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How do you wire a PCB?

Posted:03:28 PM May 10, 2021 writer: louise

As an electronic engineer, the miniaturization of electronic products and the complexity of system design make PCB design more and more complicated. Nowadays, highly integrated circuit boards are getting smaller and smaller, and the pins of packaged devices are getting denser and denser, all of which put tremendous pressure on wiring.
Wiring is the top priority of the PCB design process, which will directly affect the performance of the PCB board, and the design process is the most cumbersome and more demanding. Although many advanced EDA tools now provide automatic wiring functions and are quite intelligent, automatic wiring does not guarantee a 100% deployment rate. Therefore, many engineers are not satisfied with the results of automatic wiring. Manual wiring is still the choice of most engineers. The wiring is restricted by electrical rules to meet the signal integrity requirements.
In the PCB design process, wiring can be roughly divided into three steps:
The first is distribution, which is also the most basic requirement for PCB design. If the circuit is unavailable, then the basic function of the board is not available, it is a waste board, not to mention anything else.
The second is performance, which is a measure of the quality of a printed circuit board. After the wiring is completed, it is necessary to consider how to achieve the best performance and avoid various interferences.
The third is beautiful, smooth lines and excellent performance, but if the picture is beautiful but you dare not look at it, you should consider how to beautify the chaotic lines. The wiring is neat and uniform, which can also bring great convenience for future testing and maintenance. This is also the basic quality that a senior engineer should possess.
Wiring is far from being as simple as imagined, and it is not just a simple task, but if wiring is done as an art, there will be different gains. There are pictures and the truth, let us feel the difference in the wiring between novices and masters.
So how to wire elegantly? The wiring method can be very different. To avoid the parallel of the input end and the output end during the wiring, the reflection interference and the parasitic coupling of two adjacent wiring layers parallel to each other will affect the stability of the line, even when the interference is serious. The circuit board can't work at all. Many factors need to be considered when wiring. Now I will give you the rules of PCB wiring.
In the last article "Three Principles in PCB Design", we have introduced the 3W principle, 20H principle, five-five principle, I will not talk to more here. If you need to learn, click the page link to see.

Power and ground rules

The interference between the power supply and the bottom line directly affects the performance of the product, so the wiring of the power supply and the ground line should strive to minimize the noise interference generated 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 expressed: it is well known that the power supply and the ground wire are added Coupling capacitor. Try to widen the width of the power and ground wires, 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 smallest width can be reached 0.05~0.07mm, 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) Use a large area of ​​the copper layer as the ground wire Use, connect the unused places on the printed board to 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.

The signal line is laid in the electric (ground) layer rule

In the multi-layer printed board wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase the workload of production, and the cost will increase 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.

The common ground rule of digital circuit and analog circuit

Now PCBs are mostly composed of digital circuits and analog circuits, and mutual interference issues should be considered when wiring, especially 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 components. For the ground line, the entire 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 separated, they are not connected, but at the interface (such as plugs, etc.) connecting the PCB 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, which is determined by the system design.

Rules for the treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as ① Welding requires high-power heaters. ②It 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.

Network system rules

The grid grounding system is an effective method to reduce trace inductance and provide a return path for RF current. The grid is too dense, although the path has increased, the step is too small, and the amount of data in the field is too large, which will inevitably have higher requirements on the storage space of the device, and also the computing speed of the computer-type 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 less than an integral multiple of 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 Inches, etc.
Design rule checking (DRC) rules

After the 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 also necessary to confirm whether the rules made meet the requirements of the printed board production process. The general inspection has the following aspects:

  • 1. Whether the distance between line and line, line and component pad, line and through the hole, component pad, and hole, through-hole, and through-hole is reasonable, and whether it meets the production requirements.
  • 2. 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.
  • 3. Whether the best measures have been taken for the key signal lines, such as the shortest length, plus protection lines, and the input lines and output lines are separated.
  • 4. Whether there are separate ground wires for the analog circuit and digital circuit.
  • 5. Whether the graphics (such as icons and annotations) added to the PCB will cause a signal short circuit.
  • 6. Modify some unsatisfactory linear shapes. 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.
  • 7. Whether the outer frame edge of the power ground layer in the multilayer board is reduced. For example, the copper foil of the power ground layer is exposed to the outside of the board and it is easy to cause a short circuit.

There are many points to pay attention to in the PCB layout. The above rules are only a part of them, and there will be more problems encountered in actual work. Moreover, the layout rate of the automatic wiring depends on a good layout. The wiring rules, including the number of bends of the wiring, the number of vias, and the number of steps, need to be predetermined in advance. Therefore, if you want to truly master the PCB layout, you still need to practice hard, continue to accumulate a wealth of experience, and master more rules and skills, to perform the PCB layout elegantly and design a sophisticated and beautiful PCB board.

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