6.3 Microstrip
Microstrip refers to the traces on the outer layers of a circuit board. Some drawings show a trace resting on top of the dielectric of a given thickness. Others show the trace as partially embedded in the dielectric. The characteristic impedance Z0 for the microstrip, as shown in Figure 6.1, is given in Equation 6.1:
This equation will give good results for 0.1 < w/h < 3 and where ε R < 15.
Figure 6.1 Microstrip geometry.
It is interesting to hold the characteristic impedance constant and note the relationship between trace width and dielectric height. Figures 6.2–6.4 show curves of constant characteristic impedance, where the dielectric constant is 4. If the dielectric constant is 3.5, the characteristic impedance rises 5%.
Figure 6.2 Microstrip. Trace width versus the dielectric height needed to hold the Z0 constant. Trace thickness equals 1.5 mil.
Figure 6.3 Microstrip. Trace width versus the dielectric height needed to hold the Z0 constant. Trace thickness equals 2.0 mil.
Figure 6.4 Microstrip. Trace width versus the dielectric height needed to hold the Z0 constant. Trace thickness equals 2.7 mil.
Note that the equations are continuous, but the user faces many dimensional restrictions. For example, trace thickness standards are half 1- and 2-oz plated copper. This thickness will increase on outer layers when copper is plated in drilled holes. In practice 1/2-oz copper ends up thinner than the expected 0.7 mil. The useable thickness is between 0.5 and 0.6 mil. This reduction in thickness results from cleaning operations during manufacture. The designer should consult the manufacturer to determine the final trace thickness after manufacture.
Dielectric thicknesses are limited to the laminates or cores that are available. The equations provide some insight into just how thick the dielectric must be to bring the characteristic impedance into range. Here again, there are many thicknesses available, and there are also many tolerance problems. It is wise to work with a manufacturer and use materials that he understands and that he keeps in stock.
The basic relationship between trace width, trace thickness, and dielectric height for 50-ohm lines and a dielectric constant of 4 is shown in Figure 6.5.
Figure 6.5 Curves showing the relationship between trace thickness t, trace width w, and dielectric height h for 50-ohm microstrip lines with a dielectric constant of 4.
