Are those 'screws' actually screws or are they bolts? Usually the shorter fat screws are used to mount a fan case going through a mounting hole or grill hole and screwing into the fan.
Small mounting (as in thin, not short and fat) bolts (much finer thread then a screw has) would go through the fan to the mounting holes in the case.
Worth contacting both Phanteks and the case manufacture and find out what length, size (thickness of the bolt shaft) and thread type is needed for those mounting holes: They might send you some, or, with that information, a decent hardware store should have them.