In mixed-signal designs, substrate noise originating from the digital part can seriously influence the functionality of the analog part. As such, accurately modeling the properties of the substrate as a noise-propagator is becoming ever more important. A model can be obtained through the finite element method (FEM) or the boundary element method (BEM). The FEM performs a full 3D discretization of the substrate, which makes this method very accurate and flexible but also slow. The BEM only discretizes the contact areas on the boundary of the substrate, which makes it less flexible, but significantly faster. A combination between BEM and FEM can be efficient when we need flexibility and speed at the same time. This paper briefly describes the BEM and the FEM and their combination, but mainly concentrates on the theoretical validation of the combined method and the experimental verification through implementation in the SPACE layout to circuit extractor and comparison with commercial BEM and FEM tools.