1. What are the meanings of the different terms used in the Board Routability Guidelines? (These guidelines are a section of the Packaging and Thermal Characteristics page.)
2. Why are the Solder land Diameter (L) and Opening in Solder Mask (M) diameters numerically different?
"Component land Pad diameter" is the diameter of the exposed pad opening on the substrate side. On the substrate side, the pad is covered by solder mask with an opening to expose the land pad. This opening has a diameter of 0.40mm for BGA and 0.45 mm for FlipChips. Thus, we recommend that a board design has a land pad of 0.40 mm. This is "Solder land Diameter (L)".
The "Opening in Solder Mask (M) Diameter" is the opening of the solder mask on the board side, surrounding the land. To achieve stand-off, it should be bigger than the land.
This is why Xilinx recommends that M = 0.50 mm for BGA and 0.55 mm for Flip Chips.