Preparation method of composite conductive microspheres for ACF (anisotropic conductive film)

Abstract

The invention discloses a preparation method of composite conductive microspheres for ACF, and belongs to the technical field of conductive materials. Comprising the following steps: (1) preparing functionalized polymer microspheres with rough surfaces; (2) activating the microspheres obtained in the step (1) in a palladium chloride diluted hydrochloric acid solution to obtain activated microspheres; and (3) dispersing the activated microspheres in the step (2) in a chemical nickel plating solution to obtain the composite conductive microspheres. The surface of the microsphere is provided with density-adjustable amino functional groups, the microsphere and palladium ions in an activation reagent can form a stable complex, the activation degree is higher, the surface of the microsphere is rough, the microsphere and a plating layer have stronger binding force relative to a smooth surface, the obtained nickel plating layer is firmer and is not prone to falling off in the using process, and the preparation method is simple and easy to implement. The environment is protected.

Description

Technical Field

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

[0002] In the process of modern industrial development, information technology and related industries have developed rapidly, among which the electronics industry has developed into the world's largest industry. The packaging materials in the electronics industry have gradually changed from the original tin-lead solder to green, thin and light anisotropic conductive film (ACF). Due to its low curing temperature, good conductivity, high flexibility, and ultra-fine pitch connection, it is widely used in liquid crystal display modules, flexible circuit boards, chip packaging, high-precision wiring, wearable devices and other fields.

[0003] The conductive principle of ACF is to use conductive microspheres to connect the electrodes between the IC chip and the substrate to achieve conduction, and to avoid short circuits between two adjacent electrodes, thereby achieving conduction in the vertical direction and non-conduction in the horizontal direction. Among them, resin adhesives and conductive microspheres are the two core technologies of ACF, which play a decisive role in the performance of the product. The development of conductive microspheres has gone through three generations. The first generation is conductive metal and its oxide microspheres, such as nano silver powder, nano copper powder, etc., but the metal microspheres have a large density and are easy to settle in the resin adhesive, uneven distribution, affecting the conductivity; the second generation is an incompressible composite conductive microsphere with a hard core such as silicon dioxide, Sn, etc., and an outer layer of silver or copper plating; the third generation is a composite conductive microsphere with polymer particles as the core and a highly conductive nickel or gold shell layer on the outer layer. Because of its elasticity, the sphere is not easy to break during compression, and has good conductivity, it has become the mainstream product now. Generally, the polymer core is made of polystyrene, polymethyl methacrylate and other materials, and the outer coating is generally nickel, silver, copper, gold, etc. Conventionally, a polymer core with a smooth surface is prepared by dispersion polymerization or seed swelling method, and then a metal conductive layer is plated on its surface through steps such as roughening, sensitization, activation, and chemical plating.

[0004] In Chinese patent CN102321879A, polystyrene microspheres are first prepared, and then after coarsening and surface modification with concentrated sulfuric acid, chemical nickel plating is performed after activation with palladium colloid to obtain PS / Ni composite conductive microspheres. The process is complicated, and highly corrosive concentrated sulfuric acid is required to roughen the surface of the microspheres, which has high safety risks and a heavy burden on the environment. According to the article Synthesis of Monodisperse Nickel-Coated Polymer Particles by Electroless Plating Method Utilizing Functional Polymeric Ligands, the surface of the microspheres is functionally modified, and the steps of coarsening and sensitization can be omitted, and chemical nickel plating can be performed directly after activation. Based on this idea, in patent WO2022 / 002278, the polymer starting microspheres are treated with tris(2-aminoethyl)amine at 105°C for 5 hours, the amino groups are modified on the surface of the microspheres, and then the metal-plated layer is directly activated to obtain conductive microspheres. The step of using concentrated sulfuric acid for roughening is omitted, but the process still requires long-term high-temperature treatment to functionalize the surface, the operation is complicated and the cost of scale-up equipment is high. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a method for preparing composite conductive microspheres for ACF. The surface of the microspheres has amino functional groups with adjustable density, which can form stable complexes with palladium ions in an activation reagent, and have a higher degree of activation. The surface of the microspheres is rough, and has a stronger bonding force with the coating than a smooth surface. The obtained nickel coating is more solid and not easy to fall off during use. The preparation method omits the process of roughening and sensitizing with strong acid, and the method is simple and green and environmentally friendly.

[0006] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme: a method for preparing composite conductive microspheres for ACF, comprising the following steps:

[0007] Step (1): Add a dispersant and a dispersion medium into a reaction container, stir evenly at a temperature of 20-30°C, add a styrene monomer and an initiator in a nitrogen atmosphere, heat to 65-80°C, and polymerize for 1-3 hours, then add a crosslinker and a functionalizing agent, react for 6-24 hours, cool to room temperature, and centrifuge and wash to obtain rough and functionalized polymer microspheres.

[0008] Step (2): dispersing the polymer microspheres obtained in step (1) in pure water, and then adding palladium chloride dilute hydrochloric acid solution to activate them to obtain activated microspheres;

[0009] Step (3): dispersing the activated microspheres in step (2) in a chemical nickel plating solution to obtain composite conductive microspheres.

[0010] Furthermore, in the step (1), the stirring speed is 100 to 500 rpm.

[0011] Furthermore, the dispersion medium in step (1) is one or a mixture of two or more of methanol, ethanol, and isopropanol.

[0012] Preferably, the dispersion medium is anhydrous ethanol.

[0013] Preferably, the dispersion medium is a mixture of anhydrous ethanol and water.

[0014] Furthermore, the dispersant in step (1) is sodium styrene sulfonate, polyvinyl pyrrolidone, sodium dodecylbenzene sulfonate or sodium dodecyl sulfate; preferably, the dispersant is polyvinyl pyrrolidone.

[0015] Furthermore, the mass percentage of the dispersant to the dispersion medium is 0.1%-5%.

[0016] Furthermore, in the step (1), the mass ratio of the dispersant to the styrene monomer is 1:(2-20).

[0017] Furthermore, the initiator in step (1) is dibenzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisovaleronitrile, potassium persulfate or ammonium persulfate; preferably, the initiator is azobisisobutyronitrile.

[0018] Furthermore, the mass percentage of the initiator to the styrene monomer is 1%-3%.

[0019] Furthermore, the crosslinking agent in step (1) is divinylbenzene or ethylene glycol dimethacrylate; preferably, the crosslinking agent is divinylbenzene.

[0020] Furthermore, the mass percentage of the cross-linking agent to the styrene monomer is 0.01%-20%.

[0021] Furthermore, the functionalizing agent in step (1) is polyethyleneimine; the mass percentage of the functionalizing agent in the dispersion medium is 0.1%-10%.

[0022] Furthermore, the concentration of the dilute hydrochloric acid solution of palladium chloride in step (2) is 3-7 g / L.

[0023] Furthermore, the mass volume ratio of the polymer microspheres to the dilute hydrochloric acid solution of palladium chloride is 1:(1-20) (g / ml).

[0024] Furthermore, the volume ratio of the pure water to the dilute hydrochloric acid solution of palladium chloride is (1-20):1.

[0025] Furthermore, the composition of the chemical nickel plating solution in step (3) is: 0.1M nickel sulfate, 0.24M sodium dihydrogen phosphate, 0.15M sodium acetate and 0.05M sodium citrate, and the pH value is 5.

[0026] Furthermore, in step (3), the mass volume ratio of the activated microspheres to the chemical nickel plating solution is 1:(5-99) (g / ml).

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The synthesized microspheres of the present invention can save the process of roughening and sensitizing with strong acid in the traditional preparation method of conductive microspheres, and the method is simple and green and environmentally friendly; the surface of the microspheres has amino functional groups with adjustable density, which can form a stable complex with the palladium ions in the activation reagent palladium chloride. Compared with microspheres without functional groups, the activation degree is higher, so the coating formed in the process of chemical nickel plating is more compact and not easy to fall off, and the activation degree of the microspheres can be controlled by controlling the amino density on the surface of the microspheres, thereby controlling the thickness of the coating.

[0029] 2. The present invention adds a crosslinking agent and a functionalizing agent, and because of the phase separation effect, polymer microspheres with rough surfaces are obtained. The rough surface has a stronger binding force than a smooth surface in the subsequent activation and chemical plating stages, and the obtained coating is more solid. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0031] Figure 1 This is the SEM image of the polystyrene microspheres with 2% cross-linked surface amino groups in Example 1.

[0032] Figure 2 This is the SEM image of the composite conductive microspheres with nickel plated on the surface in Example 1.

[0033] Figure 3 This is the SEM image of the polystyrene microspheres with 3% cross-linked surface amino groups in Example 2.

[0034] Figure 4 This is the SEM image of the polystyrene microspheres with 5% cross-linked surface amino groups in Example 3.

[0035] Figure 5 This is the SEM image of the polystyrene microspheres with smooth surface in Comparative Example 1.

[0036] Figure 6 This is the SEM image of the composite conductive microspheres with nickel plating on the surface in Comparative Example 1. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0038] Example 1

[0039] The embodiment of the present invention provides a method for preparing composite conductive microspheres for ACF, comprising the following steps:

[0040] (1) Synthesis of amino polymer microspheres with rough surface: 14.4 g polyvinyl pyrrolidone (PVP, K30) and 480 ml anhydrous ethanol were added to a 1000 mL three-necked flask, and the mixture was stirred at 25°C and 300 rpm to mix evenly, and nitrogen was passed through the system to remove oxygen. Then 57.6 g styrene (ST) and 1.06 g azobisisobutyronitrile (AIBN) were added to the system, and the temperature was raised to 70°C. The polymerization reaction was carried out for 2 h. 1.2 g divinylbenzene (DVB) and 4.8 g polyethyleneimine (PEI 1800) were added to the system, and the polymerization reaction was continued for 16 h. After the reaction was completed, the system was cooled to room temperature, centrifuged, and washed alternately with ethanol and water for 6 times to obtain 2% cross-linked PS-DVB microspheres with a particle size of 2.2 μm and a surface amino group density of 26 mmol / g. Figure 1 shown.

[0041] (2) Activating microspheres: 1 g of the amino polymer microspheres obtained in step 1 was ultrasonically dispersed in 40 mL of pure water, and then added into 10 mL of a 5 g / L palladium chloride dilute hydrochloric acid solution, ultrasonically mixed for 0.5 h, washed with pure water three times, and dried for later use to obtain activated polymer microspheres.

[0042] (3) Composite conductive microspheres: 1 g of activated polymer microspheres were dispersed in 50 mL of chemical plating solution, stirred at 40 °C for 1 h, washed, and dried to obtain composite conductive microspheres with a surface layer of nickel plating and a particle size of 2.5 μm. Figure 2 The chemical plating solution comprises: main salt: nickel sulfate 0.1M, reducing agent: sodium dihydrogen phosphate 0.24M, buffer: sodium acetate 0.15M, complexing agent: sodium citrate 0.05M, pH=5.

[0043] Example 2

[0044] The embodiment of the present invention provides a method for preparing composite conductive microspheres for ACF, comprising the following steps:

[0045] (1) Synthesis of amino polymer microspheres with moderately rough surface: 14.4 g polyvinyl pyrrolidone (PVP, K30) and 480 ml anhydrous ethanol were added to a 1000 mL three-necked flask, and the mixture was stirred at 25°C and 300 rpm to mix evenly, and nitrogen was passed through the system to remove oxygen. Then 40 g styrene (ST) and 0.75 g azobisisobutyronitrile (AIBN) were added to the system, and the temperature was raised to 75°C. The polymerization reaction was carried out for 2 h. 1.3 g divinylbenzene (DVB) and 7.2 g polyethyleneimine (PEI 1800) were added to the system, and the polymerization was continued for 12 h. After the reaction was completed, the system was cooled to room temperature, centrifuged, and washed alternately with ethanol and water for 6 times to obtain 3% cross-linked PS-DVB microspheres with a particle size of 1.5 μm and a surface amino group density of 35 mmol / g. Figure 3 shown.

[0046] (2) Activated microspheres: same as in Example 1.

[0047] (3) Composite conductive microspheres: same as in Example 1.

[0048] Example 3

[0049] The embodiment of the present invention provides a method for preparing composite conductive microspheres for ACF, comprising the following steps:

[0050] (1) Synthesis of amino polymer microspheres with moderately rough surface: 14.4 g polyvinyl pyrrolidone (PVP, K30) and 480 ml anhydrous ethanol were added to a 1000 mL three-necked flask, and the mixture was stirred at 25°C and 300 rpm to mix evenly, and nitrogen was passed through the system to remove oxygen. Then 48 g styrene (ST) and 0.91 g azobisisobutyronitrile (AIBN) were added to the system, and the temperature was raised to 70°C. After the polymerization reaction for 2 hours, 2.6 g divinylbenzene (DVB) and 9.6 g polyethyleneimine (PEI 1800) were added to the system and the reaction was continued for 12 hours. After the reaction was completed, the mixture was cooled to room temperature, centrifuged and washed alternately with ethanol and water for 6 times to obtain 5% cross-linked PS-DVB microspheres with a particle size of 1.7 μm and a surface amino group density of 39 mmol / g. Figure 4 shown.

[0051] (2) Activated microspheres: same as in Example 1.

[0052] (3) Composite conductive microspheres: same as in Example 1.

[0053] Comparative Example 1

[0054] The comparative example of the present invention provides a method for preparing conventional conductive microspheres, comprising the following steps:

[0055] (1) Synthesis of polymer microspheres with smooth surface: 14.4 g polyvinyl pyrrolidone (PVP, K30) and 480 ml anhydrous ethanol were added to a 1000 mL three-necked flask, and the mixture was stirred at 25°C and 300 rpm to mix evenly, and nitrogen was passed through the system to remove oxygen. Then 72 g styrene (ST), 1.5 g divinyl benzene (DVB), and 1.33 g azobisisobutyronitrile (AIBN) were added to the system, and the temperature was raised to 70°C and the polymerization reaction was carried out for 16 hours. After the reaction was completed, the mixture was cooled to room temperature, centrifuged and washed alternately with ethanol and water for 6 times to obtain 2% cross-linked PS-DVB microspheres with a particle size of 3 μm, as shown in FIG. Figure 5 shown.

[0056] (2) Nickel deposition on the outer layer of smooth polymer microspheres:

[0057] Coarsening: Take 1 g of the polymer microspheres obtained in step 1, add 10 mL of concentrated sulfuric acid, stir for 60 min, and wash with pure water 5 times;

[0058] Sensitization: 1g of the roughened microspheres were ultrasonically dispersed in 20ml of pure water, and then 20mL of a mixture of 10g / L stannous chloride and hydrochloric acid was added, stirred for 30min, and washed with pure water for 5 times;

[0059] Activation: 1 g of sensitized microspheres were ultrasonically dispersed in 40 mL of pure water, and then added into 10 mL of 5 g / L palladium chloride dilute hydrochloric acid solution, ultrasonically mixed for 0.5 h, washed with pure water 3 times, and dried for use to obtain activated polymer microspheres;

[0060] Chemical plating: 1 g of activated polymer microspheres were dispersed in 50 mL of chemical plating solution, stirred at 40 °C for 1 h, washed, and dried to obtain composite conductive microspheres with a surface layer of nickel plating and a particle size of 3.2 μm. Figure 6 shown.

[0061] Depend on Figure 1 , Figure 3 and Figure 4 (Corresponding to the polymer microspheres synthesized in Examples 1, 2, and 3, respectively) It can be seen that the roughness of the microsphere surface is increasing. This is because as the amount of functionalized reagent added gradually increases and the degree of crosslinking increases, the phase separation phenomenon that occurs during polymerization becomes increasingly obvious. As the surface roughness of the microsphere gradually increases and the specific surface area increases, the corresponding amino density on the surface of the microsphere also gradually increases. Figure 2 and Figure 6 It can be seen that the coating formed by the rough amino polymer microspheres during the chemical nickel plating process is more compact and less likely to fall off than the coating formed by the microspheres with smooth surfaces.

[0062] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing composite conductive microspheres for ACF, characterized in that: The steps include: Step (1): adding a dispersant and a dispersion medium to a reaction container, stirring at a temperature of 20-30° C., adding a styrene monomer and an initiator in a nitrogen atmosphere, heating to 65-80° C., polymerizing for 1-3 hours, adding a crosslinking agent and a functionalizing agent, reacting for 6-24 hours, cooling to room temperature, and centrifuging and washing to obtain a rough and functionalized polymer microsphere; Step (2): dispersing the polymer microspheres obtained in step (1) in pure water, and then adding a dilute hydrochloric acid solution of palladium chloride to activate them, thereby obtaining activated microspheres; Step (3): dispersing the activated microspheres in step (2) in a chemical nickel plating solution to obtain composite conductive microspheres.

2. A method for preparing composite conductive microspheres for ACF, characterized in that: The dispersion medium in step (1) is one of methanol, ethanol, isopropanol or a mixture of two or more thereof.

3. According to the preparation method of composite conductive microspheres for ACF, it is characterized in that: The dispersant in step (1) is sodium styrene sulfonate, polyvinyl pyrrolidone, sodium dodecylbenzene sulfonate or sodium dodecyl sulfate; the mass percentage of the dispersant in the dispersion medium is 0.1%-5%.

4. A method for preparing composite conductive microspheres for ACF, characterized in that: In the step (1), the mass ratio of the dispersant to the styrene monomer is 1:(2-20).

5. A method for preparing composite conductive microspheres for ACF, characterized in that: The initiator in step (1) is dibenzoyl peroxide, lauroyl peroxide, azobisisobutyronitrile, azobisisovaleronitrile, potassium persulfate or ammonium persulfate; the mass percentage of the initiator to the styrene monomer is 1%-3%.

6. A method for preparing composite conductive microspheres for ACF, characterized in that: The cross-linking agent in step (1) is divinylbenzene or ethylene glycol dimethacrylate; the mass percentage of the cross-linking agent to the styrene monomer is 0.01%-20%.

7. A method for preparing composite conductive microspheres for ACF, characterized in that: The functionalizing agent in step (1) is polyethyleneimine; the mass percentage of the functionalizing agent in the dispersion medium is 0.1%-10%.

8. A method for preparing composite conductive microspheres for ACF, characterized in that: The concentration of the dilute hydrochloric acid solution of palladium chloride in step (2) is 3-7 g / L; the mass volume ratio of the polymer microspheres to the dilute hydrochloric acid solution of palladium chloride is 1:(1-20)(g / ml); the volume ratio of the pure water to the dilute hydrochloric acid solution of palladium chloride is (1-20):

1.

9. A method for preparing composite conductive microspheres for ACF, characterized in that: The chemical nickel plating solution in step (3) comprises: 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 value is 5.

10. A method for preparing composite conductive microspheres for ACF, characterized in that: In the step (3), the mass volume ratio of the activated microspheres to the chemical nickel plating solution is 1:(5-99) (g / ml).

Images