An SI engineer who does not understand processing cannot be called Mr. High Speed

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An SI engineer who does not understand processing cannot be called Mr. High Speed [Copy link]

Author: Huang Gang, a member of Yibo Technology Expressway Media

Simulation is the simulation of PCB design, and PCB design is the design used for processing and production. As the error factors brought by processing to the design increase, is simulation the simulation of PCB design or the simulation after production and processing?
Mr. Gaosuo has shared many of the effects of processing errors on PCB design in the field of high-speed serial links in previous articles, as well as the differences in simulation results with and without considering processing factors. Of course, we all know that as the speed increases, processing factors will have a greater impact on high-speed signal performance, such as the stub of the via, the diameter of the drill hole, the layer bias, etching and other factors, which we have introduced and shared cases in previous articles.

Since we have talked a lot about the processing factors in high speed, let's talk about a simpler one today, that is, the simulation design of power supply voltage drop, which is often called DC simulation. As for its simulation principle, it may be the simplest simulation theory, and the following picture can explain it clearly.

Therefore, as long as we get the voltage value and maximum current value of the power network, we can simulate the voltage drop of the PCB power link. Just like the example below, the power link from VRM to sink end has a voltage of 0.85V and a current of 5A. After knowing these parameters, we can do it easily.

The following is the simulation result of the power link. It seems that both the voltage at the load end and the current density of the entire link can meet the requirements. That is to say, the PCB power design after simulation is ok!

This is of course a very objective result, which objectively and accurately reflects that the PCB design of this power link meets the requirements. Basically, the simulation ends here.

Suddenly one day, Mr. Gaosuo thought that the PCB link of the power supply might also be affected by some processing factors like the high-speed signal. Then Mr. Gaosuo looked up some IPC standards to see if there are any processing tolerances that will affect the design of the power supply. Finally, he found the standard for copper thickness processing in a certain chapter, which is the picture below.

As can be seen from the figure, the copper thickness of 0.5oz is generally 0.6mil, but the extreme limit is 0.449mil. Since it is listed in the IPC standard, it means that even if the 0.5oz copper is really made to be only 0.449mil thick after processing, it still meets the requirements!

Mr. Gaosuo suddenly thought, if the copper thickness of the case mentioned above is really only 0.449mil after processing, will the voltage drop be...

While everyone was still worried, Mr. Gaosuo had already changed the copper thickness from 0.6mil to 0.449mil on the original case for simulation verification! It was found that such a change has a significant deterioration in both voltage drop and current density!

From the simulation results of 0.449mil, the voltage drop increased by about 7mV, and the current density deteriorated by 33A/mm, which is a level that cannot be ignored.

Of course, after realizing the "seriousness" of the problem, Mr. Gaosuo also communicated with our DFM engineers and colleagues at the board factory to find out whether the probability of making such an extreme copper thickness is high. Fortunately, they said that basically there has been no such situation, and the main thickness is still around 0.6mil. The following are some PCB copper thickness slices they provided, which are consistent with their statement.

Although this is just some of Mr. Gaoshou's whim and simulation verification, and he has not actually made such a thick copper board, but from a general perspective, learning more about processing knowledge, especially the impact of processing on design and simulation, is very helpful for hardware engineers, PCB engineers and SI engineers!

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What software is used to simulate high-speed display?