Electronic Design - How to make your PCB layout fast and look efficient

How to make your PCB layout fast and look efficient

PCB wiring is very important in the entire PCB planning. How to achieve fast and efficient wiring and make your PCB wiring look tall is worth studying.

This article sorts out several aspects that need to be paid attention to in PCB wiring, come to check the missing parts!

Digital circuit and analog circuit

Common ground processing There are now many PCBs that are no longer a single functional circuit, but are composed of a mixture of digital circuits 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 equipment. 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 and not connected to each other, but at the interface (such as the plug number) where the PCB connects with the outside world.

There is a short connection between the digital ground and the imitation ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system planning. When the signal line is laid on the power layer or ground layer and wired on the multilayer printed board, because there are not many lines left in the signal line layer that have not been laid out, adding more layers will constitute a waste and give a certain amount of work for production and addition., The cost has also been increased accordingly, in order to solve this opposition, you can consider wiring on the power layer or 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.

Handling of connecting legs in large area conductors In large-area grounding or power supplies, the legs of common components are connected to them. The treatment of connecting legs needs to be considered in general. In terms of electrical performance, the pads of the component legs are connected to the copper surface. Full connection is good, but there are some undesirable dangers for the welding device of the component, such as: welding requires a high-power heater; simply constitute a virtual welding spot.

Therefore, the electrical performance and process requirements are coordinated, and cross-patterned pads are made, which are called thermal barriers, commonly known as thermal pads. In this way, the possibility of virtual solder joints due to excessive heat dissipation during welding can be greatly reduced.

The processing of the power connection or the ground leg of the multilayer board is the same. The role of the network system in wiring In many CAD systems, the wiring is determined by the network system.

The grid is too dense, although the path has been added, the step is too small, and the data volume of the field is too large. This will inevitably have higher requirements on the storage space of the device. At the same time, the computing speed of the target computer electronic products is also Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by device holes and fixed holes.

Too sparse grids and too few channels have a great impact on the distribution rate. So there must be a reasonable grid system to support the wiring.

The distance between the legs of standard equipment is 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches, etc.

The processing of the power supply and ground wire makes the wiring in the entire PCB board very well completed, but the disturbance caused by the 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 power supply and ground wire should be treated carefully, and the noise disturbance generated by the power supply and ground wire should be minimized to ensure the quality of the product.

Every engineer engaged in electronic product planning understands the cause of the noise between the ground wire and the power cord, and now only expresses the reduced noise suppression: it is well known to add the noise between the power supply and the ground wire. Lotus capacitor.

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 most precise width can reach 0.05 ～0.07mm, the power cord is 1.2～2.5mm. For the digital circuit PCB, a wide ground wire can be used to form a loop, that is, a ground net can be used. The ground that imitates the circuit cannot be used in this way. A large area of copper layer is used as a ground wire, which is not used on the printed board. All places are connected with the ground as a ground wire. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

Planning Rules Check (DRC) After the wiring plan is completed, it is necessary to carefully check whether the wiring plan conforms to the rules set by the planner. At the same time, it is also necessary to acknowledge whether the established rules meet the requirements of the printed board production process. Generally, check the following aspects :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. Whether the width of the power line and the ground line are appropriate. Whether the power supply and the ground wire are tightly coupled.

Is there any place in the PCB where the ground wire can be widened? Regarding the critical signal line, whether the best measures have been taken, such as the shortest length, the maintenance line is added, and the input line and output line are clearly separated. Whether there are separate ground wires for the analog circuit and the digital circuit. Whether the graphics (such as icons and annotations) added to the PCB will constitute a signal short circuit. Modify some undesirable line shapes. Whether there is 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 pad of the equipment to avoid affecting the quality of the electrical equipment. Whether the outer frame edge of the power ground layer in the multi-layer board is reduced, if the copper foil of the power ground layer is exposed outside the board, it will simply constitute a short circuit.

Must-know skills for PCB planning

1. What is the relationship between the line width and the size of the vias on the PCB board and the size of the current passing through?

The thickness of the copper foil of a general PCB is 1 ounce, and if it is about 1.4 mil, the maximum current allowed by a line width of approximately 1 mm is 1A. The via hole is more complicated. In addition to the size of the via pad, it is also related to the thickness of the hole wall sinking copper during the process of electroplating.

2. How to improve the distribution rate?

The completion of the planning of a printed board diagram generally requires the process of schematic input-net table generation-definition of Keepout Layer-net table (component) loading-component layout (manual) wiring and so on.