A surface mount component can be considered a complex, strip-like transmission line with the PCB back plane serving as its ground. Variations in the substrate parameters have a dramatic effect on the frequency response.
Modelithics models incorporate features that enable accurate accounting of substrate-related effects, by appropriately scaling affected model parameters. Each model is generated from multiple sets of S-parameter measurements, made with the parts mounted in different PCB test fixtures.
The models are valid over a continuous range of substrate properties, within limits that are determined by the substrates used in the model extraction process, for the same or similar dielectric constant (e.g. 5-59 mil thick substrate, Er = 4.3 +/- 0.5). Many of the models in the Modelithics library are extracted from measurements using various thicknesses of FR4, Taconic or Rogers soft board materials, and in some cases Alumina or Quartz thin film test fixtures.
The models are valid over a continuous range of substrate thickness (H) and dielectric constant (Er) properties derived from the boards used in the model extraction (e.g. 5-59 mil thick substrate, Er = 4.3 +/- 0.5). The valid range of H/Er varies from model to model and is specified in each model data sheet.