Via length:How long is long?

Via Stubs Most PCBs made today are cost sensitive and use traditionally drilled vias. In a multilayer PCB, these vias are mechanically drilled after the layers are bonded together to form a complete PCB. On PCBs that are 1.6mm thick and have less number of layers, the vias that travel across the top and bottom layers of the PCB do not have enough stub length to create problems at reasonably high frequencies. But on PCBs with higher layers which can have a total PCB thickness of about 5mm and above, any vias present, will have a definite impact on signal integrity. So how do we tackle this problem? Welcome to the concept of back drilling!