Slide 57 of 64
This shows some RPI simulated pulse propagation for 10 GHz clock signals. One can observe very significant amplitude degradation if the wire is up close to the substrate. But this result is for an isolated wire over silicon. In our systems we use differential wiring everywhere. Field lines which normally would terminate in the lossy Silicon are terminated on the complementary wire partner. This effect is also observed for coplanar striplines over silicon. Even other nearby conductors can screen some of the coupling to the substrate. Unfortunately there is no good CAD tool to predict this attenuation yet.