There are two ways to apply solder masks on PCBs - UV light solder mask curing and solder mask exposure development.
UV light solder mask curing
UV curable solder mask, a solvent-free ink, is the earliest solder mask for PCB manufacturing. It is applied on PCBs by screen printing. Then the UV light sheds on the circuit boards, and the solder mask ink gets dry and cures.
UV light solder mask curing is easy but it is not precise. The image tolerance is 0.2mm, and the solder mask is also thick. So it is used only for simple PCBs, such as FR-1 PCBs and XPC paper PCBs.
Solder mask exposure-development
The solder mask ink for exposure development is a solvent-based ink that will cure when exposed under UV light. Under the UV light, the solder mask ink is printed on PCBs through a film in the PCB pads and holes pattern. In this way, the film covers the pads and holes, and the solder mask is only applied and gets dry on the blank areas and copper traces. Then the PCBs need to be baked to totally cure the solder mask.
The solder mask exposure-development process is precise, and the tolerance is 0.05mm. This solder mask applying method is appropriate for PCBs with higher requirements. Especially, a solder mask bridge must be applied in this way.
For thick-copper PCBs, a solder mask can't be printed but sprayed on PCBs in case the solder mask in the corners is too thick. At PCBHERO, solder masks are applied on thick-copper PCBs by electrostatic spraying. Electrostatic spraying develops from exposure-development solder mask printing, and it is finished in one-time. After spraying, the circuit boards are also baked. The only downside of electrostatic spraying solder masks is that it consumes plenty of solder masks.