Embedded microstrip

The commonly used PWB transmission line structures are microstrip, embedded microstrip, striphne, and dual striphne, whose cross sections are shown in Fig. 11.6. The empirical CAD oriented expressions for transmission Hne parameters, and the models for wires, ribbons, and vias are given in Table 11.3. 

In the microstrip configuration, the signal conductor is surrounded by air (except on the dielectric surface). If the conductor is coated by another dielectric, such as a glaze or encapsulant, it is referred to as embedded or coated microstrip. If the dielectric constant of the coating is close to that of the supporting dielectric, the impedance of the line will be lowered by 10-20%, and the signal propagation delay will be increased by about 20%. 

Both thin- and thick-film technology allow planar microstrip or coplanar transmission lines. However, requirements on line and coupling-gap accuracy do not always permit traditional screen printing. Etching techniques or photosensitive paste systems offer a potential solution. LTCC provides more design freedom. Impedance-matched line transitions (e.g., from microstrip to stripline or to an embedded waveguide) are possible. 

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