How big is the difference between the vias you want and the processed vias?

     How big is the difference between the vias you want and the processed vias?

    In high-speed PCB design, the influence of vias is self-evident. A poorly optimized via can reduce the performance of the entire link by one level!More importantly, if you think that you have optimized the PCB design, it will look like this after processing, then you.....

   What we are going to talk about today is the design and processing errors. To be more specific, today we are talking about the difference between the design aperture of the via and the actual processed aperture, because according to the article we have said before, the aperture has an effect on the impedance of the via.

   The influence is also great, and the larger the aperture of the via, the lower the impedance of the via.

It is already difficult to achieve a single-ended 50 ohm or a differential 100 ohm impedance for the via, and it is even more difficult after processing.

Therefore, through the simulation and test of a test board we made today, we will let everyone know that there is actually a difference between the designed aperture and the processed aperture!

our test board is designed with four apertures of 8mil, 10mil, 12mil and 14mil respectively, as shown below: In order to ensure the unity of the variables, we put the anti-pad size of the via and the position of the via next to it.

The distances are all made the same, and the only variable is the aperture of the via (and the pad for the via).

   then how can we know the error of design and processing?

   The method is also very simple. Let's take 14mil vias as an example to illustrate our verification method.

We first know that the 14mil via on the PCB design actually refers to the 14mil aperture after processing and electroplating, as shown below:

   You must also know that if you want to achieve a finished aperture of 14 mils after processing, the hole diameter must be greater than 14 mils, because after the drilling is completed, the hole wall needs to be electroplated to ensure vertical connectivity.

   Then the size of the borehole is the parameter we care about, and it is directly linked to our impedance,Because we can simulate the size of the drill hole to scan and simulate the via model

Then we can get the impedance of this via at different hole diameters through simulation, as shown below: from 0.35mm 14mil to 0.55mm 22mil hole impedance when the hole diameter is 22mil.It can be seen from the simulation results that the impedance of the via is also not a small change when the 2mil hole diameter is changed.The original 14mil design via impedance can just reach 50 ohms. As the processed aperture becomes larger and larger, the real impedance gradually decreases.

    The original 14mil design via impedance can just reach 50 ohms. As the processed aperture becomes larger and larger, the real impedance gradually decreases.

So what size drill will the board factory use to drill the 14mil design aperture that you are curious about by default?

    Now that Mr. Gao has made this test board, it must be its test data. We compared the simulation results with the test results one by one, and found that the simulation results of the 0.45mm borehole diameter and the real test results can be better simulated.

   In other words, for this design aperture 14mil via hole, the factory will use a 0.45mm or 18mil drill to drill the hole, and then electroplating into a 14mil complete aperture!

In other words, for this design aperture 14mil via hole, the factory will use a 0.45mm or 18mil drill to drill the hole, and then electroplating into a 14mil complete aperture!

   Of course, for the vias with other apertures, Mr. Gao also did similar simulation tests to prove the size of the drill used by the board factory for different design apertures. Because of the space, we will not list them one by one here.What Mr. High Speed wants to tell you through this article is that when we are designing via PCB, we must estimate in advance some changes after processing, especially in high-speed design, its processing changes affect the performance of the channel.It will have a lot of impact. It is an excellent PCB design to reserve this part of the margin!