The author: Mr Bo science and technology high speed from the members of the media Zhou Wei recently, new progress of SI siege lion redpower a little bad luck, had just endured Pin delay, a strange thing happened to now, a customer product performance has a problem, after trying many times, impedance test results found that the plate is not up to standard, but the plate factory delivery report, but according to the impedance values. This lets the thunder leopard make difficult, in the end should believe who? Alas! Is really the house partial to meet all night rain, ship late and hit the head wind, just turn SI soon, but the problem is concentrated outbreak, is the problem will deceive?

Fortunately, Thunder Leopard did not give up on themselves, and was not intimidated by the problem, because he had his master behind him, for him to shelter from the wind and rain, solve problems. Then he showed the teacher the impedance test picture sent by the customer, and the impedance result was as follows:

The master took a close look, then gently turned his head, glanced at him, and asked, “Are you sure this customer will test his impedance?”

As a newcomer, he has not started to contact the training of impedance testing, nor has he touched the impedance tester (TDR). However, when he was doing PCB design and communicating with the EQ engineer of the factory about impedance, he heard the man say that impedance test is:

One sleeve pull the probe a little data display screenshot to submit the document

But is it really like this? Thunder Leopard dare not say, for fear of saying the wrong lead to the teacher’s thunder.

So he shook his head.

Fortunately, he was always diligent and studious, and his attitude was correct. The teacher liked him deeply. Before Leopard opened his mouth, the teacher first said: “Any test is a standard of judgment, impedance test is not literally a graphical measurement can show with and without question, it remains to be seen whether the test itself graphics specification, if not in accordance with the requirements for test, measured results might not have any meaning, sometimes even have a negative impact, lead to problems of positioning moving in the wrong direction, Finally, we run into a dead end, and if we can’t find the problem, it will delay a lot of time. This is a big no-no for debug.”

The master looked at Thunder Leopard fascinated, picked up the teacup, slowly sipped a mouthful of water, and continued:

“Impedance testing standards or specifications, in fact, there is no recognized standards organization to develop, currently the most widely circulated is the IPC test standard, the other is the Intel test method.



In short, the IPC standard is that the impedance line of the line to be tested shall be completely displayed on the instrument screen. In terms of time, from the probe contact point to the final position of the open circuit, it shall be completely displayed on the TDR screen. The measurement interval is from 30% to 70% of the whole time period. That is to say, the oscilloscope reading is the maximum and minimum value in this area, as shown in the figure below.

Intel, on the other hand, uses a more even approach, which seems to require a wider range of measurements, ranging from 50% to 70% of the entire line time, as shown in the figure below.



Combined with TDR instrument of Tektek, the test was further standardized. At the same time, certain requirements were also made for the display of the starting point and the end open-circuit point. The measured stationary state was basically on the screen interval of 20%~90%, and the value was based on the screen interval of 50%~70%, as shown in the figure below.



Intel of this method is accepted by most of the PCB factory, so now the mainstream of domestic plate factory test in the factory report of the impedance values are based on the range 50% ~ 70%, as long as the impedance is not overweight, so the system board is to meet the requirements of plate factory, and because of intraplate impedance line number, it is difficult to the measured one by one, So the PCB factory tested the line is not the actual line inside the board, but the impedance strip attached to the board edge, which is somewhat different from the actual situation inside the board.”

Thunder Leopard was deeply shocked by the master’s inscrutable knowledge, completely immersed in the master such as the surging flood of respect and unable to extricate themselves. He looked at the teacher, absolutely a pious disciple.

The master looked at his expression, and then said, “Now do you know what I first asked you for?”

Thunder Leopard quickly woke up from a stupor, thought it was good to have a quick response to accept the ability to strong, the teacher gently touched, really understand a lot, so very confident back: “For now, it seems that this customer do impedance test is not very good, from the point of testing pictures, line time no distribution in the whole screen, the other no measurement range, measurement time and impedance scale is very big, can’t see the specific values, can say doesn’t make any sense to the test results, like in front of the teacher said, This result can be misleading to find the problem, the correct test result should be like the image below.”



The master listened to Lei Bao’s answer, his heart was filled with joy, he secretly knew that there was something to teach him, and he was worried that his apprentice was conceited and complacent. So he asked him with quiet joy and anger, “Since the factory report is to meet the requirements, in terms of actual engineering, is there no need to doubt the problem of impedance?”

The teacher asked, it was a little confused before the Leopard place, “really know my master also”,

Thunder Leopard secretly marveled at the master of omniscient, thought later what matter or not to do little tricks to play clever good, estimate is not from the master of the eye, but already can not in front of the master cowardly, had to bite the head and say:

“I guess the impedance test plate factory can only guarantee the screen shows that 50% ~ 70% that a line impedance, and the actual product is the need to look at the whole period of the impedance, a lot of times even board factory is meet the requirements of data, but outside the scope of the impedance values may be overweight, and this part of the excessive amounts of data is not shown in the inside of the impedance of the factory report, So I think even if plate factory test data is ok, there is no guarantee that the actual line impedance must be no problem, such as most of the time connectors or BGA line will have a hole, and the through hole is usually in the ends of the line, this time via the impedance will not enter the impedance test range, even low impedance beyond the requirements, The test data from the board factory will not be displayed either, as shown in the picture below of the actual product impedance test with the chip. The diagram below:



Although the results of the measurement interval meet the requirements, the two marked parts of M1 and M3 in the figure should be through hole or chip package. Since the impedance is not optimized properly, the impedance of these two parts is much lower, resulting in the final impedance mismatch of the whole circuit, which leads to relatively large signal reflection. Finally affect the quality of the signal and system work instability such as packet loss and other phenomena. I don’t know if my idea is right or not, but I still hope the master can give me some guidance.”

This is the Leopard master more satisfied with the place, really is the talent is more intelligent, is born to learn material, also might as well the master had insisted on the leadership, from hundreds of design engineers inside the special selection of a pain-taking. The answer to the question was almost in place, but there was one key point that the master felt it necessary to add. So the master smiled and said with satisfaction:

To acquire almost “, but still a little too ideal, I actually mentioned, in front of the plate factory factory report the basic measurement is edge article Coupon, there won’t be on board within the line impedance test, picture unless the customer has put forward a clear need which position on the board which point test (there are very few customers have this requirement, Kanban factory is willing to cooperate), otherwise the plate factory is can’t find the corresponding line of plate was measured, because we usually offer board factory is Gerber light painting file, after the board to, through the light painting file can’t find the specific line, so only at the edge of the plate to simulate a board factory and intraplate actual line width consistent standard impedance to test, In this way, the impedance line is actually relatively ideal, and there is no impact of holes and other factors in the middle. The test result is certainly relatively ideal, which is why the factory report meets the requirements in many cases, but there is a big deviation in the actual impedance of the line on the board. You can see the difference between the impedance bar and the impedance in the board in the picture below.”



Leopard secretly sighed: “really listen to the teacher a words, win ten years.” But there were still some doubts in my heart, but I felt embarrassed to ask my teacher. His question was:

Why can’t the impedance test range be the whole line segment but 30% (50%) ~70%?