PCB Routing Principles

2019/1/23 17:39:54

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparatory work is done for it. In the whole PCB, wiring design process is the most limited, the most detailed technique and the largest workload. PCB wiring has single side wiring, double side wiring and multi-layer wiring. There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic wiring, interactive pre-wiring can be used for the more stringent requirements of the line, the input and output side lines should avoid adjacent parallel, so as to avoid reflection interference. Ground wire isolation should be added when necessary. The wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is likely to occur in parallel.


The routing rate of automatic wiring depends on a good layout. The wiring rules can be set in advance, including the number of bending times, the number of through holes and the number of steps. Generally, exploratory warping is carried out first to quickly connect the short lines, and then maze-type wiring is carried out. First, the wiring to be distributed is optimized for the global wiring path, which can disconnect the already distributed lines according to the needs. Try rewiring to improve the overall effect.


For high density PCB design has been felt not adapted to the through hole, it wasted a lot of valuable wiring channel, in order to solve this contradiction, the blind and buried hole technology, it not only completed the guide hole, also save a lot of wiring channel make wiring process more convenient, more smooth, more perfect, the PCB design process is a complex and simple process, to master it well, still need the electronic engineering design personnel to their experience, to get the truth.


1. Processing of power supply and ground wire


Even if the wiring in the entire PCB board is completed very well, but due to the power supply, ground wire is not considered carefully and caused by interference, will make the performance of the product decline, sometimes even affect the success rate of the product. So the wiring of power supply, ground wire should treat seriously, the noise interference that place of power supply, ground wire produces drops minimum limit, in order to assure the quality of the product.

For every engineer engaged in the design of electronic products, the causes of noise generated between ground and power lines are well understood. The following is the description of noise reduction and suppression:


It is well known that decoupling capacitance is added between power supply and ground wire.

Try to widen the width of power supply and ground wire, preferably the ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually the width of the signal line is: 0.2 ~ 0.3mm, the most fine width of 0.05 ~ 0.07mm, the power line is 1.2 ~ 2.5 mm.


The PCB of digital circuit can be used to form a circuit with wide ground conductors, that is, a ground network (the ground of analog circuit cannot be used in this way).

Use large area copper layer as ground wire, on the printed board is not used to connect the place with the ground as ground wire. Or make it into a multilayer board, power supply, ground wire each occupies a layer.


2. Digital circuit and analog circuit common ground processing


Many PCBS today are no longer single-function circuits (digital or analog), but a mix of digital and analog circuits. Therefore, it is necessary to consider the interference between them when wiring, especially the noise interference on the ground.


The sensitivity of the high frequency digital circuits, analog circuits, the signal wire, high-frequency signal lines as far as possible away from the sensitive analog devices, for the ground, moving the PCB to the outside world is only one node, so must be within the PCB processing, mold has problem, and inside the plate to digital and analog is actually are divided between them, just connected with the outside world in PCB interface (e.g., plugs, etc.). There is a short connection between digital and analog, note that there is only one connection point. There are also differences on PCBS, which are determined by the system design.


3. The signal line is placed on the electrical (ground) layer


In the multi-layer printed circuit board wiring, because the signal line layer did not finish the wiring is not much left, and more layers will cause waste will also increase the production of a certain amount of work, the cost is also increased, in order to solve this contradiction, you can consider the electrical (ground) layer wiring. The power layer should be considered first, followed by the formation. Because it's best to preserve the integrity of the formation.


4. Processing of connecting legs in large-area conductors


In a large area of grounding (electricity), the legs of common components and their connection, the treatment of the connection leg needs to be taken into comprehensive consideration, in terms of electrical performance, the welding disk of the component legs and copper surface full connection is good, but there are some bad hidden dangers for the welding and assembly of components such as welding needs a high power heater.


Therefore, considering both the electrical properties and process needs, it is necessary to make cross-flower welding plate, which is called heat shield and also called Thermal shield. In this way, it can greatly reduce the possibility of producing virtual solder joints due to excessive heat dissipation of cross section during welding. The electrical (ground) leg of the multilayer plate is treated the same.


5. The role of network system in wiring


In many CAD systems, wiring is determined by the network system. The grid is too dense, the path is increased, but the step is too small, and the data volume of the graph field is too large, which must have higher requirements on the storage space of the equipment, and at the same time, the computing speed of the object computer electronic products has a great impact. Some paths are not available, such as those occupied by the welding pad of the component leg or by the mounting hole or the fixing hole. The grid is too sparse and the path is too few, which has great influence on the distribution rate. Therefore, it is necessary to have a reasonably dense grid system to support wiring.


The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of grid system is generally 0.1 inches (2.54mm) or less than the integer multiples of 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 inches and so on.


6. Design rule check (DRC)


After the completion of wiring design, 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 PCB production process. Generally, the following aspects are checked:

Whether the distance between line and line, line and element welding plate, line and through-hole, element welding plate and through-hole, through-hole and through-hole are reasonable and meet the production requirements.


Whether the best measures are taken for critical signal lines, such as the shortest length, protection lines, and obvious separation of input and output lines.

Analog circuit and digital circuit parts, whether there are independent ground wire.

Whether the graphs (such as ICONS and labels) added in PCB will cause short circuit of signals.


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