In
PCB board 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 board, the design process of wiring is highly limited, skillful, and workload heavy. PCB board wiring includes single-sided wiring, double-sided wiring and multi-layer wiring. There are also two ways of routing: automatic routing and interactive routing. Before automatic routing, you can use interactive pre-routing for lines with stricter requirements. The edges of the input end and the output end should avoid adjacent parallels to avoid reflection interference. If necessary, ground wire isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling will easily occur in parallel. The routing rate of automatic routing depends on a good layout. The routing rules can be preset, including the number of bends of the traces, the number of vias, and the number of steps. Generally, exploratory wiring is performed first, and short lines are quickly connected, and then labyrinth wiring is performed. And try re-wiring to improve the overall effect. The current high-density PCB board design has felt that the through hole is not suitable. It wastes many valuable wiring channels. It also saves many wiring channels to make the wiring process more convenient, smoother and more complete. The design process of PCB boards is a complex and simple process. Engineering designers can only get the true meaning of it by experiencing it for themselves.
1. Treatment of power supply and ground wire
Even if the wiring in the entire
PCB board is well completed, the interference caused by the lack of thoughtful 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 and ground wires should be taken seriously, and the noise interference generated by the power and ground wires should be reduced to a minimum to ensure the quality of the products. Every engineer who is engaged in the design of electronic products understands the reasons for the noise between the ground wire and the power wire. Now only the reduced noise suppression is expressed:
1) It is well known to add decoupling capacitors between the power supply and the ground wire.
2) Try to widen the width of the power and ground wires, preferably the ground wire is wider than the power wire. ～0.07mm, the power line is 1.2～2.5mm, and the PCB board of the digital circuit can be used to form a loop with a wide ground wire, that is, a ground net can be used (the ground of the analog circuit cannot be used in this way).
3) Use a large-area copper layer as a ground wire, and connect the unused places on the printed board to the ground as a ground wire. Or make a multi-layer board, power supply, ground wire each occupy one layer.
2. Common ground processing of digital circuits and analog circuits
Now there are many PCB boards that are no longer a single function circuit (digital or analog circuit), 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 kept away from the sensitive analog circuit devices as much as possible. For the ground line, the whole PCB board has only one node to the outside world. Therefore, the problem of digital and analog common ground must be dealt with inside the PCB board, and the digital ground and the analog ground are actually separated inside the board, and they are not connected to each other, only at the interface between the PCB board and the outside world (such as plugs). Wait). The digital ground is a little shorted to the analog ground, note that there is only one connection point. There are also no common ground on the PCB board, which is determined by the system design.
3. Signal lines are routed on the electrical (ground) layer
In the wiring of multi-layer printed boards, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, we can consider wiring on the electrical (ground) layer. The power plane should be considered first, followed by the ground plane. Because the best is to preserve the integrity of the formation.
4. Handling of connecting legs in large area conductors
In a large area of grounding (electricity), the legs of commonly used components are connected to it, and the handling of the connecting legs needs to be comprehensively considered. There are some hidden dangers in the welding assembly of components, such as: 1) Welding requires high-power heaters. 2) It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, cross-shaped pads are made, which are called thermal isolation and commonly known as thermal pads. In this way, the possibility of virtual solder joints caused by excessive heat dissipation of the cross-section during welding can be greatly reduced. The electrical (ground) leg of a multilayer board is treated the same way.
5. The role of the network system in wiring
In many CAD systems, wiring is determined by the network system. If the grid is too dense, although the number of channels is increased, the step is too small, and the amount of data in the image field is too large, which must have higher requirements on the storage space of the equipment, and also affect the computing speed of computer electronic products. great influence. And some vias are invalid, such as those occupied by pads of component legs or occupied by mounting holes, positioning holes, etc. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a grid system with reasonable density 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 (2.54mm) or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches etc.
6. Design Rule Checking (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules formulated by the designer, and it is also necessary to confirm whether the rules formulated meet the requirements of the printed board production process. Generally, the following aspects are checked:
1) Whether the distance between wire and wire, wire and component pad, wire and through hole, component pad and through hole, and through hole and through hole is reasonable, and whether it meets the production requirements.
2) Are the widths of the power and ground wires appropriate, and are the power and ground wires tightly coupled (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?
3) Whether the best measures have been taken for the key signal lines, such as short lengths, protective lines, and the input lines and output lines are clearly separated.
4) Whether the analog circuit and the digital circuit part have their own independent ground wires.
5) Whether the graphics (such as icons, labels) added to the PCB board will cause a short circuit of the signal.
6) Modify some unsatisfactory line shapes.
7) Is there a process line on the PCB board? 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 electrical quality.
8) 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, it is easy to cause a short circuit.
High-speed circuit design is a very complex design process. The method described in this paper is specifically aimed at solving these high-speed circuit design problems. In addition, there are several factors that need to be considered when designing high-speed circuits, and these factors are sometimes in opposition to each other. If high-speed devices are placed close together, delays can be reduced, but crosstalk and significant thermal effects can occur. Therefore, in the design, it is necessary to weigh various factors and make a comprehensive compromise consideration; it not only meets the design requirements, but also reduces the design complexity. The use of high-speed
PCB board design methods constitutes the controllability of the design process. Only controllable ones are reliable and successful.
