and also have a great impact on the computing speed of computer-related electronic products. Some paths are invalid, such as those occupied by the pads of the component legs or by the mounting holes and fixed holes. Too sparse grids and too few paths have a great impact on the routing rate. Therefore, a grid system with reasonable density is required to support the routing. The distance between the two legs of standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches, etc. 6. Design Rule Check (DRC): After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer, and also to confirm whether the rules set meet the requirements of the printed circuit board production process. The general inspection includes the following aspects: Whether the distance between lines, lines and component pads, lines and through holes, component pads and through holes, and through holes is reasonable and meets production requirements. Are the widths of the power and ground lines appropriate? Are the power and ground lines tightly coupled (low wave impedance)? Is there any place in the PCB where the ground line can be widened? Have the best measures been taken for key signal lines, such as the shortest length, adding protection lines, and clearly separating the input and output lines? Do the analog and digital circuit parts have their own independent ground lines? Will the graphics added to the PCB later (such as icons, annotations) cause signal short circuits? Modify some undesirable line shapes. Are process lines added to the PCB? Does the solder mask meet the requirements of the production process? Is the solder mask size appropriate? Are the character logos pressed on the device pads to avoid affecting the quality of the electrical equipment? Is the outer frame edge of the power ground layer in the multilayer board reduced? If the copper foil of the power ground layer is exposed outside the board, it is easy to cause a short circuit.

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In addition to the Top layer and the Bottom layer, what are the other two layers of a four-layer board? Are they mechanical layers 1 and 2?

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OP, what is interactive wiring?

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What is the number of steps?

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What is the cause of the noise between the ground wire and the power wire?