Preparation method of conductive microspheres with strong binding force for ACF (anisotropic conductive film)

Abstract

The invention discloses a preparation method of high-binding-force conductive microspheres for ACF (anisotropic conductive film), and belongs to the technical field of conductive materials. The method comprises the following steps: introducing a bifunctional sulfydryl functional reagent with sulfydryl and double bonds in a polymerization reaction process, and firmly combining the sulfydryl to the surface of a polymer microsphere; carrying out surface activation treatment by utilizing the strong affinity of sulfydryl to palladium ions; and forming a metal coating on the surface of the microsphere through chemical plating. Sulfydryl is stabilized through double-bond copolymerization, palladium ion adsorption and coating binding force are enhanced through the sulfydryl, and the binding strength of the conductive microsphere and a connecting substrate, the adhesive force of a chemical coating and a microsphere matrix and the conductive performance and long-term reliability of ACF are remarkably improved. The conductive microsphere prepared by the invention has the advantages of simple process, controllable cost, excellent performance and the like, is suitable for high-density and high-reliability electronic interconnection, and can be applied to the fields of preparation of anisotropic conductive adhesive films and packaging of related electronic products.

Description

Technical Field

[0001] The invention belongs to the technical field of conductive materials, and in particular relates to a method for preparing conductive microspheres with strong binding force for ACF. Background Art

[0002] Anisotropic Conductive Film (ACF) is a key interconnect material widely used in liquid crystal display (LCD), flexible electronics, chip packaging and other fields. ACF achieves electrical connection in the Z direction by dispersing conductive microspheres in a polymer matrix while maintaining insulation in the X and Y directions, thereby meeting the needs of high-density, fine-pitch interconnection. As electronic products develop towards thinness, miniaturization and high performance, higher requirements are placed on the conductivity, connection strength and long-term reliability of ACF.

[0003] At present, the mainstream solution of ACF is to use composite polymer microspheres as conductive particles, whose surface is coated with metal coating. However, traditional conductive microspheres have problems such as insufficient bonding strength and easy detachment of the coating. Especially in high temperature and high humidity environments, the conductive microspheres are easily separated from the connection interface, resulting in increased contact resistance and even connection failure. In order to improve the bonding strength between the conductive microspheres and the connection substrate, the existing technology usually adopts surface roughening or chemical modification methods, but these methods often have disadvantages such as complex processes and unclear effects.

[0004] Therefore, developing an ACF interconnect material that can effectively improve the bonding strength of conductive microspheres and the adhesion of the coating, and has a simple process and controllable costs is a key technical problem that needs to be urgently solved in this field. Summary of the invention

[0005] The invention overcomes the shortcomings of the prior art and provides a method for preparing conductive microspheres with strong binding force for ACF.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing a strong-binding conductive microsphere for ACF, comprising the following steps:

[0007] S1, adding a polymerization monomer, an initiator and a dispersant to a dispersion medium, and initiating a polymerization reaction after mixing evenly, and adding a crosslinking agent and a thiol functionalization agent during the polymerization reaction to obtain a first conductive microsphere intermediate;

[0008] S2, dispersing the first conductive microsphere intermediate in a dilute hydrochloric acid solution of palladium chloride for surface activation treatment to obtain a second conductive microsphere intermediate;

[0009] S3, dispersing the second conductive microsphere intermediate in a chemical plating solution, and performing chemical metal plating treatment to obtain conductive microspheres.

[0010] In a preferred embodiment of the present invention, step S1 includes:

[0011] S11. Stir and mix a dispersant and a dispersion medium to form a system, and introduce nitrogen into the system;

[0012] S12. Add a polymerization monomer and an initiator to the system in step S11, and raise the temperature for polymerization for 0 - 2 h;

[0013] S13. Add a crosslinking agent and a mercapto-functionalized reagent to the system in step S12, and continue polymerization for 6 - 24 h;

[0014] S14. After filtration, washing and drying, a first conductive microsphere intermediate is obtained.

[0015] In a preferred embodiment of the present invention, the polymerization monomer in step S1 includes styrene.

[0016] In a preferred embodiment of the present invention, the initiator in step S1 includes one or more of benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azodiisovaleronitrile, potassium persulfate and ammonium persulfate.

[0017] In a preferred embodiment of the present invention, the mass percentage of the initiator in the polymerization monomer is 0.5% - 5%.

[0018] In a preferred embodiment of the present invention, the dispersant includes one or more of polyvinylpyrrolidone, sodium styrenesulfonate and sodium dodecyl sulfate.

[0019] In a preferred embodiment of the present invention, the mass percentage of the dispersant in the dispersion medium is 0.1% - 5%.

[0020] In a preferred embodiment of the present invention, the dispersion medium in step S1 includes one or more of methanol, ethanol and isopropanol.

[0021] In a preferred embodiment of the present invention, the dispersion medium in step S1 includes a mixture of one or more of methanol, ethanol and isopropanol and water.

[0022] In a preferred embodiment of the present invention, the crosslinking agent in step S1 includes divinylbenzene or ethylene glycol dimethacrylate.

[0023] In a preferred embodiment of the present invention, the mass percentage of the crosslinking agent in the polymerization monomer is 0.01% - 20%.

[0024] In a preferred embodiment of the present invention, the mercapto-functionalized reagent in step S1 includes at least one of 3-(4-mercaptophenyl)acrylic acid, 2-propene-1-thiol, 4,4'-dimercapto stilbene, 4-mercapto cinnamic acid, ethyl 4-mercapto cinnamate or crotyl mercaptan.

[0025] In a preferred embodiment of the present invention, the concentration of the palladium chloride dilute hydrochloric acid solution in step S2 is 3-7 g / L.

[0026] In a preferred embodiment of the present invention, the mass-volume ratio of the first conductive microsphere intermediate to the palladium chloride dilute hydrochloric acid solution is 1:1-50.

[0027] In a preferred embodiment of the present invention, the electroless plating solution in step S3 includes 0.05-0.15 M nickel sulfate, 0.18-0.30 M sodium dihydrogen phosphate, 0.10-0.20 M sodium acetate, and 0.03-0.07 M sodium citrate, and the pH value is 4.5-5.5.

[0028] Another technical solution provided by the present invention: the conductive microspheres obtained by the method for preparing strongly bonded conductive microspheres for ACF as described above.

[0029] Another technical solution provided by the present invention: the application of the above conductive microspheres in the preparation of anisotropic conductive adhesive films.

[0030] The present invention solves the defects existing in the background technology, and the present invention has the following beneficial effects:

[0031] (1) By introducing a mercapto-functionalized reagent with both mercapto and double bond bifunctional groups during the preparation process of the conductive microspheres, and combining surface palladium activation and electroless metal plating processes, the present invention effectively regulates the surface chemical properties of the conductive microspheres and significantly enhances the bonding strength of the metal coating, improves the bonding strength between the conductive microspheres and the object to be connected, enhances the adhesion of the chemical coating to the microsphere matrix, thereby improving the conductive performance and reliability of the ACF.

[0032] (2) By introducing mercapto functional groups on the surface of the microspheres, strong chemical bonds or coordination bonds can be formed with metal surfaces, oxide surfaces, and other polar surfaces, thereby improving the adhesion between the microspheres and these materials, directly enhancing the interfacial bonding strength between the ACF conductive microspheres and the connection substrate, reducing the contact resistance, and ensuring the stable and reliable electrical connection. Compared with the connection methods of traditional conductive microspheres that rely on physical embedding or weak interactions, the present invention can form stronger chemical bonds, reduce the risk of microsphere detachment, and reduce failures caused by poor contact, especially in high-density and fine-pitch connection application scenarios.

[0033] (3) The present invention utilizes the participation of double bonds in copolymerization reactions to fix sulfhydryl groups inside the microspheres in the form of covalent bonds, solving the problem of easy detachment of sulfhydryl groups and greatly increasing the stability and density of sulfhydryl groups on the surface of the microspheres. A large number of stably existing sulfhydryl groups can effectively adsorb palladium ions, forming high-quality catalytic active centers, improving the uniformity and bonding strength of the electroless metal plating layer, and reducing the risk of coating peeling. Compared with traditional conductive microspheres, the microspheres prepared by the present invention have a more solid metal coating, can withstand greater mechanical stress, and improve the compressive resistance and durability of the conductive microspheres in ACF applications.

[0034] (4) The present invention combines the strong adhesion characteristics of sulfhydryl groups with the copolymerization ability of double bonds, firmly embedding the sulfhydryl groups into the microsphere polymer matrix through double bonds, ensuring the stable and uniform distribution of sulfhydryl groups on the surface, and significantly enhancing the adsorption effect of palladium ions and the bonding strength of electroless metals. This combination enables the microspheres to form strong chemical bonds with the connecting substrate and also ensures a tight mechanical interlock between the coating and the microsphere matrix, thereby improving the overall performance of the ACF. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0036] Figure 1 is a flowchart of a preparation method of composite conductive microspheres for ACF;

[0037] Figure 2 is a flowchart of step S1 in a preparation method of composite conductive microspheres for ACF;

[0038] Figure 3 is an SEM image of the first conductive microsphere intermediate in Example 1;

[0039] Figure 4 is an SEM image of the conductive microspheres in Example 1;

[0040] Figure 5 is an SEM image of the first conductive microsphere intermediate in Example 2;

[0041] Figure 6 is an SEM image of the conductive microspheres in Example 2;

[0042] Figure 7 is an SEM image of the first conductive microsphere intermediate in Example 3;

[0043] Figure 8It is the SEM image of the conductive microspheres in Example 3;

[0044] Figure 9 It is the SEM image of the first conductive microsphere intermediate in Comparative Example 1;

[0045] Figure 10 It is the SEM image of the conductive microspheres in Comparative Example 1;

[0046] Figure 11 It is the SEM image of the first conductive microsphere intermediate in Comparative Example 2;

[0047] Figure 12 It is the SEM image of the conductive microspheres in Comparative Example 2. Detailed implementation manners

[0048] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0049] In the following description, many specific details are set forth to facilitate a thorough understanding of the present invention. However, the present invention may be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0050] In the description of the present application, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as limiting the protection scope of the present application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Therefore, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "plurality" is two or more.

[0051] In the description of the present application, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific situations.

[0052] Exemplary method:

[0053] As Figure 1 shown, a preparation method of strongly bonded conductive microspheres for ACF includes the following steps:

[0054] S1. Add a polymerization monomer, an initiator, and a dispersant into a dispersion medium, mix them evenly, and then initiate a polymerization reaction. During the polymerization reaction, add a crosslinking agent and a mercapto-functionalized reagent; after the reaction ends, filter, wash, and dry to obtain a first conductive microsphere intermediate.

[0055] S2. Disperse the first conductive microsphere intermediate in a dilute hydrochloric acid solution of palladium chloride for surface activation treatment; after the reaction ends, filter, wash, and dry to obtain a second conductive microsphere intermediate.

[0056] S3. Disperse the second conductive microsphere intermediate in an electroless plating solution for electroless plating of a metal layer; after the reaction ends, filter, wash, and dry to obtain conductive microspheres.

[0057] Next, each step will be described in detail.

[0058] As Figure 2 shown, step S1 specifically includes:

[0059] S11. Stir and mix the dispersant and the dispersion medium to form a system, and introduce nitrogen into the system.

[0060] S12. Add the polymerization monomer and the initiator into the system in step S11, and raise the temperature to 60 - 80 °C for polymerization for 0 - 2 h.

[0061] S13. Add the crosslinking agent and the mercapto-functionalized reagent into the system in step S12, and continue polymerization for 6 - 24 h.

[0062] S14. Filter, wash, and dry to obtain a first conductive microsphere intermediate.

[0063] In the polymerization reaction, the initiator decomposes to generate free radicals, which initiate the polymerization of monomers. Oxygen is a free radical scavenger (polymerization inhibitor) that can react with free radicals, consume free radicals, reduce the concentration of free radicals, and thus inhibit or terminate the polymerization reaction. This will lead to a decrease in the polymerization rate, an extension of the polymerization time, and even polymerization failure. In step S11, by introducing nitrogen, the oxygen in the system is removed to ensure the smooth progress of the polymerization reaction, increase the polymerization rate and the molecular weight of the polymer, and prevent the reactants from being oxidized.

[0064] In step S11, a dispersant and a dispersion medium are added to construct a uniform and stable reaction system, preventing the monomers from agglomerating in the early stage of polymerization and ensuring the homogeneity of the subsequent reaction.

[0065] In step S12, polymerization monomers and an initiator are added, and the temperature is raised to initiate the polymerization reaction, inducing the monomers to polymerize under preset conditions to form polymer segments with a specific molecular weight.

[0066] In step S13, a crosslinking agent and a mercapto-functionalized reagent are added to form a denser crosslinking network inside the microspheres, enhancing the overall strength and solvent resistance of the microspheres; it is beneficial to form a uniform distribution on the surface of the microspheres, improve the adsorption capacity for palladium ions, and enhance the bonding strength of the subsequent chemical plating layer.

[0067] In step S12, the temperature is raised for polymerization for 0 - 2 h. Through the short-term temperature-raising polymerization in the early stage, a certain number of polymer micro-nuclei are induced to form in the monomer molecules, realizing effective control of the microsphere nucleation process. Specifically, by restricting the nucleation time, the over-diffusion of the nucleation process is avoided, so as to obtain a microsphere population with a more concentrated particle size distribution; at the same time, the pre-formed micro-nuclei can serve as the growth centers for the subsequent polymerization reaction, reducing the number of newly formed micro-nuclei during the later reaction process, decreasing the dispersion degree of the particle size distribution, and improving the overall uniformity of the microspheres.

[0068] In step S1, the polymerization monomers include styrene. Styrene has good polymerization activity and is easy to carry out free radical polymerization reaction to form high molecular weight polymers. Its polymerization process has good controllability, which is beneficial to obtaining microspheres with specific particle sizes and morphologies.

[0069] In step S1, the initiator includes one or more of benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisovaleronitrile, potassium persulfate, and ammonium persulfate.

[0070] Furthermore, the mass percentage of the initiator in the polymerization monomers is 0.5% - 5%.

[0071] Benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisovaleronitrile, potassium persulfate, and ammonium persulfate are all free radical initiators that generate free radicals through thermal decomposition or redox decomposition. The above initiators are suitable for the suspension polymerization of styrene, with a decomposition temperature in the range of 60 - 80 °C, which is suitable for the polymerization reaction of styrene. Among them, potassium persulfate and ammonium persulfate are water-soluble initiators, suitable for emulsion polymerization; benzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, and azobisisovaleronitrile are oil-soluble initiators, suitable for suspension polymerization.

[0072] In step S1, the dispersant includes one or more of polyvinylpyrrolidone, sodium styrenesulfonate, and sodium dodecyl sulfate.

[0073] Furthermore, the mass percentage of the dispersant in the dispersion medium is 0.1% - 5%.

[0074] Polyvinylpyrrolidone, sodium styrenesulfonate, and sodium dodecyl sulfate are all surfactants, each having a hydrophilic group and a hydrophobic group. They can reduce the surface tension in the dispersion medium, stabilize the dispersion system, and can adsorb on the surface of the microspheres to form a protective layer to prevent the microspheres from aggregating. Specifically, polyvinylpyrrolidone (PVP) is a polymeric dispersant that can stabilize the microsphere dispersion system through steric hindrance effects; sodium styrenesulfonate (SSS) and sodium dodecyl sulfate (SDS) are ionic surfactants that can stabilize the microsphere dispersion system through electrostatic repulsion effects.

[0075] In step S1, the dispersion medium includes one or more of methanol, ethanol, and isopropanol.

[0076] Furthermore, in step S1, the dispersion medium includes a mixture of one or more of methanol, ethanol, and isopropanol and water.

[0077] The above dispersion medium can form a good dispersion system with the selected dispersant to stabilize the dispersion state of the microspheres. Alcohols and water have different polarities, and by adjusting the ratio of alcohols to water, the polarity of the reaction system can be controlled, affecting the particle size and morphology of the microspheres.

[0078] In step S1, the crosslinking agent includes divinylbenzene or ethylene glycol dimethacrylate.

[0079] Furthermore, the mass percentage of the crosslinking agent in the polymerization monomer is 0.01% - 20%.

[0080] Divinylbenzene and ethylene glycol dimethacrylate are both polyfunctional monomers, having two or more polymerizable functional groups (such as vinyl or acryloyl), and can copolymerize with monomer molecules during the polymerization process to form a crosslinked structure. Specifically, divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDMA) can copolymerize with styrene to form a crosslinked polymer. These crosslinking agents have a relatively high crosslinking efficiency and a reaction activity similar to that of styrene, and can effectively improve the strength of the polymer at a relatively low dosage.

[0081] In step S1, the mercapto-functionalizing reagent includes at least one of 3-(4-mercaptophenyl)acrylic acid, 2-propen-1-thiol, 4,4'-dimercapto stilbene, 4-mercaptophenyl cinnamic acid, ethyl 4-mercaptophenyl cinnamate or crotyl mercaptan.

[0082] The above-mentioned 3-(4-mercaptophenyl)acrylic acid, 2-propen-1-thiol, 4,4'-dimercapto stilbene, 4-mercaptophenyl cinnamic acid, ethyl 4-mercaptophenyl cinnamate and crotyl mercaptan all have a mercapto (-SH) and a double bond (C=C) at the same time.

[0083] The mercapto group is a reactive group with a strong ability to bind to metals, especially a strong adsorption effect on noble metals such as palladium (Pd), making the microspheres more easily activated by palladium ions. The double bond is a polymerizable group. During the polymerization reaction, these molecules can participate in the construction of the microspheres through the double bond, effectively connecting the mercapto group to the polymer skeleton of the microspheres, covalently bonding inside the microspheres, and enhancing the stability of the mercapto groups on the microsphere surface.

[0084] In step S2, the concentration of the palladium chloride dilute hydrochloric acid solution is 3-7 g / L.

[0085] Furthermore, the mass-volume ratio of the first conductive microsphere intermediate to the palladium chloride dilute hydrochloric acid solution is 1:1-50.

[0086] By using the palladium chloride dilute hydrochloric acid solution, the surface of the microspheres is activated for subsequent electroless plating of metals. Specifically, palladium ions can coordinate with the mercapto groups on the microsphere surface and adsorb on the microsphere surface. The palladium ions adsorbed on the microsphere surface have catalytic activity and can catalyze the progress of the electroless plating reaction, enabling metal ions to be reduced and deposited on the microsphere surface to form a metal coating. Hydrochloric acid can dissolve palladium chloride and maintain the stable state of palladium ions.

[0087] The electroless plating solution in step S3 includes 0.05-0.15 M nickel sulfate, 0.18-0.30 M sodium dihydrogen phosphate, 0.10-0.20 M sodium acetate and 0.03-0.07 M sodium citrate, and the pH value is 4.5-5.5.

[0088] Exemplary conductive microspheres:

[0089] A strongly binding conductive microsphere for ACF, prepared by the above exemplary method.

[0090] Exemplary application:

[0091] Application of a strongly binding conductive microsphere for ACF in preparing an anisotropic conductive adhesive film.

[0092] Example 1

[0093] A preparation method of a strongly binding conductive microsphere for ACF, comprising the following steps:

[0094] S1: Mix 14.4 g of polyvinylpyrrolidone (PVP, K30), 456 ml of absolute ethanol and 24 ml of pure water under the condition of 25 °C with stirring at 300 rpm to form a system, and introduce nitrogen into the system; add 72 g of styrene (st) and 1.47 g of azobisisobutyronitrile (AIBN) into the system, heat up to 70 °C, and react for 2 h; add 1.5 g of divinylbenzene (DVB) and 3.6 g of 3-(4-mercaptophenyl) acrylic acid into the system, and continue the polymerization reaction for 16 h; after the reaction is completed, cool to room temperature, and wash 6 times alternately with ethanol and water to obtain a 3.1-μm first conductive microsphere intermediate with a mercapto functional group, as Figure 3 shown.

[0095] S2: Ultrasonically disperse 1 g of the first conductive microsphere intermediate obtained in step S1 in 40 mL of pure water, add 15 mL of a dilute hydrochloric acid solution of palladium chloride with a concentration of 5 g / L, ultrasonically mix for 0.5 h, wash 3 times with pure water, and dry to obtain a second conductive microsphere intermediate.

[0096] S3: Disperse 1 g of the second conductive microsphere intermediate in 50 mL of electroless plating solution, stir at 40 °C for 1 h, wash after cooling to room temperature, and dry in vacuum to obtain a conductive microsphere with a particle size of 3.3 μm, as Figure 4 shown. Among them, the components of the electroless plating solution include 0.1 M of nickel sulfate, 0.24 M of sodium dihydrogen phosphate, 0.15 M of sodium acetate, 0.05 M of sodium citrate, and the pH of the solution is 5.

[0097] Example 2

[0098] A preparation method of a strongly binding conductive microsphere for ACF, comprising the following steps:

[0099] S1: Mix 14.4 g of polyvinylpyrrolidone (PVP, K30) and 480 ml of absolute ethanol under the condition of 25 °C with stirring at 300 rpm to form a system, and introduce nitrogen into the system; add 72 g of styrene (st) and 1.47 g of azobisisobutyronitrile (AIBN) into the system, heat up to 70 °C, and react for 2 h; add 1.5 g of divinylbenzene (DVB) and 3.6 g of 2 - propene - 1 - thiol into the system, and continue the polymerization reaction for 12 h; after the reaction is completed, cool to room temperature, and wash alternately with ethanol and water for 6 times to obtain a 2.9 - μm first conductive microsphere intermediate with thiol functional groups, as Figure 5 shown.

[0100] S2: Ultrasonically disperse 1 g of the first conductive microsphere intermediate obtained in step S1 in 40 mL of pure water, add 15 mL of a dilute hydrochloric acid solution of palladium chloride with a concentration of 5 g / L, ultrasonically mix for 0.5 h, wash 3 times with pure water, and dry to obtain a second conductive microsphere intermediate.

[0101] S3: Disperse 1 g of the second conductive microsphere intermediate in 50 mL of electroless plating solution, stir at 40 °C for 1 h, wash after cooling to room temperature, and dry in vacuum to obtain conductive microspheres with a particle size of 3.3 μm, as Figure 6 shown. Among them, the components of the electroless plating solution include 0.1 M of nickel sulfate, 0.24 M of sodium dihydrogen phosphate, 0.15 M of sodium acetate, 0.05 M of sodium citrate, and the pH of the solution is 5.

[0102] Example 3

[0103] A preparation method of strongly - binding conductive microspheres for ACF, comprising the following steps:

[0104] S1: Mix 14.4 g of polyvinylpyrrolidone (PVP, K30), 456 ml of absolute ethanol and 24 ml of pure water under the condition of 25 °C with stirring at 300 rpm to form a system, and introduce nitrogen into the system; add 57.6 g of styrene (st) and 1.18 g of azobisisobutyronitrile (AIBN) into the system, heat up to 70 °C, and react for 2 h; add 1.2 g of divinylbenzene (DVB) and 2.88 g of 4,4′ - dimercapto stilbene into the system, and continue the polymerization reaction for 8 h; after the reaction is completed, cool to room temperature, and wash alternately with ethanol and water for 6 times to obtain a 2.6 - μm first conductive microsphere intermediate with thiol functional groups, as Figure 7 shown.

[0105] S2: Ultrasonically disperse 1 g of the first conductive microsphere intermediate obtained in step S1 in 40 mL of pure water, add 15 mL of a dilute hydrochloric acid solution of palladium chloride with a concentration of 5 g / L, ultrasonically mix for 0.5 h, wash 3 times with pure water, and dry to obtain a second conductive microsphere intermediate.

[0106] S3: Disperse 1 g of the second conductive microsphere intermediate in 50 mL of electroless plating solution, stir at 40 °C for 1 h, wash after cooling to room temperature, and dry in vacuum to obtain conductive microspheres with a particle size of 3.3 μm, as Figure 8 shown. Among them, the components of the electroless plating solution include 0.1 M nickel sulfate, 0.24 M sodium dihydrogen phosphate, 0.15 M sodium acetate, and 0.05 M sodium citrate, and the pH of the solution is 5.

[0107] Comparative Example 1

[0108] A preparation method of conventional conductive microspheres includes the following steps:

[0109] (1) Mix 14.4 g of polyvinylpyrrolidone (PVP, K30) and 480 ml of absolute ethanol, stir and mix at 25 °C at 300 rpm to form a system, and introduce nitrogen into the system; add 72 g of styrene (st), 1.5 g of divinylbenzene (DVB), and 1.47 g of azobisisobutyronitrile (AIBN) into the system, heat up to 70 °C, and carry out a polymerization reaction for 16 h; after the reaction is completed, cool to room temperature, centrifuge and wash alternately with ethanol and water 6 times to obtain a first conductive microsphere intermediate with a particle size of 3 μm, as Figure 9 shown.

[0110] (2) Take 1 g of the polymer microspheres obtained in step 1, add 10 mL of concentrated sulfuric acid, stir for 60 min, and wash 5 times with pure water;

[0111] Ultrasonically disperse 1 g of the roughened microspheres in 20 ml of pure water, add 20 mL of a mixed solution of stannous chloride and hydrochloric acid with a concentration of 10 g / L, stir and react for 30 min, and wash 5 times with pure water;

[0112] Ultrasonically disperse 1 g of the sensitized microspheres in 40 mL of pure water, add them to 15 mL of a dilute hydrochloric acid solution of palladium chloride with a concentration of 5 g / L, ultrasonically mix for 0.5 h, wash 3 times with pure water, and dry for standby to obtain activated polymer microspheres;

[0113] Disperse 1 g of the activated polymer microspheres in 50 mL of electroless plating solution, stir at 40 °C for 1 h, wash, and dry to obtain conductive microspheres with a nickel-plated surface layer and a particle size of 3.2 μm, as Figure 10 shown.

[0114] Comparative Example 2

[0115] This example provides a preparation method of composite conductive microspheres for ACF, including the following steps:

[0116] (1) Mix 14.4 g of polyvinylpyrrolidone (PVP, K30) and 480 ml of absolute ethanol evenly under stirring at 300 rpm at 25 °C to form a system, and introduce nitrogen into the system; add 57.6 g of styrene (st) and 1.06 g of azobisisobutyronitrile (AIBN) into the system, heat up to 70 °C, and carry out a polymerization reaction for 2 h. Then add 1.2 g of divinylbenzene (DVB) and 4.8 g of polyethyleneimine (PEI1800) into the system, and continue the polymerization reaction for 16 h; after the reaction is completed, cool to room temperature, and wash alternately with ethanol and water by centrifugation 6 times to obtain a first conductive microsphere intermediate with a particle size of 2.2 μm and a surface amino density of 26 mmol / g, as Figure 11 shown.

[0117] (2) Ultrasonically disperse 1 g of the amino-functionalized polymer microspheres obtained in step (1) in 40 mL of pure water, then add it to 10 mL of a 5 g / L palladium chloride dilute hydrochloric acid solution, ultrasonically mix for 0.5 h, wash with pure water 3 times, and dry for standby to obtain a second conductive microsphere intermediate.

[0118] (3) Disperse 1 g of the activated polymer microspheres in 50 mL of electroless plating solution, stir at 40 °C for 1 h, wash, and dry to obtain conductive microspheres with a nickel-plated surface layer, with a particle size of 2.5 μm, as Figure 12 shown. Among them, the components of the electroless plating solution include 0.1 M nickel sulfate, 0.24 M sodium dihydrogen phosphate, 0.15 M sodium acetate, 0.05 M sodium citrate, and the pH of the solution is 5.

[0119] Experimental Example 1

[0120] (1) Preparation of ACF:

[0121] Dissolve 50 g of epoxy resin (JER157S70) in 50 g of methyl ethyl ketone solvent, add 2.5 g of curing agent (2E4MZ) and 1 g of fumed silica (AEROSIL200) dispersant, and stir at 500 rpm for 30 minutes to ensure uniform mixing.

[0122] Add 0.75 g of conductive microspheres (Examples 1-3, Comparative Examples 1-2) to the above resin mixture respectively, and use an ultrasonic cleaner to ultrasonically disperse for 30 minutes to ensure uniform dispersion of the conductive microspheres. The ultrasonic power is 100 W and the frequency is 40 kHz.

[0123] Use the doctor blade coating method to coat the evenly dispersed mixture on the release film, and control the coating thickness by adjusting the height of the doctor blade to make the thickness after curing 25 μm.

[0124] Put the coated release film into a vacuum drying oven, dry at 80 °C for 2 hours to remove the solvent, and obtain an ACF film. The vacuum degree is maintained at 0.05 MPa.

[0125] (2) Preparation of ACF connection (refer to JIS C3286-1998):

[0126] Ultrasonically clean the ITO glass substrate and FPC with acetone, ethanol, and deionized water for 10 minutes respectively, and then dry them with nitrogen. The ultrasonic power is 100W and the frequency is 40kHz; use an ACF applicator to attach ACF films containing different conductive microspheres to the ITO glass substrate respectively, with an attachment pressure of 0.5MPa and an attachment time of 5 seconds; perform thermocompression bonding, and finally determine the thermocompression conditions as a pressure of 3MPa, a temperature of 180°C, and a time of 30 seconds.

[0127] (3) Test of the bonding force of conductive microspheres (refer to JIS C3286-1998):

[0128] Use a micro push-pull force gauge to perform a pull-out test on the conductive microspheres in the vertical direction, measure the maximum pull force value (μN) when the microspheres fall off from the ITO layer, and the test speed is 1μm / s; randomly select 30 conductive microspheres from each sample for testing, record the pull force value of each conductive microsphere, and calculate the average value and standard deviation.

[0129] (4) Test of contact resistance (refer to JIS C3286-1998):

[0130] Use a four-probe tester to accurately measure the contact resistance (mΩ) between the ITO glass and FPC after thermocompression bonding; select 10 different positions on the FPC circuit as test points to ensure that the test points are evenly distributed in the connection area; perform 3 repeated measurements at each test point, and calculate the average value, and record the value of the contact resistance.

[0131] (5) Reliability test:

[0132] Place the samples after thermocompression bonding in a thermo-hygrostat chamber, set the temperature to 85°C and the humidity to 85%RH; at 24h, 48h, 96h, 168h, and 500h, take out the samples, measure the contact resistance and the bonding force of the conductive microspheres respectively, and record the data.

[0133] The experimental results are shown in the following table:

[0134] Table 1 Bonding force of conductive microspheres

[0135]

[0136] Table 2 Contact resistance

[0137]

[0138] As can be seen from the data in Table 1 and Table 2, compared with Comparative Example 1 and Comparative Example 2, the ACF of the conductive microspheres prepared using Examples 1-3 exhibited higher initial tensile strength values, lower initial contact resistance, and smaller rates of decrease in tensile strength and increase in resistance. Specifically, the performance indicators of Comparative Example 1 were the worst, and the performance of Comparative Example 2 was better than that of Comparative Example 1 but still significantly lower than that of Examples 1-3. Example 1 of the present invention showed the best performance, having the highest initial tensile strength value and the lowest initial contact resistance, and the smallest performance degradation after 500 hours of high-temperature and high-humidity aging test. This is because Examples 1-3 used a bifunctional mercapto-functionalized reagent with a mercapto group (-SH) and a double bond (C=C).

[0139] Specifically, the mercapto group directly enhances the binding force between the microspheres and the substrate interface through strong chemical bonds and reduces the contact resistance; the double bond covalently anchors the mercapto group in the microsphere skeleton through copolymerization reaction to prevent it from falling off, thereby maintaining stable performance during long-term aging. In contrast, Comparative Example 1 was not functionalized and relied only on physical adsorption or weak van der Waals forces, which was prone to interface separation due to environmental stress; Comparative Example 2 used amino-functionalization, although there was a certain binding force, the metal affinity of the amino group was weaker than that of the mercapto group, and the amino group was prone to oxidation or hydrolysis, resulting in long-term performance decline.

[0140] Furthermore, the copolymerization effect of the double bond enables the mercapto groups to be uniformly distributed on the surface of the microspheres and stably exist, forming a high density of active sites. These mercapto groups efficiently adsorb palladium ions during the surface activation stage, forming a uniform catalytic active center, inducing dense deposition of the chemical plating layer, and achieving strong mechanical interlocking between the plating layer and the microsphere matrix. In addition, the covalently fixed mercapto groups are not easily migrated or degraded in a humid and hot environment, maintaining the stability of palladium ion adsorption and plating layer binding. In contrast, the surface of the microspheres in Comparative Example 1 lacked active groups, the palladium ions were unevenly adsorbed, and the plating layer was easily peeled off; although the amino group in Comparative Example 2 could adsorb palladium ions, its binding strength was low and it was easily affected by the environment, resulting in insufficient binding force of the plating layer and ultimately poor performance.

[0141] Generally speaking, as a polymerizable group, the double bond undergoes a copolymerization reaction with styrene monomers during the free radical polymerization process, covalently embedding the mercapto-functionalized reagent into the skeleton of the polymer microspheres. This copolymerization forms a stable cross-linked network, enabling the mercapto groups to be uniformly distributed on the surface and inside of the microspheres and preventing the mercapto groups from falling off during subsequent activation, plating, or humid and hot environments. The mercapto groups can form stable coordination bonds or chemical bonds with palladium ions, enabling the surface of the microspheres to adsorb a high density of palladium ions. This adsorption not only forms a uniform catalytic active center but also induces dense deposition of the metal layer through the subsequent electroless plating process, achieving strong mechanical interlocking between the plating layer and the microsphere matrix.

[0142] In summary, by introducing a bifunctional mercapto-functionalizing reagent with both mercapto and double bond groups, the present invention realizes effective regulation of the surface properties of conductive microspheres, fully exerts the synergistic effect of mercapto and double bond groups, and thus significantly improves the binding force, conductivity and reliability of ACF.

[0143] Based on the ideal embodiments of the present invention as the inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and must be determined according to the scope of the claims.

Claims

1. A method for preparing highly binding conductive microspheres for ACF, characterized in that: The following steps are involved: S1, adding polymerization monomer, initiator and dispersant to the dispersion medium, mixing them evenly and initiating polymerization reaction; During the polymerization reaction, a crosslinking agent and a thiol functionalization agent are added to obtain a first conductive microsphere intermediate; S2, dispersing the first conductive microsphere intermediate in a dilute hydrochloric acid solution of palladium chloride for surface activation treatment to obtain a second conductive microsphere intermediate; S3, dispersing the second conductive microsphere intermediate in a chemical plating solution, and performing chemical metal plating treatment to obtain conductive microspheres.

2. The preparation method according to claim 1, characterized in that: Step S1 includes: S11, stirring and mixing the dispersant and the dispersion medium to form a system, and passing nitrogen gas into the system; S12, adding polymerization monomers and initiators to the system in step S11, heating and polymerizing for 0-2h; S13, adding a crosslinking agent and a thiol functionalization reagent to the system in step S12, and continuing polymerization for 6-24 hours; S14, filtering, washing and drying to obtain a first conductive microsphere intermediate.

3. The preparation method according to claim 1, characterized in that: The polymerizable monomer in step S1 includes styrene.

4. The preparation method according to claim 1, characterized in that: In step S1, the initiator includes one or more of dibenzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisovaleronitrile, potassium persulfate and ammonium persulfate; And / or, the mass percentage of the initiator to the polymerization monomer is 0.5%-5%.

5. The preparation method according to claim 1, characterized in that: The dispersant includes one or more of polyvinyl pyrrolidone, sodium styrene sulfonate and sodium lauryl sulfate; And / or, the mass percentage of the dispersant in the dispersion medium is 0.1%-5%.

6. The preparation method according to claim 1, characterized in that: In the step S1, the dispersion medium includes at least one of methanol, ethanol, and isopropanol, or a mixture of at least one of methanol, ethanol, and isopropanol and water.

7. The preparation method according to claim 1, characterized in that: The cross-linking agent in step S1 includes one or more of divinylbenzene and ethylene glycol dimethacrylate; And / or, the cross-linking agent accounts for 0.01%-20% by mass of the polymerized monomers.

8. The preparation method according to claim 1, characterized in that: In the step S1, the thiol functionalization reagent includes one or more of 3-(4-mercaptophenyl) acrylic acid, 2-propylene-1-thiol, 4,4'-dimercaptostilbene, 4-mercaptocinnamic acid, ethyl 4-mercaptocinnamate and crotonylmercaptan.

9. The preparation method according to claim 1, characterized in that: The concentration of the dilute hydrochloric acid solution of palladium chloride in step S2 is 3-7 g / L; And / or, the mass volume ratio of the first conductive microsphere intermediate to the dilute hydrochloric acid solution of palladium chloride is 1:1-50.

10. The preparation method according to claim 1, characterized in that: The chemical plating solution in step S3 includes 0.05-0.15M nickel sulfate, 0.18-0.30M sodium dihydrogen phosphate, 0.10-0.20M sodium acetate and 0.03-0.07M sodium citrate, and the pH value is 4.5-5.5.

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