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Methodology of Physics-Based Via Model Development for Differential Vias
Keywords: physics based via modeling, equivalent circuit, methodology of circuit topology for via models
Signal transition through vias is critical in printed circuit boards (PCBs) and packages for high-speed interconnects. When the data rate of signals is above hundreds of Mb/s or especially in the range of Gb/s on PCBs or packages, signals due to via transitions may be distorted and degraded. This signal degradation is caused by geometry variation from a planar trace to a cylindrical via barrel and back to a planar trace again. Since the characteristic impedance of a via is usually different from the characteristic impedance of a trace, this difference will result in multi-reflection in the signal channel, and distort signals as well. In addition, the open stub of a via is actually a shorted load of the via at a certain frequency, which will stop signal transmitting through the via within a certain frequency span. Many efforts have contributed to via modeling for more than ten years. Full-wave numerical approach and equivalent circuit method are the two main categories in via modeling. Since a full-wave via model is complicated to build up, takes long time to run, and sometime is limited by the computational resources such as a huge memory requirement, the equivalent circuit method is strongly desired due to the advantage of fast to run and easy to implement in channel simulations. Many equivalent circuits for via modeling can be found in literature, which are studied case by case for most of them. A general methodology for physics-based equivalent circuit via model is proposed in this paper. It is not only to deal with how to develop the equivalent circuit via model, but also to investigate port condition and TEM reference determination. Good agreement of S-parameter comparison between the physics based via model and full-wave modeling has been achieved up to 20 GHz based on the proposed method.
Jianmin Zhang, Sr. SI Engineer
Cisco Systems, Inc.
San Jose, CA

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