While vias are widely used in PCB, the complexity of their internal structure can lead to some misconceptions, and one such issue is the via stub. In circuit design, designers often strive to place high-speed or RF signal traces on inner or outer layers without considering the actual layer hierarchy. They typically assume that the performance of the traces is the same regardless of the layer they are on. But is this assumption valid? This article will answer this question through a case study.
Keep Via Stubs
The initial design included via stubs with a maximum length of 72 mils. Through circuit simulations, we obtained two sets of trend graphs. It is evident that both the insertion loss and return loss were quite poor, with an impedance of only 61 ohms.
Remove via stubs
However, when we removed the via stubs from the design and re-simulated it, the results were dramatically different. At around 14 GHz, the insertion loss improved by approximately 40 dB, the return loss improved by about 13 dB, and the impedance increased by 20 ohms. This represents a significant improvement for high-speed PCB design!
Additionally, we compared eye diagrams obtained at a signal rate of 16 Gbps. It is clear that with the stubs present, the eye diagram was completely closed, but after removing the stubs, the eye diagram became much more open.
Summarize
Therefore, we can conclude that via stubs in high-speed PCB design directly impact signal transmission performance. To address this issue, TechSparks recommends considering the use of backdrill processes or blind buried vias when dealing with higher signal rates and longer stubs.
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