In the design of high-speed PCB, the influence of pass is self-evident. A poorly optimized pass can reduce the performance of the whole link by one level! More importantly, if you think you have optimized the PCB design, the processing must be like this, then you…
In fact, Mr. Gao has written many articles about the effect of the hole, including how the structure of each part of it affects the impedance; How its perforated stubs can be fatal to the signal; It in different layers of the output will also have different impedance performance and so on the article, we can go forward to read again.
But today we want to talk about the error of design and processing, and then detailed, today is about the difference between the design aperture and the actual processing aperture, because according to the article we said before, the impact of aperture on the impedance of the hole is also great, and the larger the aperture, the lower the impedance of the hole. It is already difficult to make the impedance through the hole to single end 50 or difference 100 ohm, coupled with the processing is more difficult. So today we do a test board through their own simulation and testing to let you know that the design aperture and the aperture of the processing is actually different!
Our test boards are designed with four holes of 8mil, 10mil, 12mil and 14mil respectively, as shown below: In order to ensure the uniformity of the variables, we made the size of the back pad of the hole and the position distance between the hole and the side hole the same, the only variable is the hole diameter of the hole (and the pad of the hole to the hole).
So how can we know the error between design and machining? The method is also very simple. Let’s take 14mil through hole as an example to illustrate our verification method.
First of all, we know that the 14mil through hole in PCB design actually refers to the aperture of 14mil after electroplating, as shown below:
As you must know, if you want to achieve the completion aperture of 14mil after machining, the hole aperture must be greater than 14mil, because after drilling is completed, the hole wall needs to be electroplated to ensure vertical connectivity.
So the size of the drill hole is the parameter we care about, and it is directly related to our impedance. Because we can simulate the size of the drilling hole and scan the model through the hole.
Then we can get the impedance of the hole through different hole diameter through simulation, as shown below: from 0.35mm of 14mil to 0.55mm of 22mil hole diameter. It can be seen from the simulation results that when the hole diameter is changed by 2mil, the impedance of the hole is also a small change. The original design of 14mil through hole impedance can just reach 50 ohm, with the larger and larger processing aperture, the real impedance is also slowly decreased.
So for the design aperture of 14mil that you are curious about, what is the default size of the drill cutter used by the plate factory to drill? 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 0.45mm drilling aperture could be better fitted with the real test results. That is to say, for this hole with a design aperture of 14mil, the factory will drill the hole with a drill tool of 0.45mm, that is, 18mil, and then electroplate it into a complete aperture of 14mil!
Of course, other aperture through the hole, high-speed Mr. Also did similar simulation test verification, to prove that the plate factory for different design aperture used by the size of the drill, here because of the relationship between space is not a list. Mr High-speed through this article want to tell you, when we have had holes in PCB design, be sure to estimate in advance to its changes after processing, especially in the design of high speed, its processing change will have a nontrivial effect on the performance of channel, to set aside this part of the allowance is a good PCB design!
