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October 2017 • The PCB Design Magazine 23 Multilayer PCBs are ideally suited for provid- ing interconnection wiring that is specifically designed to provide desired levels of impedance control. Techniques commonly referred to as mi- crostrip and stripline are employed for imped- ance control. There are four basic types of trans- mission line constructions, as shown in Figure 4: 1. Microstrip 2. Embedded microstrip (with solder mask or conformal coating) 3. Balanced stripline (aka symmetric stripline) 4. Unbalanced or dual embedded stripline (aka asymmetric stripline) WHEN DO TRACES BECOME TRANSMISSION LINES? Figure 3: A 50Ω, 1-inch trace measured with a vector network analyzer (source: Eric Bogatin). Figure 4: The four main configurations of impedance structures (source: IPC-2251).

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