High Speed PCB Design Guide PCB Layout

In PCB design, wiring is an important step in completing product design. It can be said that the previous preparations are all done for it. In the entire PCB, the wiring design process has the highest limits, the most detailed skills, and the largest workload. PCB wiring includes single-sided wiring, double-sided wiring and multi-layer wiring.

There are two ways of wiring: automatic wiring and interactive wiring.

Before automatic wiring, you can use interactive to pre-wire lines with strict requirements. The edges of the input end and the output end should avoid being adjacent and parallel to avoid reflection interference. If necessary, ground wire isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur when parallel.
The routing rate of automatic wiring depends on good layout. The wiring rules can be preset, including the number of bends of the traces, the number of via holes, the number of steps, etc. Generally, exploratory warp routing is performed first to quickly connect short wires, and then labyrinth routing is performed to optimize the global routing path of the wires to be wired. It can disconnect the wires that have been wired as needed. And try rerouting it to improve the overall effect.
The current high-density PCB design has felt that through holes are not suitable. It wastes many precious wiring channels. To solve this contradiction, blind hole and buried hole technologies have emerged, which not only complete the role of through holes. , and also saves many wiring channels, making the wiring process more convenient, smoother and more complete. The design process of PCB board is a complex yet simple process. To master it well, it requires a large number of electronic engineering designers. Only when people experience it themselves can they get the true meaning.

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1. Handling of power supply and ground wires

Even if the wiring in the entire PCB is completed well, interference caused by insufficient consideration of power and ground wires will degrade the performance of the product and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wires must be taken seriously to minimize the noise interference generated by electricity and ground wires to ensure the quality of the product.

Every engineer who is engaged in the design of electronic products understands the causes of noise between the ground wire and the power wire. Now we only describe the reduction type noise suppression:

(1) It is well known that decoupling capacitors are added between the power supply and ground wires.
(2) Try to widen the width of the power and ground wires. It is best to make the ground wire wider than the power wire.

Their relationship is: ground wire > power wire > signal wire. Usually the signal wire width is: 0.2 ~ 0.3mm, and the thinnest The width can reach 0.05~0.07mm, and the power cord is 1.2~2.5 mm
For digital circuit PCBs, wide ground wires can be used to form a loop, that is, to form a ground network (the ground of analog circuits cannot be used in this way)
(3) Use a large area of copper layer as a ground wire, and connect all unused areas on the printed board to the ground as a ground wire. Or it can be made into a multi-layer board, with power supply and ground wires occupying one layer each.

2. Common ground processing of digital circuits and analog circuits

Nowadays, many PCBs are no longer single-functional circuits (digital or analog circuits), 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 line.
The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground lines, 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. However, the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but are only at the interface where the PCB connects to the outside world (such as plugs, etc.). The digital ground is shorted a little bit to the analog ground, note that there is only one connection point. There are also different ground on the PCB, which is determined by the system design.

3. Signal lines are laid on the electrical (ground) layer

When wiring multi-layer printed boards, there are not many unfinished lines left on the signal line layer. Adding more layers will cause waste and increase the workload of production, and the cost will also increase accordingly. To resolve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, followed by the ground layer. Because it is best to preserve the integrity of the formation

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4. Treatment of connecting legs in large-area conductors

In large-area grounding (electricity), the legs of commonly used components are connected to it. The handling of the connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface,

but for There are some hidden dangers in the welding assembly of components, such as:

① Welding requires a high-power heater.

②It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process requirements, a cross-shaped solder pad is made, which is called heat shield, commonly known as thermal pad (Thermal). In this way, the possibility of virtual solder joints due to excessive cross-section heat dissipation during welding can be eliminated. Sex is greatly reduced. The treatment of the power (ground) layer legs of multi-layer boards is the same.

5. The role of network system in wiring

In many CAD systems, routing is determined based on the network system. If the grid is too dense, although the number of channels is increased, the steps are too small and the amount of data in the image field is too large. This will inevitably have higher requirements for the storage space of the device, and it will also affect the computing speed of computer electronic products. great impact. Some paths are invalid, such as those occupied by the pads of component legs or occupied by mounting holes and mounting holes. Too sparse mesh and too few channels will have a great impact on the routing rate. Therefore, there must be a grid system with reasonable density to support wiring.
The distance between the legs of a standard component is 0.1 inches (2.54mm), 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. Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. It is also necessary to confirm whether the rules set meet the needs of the printed board production process. General inspections include the following aspects:
(1) Whether the distance between wires and wires, wires and component pads, wires and through holes, component pads and through holes, and through holes and through holes is reasonable and meets production requirements.
(2) Are the widths of the power and ground wires appropriate, and are the power and ground wires tightly coupled (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?
(3) Whether the best measures have been taken for key signal lines, such as keeping them to the shortest length, adding protective lines, and clearly separating input lines and output lines.
(4) Whether the analog circuit and digital circuit parts have independent ground wires.
(5) Whether graphics (such as icons, labels) added to the PCB later will cause signal short circuits.
(6) Modify some unideal line shapes.
(7) Are there process lines added to the PCB? Whether the solder mask meets the requirements of the production process, whether the size of the solder mask is appropriate, and whether the character mark is pressed on the device pad to avoid affecting the quality of the electrical assembly.
(8) Whether the edge of the outer frame of the power supply ground layer in the multilayer board is reduced. If the copper foil of the power supply ground layer is exposed outside the board, it may easily cause a short circuit.

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