As Ball Grid Array (BGA) packages become increasingly popular and become more populated across the array with higher pin count and smaller pitch, it is important to understand how they are affected by various board layout techniques. This document provides a brief overview of PCB layout considerations when working with BGA packages. It outlines some of the most common problems and provides tips for avoiding them at the design stage. A key challenge of adopting fine-pitch (0.8 mm or less) BGA packages is the design of a route fanout pattern that maximizes I/O utilization while minimizing fabrication cost.
Lattice (R) presented quite good guide for selecting a proper pads, openings and overall recomendations to have a deal with BGAs
See in Google for PCBLayoutRecommendationsforBGAPackages.pdf
or check a direct link below https://www.latticesemi.com/-/media/LatticeSemi/Documents/ApplicationNotes/PT/PCBLayoutRecommendationsforBGAPackages.ashx?la=en