For instance, in case of high speed single-ended signals, the ground return must also have to be tracked alongside the signal if the signal does not track over a low impedance ground plane (figure 1). The narrow GND track from below, which interconnect the 2 integrated circuits does not represent a good layout solution due to the fact that it leads to a very large signal loop area. The good solution is to route a GND track directly between the 2 ICs, alongside the signal track.

 

Fig. 1 At high speed single-ended signals the ground return path has to be routed alongside the signal (in case of no low impedance ground plane)