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How to design PCB wiring

How to design PCB wiring

2019-07-26

In PCB design, wiring is an important step to complete product design. It can be said that the preparation work is done for it. In the whole PCB, the design process of wiring is the highest, the skills are the finest and the workload is the largest.

PCB 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, more stringent requirements can be pre-routed by interactive routing. The input end and the output end should avoid adjacent parallel lines, so as to avoid reflective interference. If necessary, the ground wire should be isolated, and the wiring of two adjacent layers should be perpendicular to each other, so parasitic coupling can easily occur in parallel. The routing rate of automatic routing depends on a good layout. The routing rules can be set in advance, including the number of bends, the number of through holes, the number of steps, etc.

At present, the design of high density PCB has not been adapted to the through hole. It wastes a lot of valuable wiring channels. To solve this contradiction, blind hole and buried hole technology have appeared. It not only completes the function of through hole, but also saves many wiring channels to make the wiring process more convenient, more smooth and more perfect. PCB board design process is a complex and simple process, in order to grasp it well, it still needs the majority of electronic engineering designers to experience, in order to get the true meaning of it.

Processing of Power Supply and Ground Wire

Even though the wiring in the whole PCB board is very good, the interference caused by the improper consideration of power supply and ground wire will degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of power and ground wires should be taken seriously, and the noise interference caused by power and ground wires should be reduced to a minimum, so as to ensure the product quality.
1, It is well known that decoupling capacitors are added between power supply and ground wire.
2, To widen the width of power supply and ground wire as far as possible, preferably the ground wire is wider than the power line. Their relationship is: ground wire > power line > signal line. Usually, the signal line width is 0.2-0.3 mm, the longest narrow width is 0.05-0.07 mm, and the power line is 1.2-2.5 mm.
3, The PCB of digital circuits uses wide ground wires to form a loop, i.e. a ground network for use (analog circuits cannot be used in this way).
4, Use large copper layers as ground wires, and connect unused places to ground wires on printed boards. Or it can be made into multi-layer board, power supply, ground wire each occupies one layer.

Common Ground Processing of Digital Circuit and Analog Circuit

Nowadays, many PCBs are no longer single functional circuits (digital or analog circuits), but composed of digital and analog circuits. Therefore, when wiring, it is necessary to consider the interference between them, especially the noise interference on the ground line.
The frequency of digital circuit is high and the sensitivity of analog circuit is strong. For signal line, the high frequency signal line is as far away from the sensitive analog circuit device as possible. For ground line, PCB has only one node to the outside. Therefore, it is necessary to deal with the problem of digital and analog common ground in PCB, while in board, it is practically digital and analog. They are separated from each other, but only at the interface between PCB and the outside world (such as plugs). There is a little short connection between digital and analog. Please note that there is only one connection point. There are also differences in PCB, which is determined by system design.


Signal circuits are laid on the electric (ground) layer

In the multi-layer PCB routing, because there are not enough lines left in the signal layer, adding more layers will result in waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, we can consider wiring on the electric (ground) layer. First of all, the power supply layer should be considered, and then the ground, because it is best to preserve the integrity of the ground.

Processing of Connecting Legs in Large Area Conductors

In large area grounding (electricity), the legs of common components are connected with them, and the treatment of the connecting legs needs comprehensive consideration. As far as the electrical performance is concerned, the welding pad of the component legs is better to be fully connected with the copper surface, but there are some hidden dangers in the soldering assembly of the components, such as: 1) Soldering requires a high-power heater. (2) It is easy to cause faulty soldered joints. Therefore, considering the both electrical performance and technological requirements, a cross-shaped pad, known as heat shield, commonly known as thermal pad, can greatly reduce the possibility of cold solder joints due to excessive cross-section heat dissipation during welding. The legs of the multi-layer board are treated the same.

The Role of Network System in wiring

In many CAD systems, wiring is determined by the network system. Although the grid is too dense, the path is increased, but the step is too small and the amount of data in the field is too large. This will inevitably require higher storage space of the equipment, but also has a great impact on the computing speed of object computer electronic products. Some paths are invalid, such as those occupied by pads of component legs or by mounting holes, fixed holes, etc. Over-sparse grids and too few paths have a great impact on the routing rate. So we need a dense and reasonable grid system to support wiring.
The distance between the legs of standard components is 0.1 inch (2.54 mm), so the basis of grid system is usually 0.1 inch (2.54 mm) or an integral multiple of less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.

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. At the same time, it is also necessary to confirm whether the rules formulated conform to the requirements of the production process of printed circuit boards. Generally, the following aspects are inspected:

1. Whether the distance between wire and wire, wire and component pad, wire and through hole, component pad and through hole, through hole and through hole is reasonable or not, and whether it meets the production requirements.
2. Are the widths of the power and ground wires appropriate, and are they tightly coupled (low wave impedance)? Is there any room in the PCB to widen the ground wire?
3. Whether the best measures have been taken for the key signal lines, such as the shortest length, the addition of protective lines, the obvious separation of input lines and output lines.
4. Whether the analog and digital circuits have separate ground wires.
5. Whether the graphics (such as icons and annotations) added to PCB will cause short circuit of the signal.
6. Modify some unsatisfactory lines.
7. Is there a process line on the PCB? Whether the welding resistance meets the requirements of the production process, whether the size of the welding resistance is appropriate, and whether the character mark is pressed on the pad of the device, in order to avoid affecting the installation quality.
8. Whether the edge of the outer frame of the power supply layer in the multilayer board is narrowed or not, for example, the copper foil of the power supply layer is prone to short circuit outside the exposed plate.

In short, the above skills and methods are all based on experience. In the process of designing PCB drawings, besides skilled use of drawing tools and software, you also need solid theoretical knowledge and rich practical experience, which can help you complete your PCB drawings quickly and effectively. But it's also very important to be careful, because a small error may lead to your final product becoming a waste, so we prefer to spend more time in the process of drawing carefully checking the details rather than going back to check if something goes wrong, which may take more time.


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