Investigation and correlation study for board-level multi-GHz interconnects with non-ideal return paths

Dense, complex, and cost conscious board designs often make it impractical to avoid using non-ideal return paths in board-level transmission line interconnections. For example, vertical transitions without specific local return vias and split reference plane constructions are frequently used. The use of non-ideal return paths is likely to result in signal power loss and/or unwanted coupling excited from other nearby designs. This ultimately leads to reductions in signal integrity margin such as smaller voltage margins and increased jitter. Compared to interconnections with a well-defined reference plane, it is relatively difficult to accurately simulate interconnections with non-ideal return paths because the electromagnetic field pattern is more complex and is dispersed widely over the structure, requiring modeling large regions of the board. To help build confidence in electrical simulation this paper tackles three different design issues related to return paths: multiple layer transitions, split reference planes, and stitching vias between planes. Dedicated test structures with different design variables for each issue were fabricated and measured for their frequency responses as well as simulated using a commercially available 3D tool. The measured and simulated results are presented in terms of correlation between results produced by the two approaches and the design guidance that can be inferred from design variations.