Digital Signal Processing Reference
In-Depth Information
−
6
−
6.5
Day 0, sample is preconditioned to be dry
−
7
−
7.5
−
8
Sample stored in an environmental chamber at
38
°
C and 95% relative humidity for 55 days
−
8.5
−
9
−
9.5
0
10
20
30
40
50
60
Days
Figure 6-23
Insertion loss at 10 GHz of a transmission line built on 7628-FR4 dielectric,
showing how losses change with time exposed to a humid environment. (Adapted from
Hamilton et al. [2007].)
conditions in an environmental chamber and measured at specific intervals for
55 days. In this example, the dielectric transmission-line losses at 10 GHz were
−
0. When the structure was exposed to
a humid environment, the dielectric became almost fully saturated within the
first 7 days, increasing the loss to
6
.
3 dB under dry conditions at
t
=
−
8
.
9 dB. Finally, the transmission-line loss
stabilized to a value of
48 days.
Stripline transmission lines tend to absorb moisture at a significantly slower
rate than microstrip lines because there is much less dielectric surface area
exposed to the surrounding environment. Hamilton et al. [2007] report exper-
imental data for striplines built on 7628-FR4 dielectric with 90% coverage of the
dielectric material by copper planes, showing that it takes about 5 months for
the stripline to reach the same loss value as a microstrip does in 7 days under
identical conditions. This means that striplines are not immune to environmental
conditions; however, they absorb moisture at a significantly slower rate.
−
9
.
3 dB of loss at
t
=
6.6.2 Mitigation
Unfortunately, there are very few practical methods for mitigating the effect of
the environment on transmission-line performance. In consumer products such
as laptop or desktop mother boards, the full range of environmental conditions
must be accounted for in the design to ensure that performance specifications are
maintained in both dry and humid environments (such as a dry Arizona winter
or a humid Florida summer). This means that an FR4-based transmission-line
model used in the design must comprehend as much as a 50% or more variation
in the loss tangent, which can significantly reduce solution spaces, particularly for
long bus lengths and high data rates. Generally, the use of stripline transmission
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