Protel PCB cabling Essence

 

Article 1 PCB cabling
In PCB design, wiring is an important step to complete the product design. It can be said that the previous preparations are for it. In the entire PCB, wiring is the most restrictive in the design process, the skills are the finest and the workload is the greatest. PCB wiring includes single-sided wiring, double-sided wiring, and Multilayer Wiring. There are also two ways of wiring: Automatic wiring and interactive wiring. Before Automatic wiring, you can use interactive wiring to pre-arrange lines with strict requirements, edge lines at the input end and output end should be parallel to avoid reflection interference. Ground Line isolation should be applied when necessary. The wiring of two adjacent layers must be perpendicular to each other, and parallel cables are prone to parasitic coupling.
The distribution rate of automatic cabling depends on a good layout. The wiring rules can be preset, including the number of bending times of cabling, the number of guide holes, and the number of steps. Generally, you can perform exploratory cabling first, quickly connect short-term cabling, perform labyrinth cabling, and first optimize the cabling path, it can disconnect the laid line as needed. Try re-wiring to improve the overall effect.
The current high-density PCB design has been unable to adapt to the penetration hole, which wastes many valuable wiring channels. To solve this problem, blind holes and buried holes have emerged, it not only completes the via hole function, but also saves many cabling channels to make the cabling process more convenient, smoother, and more complete, the design process of the PCB Board is a complex and simple process. To grasp it well, the majority of electronic engineering designers should learn from their own experiences to get the true meaning.
1. Power Supply and Ground Line Processing
The cabling in the entire PCB Board is well completed, but the interference caused by poor consideration of power supplies and ground wires may degrade the performance of the product and sometimes even affect the success rate of the product. Therefore, the cabling of electricity and Ground Wires should be taken seriously, and noise interference caused by electricity and Ground Wires should be minimized to ensure product quality.
For every engineer engaged in electronic product design, he understands the reason for noise between the ground and the power cord. Now he only expresses the noise reduction:
(1) It is well known that decoupling capacitors are added between power supplies and ground wires.
(2) try to widen the power supply and ground wire width, preferably the ground wire is wider than the power cord. Their relationship is: Ground Wire> power cord> signal line. Generally, the signal line width is 0.2 ~ 0.3mm, up to 0.05 ~ 0.07mm, power cord 1.2 ~ 2.5
A circuit is formed for the PCB of a digital circuit, that is, a ground network is formed for use (the ground of a analog circuit cannot be used in this way)
(3) Use a large area copper layer as the ground wire, and connect unused areas with the ground on the printed board as the ground wire. Or make a multi-layer board, power supply, ground each occupies a layer.
2. Co-location processing of digital and analog circuits
Now many PCB are no longer a single functional circuit (digital or analog circuit), but composed of a mix of digital and analog circuits. Therefore, when wiring, the issue of mutual interference between them must be considered, especially noise interference on the geographic line.
The frequency of a 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 sensitive analog circuit devices. For the ground line, the whole PCB has only one node for the outside world, therefore, the problem of number and mode co-location must be dealt with inside the PCB, while the numbers inside the board are actually separated from each other, it is only in the PCB and external connection interface (such as plug ). There is a short connection between numbers and analog locations. Note that there is only one connection point. There are also non-co-locations on the PCB, which is determined by the system design.
3. The signal line is laid on the electrical (ground) layer.
When wiring a multi-layer printed board, because there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload for production, the cost also increases accordingly. To solve this conflict, you can consider wiring on the electrical (ground) layer. The power supply layer should be considered first, followed by the formation. Because it is best to preserve the integrity of the formation.
4 Handling of connection leg in large-area Conductor
In a large area of grounding (electricity), the legs of commonly used components are connected to them, and the handling of the connection leg needs to be comprehensively considered. In terms of electrical performance, the welding pad of the component leg and the copper surface are fully connected, but the welding assembly of the component has some bad risks, such as: ① High-Power heater is required for welding. ② It is easy to cause virtual welding points. Therefore, taking into account the electrical performance and technological needs, it is made into a cross-flower Solder Pad, known as heat shield, commonly known as Thermal, this greatly reduces the possibility of generating virtual welding points due to excessive heat dissipation of the cross section during welding. The electrical connection (ground) layer leg of the multi-layer board is handled the same.
5. Functions of the network system in cabling
In many CAD systems, cabling is determined by the network system. Although the mesh is too dense and the path is increased, the step is too small, and the data size of the graph field is too large, which must have higher requirements on the storage space of the device, at the same time, the computing speed of the electronic products of the object computer has a great impact. Some paths are invalid, such as those occupied by the pad of the component leg, the mounting holes, and the fixed holes. The grid is too sparse, and too few paths have a great impact on the distribution rate. Therefore, it is necessary to have a grid system with reasonable density to support cabling.
The distance between the two legs of the standard component is 0.1 inch (2.54mm), so the basis of the grid system is generally set to 0.1 inch (2.54mm) or an integer less than 0.1 inch, such: 0.05 inch, 0.025 inch, 0.02 inch, and so on.
6. Design Rule Check (DRC)
After the wiring design is complete, check whether the wiring design complies with the rules set by the designer. At the same time, check whether the rules meet the requirements of the printing board production process, the general check has the following aspects:
(1) Whether the distance between wire and wire, wire and component pad, wire and link hole, component pad and link hole, link hole and link hole are reasonable and meet the production requirements.
(2) is the width of the power cord and ground wire suitable, and is the power source tightly coupled with the ground wire (low impedance )? Whether the ground can be widened in the PCB.
(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 obviously separated.
(4) Whether the analog circuit and digital circuit have independent ground wires.
(5) Whether the image (mark, note) added in the PCB will lead to short-circuit signal.
(6) modify some unsatisfactory line shapes.
(7) Is there a craft line on the PCB? Whether the solder mask meets the requirements of the production process, whether the Solder Mask size is suitable, and whether the Character Mark is pressed on the device pad to avoid affecting the electrical assembly quality.
(8) whether the outer frame edge of the power source formation in the multi-layer board is reduced, such as the copper foil exposed in the power source formation is prone to short circuit.