Some thoughts on High Speed PCB Design PART2: Signal and Circuit Processing

Hi guys today we continue sharing you with the signal and circuit Processing of Controllability and Electromagnetic Compatibility High Speed PCB Design .

High Speed PCB Design tips

Handling of power supply and ground wire

Even if the wiring in the entire PCB board is well completed,
However, due to the disturbance caused by the power supply and the ground wire being inconsiderate,
It 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 should be treated carefully.
Reduce the noise interference generated by electricity and ground wires to the lowest limit to ensure the quality of the product.

Every engineer engaged in electronic product planning understands the cause of noise between ground and power lines,
Now only the reduced noise suppression is expressed:

(1) It is well known that a decoupling capacitor is added between the power supply and the ground.
(2) Widen the width of the power supply and ground wire as much as possible. Most preferably, the ground wire is wider than the power wire.
Their relationship is: ground wire>power wire>signal wire,
The general signal line width is: 0.2 ~ 0.3mm, the most thinner width can reach 0.05 ~ 0.07mm,
The power line is 1.2 ~ 2.5 mm, and the PCB of the digital circuit can form a loop with a wide ground wire.
That is to form a ground network to use (the ground of the analog circuit cannot be used in this way)
(3) Use a large area of copper layer as the ground wire, and connect the unused places to the ground on the printed board as the ground wire.
Or make a multi-layer board, the power supply and the ground wire each occupy one layer.

Common ground processing of digital circuit and analog circuit

Now there are many PCBs that are no longer single-function circuits (digital or analog circuits),
Instead, it is 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.

The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line,
Keep high-frequency signal lines as far away from sensitive analog circuit equipment as possible.
For the ground wire, the entire PCB only needs one node to the outside world.
Therefore, the problem of digital and analog common ground must be dealt with inside the PCB.
However, the digital ground and the analog ground are actually separated inside the board, and they are not connected to each other.
It is only at the interface where the PCB connects with the outside world (such as the plug number).
There is a short circuit between the digital ground and the analog ground, please note that there is only one connection point.
There are also different grounds on the PCB, which is determined by the system design.

Wiring processing of signal lines on the electrical (ground) layer

When the signal line is laid on the electrical (ground) layer when wiring in a multi-layer printed board,
Because there are not many remaining lines in the signal line layer,
Adding more layers will cause waste and increase the workload to a certain extent, and the cost will increase accordingly.
In order to deal with this contradiction, it is possible to consider wiring on the electrical (ground) layer.
The power layer should be considered first, and the ground layer second.
Because the best thing is to preserve the integrity of the formation.

Treatment of connecting legs in large-area conductors

In large-area grounding (electricity), the legs of common components are connected to it,
Comprehensively consider the processing requirements of the connecting legs,
In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface.
However, there are some bad hidden dangers in the welding device of components, such as:
① Welding requires a high-power heater.
②Simple formation of virtual solder joints. Therefore, taking into account the electrical performance and process requirements,
Made into a cross flower pad, called heat shield (heat shield), commonly known as thermal pad (Thermal),
In this way, the possibility of false solder joints due to excessive cross-section heat dissipation during welding can be greatly reduced.
The treatment of the power-connecting (ground) layer legs of the multi-layer board is the same.

Today we discuss above issues.You are welcome to express your thoughts in the comments.
Next time we will continue to discuss other aspects.If you need SMT equipment, inquiry is also very welcome.

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