A kind of UV light curing ink and its use method

Abstract

The present application discloses a UV light-curable ink and a method for using the same in manufacturing a flexible printed circuit board. Specifically, the method comprises the following steps: preparing an ink mixture; applying the ink to the flexible printed circuit board, and then sequentially drying, pre-baking, exposing, and developing the ink; and UV curing the treated flexible printed circuit board, first using ultraviolet light and visible light alternately. The method described in the present invention can solve the problem of incomplete curing in thicker coatings. The product of the present invention has good flexibility and stability in complex processing, and is particularly suitable for applications requiring thicker coatings.

Description

Technical Field

[0001] The present application relates to the technical field of printed circuit board manufacturing, and in particular to a UV light-curing ink and a method for using the same. Background Art

[0002] Flexible Printed Circuit Board (FPC) is a circuit board that uses flexible insulating materials (such as polyimide or polyester film) as the substrate. The copper foil circuit is printed or etched on the substrate through a special process to form a circuit with high flexibility and excellent electrical performance. Compared with traditional rigid circuit boards, they can be bent, folded, and even curled freely, which makes them very suitable for use in electronic devices with limited space or requiring dynamic movement.

[0003] Solder mask ink is a material used to protect circuits from environmental influences and short circuits during the circuit board manufacturing process. It is coated on all areas of the circuit board except for the parts that need to be soldered (such as pads) to prevent solder from spreading in unexpected locations and ensure electrical isolation. There have been corresponding studies in the prior art. The latest development of UV light-curable ink can undergo chemical reactions under ultraviolet light and only cure in the areas that need to be retained. The rest can be removed by developer to achieve precise graphic transfer. This technology greatly improves the positioning accuracy and production efficiency of the solder mask layer, and is suitable for large-scale production and the manufacture of high-density circuit boards.

[0004] Due to the limited penetration of UV light, for thicker coatings, UV light cannot fully cure the internal ink, resulting in incomplete curing, affecting the solder mask effect and circuit reliability. Therefore, the common coating is controlled below 20μm. However, flexible boards may require thicker ink coatings in certain specific occasions. For example, in applications that need to withstand frequent bending, twisting or high mechanical stress, thicker ink layers can provide better protection to prevent wires and pads from being damaged by mechanical stress. For example, in wearable devices, folding screen mobile phones or dynamically bent sensor applications, thick ink layers can enhance the durability of flexible circuits. In addition, in situations where electromagnetic interference (EMI) or radio frequency interference (RFI) protection is required, thicker conductive ink layers can be used as shielding layers to reduce the leakage of electromagnetic radiation and the intrusion of external interference.

[0005] In the prior art, for situations where thicker coatings are required, the solution is to adopt the method of layered coating and distributed curing. However, layered coating and distributed curing will increase the production cycle and reduce the production efficiency. In addition, multiple coating and curing may form tiny bubbles or uneven interfaces between layers, affecting the overall quality and appearance of the solder mask layer, and may even become a failure point, reducing the reliability of the circuit board. Therefore, solving the curing problem of thicker coatings is a topic worth studying. Summary of the invention

[0006] Purpose of the invention: In view of the shortcomings of the prior art, the present invention provides a UV light curing ink and a method for using the same in manufacturing a flexible printed circuit board. The present invention is particularly suitable for occasions where a thicker coating is required.

[0007] Technical solution: The present invention provides a method for using UV light-curable ink, which specifically comprises the following steps: mixing an ink main agent and an ink auxiliary agent in a ratio of 10-12:3, stirring evenly, and then standing for 15-20 minutes to ensure that the viscosity is 150-300 dPa·s, and preheating to 25±2°C in a constant temperature environment to obtain a standby ink mixture;

[0008] The clean flexible printed circuit board is placed on the carrying platform of the coating machine, and the ink mixture is coated on the flexible printed circuit board, and the coating speed is controlled to be 20-30 m / min. The coated flexible printed circuit board is dried, pre-baked, exposed, and developed in sequence;

[0009] Place the treated flexible printed circuit board into a UV curing box and first use a 365nm light source for preliminary pre-curing with a time set to 30-40s; then switch to a 395nm light source for curing for 40-60s, and then use visible light for 40-60s, alternating between ultraviolet light and visible light, with the total time controlled to not exceed 10 minutes.

[0010] Specifically, the ink main agent has the following composition and proportion: photosensitive resin: 30-40 parts of acrylate photosensitive prepolymer; flexible monomer: 10-15 parts of triethylene glycol diacrylate; photoinitiator: 1-5 parts of 2-methyl-1-[4-methylthiophenyl]-2-morpholinyl-1-propanone; filler: 2-8 parts of nano-scale silicon dioxide.

[0011] Specifically, the composition and proportion of the ink auxiliary agent are: 1-3 parts of sebacate ester; 1-2 parts of leveling agent BYK-33; 1-5 parts of pigment. The pigment is selected according to the required color, and any pigment that does not affect the flexibility of the ink can be used, such as phthalocyanine green G, phthalocyanine blue BGS, etc., and iron oxide pigments.

[0012] When the application requires a thicker ink thickness (for example, more than 30μm, or even more than 50μm), the composition and ratio of the ink adjuvant are: 1-3 parts of sebacate; 1-2 parts of leveling agent BYK-33; 1-2 parts of azobenzene (or its derivatives); 1-5 parts of pigment. For the present invention, the addition of azobenzene (or its derivatives) and the light operation during curing are more important. In the pre-curing stage, ultraviolet rays in this band can effectively excite azobenzene molecules, prompting them to transform from the cis configuration to the more stable trans configuration. This process occurs rapidly, which helps to quickly fix the basic form of the ink layer and prevent flow or collapse. Then switch to a 395nm UV light source to continue curing for 1 minute. This wavelength is selected to promote further cross-linking of the remaining uncured ink components, while maintaining azobenzene in the trans configuration to ensure complete curing and hardness improvement of the coating. Afterwards, irradiate with visible light for 1 minute. The role of this stage is to prompt the azobenzene molecules in the trans configuration to re-convert to the cis configuration (such as trans configuration) under the action of visible light. Figure 1 As shown in the figure, although cis-azobenzene is not as stable as trans-azobenzene, in the process flow, this conversion helps to release the internal stress accumulated during the curing process, reduce the risk of cracking or peeling of the coating, and may promote further reaction of certain ink components to enhance the curing effect. The present invention can optimize the physical properties of the coating while ensuring the curing efficiency, such as improving the surface hardness, adhesion and stress crack resistance, by alternating irradiation of ultraviolet light and visible light.

[0013] Specifically, after standing, use a viscometer to check the viscosity of the mixed ink to ensure that the viscosity is 180±20 dPa·s. If it is not within the range, adjust it and retest it.

[0014] Specifically, the coating is applied by adjusting the roller pressure to a moderate level to ensure uniform coating, and the coating thickness is adjusted in real time by monitoring with a laser distance sensor to ensure that the uniformity error is within ±2 μm. The coating thickness will directly affect the subsequent curing effect and durability.

[0015] Specifically, the drying process is to place the flexible printed circuit board into a preset drying chamber, control the temperature at 40°C and maintain it stably, and dry it for 5 minutes to avoid deformation of the board surface due to rapid temperature rise.

[0016] Specifically, the pre-baking temperature is between 70±5°C and the time is 5-15 minutes. The solvent in the ink is volatilized by pre-baking so that the coating will not adhere to the film during exposure. After the pre-baking is completed, the flexible printed circuit board is allowed to cool naturally to room temperature or air-cooled to accelerate the cooling to avoid stress problems caused by large temperature differences.

[0017] Specifically, the exposure is to precisely align the cooled flexible printed circuit board onto the exposure machine platform, cover and fix the pre-prepared precision mask to ensure that the pattern is completely aligned with the board surface; start the exposure program to ensure that the pattern transfer accuracy reaches ±5μm. More specifically, the exposure parameters are set as: a UV light source with a wavelength of 405nm, a light intensity controlled at 600mW / cm2, and an exposure time accurate to 80-100 seconds to achieve high-quality pattern transfer.

[0018] Specifically, the development is as follows: the exposed flexible printed circuit board is immediately transferred to the developing tank, the developer temperature is maintained at 35±1°C, circulated, and the developing time is set to 2-3 minutes. During this period, the pH value and concentration of the developer are monitored to ensure the developing effect. After the development is completed, it is rinsed with deionized water and then dried. In the development step, the developer is used to dissolve and remove the coating film of the unexposed part, while retaining the exposed part. Insufficient development will result in residual uncleared ink, while excessive development may cause the coating film to peel off.

[0019] Beneficial effects: Compared with the prior art, the present invention combines light irradiation strategies of different wavelengths to accelerate the curing reaction and optimize the microstructure of the cured product through the special properties of azobenzene, thereby improving the comprehensive performance of the flexible printed circuit board coating while ensuring production efficiency.

[0020] The cis-trans isomerization of azobenzene is not only an energy conversion process, but also an effective means of regulating the curing rate and material properties. Through light-induced structural changes, the rate and extent of the reaction in the ink can be controlled, so that thick coatings can be cured evenly, avoiding the problem of incomplete internal curing due to thickness.

[0021] The reversible isomerization of azobenzene also helps to release internal stress during and after curing, which is especially important for thick coatings, reducing cracks and deformation caused by curing shrinkage and improving the quality and reliability of the finished product. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the isomerization principle of azobenzene under alternating irradiation of ultraviolet light and visible light. DETAILED DESCRIPTION

[0023] The technical solution of the present application is described in detail below through examples, but the protection scope of the present application is not limited to the examples. The parts, proportions and contents described in the present invention are all in units of mass unless otherwise specified.

[0024] Example 1

[0025] The ink main agent has the following composition and proportion: photosensitive resin: 35 parts of acrylate photosensitive prepolymer; flexible monomer: 12 parts of triethylene glycol diacrylate; photoinitiator: 3 parts of 2-methyl-1-[4-methylthiophenyl]-2-morpholinyl-1-propanone; filler: 5 parts of nano-scale silicon dioxide.

[0026] The ink auxiliary agent has the following composition and proportion: 2 parts of sebacate ester; 1 part of leveling agent BYK-33; and 2 parts of pigment.

[0027] Mix the ink main agent and ink auxiliary agent in a ratio of 10:3, and stir with an electric stirrer for at least 5 minutes to ensure that there are no particles and the mixture is fully mixed. Then let it stand for 20 minutes to discharge excess gas. Use a viscometer to check the viscosity of the mixed ink and ensure that it is within the range of 180±20dPa·s. If it is not within the range, it should be adjusted appropriately and retested. Before use, the mixed ink needs to be preheated to 25±2℃ in a constant temperature environment.

[0028] Place the cleaned and dried flexible printed circuit board steadily on the loading platform of the coating machine to ensure that the board surface is dust-free and flat. Set coating parameters: the coating thickness target value is 15μm, and the roller pressure is adjusted to a moderate level to ensure uniform coating, and the coating speed is controlled to 20 meters / minute. At the same time, the laser range sensor is used for monitoring and real-time adjustment of the coating thickness to ensure that the uniformity error is within ±2μm.

[0029] After coating, the flexible printed circuit board is sent to the preset drying room, the temperature is controlled and maintained at 40°C, and the drying time is 5 minutes. After drying, pre-baking is performed at a temperature between 70±5°C and the time is 10 minutes. After pre-baking, it is cooled to room temperature.

[0030] The cooled flexible printed circuit board is precisely aligned to the exposure machine platform, covered and fixed with a pre-prepared precision mask to ensure that the pattern is completely aligned with the board surface. The exposure parameters are set as follows: a UV light source with a wavelength of 405nm, a light intensity controlled at 600mW / cm2, and an exposure time of 90 seconds to achieve high-quality pattern transfer. Before starting the exposure program, confirm that all settings are correct again. Then start the exposure program to ensure that the pattern transfer accuracy reaches ±5μm.

[0031] The exposed flexible printed circuit board is immediately transferred to the developing tank, and the developer temperature is maintained at 35±1℃ and circulated. The developing time is set to 2 minutes, and the pH value and concentration of the developer are monitored during the period to ensure the developing effect. After the development is completed, it is rinsed with deionized water and then dried. The developing step uses a developer to dissolve and remove the coating film of the unexposed part, while retaining the exposed part.

[0032] Place the cleaned and dried flexible printed circuit board into the UV curing box. Use a 365nm light source for preliminary pre-curing, set the time to 1 minute, then switch to a 395nm light source for deep curing for 3 minutes, and the total curing time is controlled to be no more than 4 minutes. During the curing process, the ambient temperature is ensured to be within the room temperature range through temperature monitoring to avoid thermal stress affecting the circuit quality.

[0033] Example 2

[0034] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 25±2 μm.

[0035] Example 3

[0036] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 35±2 μm.

[0037] Example 4

[0038] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 45±2 μm.

[0039] Example 5

[0040] The corresponding tests were carried out on Examples 1-3, and the specific contents are as follows:

[0041] Bending resistance: Bend 180° with the solder mask film on the outside and evaluate based on the following criteria:

[0042] √: No cracks or peeling on the film; ×: There are cracks or peeling on the film.

[0043] Adhesion: Use a needle tip to make an "X" on the film, then stick a cellophane tape on the cut and pull it, and evaluate based on the following criteria:

[0044] √: not torn off; ×: a lot of torn off.

[0045] Chemical stability: At 20°C, immerse the PCB coated with solder mask ink in a 10wt% sulfuric acid solution or a 10wt% sodium hydroxide solution, take it out after 30 minutes, and evaluate the state and adhesion of the coating. The judgment criteria are as follows:

[0046] √: No change or slight change was found; ×: There was obvious expansion or swelling and shedding on the coating film.

[0047] Stability in hot and cold environments: First, pre-treat the device: place it at 125°C for 24 hours, then place it at 60°C and 60% RH for 52 hours; then cycle the device under hot and cold conditions: place it at -65°C for 15 minutes, then place it at 150°C for 15 minutes, and repeat this process. The switching time is <10s and the number of cycles is 1000.

[0048] The criteria for determination are as follows:

[0049] √: No cracking or falling off; ×: Cracks or falling off occur.

[0050] The test results are shown in the following table:

[0051]

[0052] From the data of Examples 1-4, it can be seen that when the thickness is below 15 μm, there is no problem with flexibility and stability. When it exceeds 20 μm, there is no problem with flexibility but the stability decreases; when it exceeds 30 μm, the adhesion and stability are both unqualified. The reason is that the thickness is too thick and the curing is incomplete; when it exceeds 40 μm, the appearance also changes due to uneven curing, affecting the product quality.

[0053] Example 6

[0054] The ink main agent has the following composition and proportion: photosensitive resin: 35 parts of acrylate photosensitive prepolymer; flexible monomer: 12 parts of triethylene glycol diacrylate; photoinitiator: 3 parts of 2-methyl-1-[4-methylthiophenyl]-2-morpholinyl-1-propanone; filler: 5 parts of nano-scale silicon dioxide.

[0055] The composition and proportion of the ink auxiliary agent are: 2 parts of sebacate; 1 part of leveling agent BYK-33; 1 part of azobenzene; and 2 parts of pigment.

[0056] Mix the ink main agent and ink auxiliary agent in a ratio of 10:3, and stir with an electric stirrer for at least 5 minutes to ensure that there are no particles and the mixture is fully mixed, then let it stand for 20 minutes to exhaust excess gas. Use a viscometer to check the viscosity of the mixed ink to ensure that it is within the range of 180±2cps. If it is not within the range, it should be adjusted appropriately and retested. Before use, the mixed ink needs to be preheated to 25±2℃ in a constant temperature environment.

[0057] Place the cleaned and dried flexible printed circuit board steadily on the loading platform of the coating machine to ensure that the board surface is dust-free and flat. Set coating parameters: the coating thickness target value is 25μm, and the roller pressure is adjusted to a moderate level to ensure uniform coating, and the coating speed is controlled to 20 meters / minute. At the same time, the laser range sensor is used for monitoring and real-time adjustment of the coating thickness to ensure that the uniformity error is within ±2μm.

[0058] After coating, the flexible printed circuit board is sent to the preset drying room, the temperature is controlled and maintained at 40°C, and the drying time is 5 minutes. After drying, pre-baking is performed at a temperature between 70±5°C and the time is 10 minutes. After pre-baking, it is cooled to room temperature.

[0059] The cooled flexible printed circuit board is precisely aligned to the exposure machine platform, covered and fixed with a pre-prepared precision mask to ensure that the pattern is completely aligned with the board surface. The exposure parameters are set as follows: a UV light source with a wavelength of 405nm, a light intensity controlled at 600mW / cm2, and an exposure time of 90 seconds to achieve high-quality pattern transfer. Before starting the exposure program, confirm that all settings are correct again. Then start the exposure program to ensure that the pattern transfer accuracy reaches ±5μm.

[0060] The exposed flexible printed circuit board is immediately transferred to the developing tank, and the developer temperature is maintained at 35±1℃ and circulated. The developing time is set to 2 minutes, and the pH value and concentration of the developer are monitored during the period to ensure the developing effect. After the development is completed, it is rinsed with deionized water and then dried. The developing step uses a developer to dissolve and remove the coating film of the unexposed part, while retaining the exposed part.

[0061] Place the cleaned and dried flexible printed circuit board into the UV curing box. Use a 365nm light source for preliminary pre-curing, with the time set to 30-40s; then switch to a 395nm light source for curing for 40-60s, and then use visible light for 40-60s, alternating between ultraviolet light and visible light, with the total time controlled to no more than 10 minutes. During the curing process, temperature monitoring is used to ensure that the ambient temperature is within the room temperature range to avoid thermal stress affecting the circuit quality.

[0062] Example 7

[0063] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 35±2 μm.

[0064] Example 8

[0065] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 45±2 μm.

[0066] Example 9

[0067] The main steps of this embodiment are substantially the same as those of embodiment 1, except that the thickness of the coating is controlled to be 55±2 μm.

[0068] Example 10

[0069] Corresponding tests were performed on Examples 6-9, and the test contents were the same as those of Example 5:

[0070] The test results are shown in the following table:

[0071]

[0072] For flexible printed circuit boards, the solder resist ink should have not only conventional properties, but also good adhesion, flexibility, bending resistance and stability during complex processing. From the data of Examples 6-9, it can be seen that the present invention has not only good flexibility but also good stability even when the coating is thick.

[0073] The above description is only a preferred embodiment of the present application and is not intended to limit the present application.

Claims

1. A method for using UV light-curing ink, characterized in that: Follow these steps: The ink main agent and the ink auxiliary agent are mixed in a ratio of 10-12:3, stirred evenly, and then allowed to stand for 15-20 minutes to ensure that the viscosity is 150-300 dPa·s, and preheated to 25±2°C in a constant temperature environment to obtain a ready-to-use ink mixture; The clean flexible printed circuit board is placed on the carrying platform of the coating machine, and the ink mixture is coated on the flexible printed circuit board, and the coating speed is controlled to be 20-30 m / min. The coated flexible printed circuit board is dried, pre-baked, exposed, and developed in sequence; The treated flexible printed circuit board is placed in a UV curing box, and a 365nm light source is first used for preliminary pre-curing, and the time is set to 30-40s; then the 395nm light source is switched to cure for 40-60s, and then the visible light is used for 40-60s, and the ultraviolet light and visible light are alternately used, and the total time is controlled to be no more than 10 minutes; The composition and proportion of the ink main agent are as follows: photosensitive resin: 30-40 parts of acrylate photosensitive prepolymer; flexible monomer: 10-15 parts of triethylene glycol diacrylate; Photoinitiator: 1-5 parts of 2-methyl-1-[4-methylthiophenyl]-2-morpholinyl-1-propanone; Filler: 2-8 parts of nano-silicon dioxide; The ink auxiliary agent has the following composition and proportion: 1-3 parts of sebacate; 1-2 parts of leveling agent BYK-33; 1-2 parts of azobenzene or its derivatives; and 1-5 parts of pigment.

2. The method for using the UV light curing ink according to claim 1, characterized in that: After standing, use a viscometer to check the viscosity of the mixed ink to ensure that it is 180±20 dPa·s. If it is not within the range, adjust it and retest.

3. The method for using the UV light curing ink according to claim 1, characterized in that: For the drying, the flexible printed circuit board is sent into a preset drying chamber, the temperature is controlled at 40° C. and maintained stably, and the drying time is 5-10 minutes.

4. The method for using the UV light curing ink according to claim 1, characterized in that: The pre-baking temperature is between 70±5°C and the time is 5-15 minutes.

5. The method for using the UV light curing ink according to claim 1, characterized in that: The exposure described above is to precisely align the cooled flexible printed circuit board onto the exposure machine platform, cover and fix the pre-prepared precision mask, and ensure that the pattern is completely aligned with the board surface; Start the exposure program and ensure that the pattern transfer accuracy reaches ±5μm.

6. The method for using the UV light curing ink according to claim 5, characterized in that: The exposure parameters were set as follows: UV light source with a wavelength of 405nm, light intensity controlled at 600mW / cm², and exposure time accurate to 80-100 seconds.

7. The method for using the UV light curing ink according to claim 1, characterized in that: The development described is as follows: the exposed flexible printed circuit board is immediately transferred to a developing tank, the developer temperature is maintained at 35±1°C, circulated, and the developing time is set to 2-3 minutes. After the development is completed, it is rinsed with deionized water and then dried.

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