for proper placing ground and power planes, see
TN1074 - PCB Layout Recommendations for BGA Packages
page 1: table 1: row 0.8 ball pitch, 756/554/381/400 caBGA
Pad copper diameter:  0.4  mm
Solder mask opening:  0.5  mm
Solder land diameter: 0.35 mm (solder paste = stencil opening)
page 32: example for BGA 381 ECP5