1 The processing of power supply and ground wire even if the wiring in the entire PCB board is completed well, but the interference caused by the insufficient consideration of the power supply and ground wire will reduce the performance of the product and sometimes even affect the product. Success rate. Therefore, the wiring of electricity and ground wire should be taken seriously, and the noise interference generated by electricity and ground wire should be minimized to ensure the quality of the product. Every engineer who is engaged in the design of electronic products understands the reason for the noise between the ground wire and the power cord, and now only expresses the reduced noise suppression: It is well known to add between the power supply and the ground wire Coupling capacitor. Widen the width of power supply and ground wire as much as possible. It is better that 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 line is 1.2 ～ 2.5 mm For the digital circuit PCB, a wide ground wire can be used to form a loop, that is, a ground net can be used (the analog circuit ground can not be used in this way). Use, connect all the unused places to the ground as a ground wire on the printed board. Or it can be made into a multi-layer board, with one layer for power and ground.
2 Common ground processing of digital circuits and analog circuits There are now many PCBs that are no longer single-function circuits (digital or analog circuits), but are composed of digital and analog circuits. Therefore, when wiring, you need to consider the mutual interference between them, especially the noise interference on the ground. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines are as far away as possible from sensitive analog circuit devices. For ground lines, the entire PCB has only one node to the outside world, so It is necessary to deal with the problem of digital and analog common ground inside the PCB, and the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but only at the interface between the PCB and the outside world (such as plugs). There is a short circuit between digital ground and analog ground. Please note that there is only one connection point. There are also common grounds on the PCB, which is determined by the system design.
3 When the signal lines are laid on the electrical (ground) layer when wiring on the multi-layer printed board, because there are not many lines left on the signal line layer, the additional layers will cause waste and will increase production by a certain amount. The workload and cost have increased accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, followed by the ground layer. Because it is best to preserve the integrity of the formation.
4 Treatment of connecting legs in large-area conductors In large-area grounding (electricity), the legs of commonly used components are connected to them. The processing of connecting legs needs to be comprehensively considered. In terms of electrical performance, the pads of the component legs are The copper surface is fully connected, but there are some hidden dangers to the welding assembly of the components such as: ① welding requires a high-power heater. ② It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, made a cross-shaped pad, called heat shield (heat shield) commonly known as thermal pad (Thermal), so that during welding due to excessive cross-section heat dissipation may produce virtual solder joints Sexuality is greatly reduced. The handling of the grounding (ground) legs of the multilayer board is the same.
5 The role of network systems in wiring In many CAD systems, wiring is determined by the network system. The grid is too dense, although the channel has increased, but the stepping is too small, and the amount of data in the field is too large, which will inevitably have higher requirements for the storage space of the device, and also the computing speed of computer electronic products Great influence. And some channels are invalid, such as occupied by the pads of the component legs or occupied by mounting holes and fixed holes. The grid is too sparse, and too few channels have a great influence on the deployment rate. Therefore, a grid system with reasonable density should be used to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of fewer 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 meets the rules formulated by the designer, and also to confirm whether the established rules meet the requirements of the printed board production process. Aspect: Whether the distance between the wire and the wire, the wire and the component pad, the wire and the through-hole, the component pad and the through-hole, the through-hole and the through-hole are reasonable, and whether they meet the production requirements. Is the width of the power line and ground line appropriate, and are the power source and ground line tightly coupled (low wave impedance)? Is there any place in the PCB where the ground wire can be widened. Have you taken the best measures for the key signal lines, such as the shortest length, adding protection lines, the input lines, and output lines are clearly separated? Whether the analog circuit and the digital circuit have their own independent grounds. Will the graphics added to the PCB (such as icons and annotations) cause a 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 mark is pressed on the device pad to avoid affecting the quality of the electrical equipment. Whether the edge of the outer frame of the power ground layer in the multi-layer board is reduced, such as the copper foil of the power ground layer exposed outside the board is easy to cause a short circuit. Overview The purpose of this document is to explain the process and some precautions of using PCBs, the PCB design software Power PCB, to provide design specifications for the designers of a working group so that designers can communicate and check each other.
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