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60 The PCB Design Magazine • December 2017 to be higher than average. They typically do not rise to participate in the wave. Nevertheless, the wave seems to jump across the impenetra- ble barrier continuing the circuit, just as elec- tromagnetic energy is coupled between traces and components of a PCB without physically touching. Similarly, the speed of a computer does not depend intrinsically on the speed of electrons, but rather on the speed of energy transfer between electronic com- ponents. The actual veloc- ity of electrons through a conductor is very slow (~10 mm per second), however the "knock on" effect is very fast as it follows the electromagnetic field. The energy propagates as an electromagnetic wave. And, the speed of this wave var- ies depending on the layer, in the multilayer substrate, and the surrounding dielec- tric materials. A stripline is any trace sandwiched between refer- ence planes on both sides, as in Figure 2. The electric fields (blue) of a stripline are totally contained between the two solid planes, so the speed of propagation for signals guided by the trace is entirely determined by the dielectric constant of the surrounding materials. On the other hand, a microstrip is any trace fabricated on the outer layers of a PCB. A mi- crostrip has dielectric material and a plane on one side and air on the other. An embedded mi- crostrip is similar but is covered in a conformal coating such as solder mask or another dielec- tric material. In this case, the effective dielectric constant should be calculated by a field solver and represents a combination of the surround- ing materials. There are also other variants of microstrip and stripline, such as build-up mi- crostrip and dual asymmetric stripline. The electric fields surrounding the mi- crostrip exist partially within the dielectric material(s) and partially within the surround- ing air. Since air has a dielectric constant of one, which is always lower than that of FR-4 (typi- cally 4.3), mixing a little air into the equation will speed up the signal propagation. Even if the trace widths are adjusted on each layer, so as the impedance is identical, the propagation speed of microstrip is always faster than stripline, typ- SIGNAL FLIGHT TIME VARIANCE IN MULTILAYER PCBS Figure 2: Microstrip and stripline electromagnetic fields, simulated in HyperLynx. Figure 3: Relative signal propagation of microstrip and stripline, simulated in iCD Design Integrity.