PCB News - High-speed PCB wiring skills

High-speed PCB wiring skills

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB design, the wiring design process is the most limited, the skills are the smallest, and the workload is the largest. If you want to master it well, you still need the extensive experience of electronic engineering designers to get the true meaning of it. The processing of power supply and ground wire makes the wiring in the entire PCB board very well completed, but the interference caused by improper consideration of the power supply and ground wire will reduce the performance of the product, and sometimes even affect the success of the product Rate. Therefore, the wiring of the electric and ground wires must be taken seriously, and the noise interference generated by the electric and ground wires should be minimized to ensure the quality of the product. Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is described:

1) It is well known to add decoupling capacitors between the power supply and ground.

2) Whether the best measures have been taken for the key signal lines, such as the shortest length, plus the protection line, and the input line and output line are clearly separated.

3) Whether there are separate ground wires for the analog circuit and digital circuit.

4) Modify some undesirable line shapes.

5) Whether the outer frame edge of the power ground layer in the multi-layer board is reduced, for example, the copper foil of the power ground layer is exposed to the outside of the board, which may cause a short circuit

6) Whether the graphics (such as icons and annotations) added to the PCB board will cause signal short circuit.

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 logo is pressed on the device pad, so as not to affect the quality of the electrical equipment.

8). Is the width of the power line and the ground line appropriate, and is there a tight coupling between the power supply and the ground line (low wave impedance)? Is there any place in the PCB that allows the ground line to be widened.

9). Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the thinnest The width can reach 0.05～0.07mm, and the 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, to form a ground net for use (the ground of the analog circuit cannot be used in this way) (3), The large area copper layer is used as ground wire, and the unused places on the printed circuit board are connected to the ground as the ground wire. Or it can be made into a multi-layer board, and the power supply and ground wire occupy one layer each. 2 Common ground processing of digital circuit and analog circuit Nowadays, many PCBs are no longer a single functional circuit (digital or analog circuit), but are composed of 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 wire. 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 should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated, they are not connected to each other, but at the interface (such as a plug, etc.) connecting the PCB board to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB board, which is determined by the system design. 3 When the signal line is laid on the electrical (ground) layer and the multilayer printed board is laid out, there are not many wires left in the signal line layer that are not laid out. Adding more layers will cause waste and increase production. The workload and cost have also increased accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation. 4 Treatment of connecting legs in large-area conductors In large-area grounding (electricity), the legs of common components are connected to them, and the treatment of connecting legs needs to be considered comprehensively. In terms of electrical performance, the pads of the component legs and The copper surface is fully connected, but there are some undesirable hidden dangers for the welding and assembly of the components, such as: 1. High-power heaters are required for welding. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, which are called heat shields, commonly known as thermal pads. In this way, virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the electrical connection (ground) leg of the multilayer board is the same. 5 The role of the network system in the wiring In many CAD systems, the wiring is determined based on the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a well-spaced and reasonable grid system to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc. 6 After the design rule check (DRC) wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and at the same time, it is necessary to confirm whether the rules set meet the requirements of the printed board production process. The general inspections are as follows Aspects: (1) Whether the distance between line and line, line and component pad, line and through hole, component pad and through hole, through hole and through hole is reasonable, and whether it meets the production requirements.