I'm trying to understand Altera's Application Note 114 "Designing with High-Density BGA Packages for Altera Devices".
Specifically, I'm puzzled by the examples on page 19. The "through vias" example with three layers is straightforward, but the "blind vias" example talks about using two layers and routing traces below vias, which doesn't make sense to me.
This would work for a board with three or more layers, obviously, but the color-coding of the traces suggests that in some of the vias, two traces meet on the same layer.
What am I missing here?