Influence of a floating plane on effective ground plane inductance in multilayer and coplanar packages

The impact of a floating metal layer on the effective ground plane inductance has been investigated in both multilayer (ground planes) and coplanar (ground conductors) packages. For the multilayer case, both thick and thin film geometries were examined, and the results were compared to a thin film coplanar configuration. It was seen that the floating plane actually increases the ground plane inductance in the multilayer case and decreases the ground plane inductance in the lead frame case. Examining the current density in the floating and ground plane and the ground's partial self-inductance and ground-signal partial mutual inductance give a detailed explanation for this phenomenon.