PCB Glue/adhesive and glue making machine

Printed circuit boards (PCBs) are used in the electronic systems in the chassis of automobiles. As the chassis systems of automobiles encounter harsh environments such as humidity, high and low temperatures, vibration, oil mist, and salt spray in actual use, the PCBs and their components in each system may suffer from corrosion, mildew, deformation, aging, and other problems, resulting in short circuits, leakage, and device failures on the PCBs, affecting the quality of the automobile, reducing its service life, and even threatening the life safety of the user. Therefore, in the production and manufacturing process of electronic systems, it is particularly important to protect the PCBs. The most common method is to apply conformal coating on the surface of the PCBs. Conformal coating is a coating with a special formula that forms a transparent protective film after curing. It is used to protect the PCBs and their components from environmental erosion. It has the "three-proof" properties of salt spray, moisture, and mildew resistance.

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Depending on the requirements and focus of each region and manufacturer it: has many names, such as three-proof glue, coating glue, covering glue, waterproof glue, moisture-proof glue, insulating glue, protective paint, protective paint, moisture-proof paint, three-proof oil, moisture-proof oil, three-proof agent, moisture-proof agent, protective agent, conformal coating, and conformal coating.

According to different chemical compositions: it can be divided into acrylic glue, polyurethane glue, epoxy glue, organic silicone glue, polyolefin glue, synthetic rubber and other types;

According to different curing methods: it can be divided into solvent-based curing, room temperature curing, thermal curing and ultraviolet (UV) curing types of three-conformal coatings.

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Solvent-based curing is used. The solvent is generally organic solvents such as benzene. Long-term exposure is harmful to human health and induces occupational diseases. Organic solvents are usually flammable and explosive. Room temperature curing takes a long time and reduces production efficiency. Although heat curing can cure quickly, it consumes a lot of energy, and some PCB components are not resistant to high temperatures.

The UV curing method has the "5E" characteristics: efficient, economical, energy saving, environmentally friendly, and wide adaptability. UV curing conformal coating is a solvent-free, environmentally friendly material that uses UV to trigger chemically active liquid materials to rapidly polymerize and crosslink, instantly curing into solid materials. The use of UV curing conformal coating can greatly improve the coating efficiency of PCBs, and also provides convenience for PCB repair and local rapid coating protection. At present, for complex PCBs with a small amount of shadow areas, dual hybrid curing technology is used, namely light/moisture curing, light/air curing, light/heat curing, etc. First, most areas on the UV PCB are quickly cured, and then dark curing such as moisture, air, and heat are used to make a small amount of shadow areas fully cured in a short period of time in the subsequent process, which not only improves the PCB coating production efficiency, but also ensures that the conformal coating can be fully cured. This study mainly describes the research progress of polyurethane, acrylate, and silicone conformal coatings and their UV curing.

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1.Polyurethane conformal coating

1.1 Performance characteristics of polyurethane conformal coatings

Polyurethane conformal coatings have good moisture resistance, salt spray corrosion resistance, solvent resistance, wear resistance, and a wide temperature range. They are suitable for use in wet, hot, salt spray and other working conditions. The coating has good toughness, good dielectric and mechanical properties. Before coating, the PCB must be kept clean, especially to avoid the presence of moisture. Even a trace of moisture on the PCB will cause bubbles, which in turn may cause electrical failures and require expensive rework costs. Polyurethane coatings are difficult to remove and repair, and their raw materials are potentially toxic and take a long time to cure.

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1.2 Research progress of UV solid polyurethane conformal coatings

The main components of UV-curable conformal coatings include oligomers, photoinitiators, reactive diluents and various additives. Oligomers are equivalent to the resins of ordinary conformal coatings. They are all film-forming substances and play a major role in the performance of conformal coatings. Their structures must have photocurable groups, such as epoxy groups and unsaturated double bonds. The oligomers for free radical photopolymerization are mainly various acrylic resins, and the oligomers for cationic photopolymerization are mainly epoxy resins and vinyl ether resins. Reactive diluents are equivalent to the solvents of ordinary conformal coatings. They not only have the functions of dilution and viscosity adjustment, but also participate in the photocuring reaction. They must have photocurable groups in their structures. Common reactive diluents are acrylate monomers.

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Photoinitiators are equivalent to catalysts for ordinary coatings. After absorbing UV, they generate cations or free radicals, which trigger polymerization and crosslinking reactions of active diluents and oligomers. Additives (fillers, pigments and various additives) are basically the same as those of ordinary conformal coatings, but it is necessary to pay attention to the fact that the additives used should minimize the absorption of UV to avoid affecting the UV curing reaction. Free radical photopolymerization polyurethane conformal coatings are polyurethane photocurable resins with unsaturated functional groups obtained by reacting hydroxyl-containing acrylic or methacrylic monomers with polyisocyanates. Light/dark dual-curing polyurethane conformal coatings use polyhydroxy bisphenol A epoxy acrylic resin as the main resin and form a two-component system with diisocyanate-hydroxyethyl acrylate semi-adduct. The two components and active diluents can be photopolymerized, and the semi-adduct isocyanate group reacts with the hydroxyl group of the epoxy acrylic resin to achieve room temperature shadow area curing.

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The carbon-carbon double bonds in the modified polyurethane resin undergo rapid crosslinking and photocuring under UV light and photoinitiator conditions; the isocyanate groups contained therein react with moisture to achieve moisture curing (dark curing), solving the problem that the shielded area cannot be UV cured and improving the adhesion between the paint film and the PCB substrate; the silicon-oxygen bonds contained therein have high bond energy, which is beneficial to the intermolecular stability and improves the flame retardancy and water and solvent resistance of the three-proof paint. This invention proves that only when the carbon-carbon double bonds, isocyanate groups and silicon-oxygen bonds are within the appropriate content range, the prepared three-proof paint has good performance. The three-proof paint will encounter a high temperature environment when it is used, and traditional three-proof paints often show adverse performance changes after high-temperature aging. This may result in insufficient protection for PCBs or electronic components due to stress cracking when the three-proof paint is used in a high-temperature environment.

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When exposed to actinic radiation (usually UV), the photoinitiator can cause a reaction between the acrylate functional groups on the acrylated polyurethane and the acrylic monomer. When exposed to moisture (usually moisture in the air), the isocyanate functional groups can form amine groups, which can react with other isocyanate groups to form polyureas. Compared with traditional conformal coatings, the conformal coating of this invention not only achieves UV/moisture dual curing, but also improves the surface insulation resistance, elasticity and flexibility, and heat shock resistance of the paint film (still showing better performance retention in aging tests at 130°C).

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2. Acrylic conformal coating

2.1 Performance characteristics of acrylic conformal coatings

Acrylic conformal coatings are generally single-component solvent-curing conformal coatings. The viscosity has a large operating space and can be adjusted according to different construction requirements. The paint film has high transparency and fast film-forming speed. It can reach the best performance within a few minutes through solvent drying and is easy to repair. It has significant advantages in the actual production process and occupies a large market for conformal protective paint. However, acrylic conformal paint has poor resistance to high and low temperatures. Because its glass transition temperature is very low, in high temperature environments, the coating will expand and reduce stiffness. In low temperature environments, the coating will shrink, increasing Young's modulus. In addition, acrylic conformal paint has poor chemical resistance and weak resistance to organic solvents. In addition, the surface energy of the paint film of this conformal paint is high and it does not have oil and water resistance, resulting in poor oil resistance, water resistance and self-cleaning ability of the paint film.

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2.2 Research progress of UV-curable acrylic conformal protective paint

Acrylics are easier to achieve UV curing because they have photocurable reactive groups. Yang Kai and others prepared a UV/moisture dual-curing conformal paint using polymethacrylate as an oligomer. The curing speed of the conformal anti-paint provided by this invention is significantly improved, which can improve production efficiency, and has high adhesion (up to GB/T 9286-2021 standard level 0) and excellent aging resistance. Deng Jianming found that the mechanical properties of the conformal protective paint invented by Yang Kai and others were slightly poor after curing. He wanted to improve this performance by adding inorganic dispersants, and also prepared a UV dual-curing conformal protective paint. The study found that after adding calcium carbonate of two different particle sizes, the conformal anti-paint of the invention not only solidifies quickly and has excellent mechanical properties, but also significantly improves the electrical properties. Qian Mingle and others prepared a UV and moisture dual-curing conformal paint using acrylate as an oligomer. The viscosity of the conformal protective paint of this invention is ultra-low (34.4~40.9 cps), which solves the inconvenience of construction. It uses a compound photoinitiator to adapt to a wide range of UV wavelengths, fully cross-link the matrix resin and improve adhesion, while meeting the requirements of UV and moisture. Gas double curing.

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Moisture in the air is everywhere. For UV/moisture dual-curing conformal coatings, when not exposed to light, they will react with moisture in the air and cure prematurely, resulting in increased viscosity, which will affect normal use and reduce service life. Based on this, Shi Xuetang et al. prepared a highly efficient and environmentally friendly UV/dark-curing acrylic conformal coating. After UV irradiation, the UV/dark-curing acrylic conformal coating of the invention triggers a dark reaction between the acrylate group and the thiol to achieve cross-linking and curing. In the absence of light, it does not react with moisture in the air, and the shelf life is as long as 12 months. This may be because the photoinitiator and active diluent selected do not contain isocyanate groups that are easily reactive with moisture in the air. In addition, other resins are modified using acrylic monomers or compounds with acrylic functional groups to obtain epoxy resin acrylates, polyurethane acrylates, etc., and UV-curing acrylic conformal coatings are further prepared.

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The polyurethane acrylate cured film has excellent flexibility, excellent adhesion to most substrates and corrosion resistance. It is the most widely used in practical applications, but its overall synthesis cost is relatively high. Wu Jianxin et al. prepared a highly efficient and environmentally friendly UV/moisture dual-curing three-proof paint for flexible circuit boards. On the one hand, the unsaturated double bonds in the system will crosslink and cure under the action of photoinitiators and UV light; on the other hand, polyurethane-modified acrylic resin is selected, and the isocyanate in the system reacts with moisture in the air to dark cure. The UV/moisture-cured three-proof paint not only has low shrinkage, high temperature and humidity resistance, chemical corrosion resistance, strong adhesion to the PI film of the flexible circuit board, but also cures quickly, which improves production efficiency. Wang Jiangang et al. also prepared a UV/moisture dual-curing three-proof paint. The three-proof paint of the invention has moderate viscosity, fast curing, good surface drying, and can be cured using a variety of UV (mercury lamps, halogen lamps, LED light sources). Areas that cannot be illuminated by UV can be moisture-cured. The cured paint film has excellent heat resistance, aging resistance, chemical corrosion resistance, hydrolysis stability, flame retardancy and insulation properties.

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Although the photoinitiator used in the preparation of UV-curing conformal coatings accounts for a small proportion in the formula, its photolysis products often include volatile small molecules such as aldehydes and ketones, and there are still environmental pollution problems in the mass production process. Chen Xiaojian and others prepared a low-odor UV-curing conformal coating using isocyanate-modified polyurethane acrylate, polyethylene glycol acrylate, acrylate monomer, modified photoinitiator, polydimethylsiloxane, and modified polysiloxane as raw materials. Through the reaction of the isocyanate group with the hydroxyl group of the photoinitiator, the small molecule photoinitiator is grafted on the prepolymer molecular chain, which not only increases the initiation effect of the photoinitiator, but also reduces the odor of the coating. The product has low odor during construction and after curing, which is green and environmentally friendly. After isocyanate-modified polyurethane acrylate, the paint film has excellent adhesion and good toughness.

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3. Silicone conformal coating

3.1 Performance characteristics of silicone conformal coatings

The main chain of silicone is a silicon-oxygen bond, which has good flexibility and a higher bond energy than a carbon-carbon bond. Therefore, this type of conformal coating has good flexibility, high and low temperature resistance, and weather resistance. In addition, silicone conformal coatings have good dielectric properties, and the dielectric constant and dielectric loss change little with frequency and temperature. The dielectric strength is high and it is suitable for PCB protection under high frequency and high temperature. The comprehensive performance of silicone conformal coatings is excellent, but this type of conformal coating generally requires high temperature curing, and has problems such as poor adhesion to the substrate and low mechanical strength. In addition, the molecular gap between silicone molecular chains is large, and it is very easy to adsorb sulfur-containing substances in the air (such as sulfur dioxide, hydrogen sulfide, etc.), forming solder joint vulcanization phenomenon, which leads to PCB failure.

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3.2 Research progress of UV-curable silicone conformal coatings

UV-curable silicone conformal coatings are usually modified by chemical methods to obtain acrylic modified or polyurethane modified silicone conformal coatings. The former has excellent weather resistance and light resistance, while the latter has good adhesion and oil resistance. In addition, silicone conformal coatings have excellent high temperature resistance, moisture resistance and corrosion resistance, and are mostly used on high-heat dispersion components of PCBs. Zhang Guangfa et al. used acrylate-modified silicone resins, compounded diluents, compounded photoinitiators, moisture curing catalysts and accelerators to prepare a UV/moisture dual-curing silicone conformal coating. The use of acrylate-modified silicone resins overcomes the shortcomings of traditional silicone resins, such as long curing time and high temperature curing, and achieves UV light curing. Moisture curing can also be used to make up for the problem of difficult surface drying in areas that cannot be irradiated by UV.

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While achieving rapid curing, the three-proof paint also has excellent mechanical properties, weather resistance and corrosion resistance. In addition, based on the click chemistry reaction, the introduction of unsaturated bonds (such as mercapto groups) can also enable silicone three-proof paint to achieve UV curing. Zhong Laifu and others prepared a UV moisture dual-curing three-proof paint based on click chemistry reaction. Under the irradiation of LED 365 nm UV light, the three-proof paint can be quickly cured at low energy and reach a surface dry state, which not only improves production efficiency, but also solves the shortcomings of the existing UV curing three-proof paint system when the surface is inhibited by oxygen during UV irradiation curing. The three-proof paint of this invention belongs to a pure silicone system with excellent flexibility, high temperature resistance, salt spray resistance and solvent resistance. After 800 hours of cold and hot shock, the paint film on the PCB surface did not crack, peel off, obviously yellow, or fog.

4. These adhesives are used on PCB circuit boards: Epoxy glue

Acrylic glue

Silicone

Polyurethane glue

Red Glue

Thermally conductive silicone

Quick-drying glue

Potting glue

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Z-MIXER, the equipment manufacturer for the production of these glues, provides a variety of customized equipment according to different glue viscosities and production batch sizes, mainly including the following:

Mixer:

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Reactor

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Filling Machine

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5. Conclusion

As competition in the automotive industry continues to intensify, the use of UV-curable conformal coatings on PCBs is not only environmentally friendly and energy-saving, but also improves production efficiency and helps improve product competitiveness. By searching the literature on conformal coatings and investigating the conformal coating products in the market, it is found that for conventional curing methods (moisture, heat, etc.), silicone conformal coatings are more widely used because of their excellent overall performance; for UV-curable conformal coatings, polyurethane acrylate UV-curable conformal coatings are the mainstream trend in the current market, because this type of conformal coating combines the performance characteristics of polyurethane and acrylate, and its structure is easier to achieve UV curing than silicone, and its UV-curable product price is lower than that of UV-curable silicone products. Since PCB has electronic components such as plug-ins and has a small amount of shadow areas, the UV-curing conformal coatings used on PCBs all use dual hybrid curing technology (light/moisture curing, light/air curing, light/heat curing). First, most areas on the PCB are quickly cured through UV, and then dark curing is implemented in subsequent processes using moisture, air, heat, etc., to ensure that the small amount of shadow areas are fully cured in a short period of time.

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In general, UV-curing conformal coatings not only improve the efficiency of PCB coating production, but also ensure that the conformal coatings can be fully cured. However, their comprehensive performance still needs to be improved, and the difficulties of stability, aging resistance, odor irritation and construction performance of UV-curing conformal coatings still need to be solved to make their performance comparable to that of conventionally cured conformal coatings. In addition, the most commonly used light source for UV-curing conformal coatings is a high-pressure mercury lamp, which has problems such as high input voltage, mercury pollution and ozone pollution. Therefore, the replacement of mercury lamps by LEDs is an inevitable trend. However, under the current UV LED technology conditions, improving the matching degree between light sources and between light sources and formulas is still the core focus.

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