The routing rate of automatic routing depends on a good layout, and the wiring rules can be preset, including the number of bending of the traces, the number of vias, the number of steps, and the like. Generally, the exploration type warp line is firstly connected, and the short lines are quickly connected, and then the labyrinth wiring is performed. First, the wiring to be cloth is optimized for the global routing path, and the laid line can be disconnected as needed. And try to re-route to improve the overall effect.
For the current high-density PCB design, it has been felt that the through-hole is not well adapted. It wastes a lot of valuable wiring channels. To solve this contradiction, blind hole and buried hole technology have emerged, which not only completes the function of the via hole. It also saves a lot of wiring channels to make the wiring process more convenient, smoother and more perfect. The PCB board design process is a complicated and simple process. To master it well, it also needs extensive electronic engineering design. When people go to their own experience, they can get the truth.
1. Power and ground processing
Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsistency of the power supply and the ground line may degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wires should be taken seriously, and the noise interference generated by electricity and ground wires should be minimized to ensure the quality of the products.
For every engineering engineer engaged in the design of electronic products, the reason for the noise between the ground and the power line is understood. Only the reduced noise suppression is described:
(1) It is well known to add a decoupling capacitor between the power supply and the ground.
(2) Try to widen the power supply, ground line width, preferably the ground line is wider than the power line, their relationship is: ground line > power line > signal line, usually the signal line width is: 0.2 ~ 0.3mm, the most Fine width up to 0.05~0.07mm, power cord is 1.2~2.5 mm
For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, a ground net is used for use (the ground of the analog circuit cannot be used in this way)
(3) Use a large-area copper layer for grounding, and connect the unused areas to the ground on the printed board for grounding. Or make a multi-layer board, power supply, ground line each occupy a layer.
2. Co-processing of digital circuits and analog circuits
Many PCBs are no longer single-function circuits (digital or analog circuits), but rather 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.
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 is as far as possible away from the sensitive analog circuit device. For the ground line, the whole human PCB has only one node to the outside, so The number of processing and the common ground must be handled inside the PCB, and the digital ground and the analog ground inside the board are actually separated from each other, but only at the interface where the PCB is connected to the outside (such as a plug). The digital ground is slightly shorted to the analog ground. Please note that there is only one connection point. There is also no common ground on the PCB, which is determined by the system design.
3. The signal wire is placed 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, the addition of layers will cause waste and increase the workload for production, and the cost will increase accordingly. To resolve this contradiction, 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 a large area of grounding (electricity), the legs of common components are connected to them, and the treatment of the connecting legs needs to be comprehensively considered. In terms of electrical performance, the pads of the component legs are perfectly connected with the copper surface, but There are some hidden dangers in the soldering assembly of the components. For example, 1 welding requires a high-power heater. 2 is easy to cause a virtual solder joint. Therefore, taking into account the electrical performance and process needs, the cross-shaped pad is called the thermal shield, which is commonly called the thermal pad. Therefore, the possibility of creating a solder joint due to the excessive heat of the cross section during soldering can be achieved. Sex is greatly reduced. The treatment of the grounding (ground) leg of the multilayer board is the same.
5. The role of the network system in the wiring
In many CAD systems, cabling is determined by the network system. The mesh is too dense, although the path is increased, but the stepping is too small, the data volume of the field is too large, which inevitably has higher requirements on the storage space of the device, and the computing speed of the object computer electronic product is also Great impact. Some of the paths are invalid, such as occupied by the pads of the component legs or occupied by the mounting holes and the holes. The grid is too sparse, and too few paths have a great impact on the routing rate. Therefore, there must be a sparse and reasonable grid system to support the wiring.
The distance between the legs of the 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 less than 0.1 inches of multiples, such as: 0.05 inches, 0.025 inches, 0.02 Inches and so on.
6. Design Rule Check (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and also to confirm whether the established rules meet the requirements of the printed board production process. The general inspection has the following aspects:
(1) Line and line, line and component pad, wire and through hole, component pad and through hole, whether the distance between the through hole and the through hole is reasonable, and whether the production requirements are met.
(2) Is the width of the power and ground lines appropriate, and is there a tight coupling between the power supply and the ground (low wave impedance)? Is there a place in the PCB that allows the ground line to be widened?
(3) Whether the best measures are taken for the key signal lines, such as the shortest length, the protection line, the input line and the output line are clearly separated.
(4) Whether the analog circuit and the digital circuit part have separate ground lines.
(5) Whether the graphics (such as icons and annotations) added to the PCB will cause a signal short circuit.
(6) Modify some unsatisfactory line shapes.
(7) 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, so as not to affect the quality of the electrical equipment.
(8) Whether the edge of the outer frame of the power supply ground layer in the multi-layer board is reduced, for example, the copper foil of the power supply ground layer is likely to cause a short circuit outside the exposed plate.