ELECTRICAL PROPERTIES OF DIELECTRICS - Advanced Signal Integrity for High-Speed Digital Design - page 278

board (PCB) constructed with a dielectric that tends to absorb moisture (such
as FR4) is exposed to a humid environment (such as Malaysia) for a significant
amount of time, both the loss tangent and the dielectric permittivity will increase.
To demonstrate this, Figure 6-19 shows an example of the insertion losses of an
identical structure measured in a dry environment such as an Arizona winter
(15% RH and 60 ◦ F) and a humid environment such as Malaysia (95% RH and
95 ◦ F). As explained in Chapter 9, this dramatic increase in insertion losses cor-
responds to an increase in dissipated power. In linear terms, the example in
Figure 6-19 represents approximately 50% more losses due to the dielectric at
10 GHz. Figure 6-20 depicts measured values of the same dielectric material
for both low- and high-humidity environments. Note that at about 7.5 GHz, the
increase in the loss tangent is greater than 50%. Conversely, Figure 6-21 shows

0.0

− 4.0

Arizona Winter
Low RH

− 8.0

− 12.0

Malaysia Summer
High RH

− 16.0

− 20.0

0 5 10 15 20

Frequency, GHz

Figure 6-19 Example of the impact of the environment on PCB losses for a microstrip
structure constructed on an FR4 dielectric.

0.022

95% Relative humidity and 36 ° C

0.02

0.018

1.57 × increase

0.016

0.014

10% Relative humidity and 20 C

°

0.012

0.01

0 2 4 6 8 10 12

Frequency, GHz

Figure 6-20 Example of the measured variation in tan δ for 7628-FR4 dielectric for
different environmental extremes.