Analysis and extension of segmented physics-based via modeling for microstrip transitions and differential signaling

Abstract

This work revisits the framework of physics-based modeling of multilayer substrates with vias. The first part reviews the model and discuss their limitations for irregular structures, when there exist cases where high-order modes are excited because of different situations, such as non-uniform current distributions, coupling between nearby interconnections, etc. Some of these irregular cases are not covered in the current physics-based modeling framework. From there, two aspects are explored in this work. First, an extension of the physics-based via modeling is proposed, in order to handle microstrip line transitions on the bottom or top layers. This is implemented by using a simple model for the via to microstrip line transition that works up to 12 GHz. Here, the construction of a generalized network model is also discussed, and the proposed model is validated against full-wave methods, i.e. FEM, FIT; in order to evaluate the agreement between baseline cases and the proposed modeling approaches. Second, the role of asymmetries in differential structures with stub vias is studied. It is shown that mode conversion can become an important source of degradation in differential links. These remnants via stubs can be present because of multiple reasons, such as residual stubs due to process tolerances or errors during back-drilling. Different via configurations are analyzed through full-wave simulations. Results show a large impact on mode conversion with an increment of around -35 and -15 dB when the residual stub difference changes from 2 to 14 mil at the fundamental frequency of 17.5 GHz. Furthermore, an estimation approach of the amount of mode conversion as a function of via stub asymmetry is analyzed and proposed.