The primary factor that determines the intricacy of BGA routing is the pitch size. In addition, factors such as the size of the BGA array, type of solder masks used, and layer count requirements also play crucial factors.
Pitch size is defined as the distance between consecutive balls on a BGA package, measured from center-to-center, as shown in the following diagram.
Figure 1. Definition of Pitch Size
BGA Landing Pads
Xilinx recommends using non-solder mask-defined (NSMD) copper BGA landing pads for optimum board design. NSMD pads are pads that are not covered by any solder mask, as opposed to Solder Mask Defined (SMD) pads in which a small amount of solder mask covers the pad landing. The following figure illustrates the difference between NSMD and SMD pads.
Figure 2. NSMD and SMD Pads