As PCB supplier china knows that, the copper would be exposed that resulting in SMT problems if the solder mask is too thin. According to IPC standards, corner lines are typically larger than 8 microns. Otherwise, it will turn yellow when baked, causing copper to leak.
For the finished copper thickness of 35 µ m, the maximum solder mask thickness can reach up 40 µ M. For higher final copper thickness, the solder mask thickness can be as high as 80 µ M.
If the PCB solder mask layer is too thick, the parts may tilt, affecting the assembly quality and causing IC components welding problems. Thick copper always has thick resistance to welding.

Once we scrape the solder mask on the copper surface with a scraper, it is less prone to scratches. In addition, a good solder mask is not easy to wear due to its good adhesion. Only if someone on the production line is intentionally scraping the solder resist with a sharp metal blade; otherwise, the copper won’t be severely exposed, even with a slightly thinner thickness. The most important thing is whether the adhesion of the solder mask is sufficient.
So most clients will limit the ink usage. Based on the visual inspection criteria, the lower limit is not strictly restrained provided the copper is not exposed ,including disguised exposed copper.