This is UV sensitive solder mask resin, applied as thin as possible using a silk screen mesh. Afterwards it’s heated at about 90C for 10 minutes. This makes it more sensitive to UV light by evaporating most of the solvent.
It is exposed with a 405nm laser at about 250mw of power. I intentionally unfocused the laser for a spot size of about 0.5nm
After exposure the pads are easily cleaned off with some IPA.
It works as well with a 450nm diode laser. About 1W worth of laser power will ablate soldermask as well but you risk burning off really small pads because they cannot really dissipate heat anymore. Especially NC pads tend to burn off. I’ve tried it and it works reasonably well by doing the drilling as the last step.
450nm is unfortunately not good enough to cure soldermask, 405nm is much more effective.