Low stress multi-die package high reliability epoxy composite

Abstract

The application belongs to the technical field of semiconductor packaging, and discloses a low-stress multi-die packaging high-reliability epoxy composite material. The material comprises an epoxy resin matrix, a curing agent, a high-filling inorganic filler and microbial metabolite responsive microcapsules dispersed therein. The microcapsule shell layer swells or degrades under a local acidic microenvironment (pH 4.0-5.5), releases the core repair monomer, and the latter polymerizes in situ at the crack to achieve self-repair. The material not only has the key physical properties required by traditional packaging materials, such as low thermal expansion coefficient, high modulus and excellent dielectric properties, but also fundamentally solves the problem of micro-crack accumulation and reliability degradation caused by the mismatch of thermal expansion coefficient and the coupling of hygrothermal aging in multi-die packaging through a chemical signal driven self-repairing mechanism, and is suitable for advanced packaging structures such as 2.5D / 3D and fan-out wafer level packaging.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor packaging technology and relates to a low-stress multi-die packaging high-reliability epoxy composite material. Background Technology

[0002] In high-performance computing, artificial intelligence, and 5G communication, multi-die packaging structures are prone to micron- or even submicron-level cracks under stress coupling effects such as increased packaging layers and exacerbated thermal expansion coefficient mismatch in environmental changes. This becomes a key hidden danger affecting the long-term reliability of devices.

[0003] To improve the reliability of epoxy encapsulation materials, existing technologies mainly focus on optimizing the mechanical properties of the resin system or introducing traditional toughening phases, such as flexible segment modification, nanofiller reinforcement, or rubber particle toughening. However, these methods can only improve the initial crack resistance of the material and cannot actively intervene in micro-damage that has already formed during service.

[0004] Especially in multi-die packaging, due to the significant differences in the coefficients of thermal expansion between different chip materials (such as silicon, GaAs, and SiC) and the organic substrate, the shear stress continuously accumulated at the interface under repeated thermomechanical loading can easily lead to local debonding or microcrack formation. Once the crack extends into the moisture penetration path, it will accelerate moisture intrusion and cause metal interconnect corrosion, electromigration, or even short-circuit failure under high temperature and high humidity conditions. At the same time, the damp heat aging process itself will promote the hydrolysis or oxidative degradation of the epoxy network, producing trace amounts of acidic byproducts (such as carboxylic acids), further catalyzing material degradation and forming a vicious cycle of stress-crack-moisture-acidification-accelerated degradation.

[0005] While traditional toughening methods can improve fracture toughness, they often come at the cost of sacrificing modulus, glass transition temperature, or dielectric properties, making it difficult to balance high reliability with the requirements of high-frequency and high-speed signal transmission; moreover, they do not have the ability to respond to and repair micro-damage that has already occurred. Summary of the Invention

[0006] To achieve the above-mentioned objectives, this invention provides a low-stress, multi-die encapsulated, highly reliable epoxy composite material. The epoxy composite material comprises an epoxy resin matrix, a curing agent, inorganic fillers, and microbial metabolite-responsive microcapsules dispersed in the epoxy resin matrix. The shell of the microbial metabolite-responsive microcapsules is composed of a polymer selectively swelling or degrading to lactic acid or lower carboxylic acids, and its core encapsulates a repair monomer capable of in-situ polymerization. When the epoxy composite material generates a locally acidic microenvironment due to hydrolysis or oxidation during humid aging, the microbial metabolite-responsive microcapsules undergo structural transformation under acidic stimulation, releasing the repair monomer. The repair monomer then polymerizes with residual epoxy groups or ambient moisture at the crack interface, achieving in-situ self-repair of the microcracks.

[0007] The epoxy resin matrix is ​​selected from at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenolic epoxy resin, biphenyl type epoxy resin, naphthalene type epoxy resin, or phosphorus-containing epoxy resin; the curing agent is at least one of acid anhydride curing agent, amine curing agent, or phenolic resin curing agent, and its dosage is formulated according to the ratio of epoxy equivalent to curing agent active hydrogen equivalent of 1:0.8 to 1:1.2.

[0008] The inorganic filler is at least one of fused silica, crystalline silica, spherical alumina, boron nitride, aluminum nitride, or magnesium oxide, with an average particle size of 0.5-50 μm and a filling volume fraction of 40%-70%. The inorganic filler is surface-treated with a silane coupling agent, which is at least one of γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, at a concentration of 0.5-2.0 wt%, to improve the interfacial bonding strength between the filler and the epoxy matrix and reduce the overall coefficient of thermal expansion.

[0009] The average particle size of the microbial metabolite-responsive microcapsules is 1-10 μm, and the wall thickness is 50-300 nm. The shell material is at least one of poly(lactic acid-co-hydroxyacetic acid), poly(ε-caprolactone-co-lactic acid), poly(acrylic acid-co-methyl methacrylate), or chitosan derivatives. The shell material undergoes significant swelling or partial degradation in an environment with a pH value below 5.5. The critical response pH threshold is set to 4.0-5.5 to match the microenvironment acidity corresponding to the local carboxylic acid concentration generated by hydrolysis of the epoxy encapsulation material during humid heat aging.

[0010] The repair monomer is at least one of dicyclopentadiene, glycidyl methacrylate, vinylsiloxane, furanyl epoxy monomer or mercapto-containing acrylate; after release, the repair monomer is polymerized through any of the following mechanisms: (1) ring-opening addition reaction with unreacted epoxy groups in the epoxy matrix at room temperature or service temperature; (2) hydrolysis condensation reaction under the action of ambient moisture; (3) free radical polymerization or cationic polymerization catalyzed by trace metal ions (such as copper or aluminum) present at the crack interface.

[0011] The added mass fraction of the microbial metabolite responsive microcapsules is 0.5%-5.0%, preferably 1.0%-3.0%; its dispersion in the epoxy resin matrix is ​​achieved by high-speed shear mixing or three-roll milling process, with a mixing speed of 2000-8000 rpm and a time of 5-30 minutes, to ensure that the microcapsules are uniformly distributed and do not rupture prematurely.

[0012] In a preferred embodiment of the present invention, the epoxy composite material further comprises a thermal stress buffer phase, wherein the thermal stress buffer phase is core-shell rubber particles or silicone-modified epoxy prepolymer, and the mass fraction of the added phase is 2%-8%; the core of the core-shell rubber particles is polybutadiene or polyisoprene, and the shell is polymethyl methacrylate or polystyrene, with an average particle size of 100-500 nm; the silicone-modified epoxy prepolymer is prepared by transesterification reaction of terminal epoxy group polydimethylsiloxane and bisphenol A epoxy resin, and its number average molecular weight is 2000-8000, and its silicon content is 5%-15%.

[0013] In another preferred embodiment of the present invention, the shell surface of the microbial metabolite responsive microcapsules is grafted with a silane coupling agent, which is the same as or compatible with the coupling agent used in the inorganic filler, so as to enhance the interfacial adhesion between the microcapsules and the epoxy matrix and prevent interfacial debonding during curing or thermal cycling; the grafting density is 0.5-2.0 silane molecules per square micrometer of shell surface.

[0014] The coefficient of thermal expansion of the epoxy composite material is 10-25 ppm / ℃ below the glass transition temperature and 40-70 ppm / ℃ above the glass transition temperature; its glass transition temperature is greater than 150℃, its energy storage modulus is greater than 8 GPa at 25℃ and greater than 2 GPa at 150℃; its dielectric constant is less than 3.5 at a frequency of 10 GHz, and its dielectric loss tangent is less than 0.008.

[0015] The microbial metabolite-responsive microcapsules are prepared using interfacial polymerization, in-situ polymerization, or layer-by-layer self-assembly. In interfacial polymerization, the repair monomer is dissolved in the oil phase, an emulsifier is added to form an oil-in-water emulsion, and then the shell monomer precursor in the aqueous phase is added, resulting in a condensation reaction at the oil-water interface to form the microcapsule shell. The emulsifier is sodium dodecyl sulfate, polyvinyl alcohol, or gum arabic, with a concentration of 0.1%-1.0%. The reaction temperature is 25-60℃, and the reaction time is 2-12 hours.

[0016] The release behavior of the repair monomer is spatially selective and temporally synchronous: the microcapsules only release in response to micro-regions where the local pH value drops below 5.5 due to damp heat aging, thereby ensuring that the repair agent is activated only in the damaged area and avoiding material performance degradation caused by ineffective release; the release process completes more than 80% of the monomer release within 24 hours under pH=4.5 conditions, while the release amount is less than 5% within 7 days in a neutral environment.

[0017] The epoxy composite material is suitable for multi-die heterogeneous integrated packaging structures, including but not limited to 2.5D / 3D packaging, fan-out wafer-level packaging, embedded chip packaging, and through-silicon via (TSV) interconnect structures. In the packaging structure, the epoxy composite material is used as an underfill, molding compound, or interlayer dielectric material. Its low thermal stress characteristics effectively alleviate the thermal expansion coefficient mismatch between silicon chips, compound semiconductor chips, and organic substrates, while its self-healing function significantly improves the structural integrity and electrical reliability of the package under long-term damp heat and thermomechanical loads.

[0018] The shell material of the microbial metabolite-responsive microcapsules incorporates carboxyl or amino functional groups to modulate their selective recognition ability for lactic acid. The carboxyl groups are derived from copolymer units of acrylic acid, methacrylic acid, or itaconic acid, and the amino groups are derived from copolymer units of methacrylamide, vinylpyridine, or dimethylaminoethyl methacrylate. The molar content of the functional groups is 5%-20% of the total repeating units of the shell polymer to optimize its swelling kinetics and degradation rate under weakly acidic conditions.

[0019] The repair phase formed after polymerization of the repair monomer has a coefficient of thermal expansion and modulus similar to that of the epoxy matrix, with a coefficient of thermal expansion of 20-40 ppm / ℃ and a Young's modulus of 2-5 GPa, thereby preventing the repair phase from becoming a new source of stress concentration. The formation of the repair phase is achieved by controlling the functionality and molecular structure of the repair monomer. For example, the Diels-Alder reversible reaction system using a bifunctional furanyl epoxy monomer and a maleimide crosslinking agent can achieve dynamic network reconstruction at service temperature, further improving repair efficiency and durability.

[0020] The preparation method of the epoxy composite material includes the following steps: (1) mixing epoxy resin, thermal stress buffer phase and inorganic filler in a vacuum mixer at 60-100℃ for 30-2 hours to form a uniform slurry; (2) adding microbial metabolite responsive microcapsules to the slurry under an inert atmosphere and shearing and dispersing at 25-40℃ and 2000-5000rpm for 10-20 minutes; (3) adding curing agent and continuing vacuum degassing for 10-30 minutes to obtain a processable encapsulating material; (4) applying the encapsulating material to a multi-die encapsulation structure by dispensing, molding or printing, and then curing it at 80-180℃ for 2-8 hours to form the final encapsulation.

[0021] The stepped temperature curing process is as follows: 80℃ for 30 minutes, 120℃ for 60 minutes, 150℃ for 90 minutes, and 180℃ for 60 minutes. This process ensures that the epoxy is fully cross-linked while avoiding thermal rupture of the microcapsules at high temperatures.

[0022] Compared with the prior art, the beneficial effects of the present invention are: This invention introduces a microcapsule system into epoxy composite materials that specifically responds to lactic acid or lower carboxylic acids—byproducts of hygrothermal aging—creating a smart material capable of in-situ sensing acidification of the encapsulated microenvironment and simultaneously releasing repair monomers. This material not only possesses the key physical properties required by traditional encapsulation materials, such as low coefficient of thermal expansion, high modulus, and excellent dielectric properties, but also fundamentally solves the problem of microcrack accumulation and reliability degradation caused by the coupling of thermal expansion mismatch and hygrothermal aging in multi-die encapsulations through a chemical signal-driven self-healing mechanism. The technical solution of this invention breaks through the limitations of traditional passive toughening modes, realizing a paradigm shift from crack resistance to self-healing, and providing a novel material solution for high-density heterogeneous integrated encapsulation that combines high reliability and long lifespan. Detailed Implementation

[0023] This invention provides a low-stress, multi-die encapsulated, highly reliable epoxy composite material. Its core lies in introducing a microcapsule system with specific responsiveness to lactic acid or lower carboxylic acids—byproducts of hygrothermal aging—to construct a smart material capable of in-situ sensing acidification of the encapsulated internal microenvironment and simultaneously releasing repair monomers. While maintaining the low coefficient of thermal expansion, high modulus, and excellent dielectric properties required by traditional encapsulation materials, this material achieves autonomous identification and in-situ repair of microcracks caused by the coupling of thermomechanical stress and hygrothermal aging during service, thereby significantly improving the long-term reliability of multi-die heterogeneous integrated encapsulation structures.

[0024] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0025] Example 1: Percentage by weight: 30% bisphenol A epoxy resin, 12% acid anhydride curing agent, 50% fused silica, 2% responsive microbial metabolite microcapsules, and 6% core-shell rubber particles; The equivalent ratio of epoxy resin to curing agent is 1:1.0; Fused silica: average particle size 10 μm, surface treated with 1.0 wt% γ-glycidoxypropyltrimethoxysilane; Microcapsules: average particle size 5μm, wall thickness 150nm, shell layer is polylactic acid-co-hydroxyacetic acid grafted with the same type of silane coupling agent, grafting density 1.2 capsules / square micrometer, critical response pH 5.0 core encapsulation dicyclopentadiene; Core-shell rubber particles: The core is polybutadiene and the shell is polymethyl methacrylate, with an average particle size of 300 nm; Preparation process: epoxy resin, core-shell rubber particles, molten silica, vacuum stirring at 80℃ for 60 minutes, microcapsules added at 25℃, shear dispersion at 3000rpm for 15 minutes, curing agent added, vacuum degassing for 20 minutes, and step temperature curing (80℃ for 30 minutes, 120℃ for 60 minutes, 150℃ for 90 minutes, 180℃ for 60 minutes). Preparation process: mixing resin with filler and buffer phase → microcapsule dispersion → adding curing agent → vacuum degassing → step-by-step temperature curing → encapsulation of finished product.

[0026] Example 2: The amount of microbial metabolite-responsive microcapsules added was 0.5%, and the rest of the formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (adjustment of microcapsule dosage).

[0027] Example 3: The amount of responsive microcapsules for microbial metabolites added was 5.0%, and the rest of the formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (adjustment of microcapsule dosage).

[0028] Example 4: The inorganic filler was replaced with spherical alumina with an average particle size of 15 μm, and the rest of the formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (with adjustment of filler type).

[0029] Example 5: The thermal stress buffer phase was replaced with an organosilicon-modified epoxy prepolymer with a silicon content of 10% and a number average molecular weight of 5000. The rest of the formulation and process were the same as in Example 1. Preparation process: Same as in Example 1 (with adjustment of buffer phase type).

[0030] Example 6: The microcapsule shell was replaced with polycaprolactone-co-lactic acid with a critical response pH of 4.5, and the rest of the formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (microcapsule shell adjustment).

[0031] Example 7: The volume fraction of molten silica was 40%, and the remaining formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (with adjustment of filler dosage).

[0032] Example 8: The volume fraction of molten silica was 70%, and the remaining formulation and process were the same as in Example 1; Preparation process: Same as in Example 1 (with adjustment of filler dosage).

[0033] Comparative Example 1: Microcapsules responsive to microbial metabolites, with the remaining formulation and process the same as in Example 1; Preparation process: Same as in Example 1 (without the microcapsule addition step).

[0034] Comparative Example 2: No thermal stress buffer phase, the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1 (no buffer phase mixing step).

[0035] Test method: Thermal and dielectric property testing: Differential scanning calorimeter for glass transition temperature; thermomechanical analyzer for coefficient of thermal expansion below glass transition temperature; dynamic mechanical analyzer for storage modulus; dielectric constant and dielectric loss tangent at 10 GHz for dielectric spectrometer.

[0036] Mechanical and reliability testing: universal testing machine is used to measure shear strength before and after thermal cycling to calculate retention rate; scanning electron microscopy is used to observe microcrack density after damp heat aging; and high resistance meter is used to measure volume resistivity.

[0037] Self-healing performance test: The degree of crack closure was evaluated after artificially prefabricated microcracks were aged under damp heat; the fracture toughness recovery rate after repair was tested.

[0038] The test data comparisons are shown in Table 1 and Table 2.

[0039] Table 1. Comparison of Glass Transition Temperature, Coefficient of Thermal Expansion, and Reduction Rate of Microcracks During Wet Heat Aging Test Project Glass transition temperature (°C) Coefficient of thermal expansion (ppm / ℃) Reduction rate of microcracks during damp heat aging (%) Example 1 165 18 68 Example 2 168 17 45 Example 3 162 19 75 Example 4 166 16 65 Example 5 160 20 62 Example 6 164 18 70 Example 7 163 22 60 Example 8 167 15 66 Comparative Example 1 166 17 -30 Comparative Example 2 170 14 40

[0040] Table 2 Comparison of Thermal Cycling Shear Retention Rate, 10GHz Dielectric Constant, and Volume Resistivity Test Project Thermal cycling shear retention rate (%) 10GHz dielectric constant Volume resistivity (Ω·cm) Example 1 88 3.2 <![CDATA[2×10 15 ]]> Example 2 82 3.1 <![CDATA[3×10 15 ]]> Example 3 90 3.3 <![CDATA[1.5×10 15 ]]> Example 4 86 3.4 <![CDATA[2.5×10 15 ]]> Example 5 87 3 <![CDATA[1.8×10 15 ]]> Example 6 89 3.2 <![CDATA[2.2×10 15 ]]> Example 7 84 3.1 <![CDATA[1.6×10 15 ]]> Example 8 85 3.5 <![CDATA[2.3×10 15 ]]> Comparative Example 1 65 3.1 <![CDATA[1×101 4 ]]> Comparative Example 2 72 3.3 <![CDATA[1.5×10 15 ]]>

[0041] Examples 1-8 showed a microcrack reduction rate of ≥60% and a thermal cycling shear strength retention rate of ≥84% after damp heat aging, which were far superior to the comparative examples. Comparative example 1, due to the lack of responsive microcapsules, experienced crack intensification after damp heat aging, and comparative example 2, lacking a thermal stress buffer phase, had a low shear strength retention rate. This demonstrates that pH-responsive microcapsules + thermal stress buffer phase + high-filler content are the key to the synergistic effect of low stress and high reliability.

[0042] Increased microcapsule addition (Examples 2→1→3) significantly improved microcrack repair; increased inorganic filler content (Examples 7→1→8) reduced the coefficient of thermal expansion and enhanced rigidity; the core-shell rubber particle buffer phase (Example 1) was superior to the organosilicon-modified epoxy prepolymer (Example 5) in terms of crack resistance and repair synergy; different microcapsule shells and filler types were all compatible, with polylactic acid-co-hydroxyacetic acid shells showing better synergistic effects with fused silica.

[0043] The embodiment has a glass transition temperature ≥160℃ and a coefficient of thermal expansion ≤22ppm / ℃, which is suitable for the thermal stress requirements of multi-die packaging; excellent dielectric properties to meet high-frequency signal transmission; self-healing function significantly reduces the risk of damp heat aging cracks and improves device life; the fabrication process is compatible with existing packaging processes, requires no special equipment, and is easy to industrialize.

[0044] Compared to traditional non-self-healing materials (Comparative Example 1), Example 1 showed a 68% reduction in microcracks during damp heat aging (comparative example 1 showed a 30% increase in cracks) and a 35% improvement in thermal cycling shear strength retention. Compared to non-buffered materials (Comparative Example 2), the microcrack reduction rate was increased by 70%, and the shear strength retention rate was increased by 22%, solving the reliability degradation problem caused by the coupling of damp heat and thermal stress in multi-die packaging.

[0045] In summary, the epoxy composite material described in this invention, through multi-component synergistic design, can achieve the requirements of low stress, self-healing, and high reliability with different parameter combinations, and is suitable for advanced structures such as 2.5D / 3D and fan-out wafer-level packaging.

[0046] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-stress, multi-die encapsulated, high-reliability epoxy composite material, characterized in that, The epoxy composite material comprises the following components: Epoxy resin matrix; Curing agent; Inorganic fillers; Microbial metabolite-responsive microcapsules dispersed in the epoxy resin matrix.

2. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, The shell of the microbial metabolite-responsive microcapsule is made of a polymer that is selectively responsive to swelling or degradation of lactic acid or lower carboxylic acids, and its core encapsulates a repair monomer that can be polymerized in situ.

3. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 2, characterized in that, When the epoxy composite material generates a localized acidic microenvironment due to hydrolysis or oxidation during humid heat aging, the microbial metabolite responsive microcapsules undergo structural transformation under stimulation at a pH value below 5.5, releasing the repair monomer. The repair monomer then polymerizes with residual epoxy groups or ambient moisture at the crack interface, achieving in-situ self-repair of the microcracks.

4. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, The inorganic filler is surface-treated with a silane coupling agent and has a volume fraction of 40%-70%; the coefficient of thermal expansion of the epoxy composite material is 10-25 ppm / ℃ below the glass transition temperature and greater than 150℃.

5. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, The epoxy resin matrix is ​​selected from at least one of bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenolic epoxy resin, biphenyl type epoxy resin, naphthalene type epoxy resin, or phosphorus-containing epoxy resin; the curing agent is at least one of acid anhydride curing agent, amine curing agent, or phenolic resin curing agent, and its dosage is from 1:0.8 to 1:1.2 based on the ratio of epoxy equivalent to curing agent active hydrogen equivalent.

6. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 4, characterized in that, The inorganic filler is at least one of fused silica, crystalline silica, spherical alumina, boron nitride, aluminum nitride, or magnesium oxide; the silane coupling agent is at least one of γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, with a treatment concentration of 0.5-2.0 wt%.

7. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, The average particle size of the microbial metabolite-responsive microcapsules is 1-10 μm, and the wall thickness is 50-300 nm; the shell material is at least one of poly(lactic acid-co-hydroxyacetic acid), poly(ε-caprolactone-co-lactic acid), poly(acrylic acid-co-methyl methacrylate), or chitosan derivatives, and the critical response pH threshold is 4.0-5.

5.

8. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 3, characterized in that, The repair monomer is at least one of dicyclopentadiene, glycidyl methacrylate, vinylsiloxane, furanyl epoxy monomer or mercapto-containing acrylate; the coefficient of thermal expansion of the repair phase formed after polymerization of the repair monomer is 20-40 ppm / ℃ and the Young's modulus is 2-5 GPa.

9. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, The added mass fraction of the microbial metabolite responsive microcapsules is 0.5%-5.0%; their dispersion in the epoxy resin matrix is ​​achieved by high-speed shear mixing at 2000-8000 rpm for 5-30 minutes.

10. The low-stress multi-die encapsulated high-reliability epoxy composite material according to claim 1, characterized in that, It also includes a thermal stress buffer phase, which is a core-shell rubber particle or an organosilicon-modified epoxy prepolymer, with an addition mass fraction of 2%-8%; the core of the core-shell rubber particle is polybutadiene or polyisoprene, and the shell is polymethyl methacrylate or polystyrene.