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PCB wiring is very important in the entire PCB design. How to achieve fast and efficient wiring and make your PCB wiring look tall is worth studying.
1. Common ground processing of digital circuit and analog circuit
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated, they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection 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.
2. The signal line is laid on the electric (ground) layer
When wiring a multilayer printed circuit board, since there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase a certain amount of work in production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second.
Because it is best to preserve the integrity of the formation. Baineng.com is a subsidiary of Qinji Group and is a leading domestic electronic industry service platform. It provides online components, sensor procurement, PCB customization, BOM distribution, material selection and other electronic industry supply chain complete solutions, one-stop to meet the electronics industry The overall needs of small and medium-sized customers in the industry.
3. Treatment of power supply and ground wire
Even if the wiring in the entire PCB board is completed very well, the interference caused by the improper consideration of the power supply and the 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 minimized to ensure the quality of the product. Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is expressed: it is well-known to add the noise between the power supply and the ground wire. Lotus capacitor. Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the finest width can reach 0.05 ～0.07mm, the power cord is 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way) Use a large area of copper layer for the ground wire, which is not used on the printed board Connected to the ground as a ground wire in all places. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.
4. The role of the network system in cabling
In many CAD systems, wiring is determined by the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. So there must be a reasonable grid system to support the wiring. The distance between the 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 integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.
5. Treatment of connecting legs in large area conductors
In large-area grounding (electricity), the legs of common components are connected to it, and the treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers for the welding and assembly of components, such as: 1. Welding requires high-power heaters. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, which are called heat shields, commonly known as thermal pads. In this way, virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the electrical connection (ground) leg of the multilayer board is the same.
6. Design Rule Check (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and at the same time, it is necessary to confirm whether the established rules meet the requirements of the printed board production process. The general inspection has the following aspects: line and line, line Whether the distance between the component pad, line and through hole, component pad and through hole, and through hole and through hole is reasonable, and whether it meets the production requirements. Whether the width of the power line and the ground line are appropriate, whether the power supply and the ground line are tightly coupled (low wave impedance), whether there is a place in the PCB that allows the ground line to be widened. Whether the best measures have been taken for the key signal lines, such as the shortest length, the protection line is added, and the input line and output line are clearly separated. Whether there are separate ground wires for analog circuit and digital circuit. Whether the graphics (such as icons and annotations) added to the PCB will cause signal short circuit. Modify some undesirable line shapes. Is there a process line on the PCB? Whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad, so as not to affect the quality of the electrical equipment. Whether the outer frame edge of the power ground layer in the multi-layer board is reduced, if the copper foil of the power ground layer is exposed outside the board, it is easy to cause a short circuit.
7. Via design
Via is one of the important components of multilayer PCB, and the cost of drilling usually accounts for 30% to 40% of the cost of PCB manufacturing. Simply put, every hole on the PCB can be called a via. From the point of view of function, vias can be divided into two categories: one is used for electrical connections between layers; the other is used for fixing or positioning devices. In terms of process, vias are generally divided into three categories, namely blind vias, buried vias and through vias.
