ELECTRICAL PROPERTIES OF DIELECTRICS - Advanced Signal Integrity for High-Speed Digital Design - page 248

As the speed of digital systems continues to increase with Moore's law, the elec-
trical performance of the dielectric layers of the printed circuit board, package,
or multichip modules becomes significantly more important. Dielectric materi-
als that worked perfectly well for slower designs become increasingly difficult
to design with because new phenomena, such as frequency-dependent dielec-
tric permittivity and loss tangents, environmental factors, and localized interac-
tions between the electromagnetic signal and the fiber weave reinforcement of
the board, become significant and can no longer be ignored. Without properly
accounting for the high-speed dielectric phenomena, it becomes impossible to
properly predict phase delay and signal losses, leading to nonphysical behavior
of the transmission line model. In short, simulation-based digital bus designs
exceeding 3 to 5 GHz are not possible without accounting for the effects covered
in this chapter.
Dielectrics , more commonly called insulators , are substances whose charges
in the molecules and atoms are bound and therefore cannot move over macro-
scopic distances under the influence of an applied field. Ideal dielectrics do not
contain any free charge (such as in conductors), and chemical structure is macro-
scopically neutral. When a field is applied to a dielectric, the bound charges do
not move to the surface of the material as they would in a conductor, but the elec-
tron clouds associated with the atomic and molecular structures of the dielectric
can be distorted, reoriented, or displaced, inducing electric dipoles. When this
happens, the dielectric is said to be polarized . The polarizability of a dielectric
leads directly to the definition of the relative permittivity, dielectric losses, and
the relationship between energy propagation and losses.
6.1 POLARIZATION OF DIELECTRICS
For a metal, conductivity is caused by the redistribution of free charges over
a macroscopic distance. For example, Figure 5-4 shows how the current, and
therefore the charge are largely contained within one skin depth on the bottom
of the conductor. For dielectrics, the applied field only displaces a few electrons
per atom over very small, subatomic distances. In a dielectric, the electrons are
tightly bound to the atoms, and only a negligible number of electrons are available
for conduction of electric current. The difference in electrical behavior of a
conductor versus a dielectric is essentially the difference between free and bound
charges.
6.1.1 Electronic Polarization
When a dielectric material constructed of nonpolar molecules is exposed to an
external electric field, the electrons react by shifting with respect to the nucleus
opposite the applied field. This establishes numerous small electric dipoles that
align with the electric field. When the external electric field is removed, the
electric dipoles return to the neutral position, as shown in Figure 6-1a. Essentially,