HFSS Regions in SIwave is a hybrid solution technique to obtain 3D full-wave accuracy for the S-parameters of critical nets on a printed circuit board. To demonstrate this feature, a very large PCB, 60 cm long and 42 cm wide, with 20 metal layers is used in ANSYS SIwave. High-speed differential pairs are identified on the PCB and region extents are created for the 3D discontinuities. Everything else is executed in an automated fashion in SIwave and the board is simulated with and without HFSS Regions. This whitepaper will help you develop a solid understanding of this feature.