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Beyond Design: Signal Flight Time Variance in Multilayer PCBs

Energy in the form of electromagnetic waves is transferred through the dielectric materials, of the multilayer PCB substrate, through vibrations of electric and magnetic fields. A transmission line does not carry the digital signal itself but rather, guides electromagnetic energy from one point to another. Signals travel at the same speed, given the same medium. However, the microstrip (outer layer) traces are embedded in a mélange of dielectric material, solder mask (if required) and air. This lowers the effective dielectric constant and increases the propagation speed compared to that of stripline (inner layer) traces. This month, I will look at the disparity in signal propagation in multilayer PCBs.

When we watch waves rolling onto the shore, we tend to think that the water is moving towards us, but that’s not actually the case. The individual particles that make up the waves move up and down perpendicularly to the direction of the wave. But they do not move significantly themselves until they break, and hit the shore, which disperses the energy. The particles take part in the wave by bumping into one another and transferring energy. The waves travel as a transverse wave which is characterized by particle motion that is perpendicular to the wave energy.

Surfers hang out well offshore, sitting on their boards behind the break, patiently watching the horizon for the next set and the magic wave. As the wave (swell) passes, they bob up and down vertically. The waves come in sets because the amplitude of the waves is modulated by another longer wave. The first wave in a group is small, the next one is bigger and so on until the largest wave appears in the middle of the group. Then they get smaller again. Way back in my surfing days, we used to say that the third wave was always the biggest, but others say it is the seventh; it is all relative to the number of waves in the set.

If you have ever experienced a “wave” at a football stadium, you will be amazed at the actual speed at which the wave travels. It takes less than 60 seconds for the wave to complete a circuit of a typical stadium but nobody moves (apart from standing up/down). Here the medium, in which the wave propagates, is people. If the perimeter of the outer edge of the stadium is 1 km, then the wave propagates at ~60 km/hr—and nobody has to wear a safety belt.

Many stadiums have a members-only section, to which are admitted only an elite group of persons whose apathy appears to be higher than average. They typically do not rise to participate in the wave. Nevertheless, the wave seems to jump across the impenetrable barrier continuing the circuit, just as electromagnetic 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 components. The actual velocity 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 varies depending on the layer, in the multilayer substrate, and the surrounding dielectric materials.

To read this entire column, which appeared in the December 2017 issue of The PCB Design Magazine, click here.