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66 DESIGN007 MAGAZINE I SEPTEMBER 2018 Article by Yuriy Shlepnev SIMBERIAN INC. What does it take to design predictable PCB or packaging interconnects operating at tens of Gbps? Properly identified dielectric and conductor roughness models, known manu- facturer geometry adjustments, and properly validated simulation tools are necessary con- ditions. One of the sufficient conditions is the localization property; to be predictable, all ele- ments of an interconnect link must be local- ized up to a target frequency. This article intro- duces and illustrates the localization concept, with the power-flow density computed using the unique Trefftz finite element solver avail- able in Simbeor THz software. Ideally, all interconnects should look like uniform transmission lines (or wave-guiding structures) with the specified characteristic impedance. In reality, an interconnect link is typically composed with transmission lines of different types (microstrip, strip, coplanar, coaxial, etc.) and transitions between them such as vias, connectors, breakouts and so on. Transmission lines may be coupled to each other that cause crosstalk. The transitions may reflect and radiate energy due to discon- tinuities in signal and reference conductors. The crosstalk, reflections and radiation cause unwanted and sometime unpredictable signal degradation. If analysis of traces or via hole transitions is possible in isolation from the rest of the board up to a target frequency, the structure is called localized [1] . Structures with behavior that is dependent on other structures and board geometry are called not localized, and they should not be used in multi-gigabit interconnects in general. Figure 1: Localized vs. not localized.

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