The copper circuits on the printed circuit board are formed by etching the copper foil on the copper-clad laminate with ferric chloride or copper chloride. Therefore, the parts of the circuit that are not to be etched must be protected with a resist. In screen printing, a resist ink is used, which is either self-drying or light-curing, depending on the curing method. The resist ink is printed through a screen with a resist pattern and cured on the copper-clad laminate to form a resist protective film. After the copper-clad board is etched (and sometimes electroplated) to form copper circuits, the film is removed with a dilute alkali solution, and the resist film does not remain on the printed circuit board, exposing the copper circuits. Therefore, the resist ink is required to have good adhesion to the metal copper foil, be resistant to corrosion and electroplating, and be completely removed by a dilute alkali solution.
UV resist inks generally use anhydride-modified epoxy acrylic resins, high-acid polyester acrylic resins, or modified maleic anhydride resins as the main resin, along with acrylate functional monomers; photoinitiators commonly used are 651 or sulphinone photoinitiators such as 2-ethylsulphoquinone; pigments are mostly phthalocyanine blue, The amount is generally about 1%, and a large amount of filler such as talcum powder needs to be added. In order to improve the thixotropy of the ink, a certain amount of fumed silica needs to be added. In particular, it should be noted that the alkali-soluble photosensitive resin containing a certain amount of carboxyl groups must be able to resist corrosion and electroplating after curing and forming a film by cross-linking, and it must also be soluble in a 3% sodium hydroxide solution for removal.
UV curable inks and photoinitiators have a highly dependent and synergistic relationship. Photoinitiators are the core component of UV curable inks for rapid curing, and the performance of the ink (such as curing speed, adhesion, chemical resistance, etc.) is directly affected by the type, concentration and compatibility of the photoinitiator. The following is a specific link and mechanism of action between the two:
1. Photoinitiators are the “trigger” for UV ink curing
- Core function:
- After absorbing ultraviolet (UV) energy, the photoinitiator produces active free radicals or cations, which trigger the polymerization reaction of the resin (e.g. acrylate, epoxy resin) and monomer in the ink, causing the liquid ink to instantly crosslink and cure into a solid film.
- Without a photoinitiator, UV ink cannot be cured by light, and the anti-corrosion function cannot be achieved.
- Key role:
- Light energy absorption: The initiator needs to match the emission spectrum of the UV light source (e.g. mercury lamp, LED) (e.g. a 395 nm LED needs to match an initiator with an absorption wavelength of 395–405 nm).
- Energy transfer: The absorbed light energy is converted into chemical energy to trigger the cross-linking of the resin.
- Overcoming oxygen inhibition: Some initiators (e.g. hydrogen-stripping benzophenone + amine) can reduce the inhibitory effect of oxygen on the curing reaction.
2. The type of photoinitiator determines the curing characteristics of the ink
(1) Matching the type of initiator to the ink resin
- Radical initiators (e.g. TPO, Irgacure 907):
- Suitable for acrylate resin systems, fast curing speed, but may be inhibited by oxygen.
- Commonly used in PCB solder mask inks and scenarios with high surface curing requirements.
- Cationic initiators (e.g. thiuram salts):
- are suitable for epoxy resin systems. Curing is less affected by oxygen, and is suitable for deep curing.
- They are mostly used in inks that require high temperature resistance or better chemical resistance (such as some packaging materials).
(2) Initiators affect the performance of inks
- Curing depth: Deep curing initiators such as bis-acyl phosphine oxide (BAPO) can ensure complete internal curing of thick films or high-reflectance (such as white) inks.
- Yellowing tendency: Some initiators (such as ITX) may decompose after exposure to light to produce chromophores, causing the ink to change color. Low yellowing types (such as Irgacure 819) should be selected.
- Migration: Inks for food packaging or medical use require the use of low-migration initiators (such as TPO-L) to prevent residual initiators from leaching out and contaminating.
3. Synergistic optimization in formulation design
- Initiator concentration:
- If the concentration is too low, curing will be incomplete and the resistance will be poor;
- if the concentration is too high, there will be a lot of residual initiator, which may reduce adhesion or cause migration problems.
- Optimization method: The usual addition amount is 1–5% of the total mass of the ink, and the optimal ratio needs to be determined through experimentation.
- Mixed initiator strategy:
- Surface + deep curing: For example, in PCB solder mask inks, TPO (rapid surface curing) and Irgacure 819 (deep penetration) are used in combination to ensure overall curing.
- Broad spectrum response: Combining initiators with different absorption wavelengths (e.g. Irgacure 2959 + ITX) to adapt to multi-wavelength light sources (e.g. mercury lamps).
- Additive synergy:
- Amine synergists (e.g. EDAB): Improve the curing efficiency of free radical initiators in air.
- Stabilizers: Prevent premature decomposition of the initiator during ink storage.
4. Typical problems and relevance in practical applications
| Problem phenomena | Relationship with photoinitiator | Solutions |
| Incomplete curing | Mismatch between initiator absorption spectrum and light source or insufficient concentration | Replace the initiator with one of a matching wavelength or increase the concentration |
| Yellowing of the ink | Chromophores from initiator photolysis (e.g. ITX) | Switch to a low yellowing initiator (e.g. Irgacure 784) |
| Poor adhesion | Residual initiator or insufficient resin cross-linking | Optimize the initiator concentration and add a silane coupling agent |
| Uncured in shaded areas | Insufficient photoinitiator penetration | Add a deep-curing initiator (e.g. BAPO) |
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