PCB design is not a easy work. Last we talk about the Common Errors in Schematic Diagram and PCB, today we discuss the treatment of power supply and ground wire, common ground processing of digital circuit and analog circuit, the signal line is laid on the electric (ground) layer, treatment of connecting leg in large area conductor, the function of network system in wiring and design rule check (DRC).
1.Treatment of Power Supply and Ground Wire
Even if the layout in the whole PCB board is completed well, the interference caused by the thoughtless consideration of the power supply and ground wire will degrade the performance of the product, and sometimes even affect the success rate of the product. Therefore, the layout of electricity and ground wire should be taken seriously, and the noise interference caused by electricity and ground wire should be minimized to ensure the quality of products.
For every engineer who is engaged in the design of electronic products, we all know the cause of the noise between the ground wire and the power line. Now we only describe the reduced noise suppression: as we all know, the coupling capacitor is added between the power line and the ground wire.
The width of power supply and ground wire should be widened as far as possible, and the best one is that the line width of ground wire is larger than that of power supply. The relationship between them is: ground wire > power line > signal line. Generally, the line width of signal is 0.2 ~ 0.3mm, the thinnest width is 0.05 ~ 0.07mm, and the line width of power line is 1.2 ~ 2.5mm
The PCB of digital circuit can form a loop with wide ground wire, that is to form a ground network for use (the ground of analog circuit can not be used in this way). Large area copper layer is used as ground wire, and the unused places on the printed circuit board are connected with the ground as ground wire. Or make multi-layer board, power supply, ground wire each occupy one layer.
2.Common Ground Processing of Digital Circuit and Analog Circuit
Now many PCBs are no longer single function circuit (digital or analog circuit), but composed of digital circuit and analog circuit. Therefore, when layout, we need to consider the mutual interference between them, especially the noise interference on the ground wire.
The frequency of digital circuit is high, and the sensitivity of analog circuit is strong. For the signal line, the high-frequency signal line is far away from the sensitive analog circuit device as far as possible. For the ground wire, the whole PCB has only one node to the outside world, so it is necessary to deal with the problem of digital and analog common ground inside the PCB. In fact, the digital ground and analog ground inside the PCB are separated, and they are not connected with each other, Only at the interface between PCB and outside (such as plug). There is a short circuit between digital ground and analog ground. Please note that there is only one connection point. There are also non common ground on PCB, which is decided by system design.
In the layout of multi-layer printed circuit board, because there are not many wires left in the signal line layer, adding more layers will cause waste, increase the production workload, and increase the cost accordingly. In order to solve this contradiction, we can consider layout on the electrical (ground) layer. First, we should consider the use of power layer, and then the formation. Because it’s best to keep the integrity of the formation.
4. Treatment of Connecting Legs in Large Area Conductor
In the large area of grounding (electricity), the legs of common components are connected with them. The treatment of the legs needs to be considered comprehensively. As far as the electrical performance is concerned, it is better to fully connect the pads of the legs with the copper surface, but there are some hidden dangers in the welding and assembly of components, such as:
① the welding needs high-power heater.
② It is easy to cause false solder joint.
Therefore, considering both the electrical performance and the process needs, the cruciform pad is made, which is called heat shield, and commonly known as thermal pad. In this way, the possibility of false solder joints caused by excessive heat dissipation of the cross section during welding can be greatly reduced. The treatment of the grounding leg of the multi-layer board is the same.
5.The Role of Network System in Cabling
In many CAD systems, the routing is determined by the network system. If the grid is too dense, the path increases, but the step is too small, and the amount of data in the graph field is too large, which will inevitably have higher requirements for the storage space of the device, and also have a great impact on the computing speed of the object computer electronic products. However, some paths are invalid, such as those occupied by pads of component legs or by mounting holes and fixing holes. If the grid is too sparse and there are too few paths, it has a great impact on the throughput. So we should have a reasonable grid system to support the routing.
The distance between the legs of standard components is 0.1 inch (2.54 mm), so the foundation of grid system is generally set as 0.1 inch (2.54 mm) or the integer multiple less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.
6. Design Rule Check (DRC)
After the layout design is completed, it is necessary to carefully check whether the layout design conforms to the rules formulated by the designer, and at the same time, it is also necessary to confirm whether the rules formulated conform to the requirements of PCB production process.
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- Whether the distance between line and line, line and element pad, line and through hole, element pad and through hole, through hole and through hole is reasonable and meets the production requirements.
- Is the width of power line and ground wire suitable, and is the power line and ground wire tightly coupled (low wave impedance)? Is there any place in PCB where the ground wire can be widened.
- Whether the best measures have been taken for the key signal line, such as the shortest length, adding protection line, the input line and output line are obviously separated.
- Whether there are independent ground wires for analog circuit and digital circuit.
- Whether the graphics (such as icon and label) added in PCB will cause signal short circuit.
- Some unsatisfactory alignment is modified.
- Is there a process line on PCB?
Whether the resistance welding meets the requirements of production process, whether the resistance welding size is appropriate, and whether the character mark is pressed on the device pad, so as not to affect the quality of electrical installation.
Whether the edge of the outer frame of the power layer in the multilayer board shrinks, such as the copper foil of the power layer is exposed outside the board, which is easy to cause short circuit. The purpose of this article is to explain the process and some matters needing attention of PCB design using powerpcb, a PCB design software of pads, and to provide design specifications for designers of a working group, so as to facilitate communication and mutual inspection among designers.
