6.4 Centered Stripline
Stripline refers to the traces that are located between conducting planes in the inner layers of a multilayer circuit board. The characteristic impedance of centered stripline for the trace geometry is shown in Figure 6.6 and given in Equation 6.2:
where 0.1 < w/h < 2, t/h < 0.25, and ε R < 15.
Figure 6.6 Centered Stripline.
It is interesting to plot the relationship between trace width and dielectric thickness. These curves are shown in Figures 6.7–6.9. Notice that the trace thicknesses are limited to the copper clad and copper foil thicknesses that are available and to the thickness after cleaning or plating.
Figure 6.7 Centered stripline. Curves of fixed characteristic impedance versus trace width and dielectric height for a trace thickness of 1.5 mil.
Figure 6.8 Centered stripline. Curves of fixed characteristic impedance versus trace width and dielectric height for a trace thickness of 2 mil.
Figure 6.9 Centered stripline. Curves of fixed characteristic impedance versus trace width and dielectric height for a trace thickness of 2.7 mil (2-oz copper).
Note that the dielectric spacing for a 50-ohm line are about double than those of microstrip. Notice that the characteristic impedance does depend on trace thickness. This dependence is shown in Figure 6.10.
Figure 6.10 Centered stripline. Curves showing the relationship between trace thickness, trace width, and dielectric thickness for a 50-ohm line and a dielectric constant of 4.
