UV solder resist ink
With the semi-automation and automation of electronic product assembly processes and the promotion of assembly line operations, wave soldering or dip soldering processes were adopted for soldering circuit boards in the 1960s to improve production efficiency and reduce costs. In order to prevent unnecessary solder from adhering to the printed circuit board, it is necessary to apply a permanent protective film to the surface of the board to ensure that the board does not adhere to the solder during the subsequent operations of spray soldering, dip soldering and wave soldering. This can effectively prevent short circuits caused by solder bridging and achieve a high degree of automation in the production process. In addition, this permanent protective film greatly improves the electrical insulation between the circuits and the entire board surface, thereby increasing the wiring density and operational stability of the printed circuit board. It also has a preventive effect against circuit oxidation, moisture erosion, and scratches from foreign objects, thereby extending the service life of the printed circuit board. Solder mask ink is an important material developed for the production of this protective layer. Its most important function is to prevent soldering, and it should be resistant to high-temperature solder (wave soldering temperature 260°C), as well as being moisture-proof, anti-corrosion, mildew-proof, anti-oxidation, insulating and decorative. The solder mask ink coating process has become one of the main processes in printed circuit board processing
In screen printing, solder resist is applied using solder resist ink. Solder resist ink is available in two types: heat-curing and light-curing, depending on the curing method. Solder resist ink is mainly light-curing solder resist ink at present. After the solder resist ink is cured on the printed circuit board with the copper circuit already made by screen printing with a positive solder resist pattern, a solder resist protective film is formed. After the character ink is printed, the finished product is made after passing inspection. The solder mask is a permanent coating on the printed circuit board, so it must have excellent electrical and physical and mechanical properties, as well as being resistant to the high temperatures of 260°C during post-processing wave soldering, and 288°C for military products.
UV solder resist ink oligomers mainly choose resins with good heat resistance, good insulation and good adhesion to copper, such as bisphenol A epoxy acrylic resin, phenolic epoxy acrylic resin and polyurethane acrylic resin. Currently, phenolic epoxy acrylic resin is commonly used. The reactive diluent is a multifunctional acrylate combined with a monofunctional (methyl) hydroxyacrylate. The hydroxyacrylate is beneficial for improving the adhesion of the ink to copper. The photoinitiator is mainly 651 or 2-ethylthioxanthone. The pigment is mainly phthalocyanine green, and the amount generally does not exceed 1%. More fillers can be added to the ink to improve its heat resistance and reduce volume shrinkage. To improve the adhesion of the ink to the copper, 1% to 2% adhesion promoter such as monomethyl methacrylate PM-1 or dimethyl methacrylate PM-2 must be added, as well as other additives such as defoamers, leveling agents, and polymerization inhibitors in the appropriate amounts.
A practical selection route for photoinitiator-related projects
When technical buyers or formulators screen photoinitiators, the most useful decision frame is usually cure quality plus application fit: which package cures reliably, keeps appearance acceptable, and still works under the lamp, film thickness, and substrate conditions of the actual process.
- Match the package to the lamp first: mercury lamps, UV LEDs, and visible-light systems can rank the same photoinitiators very differently.
- Check depth cure and surface cure separately: a film that feels dry on top can still be weak underneath.
- Balance yellowing with reactivity: the strongest deep-cure route is not always the best commercial choice if color or migration risk becomes unacceptable.
- Use the final formula as the benchmark: pigment load, monomer package, and film thickness can all change the apparent ranking of the same initiator.
Recommended product references
- CHLUMINIT TPO-L: A strong low-yellowing reference for LED-oriented UV systems.
- CHLUMINIT 819: Useful when a formulation needs stronger absorption and deeper cure support.
- CHLUMINIT 1173: A practical comparison point for classic short-wave UV initiation.
- CHLUMINIT ITX: A useful long-wave support route in many printing-ink packages.
FAQ for buyers and formulators
Why are blended photoinitiator packages so common?
Because one product may control yellowing or lamp fit well while another improves cure depth or line-speed performance, so the full package is often stronger than any single grade.
Should incomplete cure always be solved by adding more initiator?
Not automatically. The real limitation may be the lamp, film thickness, pigment shading, or the rest of the reactive system rather than simple under-dosage.
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Quick answer: In most UV systems, photoinitiators are selected by balancing wavelength fit, through-cure, color control, and line speed. Buyers usually compare a blended package instead of one isolated product.
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