Preparation method and application of conductive microspheres for anisotropic conductive adhesive film

Abstract

The present application relates to the technical field of anisotropic conductive adhesive film, and particularly relates to a preparation method and application of conductive microspheres for anisotropic conductive adhesive film. The preparation process comprises pretreatment steps of roughening, sensitization and activation, and a chemical plating step. The chemical plating solution is composed of nickel salt, complexing agent and buffer. The activated microspheres are dispersed in the chemical plating solution, and a reducing agent sodium hypophosphite is added in three times under constant temperature water bath stirring. Finally, the required conductive microspheres are obtained by filtration, washing and drying. In the stirring process, the pH value of the chemical plating solution is controlled. The method has simple process and low nickel source dosage. By controlling the addition amount of the reducing agent sodium hypophosphite, the conductive microspheres with rough surface, i.e. large specific surface area, are prepared by one-step method. The conductive microspheres added in the anisotropic conductive adhesive film can well reduce the contact resistance between the conductive microspheres and the chip in the conductive adhesive film, and improve the electrical performance.

Description

Technical Field

[0001] This invention relates to the field of anisotropic conductive film technology, specifically to a method for preparing conductive microspheres for anisotropic conductive films and their application. Background Technology

[0002] Anisotropic conductive film (ACF), as an emerging green and environmentally friendly microelectronic packaging interconnect material, is finding increasingly widespread applications. Research has found that when a certain amount of conductive microspheres are incorporated into the ACF, under the influence of bonding temperature and pressure during the ACF softening process, the larger the contact area between the conductive microspheres and the chip bumps or substrate chassis, the superior the conductivity of the ACF.

[0003] Conductive microspheres typically refer to core / shell conductive microspheres based on polystyrene resin. Research on these microspheres originated in the early 20th century, and over decades of development, a conductive composite microsphere system combining polymer resin microspheres and metals has gradually formed. A common approach is to directly metallize the surface of the conductive microspheres, using physicochemical methods to modify the material surface and form a metal coating, creating a core / shell structured composite microsphere. This imparts metallic properties to the polymer. This core / shell structure not only exhibits high bonding strength but also fully utilizes the polymer's excellent toughness, high elastic modulus, and other mechanical properties, while also possessing low-density conductivity and ferromagnetism.

[0004] Patent CN104347192A discloses a simplified method for preparing conductive microspheres for anisotropic conductive film production by adsorbing palladium ions, simplifying steps such as adsorption, sensitization, and activation. However, this patented method uses a large amount of activator (palladium chloride), and because the prepared conductive microspheres have a fine and dense nickel plating layer, when used in ACF conductive films, they cannot significantly improve the conductivity of the ACF conductive films.

[0005] Providing a conductive microsphere that can be well applied to ACF conductive films and improve their conductivity is of great significance to the development of ACF conductive films. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention first provides a method for preparing conductive microspheres for anisotropic conductive films.

[0007] The technical solution adopted in this invention is as follows:

[0008] A method for preparing conductive microspheres for anisotropic conductive films includes the following steps:

[0009] S1. Pretreatment: After acid washing and roughening, and sensitization by soaking in stannous chloride, polystyrene microspheres are added with an activator and stirred for a set time to obtain activated microspheres;

[0010] S2. Chemical plating: Activated microspheres are dispersed in a chemical plating solution and stirred for 40-100 minutes in a constant temperature water bath at 40-70℃ to complete the chemical plating; the chemical plating solution is composed of 7-20 g / L nickel salt, 1-20 g / L complexing agent, and 10-30 g / L buffer.

[0011] During the stirring process, sodium hypophosphite is added in three stages: The first addition of sodium hypophosphite occurs after the activated microspheres have been stirred for 10–60 minutes, allowing the Ni in the chemical plating solution to reach a certain concentration. 2+ / H2PO2 -1 The ratio is 0.6–1.20; after stirring for 8–12 minutes, sodium hypophosphite is added a second time to increase the Ni concentration in the chemical plating solution. 2+ / H2PO2 -1 The ratio is 0.45–0.51; after stirring for another 8–12 minutes, sodium hypophosphite is added for the third time to adjust the Ni content in the electroless plating solution. 2+ / H2PO2 -1 The ratio is 0.28. Finally, stir for 8-12 minutes to complete the chemical plating.

[0012] S3. After the reaction is complete, filter, wash and dry to obtain the desired conductive microspheres.

[0013] Preferably, the nickel salt is any one or more of nickel sulfate, nickel chloride, nickel hypophosphite, and nickel carbonate; the complexing agent is any one or more of EDTA, sodium citrate, succinic acid, glycine, triethanolamine, ethylenediamine, and lactic acid; and the buffer is any one or more of sodium acetate, acetic acid, boric acid, and ammonia.

[0014] Preferably, the complexing agent is obtained by mixing sodium citrate and succinic acid in a molar ratio of (0.8-1.2):1.

[0015] Preferably, the acid washing and roughening process uses sulfuric acid with a mass fraction of 50-100% or a mixed solution of 10-30 g / L sulfuric acid and 5-30 g / L potassium dichromate as a roughening agent. Polystyrene microspheres are added to the roughening agent at a ratio of 1 g: 10 mL, and stirred in a constant temperature water bath at 50-70°C for 60-300 min. The roughened polystyrene microspheres are then obtained by filtration.

[0016] Preferably, the stannous chloride immersion sensitization is performed by dispersing coarsened polystyrene microspheres in a 20 g / L stannous chloride solution at a ratio of 1 g: 20 mL, stirring in a constant temperature water bath at 40 °C for 60–240 min, filtering, and drying to obtain sensitized polystyrene microspheres.

[0017] Preferably, the activator is any one or more of palladium nitrate solution, palladium sulfate solution, and palladium chloride solution, and the amount of solute in the activator is 0.5-3 g / L; the sensitized polystyrene microspheres are added to the activator at a ratio of 1 g:(20-100) mL, stirred in a constant temperature water bath at 30-60℃ for 60-240 min, and filtered to obtain activated microspheres.

[0018] Preferably, during the stirring process in step S2, the pH value of the chemical plating solution is controlled to be 7.5-9.

[0019] The present invention also provides a conductive microsphere, which is prepared by the method described above for preparing conductive microspheres for anisotropic conductive films. The conductive microsphere has a diameter of 3–10 μm and a coating thickness of 80–150 nm.

[0020] The present invention also provides the application of the above-mentioned conductive microspheres in the preparation of anisotropic conductive films.

[0021] Finally, the present invention provides an anisotropic conductive adhesive film, wherein conductive microspheres as described above are added to the anisotropic conductive adhesive film.

[0022] The beneficial effects of this invention are as follows:

[0023] Existing methods for preparing conductive microspheres often focus on improving the density and uniformity of the surface nickel layer. Currently, most conductive microspheres used in ACF conductive films are smooth. However, the inventors have found through long-term work that smooth conductive microspheres have a small surface area, leading to higher contact resistance and relatively lower conductivity in the prepared conductive film. Therefore, this invention provides a method for preparing conductive microspheres that can construct a rough surface structure, thereby increasing the specific surface area of ​​the conductive microspheres and reducing the contact resistance between the conductive microspheres and the chip in the prepared conductive film.

[0024] The method of this invention is a simple chemical plating process with low nickel source usage. By controlling the amount of reducing agent added, conductive microspheres with a large specific surface area can be prepared in one step. This method regulates the Ni content in the chemical plating solution by controlling the amount of sodium hypophosphite added in three steps. 2+ / H2PO2 -1 The ratio controls the electroless plating rate, thereby enabling the preparation of nickel-plated conductive microspheres with different thicknesses, surface roughness, and large specific surface areas.

[0025] The present invention screens and obtains a better combination of complexing agents, which can accelerate the chemical plating speed and reduce the P content in conductive microspheres. Attached Figure Description

[0026] Figure 1 SEM image of the nickel-plated conductive microspheres prepared in Example 1.

[0027] Figure 2 The image shows a SEM image of the nickel-plated conductive microspheres prepared in Comparative Example 1.

[0028] Figure 3 The image shows the XRD pattern of the nickel-plated conductive microspheres prepared in Example 2.

[0029] Figure 4 This is a comparison diagram of Example 1 and Example 2.

[0030] Figure 5 The image shows the XRD pattern of the nickel-plated conductive microspheres prepared in Example 3. Detailed Implementation

[0031] To facilitate understanding, the technical solution of the present invention will be described in more detail below with reference to the embodiments.

[0032] Example 1

[0033] S1. Preprocessing

[0034] S11. Roughening: Disperse 1g of polystyrene microspheres in 10mL of 50% sulfuric acid, stir for 120min in a constant temperature water bath at 65℃, filter the suspension, wash with deionized water until neutral, and dry at 56℃ for 4h for later use to obtain roughened microspheres.

[0035] S12. Sensitization: Disperse 1g of coarse microspheres in 20mL of deionized water, add 20mL of 20g / L stannous chloride solution, stir for 120min in a constant temperature water bath at 40℃, wash with deionized water until neutral, and dry at 56℃ for 4h to obtain sensitized microspheres.

[0036] S13. Activation: Disperse 1g of sensitized microspheres in 20mL of deionized water, add 20mL of 0.5g / L palladium chloride solution, stir in a constant temperature water bath at 40℃ for 120min, wash with deionized water until neutral, and dry at 56℃ for 4h to obtain activated microspheres.

[0037] S2. Chemical plating

[0038] 0.2 g of activated microspheres were dispersed in 100 mL of electroless plating solution. After stirring for 30 min in a constant temperature water bath at 40 °C, 3 mL of sodium hypophosphite solution was added for the first time to increase the Ni concentration in the electroless plating solution. 2+ / H2PO2 -1The ratio was 1.20; after stirring for another 10 minutes, 3 mL of sodium hypophosphite solution was added for the second time to adjust the Ni concentration in the electroless plating solution. 2+ / H2PO2 -1 The ratio was 0.51; after stirring for another 10 minutes, 4 mL of sodium hypophosphite solution was added for the third time to adjust the Ni concentration in the chemical plating solution. 2+ / H2PO2 -1 The ratio is 0.28. Finally, stir for 10 minutes to complete the chemical plating.

[0039] In this embodiment, the electroless plating solution consists of 10 g / L nickel sulfate as the main salt, 10 g / L sodium citrate as the complexing agent, and 23 g / L sodium acetate as the buffer. The concentration of the added reducing agent sodium hypophosphite solution is 0.13 g / mL. During stirring, the pH value of the electroless plating solution is adjusted with a 4 g / L sodium hydroxide solution to control the pH value to around 8.

[0040] S3. After the reaction is complete, filter, wash and dry to obtain conductive microspheres.

[0041] Example 2

[0042] The preparation process of Example 2 is the same as that of Example 1, except that in step S2, the concentration of sodium hypophosphite solution is 0.21 g / mL.

[0043] Example 3

[0044] The preparation process of Example 3 is the same as that of Example 1. The difference is that in step S2, the chemical plating solution is composed of 10 g / L nickel sulfate as the main salt, 5 g / L sodium citrate as the complexing agent, 2 g / L succinic acid as the complexing agent, and 23 g / L sodium acetate as the buffer.

[0045] Comparative Example 1

[0046] The preparation process of Comparative Example 1 is the same as that of Example 1. The difference is that in step S2, the activated microspheres are dispersed in the chemical plating solution, stirred for 30 min in a constant temperature water bath at 40°C, and then 10 mL of sodium hypophosphite solution is added at once and stirred for 30 min to complete the chemical plating.

[0047] Comparative Example 2

[0048] The preparation process of Comparative Example 2 is the same as that of Example 1, except that in step S2, the chemical plating solution is composed of 10 g / L nickel sulfate as the main salt, 4 g / L succinic acid as the complexing agent, and 23 g / L sodium acetate as the buffer.

[0049] Comparative Example 3

[0050] The preparation process of Comparative Example 3 is the same as that of Example 1, except that in step S2, the electroless plating solution is composed of nickel sulfate 10 g / L as the main salt, malic acid 2.2 g / L and succinic acid 2 g / L as complexing agents, and sodium acetate 23 g / L as the buffer.

[0051] The performance of the six groups of conductive microspheres prepared in Examples 1-3 and Comparative Examples 1-3 was tested.

[0052] Figure 1 The image shows a SEM image of the nickel-plated conductive microspheres prepared in Example 1. It can be seen that the surface of the nickel-plated conductive microsphere coating is rough and uniform. Figure 2 The image shows a SEM image of the nickel-plated conductive microspheres prepared in Comparative Example 1. As can be seen from the image, the surface of the nickel-plated conductive microspheres is smooth.

[0053] The main reason for these two phenomena is the different methods of adding the reducing agent. In Comparative Example 1, adding the reducing agent once can prepare nickel-plated conductive microspheres with a smooth surface. However, the method of adding the reducing agent multiple times, specifically proposed in this invention, prepares nickel-plated conductive microspheres with a rough surface and a larger specific surface area. Due to their larger specific surface area, the rough nickel-plated conductive microspheres can increase the contact area with chip bumps or substrate chassis in ACF applications, reduce contact resistance, and increase the conductivity of the ACF conductive film.

[0054] Figure 3 The image shows the XRD pattern of the nickel-plated conductive microspheres prepared in Example 2. As can be seen from the spectrum, the nickel-plated conductive microspheres only exhibit a diffraction peak at 2θ = 44.6°, which belongs to the (111) spectral density of metallic Ni, and the peak shape is relatively broad. This phenomenon is caused by the presence of Ni in the chemical plating solution. 2+ / H2PO2 -1 The value affects the phosphorus content of the prepared conductive microspheres. When the Ni content in the chemical plating solution... 2+ / H2PO2 -1 When the value is below 0.3, the sodium hypophosphite content of the reducing agent in the electroless plating solution is high. After the electroless plating begins, a large number of adsorbed hydrogen atoms (H) are generated, which leads to the excess adsorbed hydrogen atoms (H) reacting with hypophosphite to generate substances with high phosphorus content. Therefore, the prepared conductive microspheres have a high phosphorus content.

[0055] Figure 4 This is a comparison graph of Example 1 and Example 2. As can be seen from the graph, the nickel content of the nickel-plated conductive microspheres in Example 1 is higher than that in Example 2. This indicates that increasing the reducing agent concentration leads to a decrease in the nickel content of the nickel-plated conductive microspheres. This is because a high concentration of reducing agent lowers the pH value of the electroless plating solution, which is beneficial for the reduction of phosphorus (P) and inhibits the reduction of nickel (Ni). 2+ The reduction reaction.

[0056] In nickel sulfate main salt systems, the chelation of nickel by complexing agents can catalyze the precipitation of phosphorus in the coating. By appropriately adjusting the choice of complexing agent, the phosphorus content in the coating can be controlled. Furthermore, different combinations of complexing agents can yield conductive microspheres with varying properties. When sodium citrate is used as a complexing agent, the electroless plating solution is stable, the plating cycle is long, and the prepared conductive microsphere coating exhibits good corrosion resistance. However, the problem is that the coating has a high phosphorus content and a low nickel content. To reduce the phosphorus content and increase the nickel content in nickel-plated conductive microspheres, thereby improving their conductivity, sodium citrate and succinic acid are added to the electroless plating solution as dual complexing agents. This accelerates the electroless plating rate while effectively reducing the phosphorus content in the conductive microspheres.

[0057] Figure 5 The image shows the XRD pattern of the nickel-plated conductive microspheres prepared in Example 3. It can be seen from the spectrum that obvious diffraction peaks appear at 2θ = 44.6°, 51.7°, and 76.4°, which belong to the (111), (200), and (200) crystal planes of metallic Ni, respectively. The diffraction peak at 44.6° is narrow and sharp. Figure 5 Compared with the XRD pattern of Example 1, it can be found that the conductive microspheres prepared by the dual complexing agent have a higher nickel content, a faster electroless plating rate, and can reduce the P content in the conductive microspheres.

[0058] In Comparative Example 2, when succinic acid was used as a single complexing agent for electroless plating, adjusting the pH of the plating solution resulted in the formation of a white precipitate, indicating that succinic acid reacts with Ni. 2+ Due to its poor complexing ability, succinic acid alone is difficult to use as a complexing agent. In Comparative Example 3, malic acid and succinic acid were used as complexing agents, and the nickel-plated conductive microspheres prepared had a loose coating structure. Combining the experimental results of Example 1, Comparative Example 1, and Comparative Example 2, the conductive microspheres prepared with dual complexing agents have better performance than those prepared with single complexing agents. Combining the experimental results of Example 3 and Comparative Example 3, compounds with higher PKa values ​​are used in combination with succinic acid as complexing agents, resulting in a more uniform and dense conductive microsphere coating.

[0059] Sample preparation: Using a self-made rectangular mold, the composite microspheres were compressed into tablets using an infrared tablet press to produce a cross-sectional area of ​​6 cm². 2 Samples with a length of 3 cm were used. The surface resistance values ​​of the six groups of conductive microspheres prepared in Examples 1-3 and Comparative Examples 1-3 were measured using a four-probe surface impedance analyzer. The conductivity was then calculated, and the results are shown in Table 1.

[0060] Table 1. Conductivity Test Data Results

[0061]

[0062] Table 1 shows that the addition of sodium hypophosphite, the concentration of sodium hypophosphite, and the type and amount of complexing agent all affect the conductivity of the conductive microspheres. The conductive microspheres prepared by adding the reducing agent in batches as proposed in this invention have higher conductivity than those prepared by adding the reducing agent in a single batch as in the comparative example. The conductive microspheres prepared using dual complexing agents have better conductivity than those prepared using a single complexing agent.

[0063] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing conductive microspheres for anisotropic conductive films, characterized in that, Includes the following steps: S1. Pretreatment: After acid washing and roughening, and sensitization by soaking in stannous chloride, polystyrene microspheres are added with an activator and stirred for a set time to obtain activated microspheres; S2. Chemical plating: Activated microspheres are dispersed in a chemical plating solution and stirred in a constant temperature water bath at 40~70℃ for 40~100 min to complete the chemical plating; the chemical plating solution is composed of 7~20 g / L nickel salt, 1~20 g / L complexing agent, and 10~30 g / L buffer; the complexing agent is obtained by mixing sodium citrate and succinic acid in a molar ratio of (0.8-1.2):

1. During the stirring process in step S2, sodium hypophosphite is added in three batches: The first batch of sodium hypophosphite is added after the activated microspheres have been stirred for 10-60 minutes, so that the Ni in the chemical plating solution... 2+ / H2PO2 -1 The ratio is 0.6~1.20; after stirring for another 8~12 min, sodium hypophosphite is added for the second time to adjust the Ni content in the chemical plating solution. 2+ / H2PO2 -1 The ratio is 0.45~0.51; after stirring for another 8~12 minutes, sodium hypophosphite is added for the third time to increase the Ni concentration in the chemical plating solution. 2+ / H2PO2 -1 The ratio is 0.

28. Finally, stir for 8-12 minutes to complete the chemical plating. S3. After the reaction is complete, filter, wash and dry to obtain the desired conductive microspheres, which have a rough surface structure.

2. The method for preparing conductive microspheres for anisotropic conductive films as described in claim 1, characterized in that, The nickel salt is any one or more of nickel sulfate, nickel chloride, nickel hypophosphite, and nickel carbonate, and the buffer is any one or more of sodium acetate, acetic acid, boric acid, and ammonia.

3. The method for preparing conductive microspheres for anisotropic conductive films as described in claim 1, characterized in that, The pickling and roughening process uses sulfuric acid or a mixed solution of sulfuric acid and potassium dichromate as a roughening agent. Polystyrene microspheres are added to the roughening agent at a ratio of 1 g: 10 mL, stirred in a constant temperature water bath at 50~70℃ for 60~300 min, and filtered to obtain the roughened polystyrene microspheres.

4. The method for preparing conductive microspheres for anisotropic conductive films as described in claim 1, characterized in that, The stannous chloride sensitization process involves dispersing coarsened polystyrene microspheres in a 20 g / L stannous chloride solution at a ratio of 1 g: 20 mL, stirring at a constant temperature water bath of 40°C for 60–240 min, filtering, and drying to obtain sensitized polystyrene microspheres.

5. The method for preparing conductive microspheres for anisotropic conductive films as described in claim 1, characterized in that, The activator is any one or more of palladium nitrate solution, palladium sulfate solution, and palladium chloride solution, and the amount of solute in the activator is 0.5~3 g / L; the sensitized polystyrene microspheres are added to the activator at a ratio of 1 g: (20~100) mL, stirred in a constant temperature water bath at 30~60℃ for 60~240 min, and filtered to obtain activated microspheres.

6. The method for preparing conductive microspheres for anisotropic conductive films as described in claim 1, characterized in that, During the stirring process in step S2, the pH value of the chemical plating solution is controlled to be 7.5~9.

7. A conductive microsphere, prepared by the method for preparing anisotropic conductive films as described in any one of claims 1-6, wherein the conductive microsphere has a diameter of 3-10 μm and a coating thickness of 80-150 nm.

8. The application of the conductive microspheres as described in claim 7 in the preparation of anisotropic conductive films.

9. An anisotropic conductive adhesive film, characterized in that, The anisotropic conductive film contains conductive microspheres as described in claim 7.

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