S Parameters Optimization of High-Speed Differential Vias Model on A Multilayer PCB

Abstract

High-speed digital system with an increased data transmission rate is facing with serious signal integrity and electromagnetic compatibility problems. To reduce crosstalk and electromagnetic radiation, the components of PCB are placed on the top layer, while the routing of differential transmission lines is on the inner layer. Therefore, Vias must be used to connect components and the inner layer. Vias cause impedance discontinuity during the signal transition and significantly affect signal integrity in a high-speed system. When the signal passes through the vias, reflection may occur due to impedance mismatch. It’s necessary to model and optimize the differential vias structure. The traditional optimizing method is realized by adjusting the vias’ radius, the distance between differential vias and the anti-pad’s radius to reduce impedance mismatch. This paper proposes new optimization methods of vias structure, which has good performance in GHz digital system. In this paper, Ansys HFSS is used to simulate the three-dimensional electromagnetic model of differential vias on a 24-layer PCB and analyze the S parameters (return loss and insert loss). Several key factors affecting the performance of differential vias are analyzed, including stub, non-functional pads and ground vias. Simulation results show that the stub will make the signal resonate and shortening the stub can eliminate the resonance gradually. Removing non-functional pads and adding additional ground vias can help improve the high frequency performance of the system.