Is the impedance tested from both ends of the surface microstrip line different?

Last time Leibao tested from the capacitor pad and from the connector end, the difference in the results was more than 6 ohms. If you haven't seen it, you can check out the previous link: Is the impedance of the surface microstrip line different when tested from both ends? what's the situation! ! .

Let's keep it a little mysterious first, and then let's see if we still measure from the Top layer, and then put different objects below, such as white paper, directly on the anti-static mat, packaging foam, anti-static bags, and suspended (air), etc. (equivalent to the bottom layer The signal comes into contact with different objects above), will the result be any different? The props are shown below:

Then the measured results are shown in the figure below:

For the same test point, the test results of different items placed underneath are so different (the highest and lowest averages are almost 20 ohms). It seems that the results measured from the Top surface are questionable. As can be seen from the above results, the impedance of the bottom blue curve is relatively low, with an average of only 85~90ohm. You should be able to guess what item this curve is placed on.

That's right, it's the anti-static bag, did you guess it right? We have shared a case before. The high-speed signal line had a code scanning label attached to its surface, which caused the system to fail to run. In fact, the impact is similar to that of an anti-static bag. This anti-static bag is mainly used to enable plastics to shield electromagnetic waves. Metalizing plastics and using a composite material called antistatic agent causes the relative dielectric constant to increase. From the impedance line point of view, the impedance becomes much smaller and takes longer than other lines. This is also because of the dielectric constant. Caused by the constant being too large. Therefore, microstrip lines are still relatively susceptible to external factors. You must also pay great attention when testing, and conduct corresponding tests based on the actual application environment of the signal. Several other curves actually have certain differences, because different materials must have different characteristics (the main focus here is on the dielectric constant). Let’s look at the testing of several different substances separately.

Let’s first take a look at several different packaging foams (I found a few common ones in the laboratory), as shown in the picture below:

Then we still take the same board, stick the bottom lines on these foam pads, and then test on the Top surface. The measured results are as shown in the figure below. It can be seen that the results are basically not very different. It can be said that the results are the same. The average measured The value is close to the previous result at the capacitor pad.

In addition, let’s compare the impact of white paper and the anti-static rubber pad on the experimental table on impedance separately. There is no need to display these two items separately. The test results are shown in the figure below:

Which one has higher impedance: white paper or anti-static rubber pad? Unexpectedly, anti-static rubber pads are mainly made of static conductive materials, static dissipative materials and synthetic rubber through various processes. Unexpectedly, the dielectric constant of white paper is actually higher than that of anti-static rubber pads. The impedance can be measured in this way. Lower. Therefore, when we test the surface signal, we cannot let the surface signal come into contact with the white paper. This may cause the surface signal test value to be lower than the actual value. Seeing this, we should also know the answer to the question in the previous issue. The results of the test at the capacitor pad in the previous issue are more consistent with the impedance value of the actual line. However, when tested at the top layer via hole, because the bottom line is in contact with the white paper, the white The dielectric constant of paper is larger than that of air, resulting in lower actual test values.

So can impedance testing be performed on the Top layer? In fact, it is still possible, as long as the signal does not contact other substances. At this time, the board can be suspended with a bracket for testing. The measured impedance is basically not much different from that at the bottom capacitor pad, as shown in the following figure:

The red curve is the result of the test at the bottom capacitor pad, and the green curve is the result of the test at the Top layer via hole. It can be seen that their average value is basically close to around 111om. In fact, when we look back at the test results of the above foam pads, they are relatively close to our actual impedance in air, which shows that using foam pads has a relatively small impact on the surface impedance test.