Issue link: https://iconnect007.uberflip.com/i/1511130
56 DESIGN007 MAGAZINE I NOVEMBER 2023 signals, on the other hand, always remain sen- sitive to crosstalk. From a PCB designer's perspective, we can only optimize what we can control and that is just the placement and routing. Ensuring the route tolerance is less than 10 ps is all we can do to stabilize the design. is may sound tight and difficult to manage but I do this rou- tinely whether called for or not. Put in a little extra effort during layout and one can achieve perfect timing with every design. A stripline is any trace sandwiched between reference planes on both sides. e elec- tric fields 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 surface layers of a PCB. A microstrip has dielectric material and a plane on one side and air on the other. An embedded microstrip is similar but is covered in a con- formal coating such as solder mask or another dielectric material. In this case, the effective dielectric constant should be calculated by a field solver and represents a combination of the surrounding materials. ere are also other variants of microstrip and stripline, such as build-up microstrip and dual asymmetric stripline. e electric fields surrounding the microstrip exist partially within the dielectric material(s) and partially within the surrounding air. Since air has a dielectric constant of 1, which is always lower than that of FR-4 (typically 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 that the impedance is identical, the propagation speed of the microstrip is always faster than the strip- line, typically by 13–17%. e speed of propa- gation of digital signals is independent of trace geometry and impedance. Figure 3: Relative signal propagation for each signal layer on a 10-layer DDR3 stackup. (Source: iCD Stackup Planner)