The common used two types of solder resist ink are thermal curing and UV curing. The other type is LPI soldermask ink. The screen printing process conditions of UV curing solder resist ink is basically the same as the resist printing ink.
PCB Soldermask Ink
Compared with thermal curing solder resist ink, UV curable solder resist ink has many advantages, such as no solvent, low volatility, low toxicity, environment friendly, fast drying, adapting to automated or semi automated assembly line operations. The biggest advantage is the curing temperature is low, which has no thermal reaction for the laminate, and does not change the substrate size or cause warping phenomenon. However, the thermal curing solder resist ink has better adhesion and heat resistance on substrate than UV curing. So before UV curing there should be a stricter pre-cleaning.
Whether it is thermal curing solder resist ink or UV curing solder resist ink, it can only be used for single-sided PCB and low-end electronic products. The applying method is screen printing. With the circuit and the pad graphic lines spacing becoming much denser, the original graphic screen printing method has been unable to meet the printing requirements of the solder resist layer.
Although the dry film solder method has been popular for a few years, but because of its expensive materials, the manufacturing technology is not widely spread, and the operation process is complex, low yield, so this technology has now been substantially eliminated. Currently the LPI soldermask ink is used for a high-density, high-precision printed circuit board. It has been gradually showing its advantages in high-end PCB.