The embedded microstrip is a similar in construction to the microstrip transmission line. Modeling approximation can be used to design the embedded microstrip trace. By understanding the embedded microstrip transmission line, designers can properly build these structures to meet their needs.
An embedded microstrip is constructed with a flat conductor suspended over a ground plane. The conductor and ground plane are seperated by a dielectric. There is also a layer of dielectric material above the conductor. One case of an embedded microstrip transmision line is a microstrip trace with soldermask.
Models have been created to approximate the characteristics of the embedded microstrip transmission line. A good approximation is provided by the formulas contained in the document IPC-2141A . For an embedded (internal) microstrip, the formulas provided in paragraph 4.2.3 shall be used. The characteristic impedance is given by:
where Z0,surf can be computed as specified in Eq. 1 of the Microstrip Calculator using H2 in place of h, εr,eff can be computed as specified in Eq. 2 of the Microstrip Calculator, h=H1 and b=H2-H1.