Impact of PCB drilling and plug holes on vias - if via resistance is also +-10%

Impact of PCB drilling and plug holes on vias - if via resistance is also +-10%

We went to the factory and asked them: Can the wiring help us control the 10% impedance tolerance? They will confidently promise you: No problem! If you want to take another look and control 8%, they will think about it a little and also say something like: OK (inside)! If you ask at this time: Can via holes help me control 10%? The boat of friendship may be. . . turn!

Also as a bridge connecting the transceiver chip, why can the cable be controlled to 10% and the via hole can not? We know that the traces are actually not well controlled. The processing error affecting the trace impedance includes the etch factor, layer deviation, and surface roughness. In the microstrip line, the effect of green oil and copper thickness is added, but Some current mainstream manufacturers can guarantee 10% or 8% impedance deviation. As for the vias, with Xiaobian's knowledge of the board, there is no board factory that promises to control the 10% impedance deviation. That certainly means that there are many factors that the board can't control. . . There is no way for the mill to help guarantee it.

In this article, we first describe two processes for hole machining. One is drilling and the other is plugging. For drilling, many people know that there will be an impact, but in the end the hole processing tolerance will have much impact on the performance of the hole? The other is the plug hole, many people feel that plugging the resin to the hole or the green oil will affect the performance of the hole. We will use simulation to verify their impact.

Let's talk about drilling first. We often hear two names, drill hole diameter and finish hole diameter. In our PCB file, we design an 8mil hole. Then you think the factory will use an 8mil drill to drill this hole. ? Obviously not. We know that a via is used to connect different layers of traces. First, the via must be vertical, that is, copper must be connected in the vertical direction of the hole. This is what we call the plated hole. copper. According to the IPC standard, there is a certain requirement for the copper plating thickness of the via hole, about 18um to 20um, then the manufacturer actually guarantees that the hole diameter after the copper plating is 8mil, which is what we call the completion of the aperture. In that case, our original drilling hole diameter must be larger than 8mil, but it is big, but how much? 10mil? 12mil? This large number actually has a large impact on the performance of our vias.

According to the manufacturer's processing error standard for drilling holes, a 0.2mm (8mil) hole will use a minimum 0.25mm (10mil) drill, and if we do not specify the control, the manufacturer may also use 0.3mm (12mil). Drill knife. Everyone knows that if the borehole is large, the impedance will be low, but how much will it be lower? Our simulation results can tell you.

In fact, knowledge of having a bit of high-speed theory all knows that the signal will produce a skin effect when it is relatively high-speed. At this time, the signal goes through the outer wall of the via hole, so no matter how much the dielectric constant is added in the via hole, All good, in fact, the electromagnetic field between the signal and the reference will not go inside. Since there is no electromagnetic field inside, then no matter what kind of media you have, there will be no impact.