Conductive paste for direct-write 3D printing and method for preparing the same

Abstract

The application provides a three-proofing glue for direct writing 3D printing and a preparation method thereof. The three-proofing glue has a viscosity of 25000-60000 cP and a thixotropy index of 4-6, can be used for direct writing 3D printing with a height-width ratio of greater than or equal to 0.3, and has a water absorption of 0.3-0.7% and a grid adhesion of 4B-5B. The three-proofing glue has the characteristics of high viscosity, high thixotropy and high shape retention, can print a wire with a height-width ratio of greater than or equal to 0.3, and can also have good moisture and heat resistance, is suitable for direct writing 3D printing applications in the chip packaging and display fields, is a three-proofing glue suitable for 3D printing, high thixotropy and high shape retention, and can greatly expand the application scenarios of the glue in the packaging field, and meets the urgent requirements of the electronic industry.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, and more specifically, to a conformal adhesive for direct-write 3D printing and its preparation method. Background Technology

[0002] Currently, with the rapid updates and iterations of various electronic devices, electronic products are gradually becoming smaller, more integrated, and more multifunctional. Under this trend, the electronic packaging industry has also experienced rapid development, and the market's requirements for the safety and reliability of packaged products are becoming increasingly stringent. Electronic devices often operate in complex environments, and their core components, such as circuit boards, are susceptible to environmental factors, potentially leading to corrosion, softening, and deformation, resulting in frequent malfunctions during long-term use. Therefore, product quality and reliability have become key indicators of consumer concern, with "three-proof" performance (moisture-proof, salt spray-proof, and anti-static) being the most valued by the industry and now listed as a key technical indicator by many manufacturers. As an important protective material in the electronic packaging field, conformal adhesive, by forming a dense protective layer, can effectively block environmental corrosive factors from damaging circuit boards and related equipment. Thanks to its special formula, this material exhibits excellent resistance to high and low temperatures and damp heat after curing, providing durable and reliable protection for electronic components even under extremely harsh conditions.

[0003] However, most conformal adhesives currently in use are solvent-based products. Their advantages include low viscosity (less than 150 mPa·s), ease of application, and wide applicability. However, their disadvantages include extremely low viscosity and lack of thixotropic conformability, making them suitable only for full-area encapsulation protection as a protective coating, and unsuitable for area-selective encapsulation protection, high-thickness chip encapsulation, and damming applications. Furthermore, low-viscosity conformal adhesives often use large amounts of toxic solvents such as benzene as viscosity adjusters. During curing, these solvents produce toxic and harmful gases, seriously affecting the health of operators and potentially causing occupational diseases. Additionally, benzene solvents have low flash points, are highly volatile, and pose flammable and explosive risks, requiring careful storage and safety management during use, increasing management and production costs. Summary of the Invention

[0004] The main objective of this invention is to provide a conformal coating for direct-write 3D printing and its preparation method, in order to solve the problem that the conformal coating in the prior art has low viscosity and poor thixotropic conformability, which makes it only suitable as a protective coating and unable to be applied to chip packaging and damming directions that require high film thickness, thus making it difficult to adapt to direct-write 3D printing.

[0005] To achieve the above objectives, according to one aspect of the present invention, a conformal adhesive for direct-write 3D printing is provided, wherein the conformal adhesive has a viscosity of 25,000 to 60,000 cP, a thixotropic index of 4 to 6, and can be used for direct-write 3D printing with an aspect ratio ≥ 0.3, and the conformal adhesive has a water absorption rate of 0.3 to 0.7% and a cross-cut adhesion of 4B to 5B.

[0006] Further, by weight, the conformal coating comprises: 60-90 parts of biphenyl epoxy resin, 10-40 parts of multifunctional epoxy resin, 8-15 parts of latent curing agent, 10-40 parts of core-shell rubber modified epoxy resin, and 15-40 parts of modified filler; the modified filler includes hydrophobic fumed silica, silane modified fumed silica, and barium sulfate.

[0007] Further, by weight, the conformal coating comprises: 70-80 parts of biphenyl epoxy resin, 20-30 parts of multifunctional epoxy resin, 10 parts of latent curing agent, 15-30 parts of core-shell rubber modified epoxy resin, and 26-33 parts of modified filler.

[0008] Furthermore, the viscosity of the biphenyl epoxy resin is >4000 cP and the epoxy value is 0.35 to 0.88; and / or the viscosity of the multifunctional epoxy resin is >500 cP and the epoxy value is 0.4 to 0.9; and / or the multifunctional epoxy resin includes one or more of glycidylamine type epoxy resin, alicyclic multifunctional epoxy resin and multifunctional phenolic epoxy resin.

[0009] Furthermore, the latent curing agent includes one or more of dicyandiamide, adipic acid dihydrazide, sebacate dihydrazide, imidazole, imidazole modified compounds, and boron trifluoride complexes.

[0010] Furthermore, the viscosity of the core-shell rubber modified epoxy resin is 2000–40000 cP; and / or the core-shell rubber modified epoxy resin is formed by blending nano-rubber with epoxy resin; wherein the particle size of the nano-rubber is 10–1000 nm, and the weight ratio of nano-rubber to epoxy resin is (20–60):100.

[0011] Furthermore, the modified filler includes hydrophobic gaseous silicon, silane-modified gaseous silicon, and barium sulfate, and the weight ratio of hydrophobic gaseous silicon, silane-modified gaseous silicon, and barium sulfate is (8-15):8:(8-15).

[0012] Furthermore, by weight, the conformal coating also includes 3 to 8 parts of curing accelerator and 1 to 3 parts of additives; the curing accelerator includes one or more of modified imidazole, organic urea accelerators and modified amines; and / or the additives are adhesion-enhancing coupling agents.

[0013] According to another aspect of the present invention, a method for preparing the conformal adhesive for direct-write 3D printing described above is provided, comprising the following steps: Step S1, mixing and stirring biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler to obtain a first mixture; Step S2, adding a latent curing agent to the first mixture and stirring to obtain a second mixture; Step S3, grinding the second mixture until the particle size is ≤10μm to obtain a third mixture; Step S4, subjecting the third mixture to vacuum degassing and pressure filtration sequentially to obtain the conformal adhesive for direct-write 3D printing.

[0014] Furthermore, the vacuum degree of vacuum degassing is 40–100 mbar; and / or the pressure of pressure filtration is 0.5–1.5 kg / cm³. 2 The filter screen size is 300-400 mesh.

[0015] Furthermore, in step S2, after adding the latent curing agent to the first mixture, a curing accelerator and an additive are also added, and then the mixture is stirred.

[0016] Furthermore, the ratio of the total weight parts of biphenyl epoxy resin and multifunctional epoxy resin to the weight parts of curing accelerator is 100:(3-8); and / or the ratio of the total weight parts of biphenyl epoxy resin and multifunctional epoxy resin to the weight parts of additives is 100:(1-3).

[0017] Applying the technical solution of this invention, the conformal adhesive possesses high viscosity and a high thixotropic index, with a viscosity of 25,000–60,000 cP and a thixotropic index of 4–6. This conformal adhesive exhibits high viscosity, high thixotropy, and high conformability, enabling the printing of filaments with an aspect ratio ≥0.3. It also maintains good resistance to damp heat, making it suitable for direct-write 3D printing applications in chip packaging and displays. This invention provides a conformal adhesive suitable for 3D printing, characterized by high thixotropy and high conformability, which can significantly expand the application scenarios of such adhesives in the packaging field and meet the urgent requirements of the electronics industry. Attached Figure Description

[0018] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0019] Figure 1 A test diagram of boiling 100 squares of water according to an embodiment of the present invention is shown;

[0020] Figure 2 A top view of a printed filament according to an embodiment of the present invention is shown;

[0021] Figure 3A three-dimensional diagram of a printed filament according to an embodiment of the present invention is shown. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] As described in the background section of this invention, existing conformal adhesives suffer from low viscosity and poor thixotropic conformability, limiting their application to protective coatings and making them unsuitable for chip packaging and damming applications requiring high film thickness, thus hindering their suitability for direct-write 3D printing. To address these issues, in a typical embodiment of this invention, a conformal adhesive for direct-write 3D printing is provided. This adhesive has a viscosity of 25,000–60,000 cP, a thixotropic index of 4–6, and can be used for direct-write 3D printing with an aspect ratio ≥0.3. Furthermore, the adhesive has a water absorption rate of 0.3–0.7% and a cross-cut adhesion of 4B–5B. Those skilled in the art will understand that viscosity and thixotropic index are indicators of the conformal adhesive before curing. The curing conditions for the conformal adhesive can be 100–150°C for 20–60 minutes.

[0024] Direct-write 3D printing differs significantly from other printing technologies in its requirements for printing materials. It involves precisely controlling nozzles to directly extrude materials (such as liquids, slurries, or soft solids) onto a substrate for planar or layered printing. The material solidifies based on its physical or chemical properties, thus requiring excellent viscosity and thixotropic conformability. However, conventional low-viscosity conformal adhesives in existing technologies lack good conformability and thixotropy, making them unsuitable for planar or three-dimensional 3D printing with specific structural requirements. They also cannot achieve selective encapsulation protection, such as in specific packaging and damming applications. The conformal adhesive of this invention has a viscosity of 25,000–60,000 cP and a thixotropic index of 4–6. This high viscosity, high thixotropy, and high conformability allow for the printing of filaments with an aspect ratio ≥0.3, making it suitable for direct-write 3D printing applications in chip packaging and displays.

[0025] Conformal adhesives meeting the above-mentioned water absorption and cross-cut adhesion requirements also exhibit excellent resistance to damp heat, effectively protecting substrates from high-temperature and high-humidity environments. Furthermore, they demonstrate exceptional adhesion to inert substrates such as silicon wafers and difficult-to-bond substrates, preventing moisture penetration from the bottom and thus effectively protecting electronic devices and improving packaging reliability. This invention provides a highly thixotropic and highly conformal conformal adhesive suitable for direct-write 3D printing, significantly expanding its application scenarios in the packaging field and meeting the urgent needs of the electronics industry.

[0026] Specifically, in a preferred embodiment, the conformal coating comprises, by weight parts: 60-90 parts of biphenyl epoxy resin, 10-40 parts of multifunctional epoxy resin, 8-15 parts of latent curing agent, 10-40 parts of core-shell rubber modified epoxy resin, and 15-40 parts of modified filler; the modified filler comprises one or more of hydrophobic fumed silica, silane modified fumed silica, and barium sulfate.

[0027] This invention uses biphenyl epoxy resin and multifunctional (e.g., trifunctional / tetrafunctional) epoxy resin as matrix resins, wherein 60-90 parts of biphenyl epoxy resin are used as the main resin. Biphenyl epoxy resin is a special epoxy resin with a rigid biphenyl structure, which can effectively improve the strength, mechanical properties and moisture and heat resistance of the cured product. It also has extremely low water absorption, which can give the cured product excellent waterproof performance and is beneficial to improving the water resistance of conformal adhesives.

[0028] 10 to 40 parts of multifunctional epoxy resin can act as a crosslinking agent, reacting and crosslinking with biphenyl epoxy resin and latent curing agents, further improving the overall crosslinking degree, rigidity and strength of the cured product, which is beneficial to improving the overall waterproof performance.

[0029] In addition, the high resistivity of the cured products of epoxy resin systems gives conformal adhesives excellent electrical insulation properties, making them more suitable for electronic packaging applications.

[0030] The latent curing agent, as the main curing agent, is used to perform a curing reaction with the main resin. The latent curing agent is stable at room temperature but can be activated and rapidly participate in curing under specific conditions (such as heating or light exposure). Therefore, mixing the latent curing agent with epoxy resin to prepare a one-component epoxy adhesive reduces workflow and costs; simultaneously, the adhesive remains stable even after prolonged storage at room temperature, facilitating long-term application. The conformal adhesive of this invention exhibits minimal viscosity change over extended room temperature storage, effectively ensuring long-term material output stability and product packaging precision during mass production based on direct-write 3D printing. The amount of latent curing agent can be appropriately added based on the epoxy equivalent of the epoxy resin. The principle is that the epoxy groups of the epoxy resin react with the groups (such as amino groups) in the latent curing agent. When the amount of latent curing agent is 8-15 parts, the added amount is slightly greater than the calculated amount, ensuring complete curing of the epoxy resin and resulting in a long-term stable high viscosity performance of the conformal adhesive.

[0031] The core-shell rubber modified epoxy resin mainly acts as a toughening agent to strengthen and reinforce the material. The resin in the resin can also participate in the curing reaction, and at the same time, it works synergistically with the epoxy resin system to improve the moisture and heat resistance of the conformal coating. The rubber part can improve the toughness and prevent the resin from being too rigid and causing brittle fracture. However, if the amount is too large, it may make the cured product too plastic, affecting the glass transition temperature Tg of the material and causing a decrease in heat resistance. Therefore, the preferred amount of core-shell rubber modified epoxy resin is 10 to 40 parts.

[0032] Among the modified fillers, the surfaces of hydrophobic fumed silica, silane-modified fumed silica, and barium sulfate can all be modified to improve the adhesive properties, thereby enhancing the overall crosslinking degree and improving thixotropic shape retention. They can also play a reinforcing role and increase hardness. The dosage can be adjusted according to the thixotropic shape retention requirements. The amount of modified filler added should not be too much, as too much will lead to excessively high viscosity of the conformal adhesive and make it difficult to apply. Therefore, the preferred dosage is 15 to 40 parts.

[0033] The conformal adhesive of this invention incorporates highly cross-linked epoxy resin and core-shell rubber-modified epoxy resin, resulting in excellent adhesion to the substrate. Even after curing, the adhesion remains at 5B, and no water vapor penetrates the adhesive layer during a red ink boiling test, representing a significant improvement over conventional conformal adhesives on the market. Simultaneously, the use of core-shell rubber-modified epoxy resin enhances the strength of the cured material and reduces the impact of humidity and moisture. Combined with specially treated modified fillers, the overall thixotropy and shape retention of the material are improved, while the hardness and strength of the cured material are increased. The adhesive exhibits high cross-linking after curing, resulting in excellent waterproof performance, making it suitable for direct-write 3D printing. Furthermore, the composition of this invention does not require the use of toxic solvents such as benzene as viscosity adjusters, and no toxic or harmful gases are generated during the curing process, making it more environmentally friendly and reducing management and production costs.

[0034] For similar reasons, in order to further improve the viscosity, mechanical properties and moisture and heat resistance of the conformal coating after curing, in a preferred embodiment, the conformal coating comprises, by weight, 70-80 parts of biphenyl epoxy resin, 20-30 parts of multifunctional epoxy resin, 10 parts of latent curing agent, 15-30 parts of core-shell rubber modified epoxy resin and 26-33 parts of modified filler.

[0035] In a preferred embodiment, the biphenyl epoxy resin has a viscosity >4000 cP (e.g., 4000–40000 cP) and an epoxy value of 0.35–0.88; and / or the multifunctional epoxy resin has a viscosity >500 cP (e.g., 500–50000 cP) and an epoxy value of 0.4–0.9; and / or the multifunctional epoxy resin includes one or more of glycidylamine type epoxy resin, alicyclic multifunctional epoxy resin, and phenolic epoxy resin. The aforementioned multifunctional epoxy resin can connect multiple epoxy groups, enabling it to react and crosslink more fully with the biphenyl epoxy resin, thereby further improving the overall crosslinking degree, viscosity, and conformal properties of the conformal adhesive.

[0036] Conventional amine curing agents such as aliphatic amines and aromatic amines react slowly with epoxy resins at room temperature. If prepared as a one-component epoxy adhesive, the viscosity of the adhesive material will change uncontrollably during application or storage, making it impossible to precisely control the amount of adhesive applied, which is not conducive to the requirements of high-precision projects. In a preferred embodiment, the latent curing agent includes one or more of dicyandiamide, adipic acid dihydrazide, sebacate dihydrazide, imidazole, imidazole modified compounds, and boron trifluoride complexes. The above-mentioned latent curing agents require specific high temperatures (e.g., 60-150°C) to initiate the reaction, which can improve the viscosity stability of the conformal adhesive and is more conducive to the preparation, storage, and use of high-viscosity one-component epoxy adhesives. Using 8-15 parts of the above-mentioned latent curing agent can further ensure the controllability of the curing process, so that the curing reaction has a more suitable rate, which is beneficial to obtaining a conformal adhesive with high viscosity and high conformability.

[0037] In a preferred embodiment, the viscosity of the core-shell rubber-modified epoxy resin is 2000–40000 cP; and / or the core-shell rubber-modified epoxy resin is formed by blending nano-rubber with epoxy resin; wherein the particle size of the nano-rubber is 10–1000 nm, and the weight ratio of nano-rubber to epoxy resin is (20–60):100. When the above-mentioned core-shell rubber-modified epoxy resin is used as a toughening agent, it does not lower the glass transition temperature (Tg) of the cured product while maintaining the toughness-enhancing effect of conventional toughening agents, thus further maintaining its good heat resistance. The above-mentioned specially modified toughening agent material can further improve the strength of the cured product, increase the overall crosslinking degree, and reduce the effects of humid heat and moisture.

[0038] To further improve the crosslinking degree and weather resistance of the cured material, in a preferred embodiment, the modified filler includes hydrophobic fumed silica, silane-modified fumed silica and barium sulfate, and the weight ratio of hydrophobic fumed silica, silane-modified fumed silica and barium sulfate is (8-15):8:(8-15).

[0039] In a preferred embodiment, the conformal adhesive further comprises, by weight, 3 to 8 parts of a curing accelerator and 1 to 3 parts of an additive; preferably, the curing accelerator includes one or more of modified imidazole, organic urea accelerators, and modified amines; and / or the additive is an adhesion-enhancing coupling agent, and may also include one or more conventional additives such as leveling agents, dispersants, defoamers, and coupling agents. The above components contribute to the conformal adhesive having superior adhesion to the substrate, especially on surfaces with inertness such as silicon wafers or substrates with special inert coatings (such as silicon nitride coatings), enabling strong adhesion.

[0040] In another typical embodiment of the present invention, a method for preparing the conformal adhesive for direct-write 3D printing described above is also provided, comprising the following steps: Step S1, mixing and stirring biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler to obtain a first mixture; Step S2, adding a latent curing agent to the first mixture and stirring to obtain a second mixture; Step S3, grinding the second mixture until the particle size is ≤10μm to obtain a third mixture; Step S4, subjecting the third mixture to vacuum degassing and pressure filtration sequentially to obtain the conformal adhesive for direct-write 3D printing.

[0041] Specifically, this invention first mixes biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin, and modified filler, and then performs a first stirring. During the first stirring, the components do not react with each other, so high-speed stirring can be used, for example, using a high-speed disperser to disperse until uniformly mixed, obtaining a first mixture. Subsequently, after the mixture cools to room temperature, the stirring speed can be appropriately reduced, a latent curing agent is added to the first mixture, and then a second stirring is performed. In this process, reducing the stirring speed can prevent the resin and curing agent from reacting due to excessive temperature during stirring, obtaining a second mixture.

[0042] The second mixture is then ground, for example, 2-5 times using a high-precision three-roll mill or ball mill. Finer grinding allows for the use of smaller printing needles, preventing needle clogging due to particle size and facilitating micron-level printing. Grinding to a particle size ≤10μm yields the third mixture. Controlling particle size improves the surface quality and detail accuracy of the cured material. Within this range, it ensures that the conformal adhesive will not clog the needles due to excessively large filler size during micron-level direct-write 3D printing, while also preserving the microstructure of the cured material. Finally, the third mixture undergoes vacuum degassing and pressure filtration to reduce residual foam and remove dust and other contaminants, resulting in a conformal adhesive for direct-write 3D printing. This adhesive exhibits high viscosity and thixotropic conformability at room temperature, displaying high resistivity, low water absorption, high bond strength, and viscosity stability.

[0043] This application's conformal adhesive achieves high viscosity, high conformability, and excellent printing performance in direct-write 3D printing through specific formulation and preparation processes. Its unique component selection and proportions not only improve the mechanical strength and stability of the cured product but also significantly reduce water absorption, enhance adhesive strength, and maintain good viscosity stability, making the printing process more precise and controllable. Furthermore, the high resistivity exhibited by the cured product gives it significant advantages in electronic packaging and insulation applications, opening up new possibilities for the application of 3D printing technology in the precision electronics field. This colloidal composition is not only suitable for direct-write 3D printing but can also be adapted to other 3D printing technologies, such as photopolymerization printing and fused deposition modeling, by adjusting the formulation, broadening its application range in manufacturing. Its excellent properties, such as high viscosity and high conformability, make it an ideal choice for manufacturing high-performance, high-precision products, providing strong technical support for innovation in multiple fields such as aerospace, automotive manufacturing, and electronic packaging.

[0044] To further promote the uniform dispersion of resin, core-shell rubber modified epoxy resin, and modified filler, reduce localized uneven mixing or sediment formation, and improve the uniformity and performance consistency of the conformal coating, in a preferred embodiment, the stirring speed for obtaining the first mixture is greater than the stirring speed for obtaining the second mixture, with a difference of 800–1200 rpm. Specifically, the stirring speed for obtaining the first mixture is 1800–2200 rpm for 10–30 min; and / or the stirring speed for obtaining the second mixture is 800–1200 rpm for 5–15 min. This ensures uniform dispersion of all components while better reducing unnecessary reactions caused by excessively high stirring speeds leading to temperature increases.

[0045] In a preferred embodiment, the vacuum degree of vacuum degassing is 40–100 mbar; and / or the pressure of pressure filtration is 0.5–1.5 kg / cm³. 2 The filter size is 300-400 mesh. Under the above conditions, air bubbles in the mixing process can be removed more effectively, avoiding the formation of voids during curing that would affect the accuracy and strength of the printed parts, improving the density and strength of the cured material, and filtering impurities and uneven particles more efficiently to further improve the uniformity of the composition and ensure the consistency and reliability of the printing effect.

[0046] In order to better improve the adhesion of the conformal adhesive to the substrate, in a preferred embodiment, in step S2, after adding the latent curing agent to the first mixture, a curing accelerator and an additive are also added, and then the mixture is stirred.

[0047] Specifically, in a preferred embodiment, the total weight ratio of biphenyl epoxy resin and multifunctional epoxy resin to the weight ratio of curing accelerator is 100:(3-8); and / or the total weight ratio of biphenyl epoxy resin and multifunctional epoxy resin to the weight ratio of additives is 100:(1-3), which is more conducive to achieving firm adhesion on the surface of an inert substrate.

[0048] Typical, but not limited, conformal coatings, by weight, include: 60 parts, 65 parts, 70 parts, 75 parts, 80 parts, 85 parts, 90 parts, or any two of these values ​​of biphenyl epoxy resin; 10 parts, 15 parts, 20 parts, 25 parts, 30 parts, 35 parts, 40 parts, or any two of these values ​​of polyfunctional epoxy resin; 8 parts, 9 parts, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, or any two of these values ​​of latent curing agent; 10 parts, 15 parts, 20 parts, 25 parts, 30 parts, 35 parts, 40 parts, or any two of these values ​​of core-shell rubber modified epoxy resin; and 15 parts, 20 parts, 25 parts, 26 parts, 30 parts, 33 parts, 35 parts, 40 parts, or any two of these values ​​of modified filler.

[0049] Typical, but not limiting, epoxy values ​​of biphenyl epoxy resins are 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.88, or any two of these values; epoxy values ​​of multifunctional epoxy resins are 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, or any two of these values.

[0050] Typical, but not limited, core-shell rubber-modified epoxy resins are formed by blending nano-rubber with epoxy resin, wherein the weight ratio of nano-rubber to epoxy resin is 20:100, 30:100, 40:100, 50:100, 60:100 or any two of these values.

[0051] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0052] Example 1

[0053] The composition of conformal adhesive is shown in Table 1, and the types of each component are as follows:

[0054] Biphenyl epoxy resin: viscosity 4500 cP, epoxy value 0.54;

[0055] Multifunctional epoxy resin: Alicyclic multifunctional epoxy resin TDE-85, with a viscosity of 2000 cP and an epoxy value of 0.85;

[0056] Latent curing agent: Dicyandiamide (DICY);

[0057] Core-shell rubber modified epoxy resin: formed by uniformly blending nano-rubber with epoxy resin with a particle size of 10-1000nm, the weight ratio of nano-rubber to epoxy resin is 40:100, and the viscosity is 2000-40000cP.

[0058] Modified fillers: hydrophobic gaseous silicon, silane-modified gaseous silicon, and barium sulfate;

[0059] Curing accelerator: Organic urea accelerator UR500;

[0060] Additive: Silane coupling agent KH560.

[0061] The preparation method is as follows:

[0062] Step S1: Mix biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler in proportion, and disperse them with a high-speed disperser until they are uniformly mixed (speed is 2000 rpm, time is 20 min) to obtain the first mixture.

[0063] Step S2: Cool to room temperature (20-30℃), reduce stirring speed (1000 rpm, 10 min), add latent curing agent, curing accelerator and additives to the first mixture, and then stir a second time to obtain the second mixture;

[0064] Step S3: Cool to room temperature (20-30℃), and grind the second mixture three times using a high-precision three-roll mill until the particle size is ≤10μm to obtain the third mixture;

[0065] Step S4: The third mixture is subjected to vacuum degassing (vacuum treatment at 50 mbar) and pressure filtration (1 kg / cm³). 2 (Filtered through a 350-mesh filter under pressure) to obtain conformal adhesive for direct-write 3D printing.

[0066] Examples 2 to 9

[0067] The difference from Example 1 is that the composition of the conformal adhesive is different by weight, as shown in Table 1.

[0068] Example 10

[0069] The conformal adhesive composition is the same as in Example 1, and the preparation method is as follows:

[0070] Step S1: Mix biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler in proportion, and disperse them with a high-speed disperser until they are uniformly mixed (speed is 1800 rpm, time is 30 min) to obtain the first mixture.

[0071] Step S2: Cool to room temperature (20-30°C), reduce stirring speed (800 rpm, 15 min), add latent curing agent, curing accelerator and additives to the first mixture, and then perform a second stirring to obtain the second mixture;

[0072] Step S3: Cool to room temperature (20-30℃), and grind the second mixture twice using a high-precision three-roll mill until the particle size is ≤10μm to obtain the third mixture;

[0073] Step S4: The third mixture is subjected to vacuum degassing (vacuum treatment at 40 mbar) and pressure filtration (0.5 kg / cm³). 2 (Filtered through a 300-mesh filter under pressure) to obtain conformal adhesive for direct-write 3D printing.

[0074] Example 11

[0075] The conformal adhesive composition is the same as in Example 1, and the preparation method is as follows:

[0076] Step S1: Mix biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler in proportion, and disperse them with a high-speed disperser until they are uniformly mixed (speed is 2200 rpm, time is 10 min) to obtain the first mixture.

[0077] Step S2: Cool to room temperature (20-30℃), reduce stirring speed (1200 rpm, 5 min), add latent curing agent, curing accelerator and additives to the first mixture, and then stir a second time to obtain the second mixture;

[0078] Step S3: Cool to room temperature (20-30℃), and grind the second mixture 5 times using a high-precision three-roll mill until the particle size is ≤10μm to obtain the third mixture;

[0079] Step S4: The third mixture is subjected to vacuum degassing (vacuuming at 100 mbar) and pressure filtration (1.5 kg / cm³). 2 (Filtered through a 400-mesh filter under pressure) to obtain conformal adhesive for direct-write 3D printing.

[0080] Comparative Examples 1 to 5

[0081] The difference from Example 1 is that the composition of the conformal adhesive is different by weight, as shown in Table 1.

[0082] Comparative Example 6

[0083] The difference from Example 1 is that E51 epoxy resin is used instead of biphenyl epoxy resin and multifunctional epoxy resin.

[0084] Comparative Example 7

[0085] Technical solution of Embodiment 1 of Chinese Patent CN 105907043 A:

[0086] (1) Preparation of component A: Take 100g of epoxy resin E51, 10g of toughening agent carboxyl liquid nitrile rubber, 30g of flame retardant aluminum hydroxide, 100g of filler fumed silica, 0.001g of degassing agent SAG47 and 5g of inorganic pigment titanium dioxide, stir evenly, and then put it in a desiccator for vacuum degassing; after degassing, component A is obtained;

[0087] (2) Preparation of component B: Take 7g of curing agent ethylenediamine, 0.001g of degassing agent SAG47 and 1g of curing accelerator DMP-30, stir evenly, and then put it into a desiccator for vacuum degassing; after degassing, component B is obtained.

[0088] (3) Printing: First, print a thin layer of component A, and then spray-deposit component B onto the surface of component A and cure quickly; repeat the process of printing a thin layer of component A and spray-depositing component B onto the surface of component A, and continue printing until the model is completely printed.

[0089] (4) Post-curing: After the model is completely printed, it is placed in a 120℃ oven for post-curing.

[0090] Comparative Example 8

[0091] Technical solution of Embodiment 1 of Chinese Patent CN 114773788 A:

[0092] 30 parts of nano-silicone rubber core-shell structure polymer-toughened epoxy resin A-1, 30 parts of epoxy acrylate resin B-1, 20 parts of epoxy propylene oxide reactive diluent monomer C-1, 20 parts of epoxy butane reactive diluent monomer D-1, 5 parts of photoinitiator E-1, 2 parts of photoinitiator E-3, 1 part of defoamer F-1, 1 part of leveling agent G-1, and 1 part of antioxidant H-1 are mixed and heated to 30-100℃. After stirring and mixing evenly, the photoinitiator is added after cooling, and the mixture is stirred until uniform to obtain a white viscous liquid, which is the photosensitive resin composition sample.

[0093] The conformal adhesives prepared in the above examples and comparative examples were subjected to performance tests, and the results are shown in Table 2.

[0094] Test content and test methods:

[0095] Water absorption rate: Samples were prepared according to standard GB / T1034-2008. The mass change of the material was tested after being soaked in room temperature environment of 23℃ for one day, and the water absorption rate of the cured product was calculated.

[0096] Inert substrate water cross-cut test: First, apply the adhesive to the silicon wafer surface to form a coating of a specific thickness. After curing the coating at 120℃ for 30 minutes, place the sample in a 100℃ water bath and boil for 30 minutes. Then, prepare the sample according to the national standard GB_T9286-2021 and perform the cross-cut test.

[0097] Viscosity (viscosity stability): Using a Borlefeld viscometer, the viscosity of the adhesive was tested every week for 4 weeks at room temperature (25°C). The viscosity stability was determined by the change in viscosity value.

[0098] Thixotropic index: When testing viscosity, viscosity data are measured at two different speeds of 1 rpm and 10 rpm. The thixotropic index is characterized by the ratio of viscosity η(1 rpm) / η(10 rpm).

[0099] Aspect Ratio Calculation: Direct writing 3D printing is performed on the substrate using a dispensing machine, and the line width and line height are measured using a high-precision sensor to calculate the aspect ratio of the printed line.

[0100] Printed line morphology: The printed line was cured at 120℃ for 30 minutes, and the line shape was observed using ultra-depth of field.

[0101] See the water-boiled 100-grit test chart for Example 1. Figure 1 It can be seen that the adhesion to the silicon wafer substrate surface can reach 5B, and the conformal adhesive of the present invention has good viscosity and adhesion.

[0102] A top view of the printing filament in Example 1 is shown below. Figure 2 The visible line width is 120μm, the line height is 70μm, and the aspect ratio is 0.58. The conformal adhesive of this invention can be used for direct-write 3D printing with an aspect ratio ≥ 0.3.

[0103] The 3D diagram of the printed filament in Example 1 is shown below. Figure 3 As can be seen from the 3D perspective, the adhesive line presents a barrier shape perpendicular to the substrate. The top of the adhesive line is uniform and flat with good height consistency. It has good straightness and no bending of the adhesive line is observed.

[0104] Table 1

[0105]

[0106]

[0107] Table 2

[0108] Performance testing Viscosity Thixotropic index Water absorption rate Adhesion (Boiled 100 Cross-Section Test) Example 1 31500 4.5 0.46 5B Example 2 27300 4.1 0.52 5B Example 3 30680 4.2 0.56 5B Example 4 31800 4.3 0.36 5B Example 5 57600 5.8 0.68 5B Example 6 25350 4.0 0.69 4B Example 7 55380 4.0 0.69 4B Example 8 25640 4.0 0.68 4B Example 9 26380 4.1 0.70 4B Example 10 25280 4.0 0.70 4B Example 11 25640 4.0 0.69 4B Comparative Example 1 21800 3.7 3.23 0B Comparative Example 2 41660 4.7 2.36 0B Comparative Example 3 24600 5.7 0.89 3B Comparative Example 4 31000 4.1 2.81 5B Comparative Example 5 31700 1.3 1.19 5B Comparative Example 6 33700 4.0 5.17 4B Comparative Example 7 21800 1.2 10.91 2B Comparative Example 8 20960 1.9 9.65 2B

[0109] It is evident that the conventional E51 epoxy resin used in Comparative Example 6 has poor resistance to damp heat after curing. Furthermore, as it is a bifunctional structure, its overall crosslinking degree is lower than that of multifunctional epoxy resins, resulting in reduced crosslinking degree and poor mechanical properties and resistance to damp heat.

[0110] The resin and curing agent used in Comparative Example 7 are all conventional materials with low crosslinking degree. The printed and cured material has poor moisture resistance and does not possess conformal adhesive properties. It uses a room temperature curing agent and is a two-component system. Components A and B need to be loaded separately for printing, making it impossible to prepare a single-component adhesive. Furthermore, a dual-nozzle solution is required, resulting in high preparation and equipment costs. The two-component system reacts after mixing, leading to uncontrollable viscosity during use or storage and making precise dosage control impossible. Moreover, component B is a thin-layer spray deposition, which easily contaminates other areas and makes it difficult to control the uniformity of the entire surface, potentially causing unevenness on component A and affecting subsequent curing uniformity. Bursting may occur during the overall heat curing process. In addition, its toughening agent is conventional carboxylated liquid nitrile rubber (CTBN), which only has a toughening effect and cannot participate in the crosslinking reaction, resulting in weak cured product strength. In contrast, the embodiments of the present invention use a latent curing agent that does not react with epoxy resin during use or storage. The reaction can only occur after the activation conditions are met. Therefore, it can be prepared into a single-component adhesive and is stable for a long time at room temperature. The core-shell structure of the rubber-epoxy resin toughening agent of the present invention has a higher degree of crosslinking and greater strength of the conformal adhesive.

[0111] In Comparative Example 8, conventional photocurable materials were used, resulting in a low degree of cross-linking in the cured product and poor resistance to damp heat. Furthermore, the prepared adhesive exhibited high fluidity but lacked high thixotropic shape retention, requiring SLA (stereolithography) layer-by-layer photocuring for 3D model printing. Directly printing the model followed by photocuring was not feasible, making the overall process time-consuming and complex. Moreover, the core-shell rubber-epoxy resin toughening agent primarily improved the toughness of the cured product and reduced brittle fracture, without addressing damp heat resistance. In contrast, the embodiments of this invention utilize biphenyl-type epoxy resin and multifunctional epoxy resin to form a highly cross-linked product, synergistically modifying the core-shell rubber epoxy resin to enhance damp heat resistance, resulting in very low water absorption. The conformal adhesive of this invention possesses high thixotropic shape retention, allowing for direct printing of 3D structures through stacking, followed by a final step of complete photocuring.

[0112] As can be seen from the above, compared with the comparative example, the conformal adhesive of the present invention has the characteristics of high viscosity, high thixotropy, and high conformability. It can print wires with an aspect ratio of ≥0.3 and also has good resistance to damp heat. It is suitable for direct-write 3D printing applications in chip packaging and display fields. It is a conformal adhesive with high thixotropy and high conformability suitable for 3D printing, which can greatly expand the application scenarios of such adhesives in the packaging field and meet the urgent requirements of the electronics industry.

[0113] Furthermore, it can be seen that the overall effect is better when all process parameters are within the preferred range of the present invention.

[0114] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A conformal adhesive for direct-write 3D printing, characterized in that, The conformal adhesive has a viscosity of 25,000~60,000 cP and a thixotropic index of 4~6, and can be used for direct-write 3D printing with an aspect ratio ≥0.

3. The conformal adhesive has a water absorption rate of 0.3~0.7% and a cross-cut adhesion of 4B~5B. By weight, the conformal coating comprises: 60-90 parts of biphenyl epoxy resin, 10-40 parts of multifunctional epoxy resin, 8-15 parts of latent curing agent, 10-40 parts of core-shell rubber modified epoxy resin and 15-40 parts of modified filler. The multifunctional epoxy resin includes one or more of glycidylamine type epoxy resin, alicyclic multifunctional epoxy resin, and multifunctional phenolic epoxy resin. The modified filler includes R202 hydrophobic fumed silica, KH560 modified fumed silica, and barium sulfate.

2. The conformal adhesive for direct-write 3D printing according to claim 1, characterized in that, By weight, the conformal coating comprises: 70-80 parts of the biphenyl epoxy resin, 20-30 parts of the multifunctional epoxy resin, 10 parts of the latent curing agent, 15-30 parts of the core-shell rubber modified epoxy resin, and 26-33 parts of the modified filler.

3. The conformal adhesive for direct-write 3D printing according to claim 1 or 2, characterized in that, The biphenyl epoxy resin has a viscosity >4000 cP and an epoxy value of 0.35~0.88; and / or The multifunctional epoxy resin has a viscosity >500 cP and an epoxy value of 0.4~0.

9.

4. The conformal adhesive for direct-write 3D printing according to claim 1 or 2, characterized in that, The latent curing agent includes one or more of dicyandiamide, adipic acid dihydrazide, sebacate dihydrazide, and imidazole.

5. The conformal adhesive for direct-write 3D printing according to claim 1 or 2, characterized in that, The viscosity of the core-shell rubber-modified epoxy resin is 2000~40000 cP; and / or The core-shell rubber modified epoxy resin is formed by blending nano-rubber with epoxy resin; wherein the particle size of the nano-rubber is 10~1000nm, and the weight ratio of the nano-rubber to the epoxy resin is (20~60):

100.

6. The conformal adhesive for direct-write 3D printing according to claim 1 or 2, characterized in that, The weight ratio of the R202 hydrophobic fumed silica, the KH560 modified fumed silica and the barium sulfate is (8~15):8:(8~15).

7. The conformal adhesive for direct-write 3D printing according to claim 1 or 2, characterized in that, By weight, the conformal adhesive also includes 3 to 8 parts of curing accelerator and 1 to 3 parts of additives; The curing accelerator includes organic urea accelerators; and / or the auxiliary agent is silane coupling agent KH560.

8. The method for preparing the conformal adhesive for direct-write 3D printing according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step S1: Mix and stir the biphenyl epoxy resin, multifunctional epoxy resin, core-shell rubber modified epoxy resin and modified filler to obtain the first mixture. Step S2: Add a latent curing agent to the first mixture and stir to obtain a second mixture; Step S3: Grind the second mixture until the particle size is ≤10μm to obtain the third mixture; Step S4: The third mixture is subjected to vacuum degassing and pressure filtration in sequence to obtain the conformal adhesive for direct-write 3D printing.

9. The preparation method according to claim 8, characterized in that, The vacuum degree of the vacuum degassing is 40~100 mbar; and / or The pressure of the pressurized filter is 0.5~1.5 kg / cm². 2 The filter size is 300~400 mesh.

10. The preparation method according to claim 8, characterized in that, In step S2, after adding the latent curing agent to the first mixture, a curing accelerator and an additive are also added, and then the mixture is stirred.

11. The preparation method according to claim 10, characterized in that, The total weight ratio of the biphenyl epoxy resin and the multifunctional epoxy resin to the weight ratio of the curing accelerator is 100:(3~8); and / or The total weight ratio of the biphenyl epoxy resin and the multifunctional epoxy resin to the weight ratio of the additive is 100:(1~3).

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