EP2S90F1020C5 Datasheet, PDF (738/768 Pages) Altera Corporation – Stratix II Device Handbook, Volume 1

PCB Material Selection
where:
Q₁, Q₂ = charges
r = distance between the charges (m)
F = force (N)
 = permittivity of dielectric (F/m).
Each PCB substrate has a different relative dielectric constant. The
dielectric constant is the ratio of the permittivity of a substance to that of
free space, as follows:
ε_r =
ε
εο
where:
_r = dielectric constant
o ^{= permittivity of empty space (F/m)}
 ^{= permittivity (F/m)}
The dielectric constant compares the effect of an insulator on the
capacitance of a conductor pair, with the capacitance of the conductor
pair in a vacuum. The dielectric constant affects the impedance of a
transmission line. Signals can propagate faster in materials that have a
lower dielectric constant.
A high-frequency signal that propagates through a long line on the PCB
from driver to receiver is severely affected by the loss tangent of the
dielectric material. A large loss tangent means higher dielectric
absorption.
The most widely used dielectric material for PCBs is FR-4, a glass
laminate with epoxy resin that meets a wide variety of processing
conditions. The dielectric constant for FR-4 is between 4.1 and 4.5. GETEK
is another material that can be used in high-speed boards. GETEK is
composed of epoxy and resin (polyphenylene oxide) and has a dielectric
constant between 3.6 and 4.2.
11–2
Stratix II Device Handbook, Volume 2
Altera Corporation
May 2007