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.
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