# BGA escape routing, understanding Altera app note

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?

• I think they mean internal layers, so two internal layers plus the top surface. That makes three layers in total, and so blind vias can happen. But yes, the wording and the colour coding is very confusing. – Majenko Sep 3 '16 at 21:54
• Consider looking up HDI PCBs for a specific application of blind microvias. You can find a variety of resources from circuit board manufacturers giving some generic examples of edge-on boards. – user2943160 Sep 3 '16 at 23:13