Page 250 - System on Package_ Miniaturization of the Entire System

224 Cha pte r F o u r

Continuous Planes
Power-ground planes in electronic packages can be a major factor for noise coupling.
Excessive supply voltage fluctuations cause signal integrity problems. In addition,
noise voltage that gets coupled to the edge of the board may cause significant
electromagnetic interference. Hence, accurate estimation of the performance of power-
ground planes is critical in a mixed-signal system.
Figure 4.71 shows the simulated S parameters between two ports on the opposite
edges of a 47 mm × 47 mm board. The dielectric between the planes is FR4 with a
dielectric constant of 4.6, a loss tangent of 0.02, and a thickness of 50 μm. The magnitude
of the transmission coefficient for both a solid plane and an EBG patterned plane are
shown (EBGs are discussed in a later section). There is a large amount of coupling
between the two ports in case of the solid plane. The EBG patterned plane, on the other
hand, suppresses the coupling significantly from about 1 to 4 GHz, which can be
designated as the “stopband” of the EBG. Hence if the RF circuits attached close to one
of the ports on this board operate within this frequency range, they will be unaffected
from the noise generated by switching digital circuits attached close to the other port.

Aperture Coupling
A solid plane made of a perfect conductor of infinite lateral dimensions would
completely shield the fields on one side from the other side. Therefore, there would be

(47, 47)

Port 2 Port 2
Port 1 Port 1
10
26
y 15
(0, 0) x (0, 0)
y
47.0mm

0
Solid plane EBG
−20

mag(S12) [dB] −40

−60

−80
−100

−120
0 2 4 6 8 10
Frequency [Hz] × 10 9
FIGURE 4.71 Coupling between two points on a solid plane and an EBG patterned plane (units in
millimeters).