ELECTRICAL PROPERTIES OF DIELECTRICS - Advanced Signal Integrity for High-Speed Digital Design - page 282

lines does not cure the effect of the environment because products are generally
designed to operate under all reasonable conditions, and it must be assumed that
there will be periods where the digital design is powered down long enough
for the dielectric to absorb enough water to increase the loss tangent. How-
ever, striplines can help mitigate the problem under certain conditions because
it takes a very long time for the dielectric contained between copper layers to
absorb water.
In some cases, the product usage model may allow mitigation of the environ-
mental effects by not allowing large changes in relative humidity for normal
operation. For example, computer servers are often operated in a controlled
temperature/humidity room, allowing engineers to design for minimum environ-
mental variation, thus increasing the performance of the platform. Additionally,
some computers are often powered on continuously. In this case, the heat gener-
ated by the CPU and the movement of air by the fan will produce a low-humidity
environment inside the computer chassis. However, in this case, the engineer must
understand that if the computer is powered down for any significant amount of
time, the board dielectric material may begin to absorb moisture. The absorbed
moisture will increase the loss of the transmission lines and decrease the perfor-
mance of the high-speed buses until the absorbed moisture diffuses out, which
could take several hours to several days, depending on the dielectric mate-
rial and the environmental conditions inside the chasses during operation and
transmission-line structure.
6.6.3 Modeling the Effect of Relative Humidity on an FR4 Dielectric
The modeling method employed to simulate the effect of moisture absorption in a
dielectric material is entirely dependent on the dielectrics affinity to absorb water,
how the absorbed moisture interacts with the chemical structure of the dielectric,
and the frequency range of interest. The dielectric most commonly used in the
electronics industry is FR4, which readily absorbs moisture. To understand how
to account properly for environmental variations in the relative humidity, we
consider how to model FR4 dielectric for two cases:
1. Low frequencies (below 2 GHz). Below 2 GHz, the effect of moisture
absorption has very little effect on total transmission-line losses (at least
for FR4 dielectrics). This behavior is demonstrated in Figure 6-19, where
the difference in insertion loss is minimal until approximately 2 GHz. This
is because at low frequencies, the total loss is usually dominated by the
skin effect resistance of the signal conductor, as described in Section 5.2.
For typical transmission lines, the losses of the dielectric begin to dominate
above about 1 to 2 GHz.
2. High frequencies (2 to 50 GHz). For this frequency range, the dry
and nominal environmental corners (up to approximately 50% relative
humidity for FR4) can be modeled with the infinite-pole Debye model, as
described in Section 6.3.5 and shown in equation (6-29). However, for