ELECTRICAL PROPERTIES OF DIELECTRICS - Advanced Signal Integrity for High-Speed Digital Design - page 277

4 ∆e r, eff = 0.23

3.5

3

2.5

2

0.35

∆e r =

1.5

1

1 1.5 2 2.5 3 3.5 4 4.5 5

e r

(a)

w

w

e r = 4.6 e r = 4.95

h

h

between bundles over a bundle

(b)

Figure 6-18 (a) Relationship between ε r and ε r ,eff ; (b) equivalent cross sections for a
trace directly over a bundle and between bundles for Example 6-2.

Trace between bundles:

ε r, eff = 3 . 5 (from Figure 6-15) and ε r ∼ 4 . 6 (from Figure 6-18a)

Trace over a bundle:

ε r, eff = 3 . 72 and ε r ∼ 4 . 95

Step 2: Create the equivalent cross sections, as shown in Figure 6-18b.

6.6 ENVIRONMENTAL VARIATION IN DIELECTRIC BEHAVIOR

One problem often overlooked in high-speed digital design is the impact that the
relative humidity (RH) of the environment has on the electrical performance of a
dielectric material. The electrical properties of a dielectric are partially a function
of the amount of moisture in the material. The extent to which a material absorbs
moisture is characterized by its moisture diffusivity and saturated moisture
concentration. Moisture diffusivity describes the rate of change of a material's
moisture concentration. The saturated moisture concentration provides an
expression for the limit to the amount of moisture that a material can contain.
It is important to note that both are a function of temperature and relative
humidity. Another measure of a material's susceptibility to moisture is its
maximum moisture uptake . This is typically reported on material data sheets as %
weight and is related to the saturated moisture concentration. If a printed circuit