This extra thin printed circuit means the thickness of printed circuit board is more thin than normal PCB.
Normal thickness of PCB is from 1.0 mm to 2.0 mm, and the Min thickness
is 0.3 mm or 0.4 mm (1L or 2L). For 4L PCB, thickness will be more
about 0.6mm. This type of board always named as thin PCB, or thin board.