1.4.6. Planar Capacitance and Spreading Inductance
Power planes adjacent to ground planes benefit from embedded planar (or buried) capacitance, which aids high frequency decoupling. Because planar capacitance is inversely proportional to the dielectric thickness between the power and ground planes, choose thin dielectrics between power and ground planes to increase planar capacitance while reducing planar spreading inductance and overall board thickness. The reduction of planar spreading inductance results in a lower impedance path and increases the effectiveness of discrete capacitances (such as 0402 or 0603 capacitances) that are frequently used for decoupling on the board.
When designing power plane capacitance into the stackup, use below equation to model the resulting planar capacitance.
- is the capacitance
- is the permittivity of free space (8.85x10-12 F/m)
- is the relative permittivity of the dielectric
- is the area of overlap between the power and ground plane
- is the dielectric separation between the planes
Use below equation to determine the planar spreading inductance of the power-ground plane sandwich.
- is the planar spreading inductance
- is the permeability of free space (32 pH/mil)
- ,, and are the respective separation between the power-ground sandwich, length and width of the plane in mils