LP3991TL-1.3/NOPB is collapsing-or-noncollapsing-bga-balls ?

For larger pitch package (>0.60mm), we suggest to use 'non solder mask defined' pad (NSMD).
Solder mask opening is larger than solder pad. There will be a gap in-between to allow the solder joint wrap around the PCB pad. This is 'collapsing'.

For fine pitch package (<=0.50mm), we suggest to use 'solder mask defined' pad (SMD).
The pad is larger than the solder mask opening so the solder ball would not wrap around the pad. This is 'non-collapsing'.

YZR is considered ‘fine pitch’ (=0.5mm), and a ‘solder mask defined’ pad (SMD) is recommended.

See Page 6 Section 5 in "AN-1112 DSBGA Wafer Level Chip Scale Package" (Document# SNVA009AG):

www.ti.com/.../snva009ag.pdf