Author: Huang Gang, member of “We Media” of Mr. Yibo Technology High Speed
The so-called package package is good, impedance and crosstalk control does not worry, package package is not good, debug is really very confusing! Here to introduce a “stingy door” related to the design errors of the package.
About the package, Mr. High Speed has written a lot of articles about it, such as the spacing of the package, the punching of the package, the reference of the package. When you think we have finished about the problem of parcel land, we have a new idea. This time finally has nothing to do with our customer Debug case, it is purely a matter of Mr. High Speed’s own test board research.
When Mr. High Speed was calculating the impedance of the envelope, he saw the impedance calculation model, and suddenly a very interesting problem emerged, that is, the model specified the line width of the signal, the thickness of the signal to the reference plane below, and the distance between the signal and the envelope of the same layer next to it. In this way, the impedance value was calculated. So does the width of the ground plane of the same layer have any effect on the impedance of the signal? So Mr. High Speed wanted to make a test board to test and verify.
Just thinking about it is not Mr. High Speed’s style. When he finally found a free time, Mr. High Speed designed and produced this test board.
We made identical lines to be tested according to the structure of the impedance calculation model, and designed the structure of the parcel with different width of the parcel plane. There are four cases of the very stingy parcel land of 5mil, 10mil, 20mil and the relatively magnificent parcel land of 50mil, as shown below:
After the board came back, Mr. High Speed immediately tested the structure, and the results were not so good as expected. This variable that can not be quantified in the general impedance calculation tools actually has a great impact on the impedance and loss of the signal. Let’s take a look at the impedance comparison situation first.
It can be seen from the results of impedance that only the very stingy 5mil of the ground impedance is on the high side, and only when it reaches more than 20mil, the impedance can be maintained to a normal level, and it is consistent with the results of impedance calculation.
The results Mr High-speed have mental preparation, because from the electromagnetic field theory to analysis, signal lines within a certain range is need to have a close reference surface as the reverse flow of electromagnetic field, if the scope of the reference surface is not enough, can’t carry all the energy of the electromagnetic field, so we need to look for further reference signal to reference. From the test case, it can be simply understood that when the width of the ground covering plane of the same layer is not enough, the signal cannot be fully referenced, so more electromagnetic fields find the complete ground plane below as a reference, so the reference plane is far away, and the impedance will naturally increase.
We can also see such a problem through the impedance, the impedance fluctuation is also very severe in the case of 5mil covering the width of the plane, which may be caused by the uneven etching of the 5mil plane during processing, such impedance fluctuation will also bring the resonance of insertion loss, we can also see this from the comparison results of the loss. We can clearly see the resonance points of different programs when the packet width is not enough.
Finally, in fact, this case is not uncommon in actual products, especially in the case that some PCB boards are very dense, and design engineers need to package the same group of relatively important signals. Due to the limitation of space, in order to make room for the signal or other devices, they may be more stingy than the width of the compression package plane, so you need to pay attention to the package, if your package width is not enough, but will bring negative effects to the signal, oh, may not be as good as the package.
