Impact of Port Type in S-Parameter Extraction of Package and PCB High-Speed Interconnections

Modern digital devices are characterized by an increasing density of high-speed signal interfaces running at growing frequencies. In this context, a reliable characterization of the signal interconnections is fundamental and requires a very accurate modeling approach, relying on 3D electromagnetic simulation tools. For BGA (Ball Grid Array) packages, due to the high routing density and the small dimensions, crosstalk is becoming a very critical quantity to be verified through s-parameters extraction and compared with application targets. S-parameters require the definition of ports at the terminals of the extracted lines, and this can be done with different methods. Focus of this paper is a comparison between circuit ports and coaxial ports, with the purpose of understanding how the port type can influence the simulation results. The fact that a specific port type requires the addition of dedicated geometry elements to make it work (i.e. an additional reference plane) is a key aspect in this study, since it changes the boundary conditions versus the original design. After presenting a series of comparative tests, the port type impact is characterized, and the optimal setup is identified considering the test case of a high-speed signal bus on a BGA package. As an extension, similar tests are performed on a PCB, confirming the same findings besides some intrinsic differences.