A new type of flexible printed circuit board and camera module prepared based on a weldable low-temperature silver paste

Abstract

The application discloses a novel flexible printed circuit board and camera module prepared based on weldable low-temperature silver paste, and belongs to the technical field of electronic materials and devices. The conductor pattern in the flexible printed circuit board comprises a main circuit pattern and an auxiliary function structure formed by weldable low-temperature cured silver paste. The curing temperature of the weldable low-temperature silver paste is 87-103 DEG C. The silver paste comprises conductive fillers, a polymer resin system, ester solvents, a curing agent, a silane coupling agent, polyamide wax thixotropic agents, polysiloxane defoaming agents, modified carbon nanotubes, modified antimony tin oxide powder, modified ceramic fillers and low-stress modifiers. The polymer resin system comprises hydrogenated bisphenol A type epoxy resin, diphenyl type epoxy resin and polyacrylate-CTBN copolymer. The conductive fillers comprise silver-coated copper flaky powder, nano silver wire and low-temperature alloy powder. Through the synergistic effect of components and the innovative process, the comprehensive improvement of mechanical properties, conductive properties, high-frequency characteristics and welding reliability is realized under the premise of ultralow-temperature curing.

Description

Technical Field

[0001] This invention relates to the field of electronic materials and devices, and in particular to a novel flexible printed circuit board and camera module based on solderable low-temperature silver paste. Background Technology

[0002] Flexible printed circuit boards (PCBs), with their excellent thinness and lightness and adaptability to three-dimensional deformation, have become a key carrier for achieving high image quality, miniaturization, multi-camera collaboration, and optical image stabilization in smart camera modules. Camera modules, especially main cameras and periscope telephoto modules, place more stringent performance requirements on PCBs. These include: 1. High-precision miniaturization challenges: The compact stacking of multi-camera arrays and modules requires extremely high wiring density on PCBs, with line width / spacing ≤20μm and pad size ≤40μm×40μm to connect the image sensor and VCM driver chip; 2. Stronger mechanical reliability: The continuous micro-movements of optical image stabilization components and the impact of phone drops require PCB conductors with excellent fatigue resistance and high bonding strength at solder joints to prevent micro-cracks that could cause signal interruption; 3. Thermo-mechanical stress challenges: CMOS sensors are extremely sensitive to temperature, and the reflow soldering process requires a silver paste curing temperature ≤105℃, while also overcoming internal stress caused by thermal expansion coefficient mismatch to prevent warping and image module defocusing.

[0003] Therefore, for the manufacturing of precision flexible printed circuit boards for camera modules, silver paste must have ultra-low temperature curing properties, high resolution printing capability, high soldering reliability, and excellent resistance to mechanical and thermal stress. Summary of the Invention

[0004] The purpose of this invention is to provide a novel flexible printed circuit board and camera module based on solderable low-temperature silver paste, in order to solve the prominent problem that existing flexible printed circuit board technology cannot simultaneously meet the requirements of ultra-high precision wiring, high reliability connection and ultra-low temperature manufacturing of camera modules. It overcomes the risk of sensor thermal damage caused by the high curing temperature of traditional silver paste, eliminates the performance failure of image module caused by insufficient silver paste welding strength or thermal stress concentration, and avoids quality defects such as micro-line breakage and signal attenuation caused by poor silver paste leveling or insufficient conductivity.

[0005] To achieve the above objectives, the present invention provides the following solution:

[0006] This invention provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The key feature is that the conductor pattern includes a main circuit pattern formed by rolled copper and an auxiliary functional structure formed by solderable low-temperature silver paste in a local area of ​​the main circuit pattern. The auxiliary functional structure includes at least one of a VCM driving line area, a sensor micro pad area, and a lens mount connection area.

[0007] The curing temperature of the aforementioned solderable low-temperature silver paste is 87℃~103℃; by mass parts, the aforementioned solderable low-temperature silver paste contains 60 parts~72 parts conductive filler, 12 parts~18 parts polymer resin system, 18 parts~30 parts ester solvent, 3 parts~6 parts curing agent, 1 part~5 parts silane coupling agent, 0.8 parts~1.5 parts polyamide wax thixotropic agent, 0.1 parts~0.3 parts polysiloxane defoamer, 0.3 parts~1 part modified carbon nanotubes, 0.5 parts~1.5 parts modified antimony tin oxide powder, 1 part~2 parts modified ceramic filler, and 0.8 parts~1.5 parts low-stress modifier;

[0008] The aforementioned polymer resin system comprises hydrogenated bisphenol A type epoxy resin, biphenyl type epoxy resin, and polyacrylate-carboxyl-terminated liquid nitrile rubber copolymer in a mass ratio of 1:(0.9~1.1):(0.3~0.5); the aforementioned conductive filler comprises silver-coated copper sheet powder, silver nanowires, and low-temperature alloy powder. The aforementioned silver-coated copper sheet powder has a D50 range of 1.0μm~3.0μm and an aspect ratio >15. The aforementioned silver nanowires have a length of 25μm~45μm and a diameter of 30nm~60nm. The aforementioned low-temperature alloy powder has a D50 range of 3μm~8μm.

[0009] Specifically, the mass ratio of silver-coated copper flake powder, silver nanowires, and low-temperature alloy powder in the above-mentioned conductive filler is (1.2-1.8):1:(0.1-0.3); the above-mentioned low-temperature alloy powder is Sn57.6Bi40Ag2.4 or Sn58Bi42; the above-mentioned curing agent is a linear phenolic resin and an imidazole accelerator in a mass ratio of (4-5):1, and the above-mentioned imidazole accelerator is 2-phenylimidazoline or 2-heptadecylimidazolium; the above-mentioned silane coupling agent is a mixture of KH-560 and a silane coupling agent with anti-migration function in a mass ratio of 1:(1-2), and the above-mentioned silane coupling agent with anti-migration function is KH-590 or aniline methyltriethoxysilane; the above-mentioned polysiloxane defoamer is a non-silicone defoamer containing defoaming polysiloxane; the above-mentioned ester solvent is any one of diethylene glycol butyl ether acetate, ethylene glycol ethyl ether acetate, or butyl butyrate.

[0010] Specifically, the epoxy value of the above-mentioned biphenyl-type epoxy resin is 0.50 eq / 100g to 0.65 eq / 100g, and the viscosity at 25℃ is 5000 mPa·s to 12000 mPa·s; the preparation method of the above-mentioned polyacrylate-CTBN copolymer is as follows: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are prepared by carrying out a living free radical polymerization reaction at 75℃ to 85℃ for 3h to 5h in the presence of tert-butyl peroxide-2-ethylhexanoate initiator at a mass ratio of (4-5):(4-5):1; wherein, the carboxyl group mass fraction in the carboxyl-terminated liquid nitrile rubber is 0.5% to 0.7%, and the number average molecular weight is 3000 Da to 5000 Da; the mass of tert-butyl peroxide-2-ethylhexanoate initiator is 1.0% to 2.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0011] More specifically, the preparation method of the above-mentioned modified ceramic filler is as follows: aluminum nitride powder with a particle size of 0.05μm to 0.15μm is dispersed in anhydrous ethanol, and a mixed modifier of silane coupling agent KH-560 and polyethyleneimine with a mass ratio of 1:(2 to 3) is added. The mixture is ultrasonically stirred at 300W to 500W at 60℃ to 70℃ for 2h to 3h, filtered, and then calcined at 90℃ to 105℃ for 1h to 2h. The total mass of the mixed modifier is 4% to 6% of the aluminum nitride powder.

[0012] More specifically, the above-mentioned polyamide wax thixotropic agent is a polyamide wax formed by the condensation of castor oil acid and dimer fatty acids with ethylenediamine. Before use, the above-mentioned polyamide wax thixotropic agent needs to be activated. The specific operation of the activation treatment is as follows: add the polyamide wax to a mixed solvent of isopropanol and propylene glycol methyl ether acetate with a volume ratio of 1:1.5 to 2.0, add aminopropanol, control the activation temperature at 45℃ to 55℃, the stirring speed at 800 r / min to 1200 r / min, and disperse and activate at high speed for 15 min to 30 min; wherein, the mass ratio of polyamide wax, mixed solvent and aminopropanol is 1:(4 to 6):(0.1 to 0.2).

[0013] Furthermore, the preparation process of the above-mentioned modified carbon nanotubes, in parts by mass, is as follows:

[0014] A1. Electrolyte preparation: Disperse 15 parts of carbon nanotubes in 350 to 450 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid, add 3 to 5 parts of 3-aminopropyltriethoxysilane as a functionalizing agent, and sonicate at 500 W to 700 W for 40 to 60 minutes to form a suspension electrolyte;

[0015] A2. Electrochemical treatment: The above-mentioned suspended electrolyte is placed in an electrolytic cell, with a graphite rod as the anode and a platinum sheet as the cathode, and the electrolysis reaction is carried out for 1.5h to 2.5h under a voltage of 2.5V to 3.5V and a stirring speed of 500r / min to 700r / min.

[0016] A3. Post-reaction treatment: After the reaction is completed, the solid is filtered and washed with alternating centrifugation of ethanol and deionized water until the conductivity of the supernatant is <5μS / cm; the modified carbon nanotubes are obtained by vacuum drying at 70℃~80℃ for 8h~12h.

[0017] Furthermore, the preparation process of the above-mentioned modified antimony tin oxide powder, in parts by mass, is as follows:

[0018] B1. Preparation of suspension: Disperse 10 parts of antimony tin oxide powder with an average particle size of 20nm to 40nm in 200 to 250 parts of isopropanol, add 0.5 to 1 part of polyvinylpyrrolidone, and sonicate at 600W to 800W for 50 to 70 minutes to prepare a suspension.

[0019] B2. In-situ polymerization: Add 0.8 to 1.2 parts of 3,4-ethylenedioxythiophene monomer to the above suspension and stir for 40 to 60 minutes at 15 to 20 degrees Celsius; add 3 parts of ethanol solution of potassium persulfate with a mass concentration of 8% to 12% and control the dropping temperature at 20 to 24 degrees Celsius, and react for 8 to 10 hours.

[0020] B3. Post-reaction treatment: After the reaction is completed, filter the mixture and wash the filter cake repeatedly with ethanol and acetone until the filtrate is colorless. Dry the filter cake under vacuum at 60℃~70℃ for 24h~36h, grind it and pass it through a 1000-1250 mesh sieve to obtain the above-mentioned modified antimony tin oxide powder.

[0021] Furthermore, by weight, the aforementioned low-stress modifier is a core-shell rubber microparticle of carboxyl-terminated polybutadiene acrylonitrile with a polymethyl methacrylate shell and an average particle size range of 100 nm to 200 nm. The preparation method is as follows:

[0022] C1. Preparation of the core emulsion: 80-100 parts of carboxyl-terminated liquid nitrile rubber, 5-10 parts of acrylonitrile, 15-25 parts of deionized water, 1.0-2.5 parts of sodium allyloxyhydroxypropyl sulfonate, and 0.5-1.5 parts of pH buffer were placed in a reactor and emulsified at 65-75°C and 200-400 rpm for 30-60 minutes with stirring. Then, the temperature was raised to 125-135°C, and the pressure was maintained at 0.35-0.45 MPa for 3-5 hours. After the reaction was completed, the mixture was cooled and discharged to obtain the core emulsion.

[0023] C2. Pre-emulsification of shell monomers: 40-60 parts of methyl methacrylate, 2-5 parts of β-hydroxyethyl methacrylate, 1-3 parts of vinyltriethoxysilane, 1.0-2.0 parts of sodium allyl hydroxypropyl sulfonate and 50-70 parts of deionized water are mixed and emulsified by high-speed shearing at 1000-3000 r / min for 20-40 min to obtain a shell pre-emulsion;

[0024] C3. Seed emulsion preparation: Take 20 to 30 parts of the core emulsion prepared in step C1, add it to the reaction vessel, dilute it to a solid content of 5% to 10%, heat it to 75℃ to 80℃, purge it with nitrogen for protection, add 0.1 to 0.3 parts of potassium persulfate aqueous solution with a mass concentration of 2% to 5% and 1 to 3 parts of the shell pre-emulsion prepared in step C2 under stirring, react for 20 min to 40 min to obtain the seed emulsion;

[0025] C4. Core-shell emulsion polymerization: The remaining shell pre-emulsion prepared in step C2 and 0.5 to 1.2 parts of potassium persulfate aqueous solution with a mass concentration of 2% to 5% are added dropwise to the above seed emulsion. The dropping temperature is controlled at 78℃ to 82℃. After the dropping is completed, the mixture is kept warm and matured for 1.5 to 2.5 hours.

[0026] C5. Post-processing: Cool to room temperature, filter the material through a 100-200 mesh sieve, and after demulsification, washing, and vacuum drying, grind it through a 500-800 mesh sieve to obtain the above-mentioned carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticles.

[0027] Furthermore, the preparation method of the above-mentioned solderable low-temperature silver paste is as follows:

[0028] S1. Add ester solvent to the reactor, then add polymer resin system, stir at 60℃~80℃ until the resin dissolves, add silane coupling agent and polysiloxane defoamer, cool to room temperature, add curing agent, keep the solution temperature below 30℃, stir until all phases are completely dissolved and mixed to obtain carrier solution.

[0029] S2. Add polyamide wax thixotropic agent to the carrier solution and stir at room temperature to obtain mixed solution A;

[0030] S3. Add low-temperature alloy powder and modified ceramic filler to mixed solution A, and stir and mix at room temperature to obtain mixed solution B;

[0031] S4. Add the silver-coated copper sheet powder and silver nanowires to the mixed solution B, then add the low-stress modifier, and stir and mix at room temperature to obtain the mixed solution C.

[0032] S5. Add the modified carbon nanotubes and modified antimony tin oxide powder to the mixed solution C, stir and mix, and control the stirring temperature to not exceed 30℃ to obtain the mixed solution D;

[0033] S6. Transfer the mixed solution D to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 65μm-75μm and the gap between the middle roller and the slow roller to 25μm-35μm, and grind for 1-2 passes. Then, adjust the gap between the fast roller and the middle roller to 35μm-45μm and the gap between the middle roller and the slow roller to 5-15μm, and grind for another 1-2 passes. Grind for a total of 2-4 passes to obtain the ground silver paste.

[0034] S7. Transfer the ground silver paste to a vacuum stirring vessel and stir and mix it under a vacuum of ≤-0.095MPa. After stirring, encapsulate the silver paste to obtain the above-mentioned solderable low-temperature silver paste.

[0035] A camera module based on solderable low-temperature silver paste includes the aforementioned novel flexible printed circuit board, and an image sensor chip and / or VCM driver chip soldered onto a conductor pattern of the circuit board via chip-level packaging technology. The key feature is that the image sensor chip is soldered to the sensor micro-pad area of ​​the conductor pattern, using Sn42Bi58 low-temperature solder, with soldering parameters of: peak temperature 110℃±3℃ and time 60s±10s; the VCM driver chip is bonded to the VCM driver circuit area of ​​the conductor pattern using anisotropic conductive adhesive, with curing conditions of 90℃ / 20min.

[0036] In this invention, the polyacrylate-carboxyl-terminated liquid nitrile rubber copolymer is referred to as polyacrylate-CTBN copolymer; KH-560 is γ-glycidyl etheroxypropyltrimethoxysilane; KH-590 is mercaptopropyltrimethoxysilane.

[0037] This invention discloses the following technical effects: The novel flexible printed circuit board and camera module prepared by the solderable low-temperature silver paste provided by this invention, through the synergistic effect of each component and innovative process, achieve a comprehensive improvement in mechanical properties, conductivity, high-frequency characteristics, and soldering reliability under the premise of ultra-low temperature curing, meeting the stringent requirements of high precision and high reliability for modern smartphone camera modules. Specific beneficial effects:

[0038] First, the silver paste of this invention can be rapidly cured at ultra-low temperatures of 87℃ to 103℃, effectively avoiding the risk of thermal damage to heat-sensitive components such as CMOS image sensors and VCM voice coil motors caused by high-temperature processes. The three-dimensional network structure of the polyacrylate-CTBN copolymer formed after curing endows the silver paste circuit with excellent dynamic bending resistance, fully meeting the stringent reliability requirements of continuous micro-motion in optical image stabilization components and mobile phone drop impacts.

[0039] Secondly, the silver paste of this invention constructs a highly efficient and stable three-dimensional conductive network with low sheet resistance through multi-scale compounding of silver nanowires, silver-coated copper flake powder, and low-temperature alloy powder, as well as the innovative application of electrochemically modified carbon nanotubes and PEDOT-coated antimony tin oxide. After undergoing a rigorous temperature cycle from -20℃ to 60℃ at 5GHz, the impedance change rate is <3.2%, perfectly matching the signal integrity requirements of high-speed image data transmission in multi-camera modules, ensuring stable transmission of autofocus and optical image stabilization control signals.

[0040] Third, the filler and low-temperature alloy components treated with a specific modification process introduced in this invention achieve ultra-uniform dispersion of nanofillers in the system, enabling the direct formation of a highly active solderable layer on the cured silver paste surface. This results in a strong metallurgical bond with Sn42Bi58 solder, exhibiting high welding shear strength, with most failure modes being internal fracture at the solder joint. The welding yield exceeds 98.5%, successfully eliminating the need for pre-plating gold / tin, and providing a highly reliable and low-cost solution for the direct mounting of camera sensors and VCM driver chips.

[0041] Fourth, this invention integrates ultra-low temperature curing, ultra-high flexibility, excellent high-frequency conductivity and direct solderability into one, providing a comprehensive solution suitable for high-density interconnection, micro-line, and direct packaging of micro-pads in smartphone camera modules. It effectively solves the problems of sensor damage caused by high-temperature processes, the need for pre-plating layers for welding, and impedance matching and reliability issues caused by high-density integration in existing technologies, and significantly improves the yield and long-term reliability of camera modules. Detailed Implementation

[0042] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the invention, but rather as a more detailed description of certain aspects, features, and embodiments of the invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, regarding numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included within the scope of this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0043] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, which will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary. The terms "comprising," "including," "having," "containing," etc., as used herein are open-ended, meaning they include but are not limited to.

[0044] Example 1

[0045] This embodiment provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed from rolled copper, and auxiliary functional structures formed by a solderable low-temperature silver paste sample 1 in a local area of ​​the main circuit pattern. In this embodiment, the auxiliary functional structures include a VCM driving circuit area, a sensor micro-pad area, and a lens mount connection area.

[0046] By mass, the solderable low-temperature silver paste sample 1 of this embodiment comprises: 67 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 25 parts diethylene glycol butyl ether acetate, 4 parts curing agent sample 1, 3 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.2 parts polysiloxane defoamer BYK-055, 0.6 parts modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, 1.5 parts modified ceramic filler sample 1, and 1.0 part low-stress modifier sample 1.

[0047] The conductive filler sample 1 comprises: a silver-coated copper sheet powder sample 1 with a D50 of 2.0 μm, a silver nanowire sample 1 with an aspect ratio of 18, a length of 35 μm, and a diameter of 45 nm, and a Sn57.6Bi40Ag2.4 (representing 57.6% tin, 40% bismuth, and 2.4% silver) with a D50 of 5 μm; wherein the mass ratio of the silver-coated copper sheet powder sample 1, the silver nanowire sample 1, and the low-temperature alloy powder sample 1 is 1.5:1:0.2.

[0048] Sample 1 of the polymer resin system includes hydrogenated bisphenol A type epoxy resin, biphenyl type epoxy resin sample 1, and polyacrylate-CTBN copolymer sample 1 in a mass ratio of 1:1:0.4. Among them, the epoxy value of biphenyl type epoxy resin sample 1 is 0.58 eq / 100g, and the viscosity at 25℃ is 8000 mPa·s. The specific preparation method of polyacrylate-CTBN copolymer sample 1 is as follows: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are prepared by carrying out a living free radical polymerization reaction at 80℃ for 4h in the presence of tert-butyl peroxide-2-ethylhexanoate initiator in a mass ratio of 4.5:4.7:1. Among them, the carboxyl group mass fraction in the carboxyl-terminated liquid nitrile rubber is 0.6%, and the number average molecular weight is 4000 Da. The mass of tert-butyl peroxide-2-ethylhexanoate initiator is 1.5% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0049] Curing agent sample 1 is a linear phenolic resin and an imidazole accelerator in a mass ratio of 4.5:1. The imidazole accelerator is 2-phenylimidazoline.

[0050] Polyamide wax thixotropic agent sample 1 is a polyamide wax sample 1, specifically BYK-410, formed by the condensation of castor oil acid and dimer fatty acids with ethylenediamine. It needs to be activated before use. The specific activation procedure is as follows: add BYK-410 to a mixed solvent of isopropanol and propylene glycol methyl ether acetate in a volume ratio of 1:1.8, add aminopropanol, control the activation temperature at 50℃, stir at 1000 r / min, and disperse and activate at high speed for 20 min; wherein, the mass ratio of BYK-410, mixed solvent and aminopropanol is 1:5:0.15.

[0051] Silane coupling agent sample 1 is a mixture of KH-560 and KH-590 in a mass ratio of 1:1.5.

[0052] The modified ceramic filler sample 1 was prepared by dispersing 1 part of aluminum nitride powder with a particle size of 0.1 μm in 8 parts of anhydrous ethanol, adding a mixed modifier of silane coupling agent KH-560 and polyethyleneimine in a mass ratio of 1:2.5, reacting with ultrasonic stirring at 400 W at 65℃ for 2.5 h, filtering, and then calcining at 100℃ for 1.5 h; wherein, the total mass of the mixed modifier is 5% of the aluminum nitride powder.

[0053] The preparation process of modified carbon nanotube sample 1, by mass parts, is as follows:

[0054] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 400 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid, and 4 parts of 3-aminopropyltriethoxysilane were added as functionalizing reagent. The mixture was ultrasonically treated with 600W power for 50 minutes to form suspension electrolyte 1.

[0055] A2. Electrochemical treatment: The suspension electrolyte 1 was placed in an electrolytic cell, with a graphite rod as the anode and a platinum sheet as the cathode, and the electrolysis reaction was carried out for 2 hours under a voltage of 3.0V and a stirring speed of 600r / min.

[0056] A3. Post-reaction treatment: After the reaction, the solid was obtained by filtration. The solid was washed 5 times by alternating centrifugation with ethanol and deionized water until the conductivity of the supernatant was <5μS / cm. The solid was then vacuum dried at 75℃ for 10h to obtain modified carbon nanotube sample 1.

[0057] The preparation process of modified antimony tin oxide powder sample 1, by mass parts, is as follows:

[0058] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 30 nm were dispersed in 220 parts of isopropanol, 0.8 parts of polyvinylpyrrolidone were added, and the mixture was ultrasonically treated with 700 W power for 60 min to prepare suspension 1.

[0059] B2. In-situ polymerization: Add 1.0 part of 3,4-ethylenedioxythiophene monomer to suspension 1 and stir at 18°C ​​for 50 min; add 3 parts of ethanol solution with a mass concentration of 10% potassium persulfate dropwise, controlling the dropwise addition temperature at 20°C, and react for 9 h;

[0060] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with ethanol and acetone until the filtrate is colorless; the filter cake is vacuum dried at 65℃ for 30h, ground and passed through an 1100-mesh sieve to obtain modified antimony tin oxide powder sample 1.

[0061] The preparation process of low-stress modifier sample 1, by mass parts, is as follows:

[0062] C1. Preparation of core emulsion: 90 parts of carboxyl-terminated liquid nitrile rubber, 8 parts of acrylonitrile, 20 parts of deionized water, 2 parts of sodium allyl hydroxypropyl sulfonate and 1 part of pH buffer were placed in a reaction vessel and emulsified at 70℃ and 300 r / min for 45 min with stirring; then the temperature was raised to 130℃ and the pressure was maintained at 0.40 MPa for 4 h; after the reaction was completed, the mixture was cooled and discharged to obtain core emulsion 1.

[0063] C2. Pre-emulsification of shell monomers: 50 parts of methyl methacrylate, 3 parts of β-hydroxyethyl methacrylate, 2 parts of vinyltriethoxysilane, 1.5 parts of sodium allyl hydroxypropyl sulfonate and 60 parts of deionized water were mixed and emulsified by high-speed shearing at 2000 r / min for 30 min to obtain shell pre-emulsion 1.

[0064] C3. Seed emulsion preparation: Take 25 parts of the core emulsion 1 prepared in step C1, add it to the reaction vessel, dilute it to a solid content of 8%, heat it to 78°C, introduce nitrogen protection, add 0.2 parts of potassium persulfate aqueous solution with a mass concentration of 3.5% and 2 parts of shell pre-emulsion 1 prepared in step C2 under stirring, react for 30 min to obtain seed emulsion 1;

[0065] C4. Core-shell emulsion polymerization: The remaining shell pre-emulsion 1 prepared in step C2 and 0.8 parts of 2.5% potassium persulfate aqueous solution were added dropwise to seed emulsion 1, and the dropping temperature was controlled at 80℃. After the dropping was completed, the mixture was kept warm and matured for 2.0h.

[0066] C5. Post-processing: After cooling to room temperature, filter the material through a 150-mesh sieve. After demulsification, washing, and vacuum drying, grind the material through a 600-mesh sieve to obtain carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 1. The average particle size of carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 1 is 156 nm.

[0067] Using the above-mentioned raw materials, a solderable low-temperature silver paste sample 1 was prepared, specifically as follows:

[0068] S1. Add 25 parts of diethylene glycol butyl ether acetate to the reactor, then add 15 parts of polymer resin system sample 1. Stir at 70°C and 400 r / min for 30 min until the resin is completely dissolved. Add 3 parts of silane coupling agent sample 1 and 0.2 parts of polysiloxane defoamer BYK-055. Cool to room temperature, add 4 parts of curing agent sample 1, keep the solution temperature at 25°C, and stir until all phases are completely dissolved and mixed to obtain carrier solution 1.

[0069] S2. Add 1 part of activated polyamide wax thixotropic agent sample 1 to carrier solution 1, and stir at 500 r / min for 45 min at room temperature to obtain mixed solution A1.

[0070] S3. Add 5 parts of Sn57.6Bi40Ag2.4 low-temperature alloy powder sample 1 and 1.5 parts of modified ceramic filler sample 1 to the mixed solution A1, and stir at 500 r / min for 45 min at room temperature to obtain mixed solution B1.

[0071] S4. Add 37.2 parts of silver-coated copper sheet powder sample 1 and 24.8 parts of silver nanowire sample 1 to the mixed solution B1, and then add 1.0 part of low stress modifier sample 1. Stir at 1000 r / min for 80 min at room temperature to obtain mixed solution C1.

[0072] S5. Add 0.6 parts of modified carbon nanotube sample 1 and 1 part of modified antimony tin oxide powder sample 1 to the mixed solution C1, stir at 1200 r / min for 110 min, and control the stirring temperature at 25℃ to obtain mixed solution D1.

[0073] S6. Transfer the mixed solution D1 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 70 μm and the gap between the middle roller and the slow roller to 30 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 40 μm and the gap between the middle roller and the slow roller to 10 μm and grind once more. Grind three times to obtain the ground silver paste sample 1.

[0074] S7. Transfer the ground silver paste sample 1 to a vacuum mixing vessel and stir at a speed of 40 r / min for 30 min under a vacuum of -0.100 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 1. The curing temperature of solderable low-temperature silver paste sample 1 is 98℃.

[0075] Example 2

[0076] This embodiment provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed from rolled copper, and auxiliary functional structures formed by the solderable low-temperature silver paste sample 2 in a local area of ​​the main circuit pattern; in this embodiment, the auxiliary functional structures include a VCM driving circuit area and a sensor micropad area.

[0077] By mass, the solderable low-temperature silver paste sample 2 of this embodiment comprises: 60 parts conductive filler sample 2, 12 parts polymer resin system sample 2, 18 parts ethylene glycol ethyl ether acetate, 6 parts curing agent sample 2, 1 part silane coupling agent sample 2, 0.8 parts polyamide wax thixotropic agent sample 2, 0.3 parts polysiloxane defoamer BYK-088, 1 part modified carbon nanotube sample 2, 0.5 parts modified antimony tin oxide powder sample 2, 1 part modified ceramic filler sample 2, and 1.5 parts low-stress modifier sample 2.

[0078] The conductive filler sample 2 comprises: a silver-coated copper flake powder sample 2 with a D50 of 3.0 μm, a silver nanowire sample 2 with an aspect ratio of 22, a length of 45 μm, and a diameter of 30 nm, and a Sn58Bi42 (representing 58% tin and 42% bismuth) low-temperature alloy powder sample 2 with a D50 of 3 μm; wherein the mass ratio of the silver-coated copper flake powder sample 2, the silver nanowire sample 2, and the low-temperature alloy powder sample 2 is 1.2:1:0.3.

[0079] Sample 2 of the polymer resin system includes hydrogenated bisphenol A type epoxy resin, biphenyl type epoxy resin sample 2, and polyacrylate-CTBN copolymer sample 2, with a mass ratio of 1:1.1:0.3. Among them, the epoxy value of biphenyl type epoxy resin sample 2 is 0.65 eq / 100g, and the viscosity at 25℃ is 5000 mPa·s. The specific preparation method of polyacrylate-CTBN copolymer sample 2 is as follows: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are prepared by carrying out a living free radical polymerization reaction at 75℃ for 5h in the presence of tert-butyl peroxide-2-ethylhexanoate initiator at a mass ratio of 4:5:1. Among them, the carboxyl group mass fraction in the carboxyl-terminated liquid nitrile rubber is 0.7%, and the number average molecular weight is 3000 Da. The mass of tert-butyl peroxide-2-ethylhexanoate initiator is 2.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0080] Curing agent sample 2 is a linear phenolic resin and an imidazole accelerator in a mass ratio of 5:1. The imidazole accelerator is 2-heptadecylimidazolium.

[0081] Polyamide wax thixotropic agent sample 2 is a polyamide wax sample 2 formed by the condensation of castor oil acid and dimer fatty acids with ethylenediamine, specifically DISPARLON 6650. It requires activation before use. The specific activation procedure is as follows: DISPARLON 6650 is added to a mixed solvent of isopropanol and propylene glycol methyl ether acetate at a volume ratio of 1:2.0, aminopropanol is added, the activation temperature is controlled at 45℃, the stirring speed is 1200 r / min, and high-speed dispersion activation is performed for 30 min; wherein, the mass ratio of DISPARLON 6650, the mixed solvent, and aminopropanol is 1:4:0.2.

[0082] Silane coupling agent sample 2 is a mixture of KH-560 and aniline methyltriethoxysilane in a mass ratio of 1:1.

[0083] The modified ceramic filler sample 2 was prepared by dispersing 1 part of aluminum nitride powder with a particle size of 0.15 μm in 9 parts of anhydrous ethanol, adding a mixed modifier of silane coupling agent KH-560 and polyethyleneimine in a mass ratio of 1:3, reacting with ultrasonic stirring at 500 W at 60℃ for 2 h, filtering, and calcining at 105℃ for 2 h; wherein, the total mass of the mixed modifier was 4% of the aluminum nitride powder.

[0084] The preparation process of modified carbon nanotube sample 2, by mass parts, is as follows:

[0085] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 450 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid, and 3 parts of 3-aminopropyltriethoxysilane were added as functionalizing reagent. The mixture was ultrasonically treated with 500W power for 60 minutes to form suspension electrolyte 2.

[0086] A2. Electrochemical treatment: The suspension electrolyte 2 was placed in an electrolytic cell, with a graphite rod as the anode and a platinum sheet as the cathode, and the electrolysis reaction was carried out for 1.5 hours under a voltage of 3.5V and a stirring speed of 500r / min.

[0087] A3. Post-reaction treatment: After the reaction, the solid was obtained by filtration. The solid was washed 4 times by alternating centrifugation with ethanol and deionized water until the conductivity of the supernatant was <5μS / cm. The solid was then vacuum dried at 80℃ for 8h to obtain modified carbon nanotube sample 2.

[0088] The preparation process of modified antimony tin oxide powder sample 2, by mass parts, is as follows:

[0089] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 20 nm were dispersed in 250 parts of isopropanol, 1 part of polyvinylpyrrolidone was added, and the mixture was ultrasonically treated with 800 W power for 50 min to prepare suspension 2.

[0090] B2. In-situ polymerization: Add 0.8 parts of 3,4-ethylenedioxythiophene monomer to suspension 2 and stir at 20°C for 40 min; add 3 parts of ethanol solution with a mass concentration of 12% potassium persulfate, controlling the addition temperature at 24°C, and react for 10 h.

[0091] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with ethanol and acetone until the filtrate is colorless; the filter cake is vacuum dried at 70℃ for 24h, ground and passed through a 1000-mesh sieve to obtain modified antimony tin oxide powder sample 2.

[0092] The preparation process of low-stress modifier sample 2, by mass parts, is as follows:

[0093] C1. Preparation of core emulsion: 100 parts of carboxyl-terminated liquid nitrile rubber, 5 parts of acrylonitrile, 15 parts of deionized water, 2.5 parts of sodium allyloxyhydroxypropyl sulfonate and 0.5 parts of pH buffer were placed in a reaction vessel and emulsified at 75℃ and 200 r / min for 30 min with stirring; then the temperature was raised to 135℃ and the pressure was maintained at 0.45 MPa for 3 h; after the reaction was completed, the mixture was cooled and discharged to obtain core emulsion 2.

[0094] C2. Pre-emulsification of shell monomers: 40 parts of methyl methacrylate, 5 parts of β-hydroxyethyl methacrylate, 1 part of vinyltriethoxysilane, 2 parts of sodium allyl hydroxypropyl sulfonate and 70 parts of deionized water were mixed and emulsified by high-speed shearing at 1000 r / min for 40 min to obtain shell pre-emulsion 2.

[0095] C3. Seed emulsion preparation: Take 30 parts of the core emulsion 2 prepared in step C1, add it to the reaction vessel, dilute it to a solid content of 5%, heat it to 75°C, introduce nitrogen protection, add 0.1 parts of potassium persulfate aqueous solution with a mass concentration of 5% and 3 parts of shell pre-emulsion 2 prepared in step C2 under stirring, react for 40 min to obtain seed emulsion 2.

[0096] C4. Core-shell emulsion polymerization: The remaining shell pre-emulsion 2 prepared in step C2 and 1.2 parts of 5% potassium persulfate aqueous solution were added dropwise to seed emulsion 2, and the dropping temperature was controlled at 78℃. After the dropping was completed, the mixture was kept warm and matured for 2.5h.

[0097] C5. Post-processing: After cooling to room temperature, filter the material through a 100-mesh sieve. After demulsification, washing, and vacuum drying, grind the material through an 800-mesh sieve to obtain carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 2. The average particle size of carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 2 is 100 nm.

[0098] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 2 was prepared, specifically as follows:

[0099] S1. Add 18 parts of ethylene glycol ethyl ether acetate to the reactor, then add 12 parts of polymer resin system sample 2. Stir at 60°C and 300 r / min for 40 min until the resin is completely dissolved. Add 1 part of silane coupling agent sample 2 and 0.3 parts of polysiloxane defoamer BYK-088. Cool to room temperature, add 6 parts of curing agent sample 2, keep the solution temperature at 29°C, and stir until all phases are completely dissolved and mixed to obtain carrier solution 2.

[0100] S2. Add 0.8 parts of activated polyamide wax thixotropic agent sample 2 to carrier solution 2, and stir at 600 r / min for 30 min at room temperature to obtain mixed solution A2.

[0101] S3. Add 7.2 parts of Sn58Bi42 low-temperature alloy powder sample 2 and 1 part of modified ceramic filler sample 2 to the mixed solution A2, and stir at 600 r / min for 30 min at room temperature to obtain mixed solution B2.

[0102] S4. Add 28.8 parts of silver-coated copper sheet powder sample 2 and 24 parts of silver nanowire sample 2 to the mixed solution B2, and then add 1.5 parts of low-stress modifier sample 2. Stir at 1500 r / min for 90 min at room temperature to obtain mixed solution C2.

[0103] S5. Add 1 part of modified carbon nanotube sample 2 and 0.5 parts of modified antimony tin oxide powder sample 2 to the mixed solution C2, stir at 1500 r / min for 90 min, and control the stirring temperature at 30℃ to obtain mixed solution D2.

[0104] S6. Transfer the mixed solution D2 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 75 μm and the gap between the middle roller and the slow roller to 25 μm and grind once. Then, adjust the gap between the fast roller and the middle roller to 35 μm and the gap between the middle roller and the slow roller to 5 μm and grind once more. Grind twice to obtain the ground silver paste sample 2.

[0105] S7. Transfer the ground silver paste sample 2 to a vacuum mixing vessel and stir at 50 r / min for 20 min under a vacuum of -0.095 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 2. The curing temperature of solderable low-temperature silver paste sample 2 is 103℃.

[0106] Example 3

[0107] This embodiment provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed from rolled copper, and an auxiliary functional structure formed by a solderable low-temperature silver paste sample 3 in a local area of ​​the main circuit pattern; in this embodiment, the auxiliary function is a lens mount connection area.

[0108] By mass, the solderable low-temperature silver paste sample 3 of this embodiment contains: 72 parts conductive filler sample 3, 18 parts polymer resin system sample 3, 30 parts butyl butyrate, 3 parts curing agent sample 3, 5 parts silane coupling agent sample 3, 1.5 parts polyamide wax thixotropic agent sample 3, 0.1 parts polysiloxane defoamer TEGO Flow 300, 0.3 parts modified carbon nanotube sample 3, 1.5 parts modified antimony tin oxide powder sample 3, 2 parts modified ceramic filler sample 3, and 0.8 parts low-stress modifier sample 3.

[0109] The conductive filler sample 3 comprises: a silver-coated copper flake powder sample 3 with a D50 of 1.0 μm, a silver nanowire sample 3 with an aspect ratio of 16, a length of 25 μm, and a diameter of 60 nm, and a Sn57.6Bi40Ag2.4 low-temperature alloy powder sample 3 with a D50 of 8 μm; wherein the mass ratio of the silver-coated copper flake powder sample 3, the silver nanowire sample 3, and the low-temperature alloy powder sample 3 is 1.8:1:0.1.

[0110] Sample 3 of the polymer resin system includes hydrogenated bisphenol A type epoxy resin, biphenyl type epoxy resin, and polyacrylate-CTBN copolymer sample 3, with a mass ratio of 1:0.9:0.5. Among them, the epoxy value of the biphenyl type epoxy resin sample 3 is 0.50 eq / 100g, and the viscosity at 25℃ is 12000 mPa·s. The specific preparation method of the polyacrylate-CTBN copolymer sample 3 is as follows: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are prepared by carrying out a living free radical polymerization reaction at 85℃ for 3h in the presence of tert-butyl peroxide-2-ethylhexanoate initiator at a mass ratio of 5:4:1. Among them, the carboxyl group mass fraction in the carboxyl-terminated liquid nitrile rubber is 0.5%, and the number average molecular weight is 5000 Da. The mass of tert-butyl peroxide-2-ethylhexanoate initiator is 1.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0111] Curing agent sample 3 is a linear phenolic resin and an imidazole accelerator in a mass ratio of 4:1. The imidazole accelerator is 2-phenylimidazoline.

[0112] Polyamide wax thixotropic agent sample 3 is a polyamide wax sample 3 formed by the condensation of castor oil acid and dimer fatty acid with ethylenediamine, specifically CRAYVALLAC SUPER. It needs to be activated before use. The specific activation procedure is as follows: add CRAYVALLAC SUPER to a mixed solvent of isopropanol and propylene glycol methyl ether acetate with a volume ratio of 1:1.5, add aminopropanol, control the activation temperature at 55℃, stir at 800 r / min, and disperse and activate at high speed for 15 min; wherein, the mass ratio of CRAYVALLAC SUPER, mixed solvent and aminopropanol is 1:6:0.1.

[0113] Silane coupling agent sample 3 consists of KH-560 and KH-590 in a mass ratio of 1:2.

[0114] The modified ceramic filler sample 3 was prepared by dispersing 1 part of aluminum nitride powder with a particle size of 0.05 μm in 7 parts of anhydrous ethanol, adding a mixed modifier of silane coupling agent KH-560 and polyethyleneimine in a mass ratio of 1:2, reacting with ultrasonic stirring at 300 W at 70℃ for 2 h, filtering, and calcining at 95℃ for 1 h; wherein, the total mass of the mixed modifier is 6% of the aluminum nitride powder.

[0115] The preparation process of modified carbon nanotube sample 3, by mass parts, is as follows:

[0116] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 350 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid, and 5 parts of 3-aminopropyltriethoxysilane were added as functionalizing reagent. The mixture was ultrasonically treated with 700W power for 40 minutes to form suspension electrolyte 3.

[0117] A2. Electrochemical treatment: The suspended electrolyte 3 was placed in an electrolytic cell, with a graphite rod as the anode and a platinum sheet as the cathode, and the electrolysis reaction was carried out for 2.5 h under a voltage of 2.5 V and a stirring speed of 700 r / min.

[0118] A3. Post-reaction treatment: After the reaction, the solid was obtained by filtration. The solid was washed 5 times by alternating centrifugation with ethanol and deionized water until the conductivity of the supernatant was <5μS / cm. The solid was then vacuum dried at 70℃ for 12h to obtain modified carbon nanotube sample 3.

[0119] The preparation process of modified antimony tin oxide powder sample 3, by mass parts, is as follows:

[0120] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 40 nm were dispersed in 200 parts of isopropanol, 0.5 parts of polyvinylpyrrolidone were added, and the mixture was ultrasonically treated with 600 W power for 70 min to prepare suspension 3.

[0121] B2. In-situ polymerization: Add 1.2 parts of 3,4-ethylenedioxythiophene monomer to suspension 3 and stir at 15°C for 60 min; add 3 parts of ethanol solution with a mass concentration of 8% potassium persulfate, controlling the addition temperature at 20°C, and react for 8 h;

[0122] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with ethanol and acetone until the filtrate is colorless. The filter cake is vacuum dried at 60°C for 36 hours, ground and passed through a 1250-mesh sieve to obtain modified antimony tin oxide powder sample 3.

[0123] The preparation process of low-stress modifier sample 3, by mass parts, is as follows:

[0124] C1. Preparation of core emulsion: 80 parts of carboxyl-terminated liquid nitrile rubber, 10 parts of acrylonitrile, 25 parts of deionized water, 1.0 part of sodium allyloxyhydroxypropyl sulfonate and 1.5 parts of pH buffer were placed in a reaction vessel and emulsified at 65℃ and 400 r / min for 60 min with stirring; then the temperature was raised to 125℃ and the pressure was maintained at 0.35 MPa for 5 h; after the reaction was completed, the mixture was cooled and discharged to obtain core emulsion 3.

[0125] C2. Pre-emulsification of shell monomers: 60 parts of methyl methacrylate, 2 parts of β-hydroxyethyl methacrylate, 3 parts of vinyltriethoxysilane, 1 part of sodium allyl hydroxypropyl sulfonate and 50 parts of deionized water were mixed and emulsified by high-speed shearing at 3000 r / min for 20 min to obtain shell pre-emulsion 3.

[0126] C3. Seed emulsion preparation: Take 20 parts of the core emulsion 3 prepared in step C1, add it to the reaction vessel, dilute it to a solid content of 10%, heat it to 80℃, introduce nitrogen for protection, add 0.3 parts of potassium persulfate aqueous solution with a mass concentration of 2% and 1 part of shell pre-emulsion 3 prepared in step C2 under stirring, react for 20 min to obtain seed emulsion 3.

[0127] C4. Core-shell emulsion polymerization: The remaining shell pre-emulsion 3 prepared in step C2 and 0.5 parts of 2% potassium persulfate aqueous solution are added dropwise to the seed emulsion 3, and the dropping temperature is controlled at 82℃. After the dropping is completed, the mixture is kept warm and matured for 1.5h.

[0128] C5. Post-processing: After cooling to room temperature, filter the material through a 200-mesh sieve. After demulsification, washing, and vacuum drying, grind the material through a 500-mesh sieve to obtain carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 3. The average particle size of carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticle sample 3 is 200 nm.

[0129] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 3 was prepared, specifically as follows:

[0130] S1. Add 30 parts of butyl butyrate to the reactor, then add 18 parts of polymer resin system sample 3. Stir at 80℃ and 500r / min for 20min until the resin is completely dissolved. Add 5 parts of silane coupling agent sample 3 and 0.1 parts of polysiloxane defoamer TEGO Flow 300. Cool to room temperature, add 3 parts of curing agent sample 3, keep the solution temperature at 22℃, and stir until all phases are completely dissolved and mixed to obtain carrier solution 3.

[0131] S2. Add 1.5 parts of activated polyamide wax thixotropic agent sample 3 to carrier solution 3, and mix at room temperature and 400 r / min for 60 min to obtain mixed solution A3.

[0132] S3. Add 2.5 parts of Sn57.6Bi40Ag2.4 low-temperature alloy powder sample 3 and 2 parts of modified ceramic filler sample 3 to mixed solution A3, and stir at 400 r / min for 60 min at room temperature to obtain mixed solution B3.

[0133] S4. Add 44.7 parts of silver-coated copper sheet powder sample 3 and 24.8 parts of silver nanowire sample 3 to mixed solution B3, then add 0.8 parts of low-stress modifier sample 3, and stir at 1000 r / min for 150 min at room temperature to obtain mixed solution C3.

[0134] S5. Add 0.3 parts of modified carbon nanotube sample 3 and 1.5 parts of modified antimony tin oxide powder sample 3 to the mixed solution C3, stir at 1000 r / min for 150 min, and control the stirring temperature at 22℃ to obtain mixed solution D3.

[0135] S6. Transfer the mixed solution D3 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 65 μm and the gap between the middle roller and the slow roller to 35 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 25 μm and the gap between the middle roller and the slow roller to 15 μm and grind twice more. Grind four times to obtain the ground silver paste sample 3.

[0136] S7. Transfer the ground silver paste sample 3 to a vacuum stirring vessel and stir at 30 r / min for 40 min under a vacuum of -0.102 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 3. The curing temperature of solderable low-temperature silver paste sample 3 is 87℃.

[0137] Example 4

[0138] This embodiment provides a camera module based on solderable low-temperature silver paste, including a novel flexible printed circuit board prepared in Embodiment 1, and an image sensor chip and a VCM driver chip soldered onto the conductor pattern of the circuit board using a chip-level packaging process. The image sensor chip is soldered to the sensor micropad area of ​​the conductor pattern using Sn42Bi58 low-temperature solder, with the following soldering parameters: peak temperature 110℃±3℃, time 60s±10s. The VCM driver chip is bonded to the VCM driver circuit area of ​​the conductor pattern using anisotropic conductive adhesive, with a curing condition of 90℃ / 20min.

[0139] Example 5

[0140] This embodiment provides a camera module based on solderable low-temperature silver paste, including the novel flexible printed circuit board prepared in Embodiment 2, and an image sensor chip soldered onto the conductor pattern of the circuit board using a chip-level packaging process. The image sensor chip is soldered to the sensor micro-pad area of ​​the conductor pattern using Sn42Bi58 low-temperature solder, with the following soldering parameters: peak temperature 110℃±3℃, time 60s±10s. The VCM driver chip is bonded to the VCM driver circuit area of ​​the conductor pattern using anisotropic conductive adhesive, with a curing condition of 90℃ / 20min.

[0141] Example 6

[0142] This embodiment provides a camera module based on solderable low-temperature silver paste, including the novel flexible printed circuit board prepared in Embodiment 3, and an image sensor chip and a VCM driver chip soldered onto the conductor pattern of the circuit board using a chip-level packaging process. The image sensor chip is soldered to the sensor micro-pad area of ​​the conductor pattern using Sn42Bi58 low-temperature solder, with the following soldering parameters: peak temperature 110℃±3℃, time 60s±10s. The VCM driver chip is bonded to the VCM driver circuit area of ​​the conductor pattern using anisotropic conductive adhesive, with a curing condition of 90℃ / 20min.

[0143] Comparative Example 1

[0144] This comparative example provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed of rolled copper and auxiliary functional structures formed by solderable low-temperature silver paste reference 1 in a local area of ​​the main circuit pattern. The auxiliary functional structures in this comparative example include a VCM drive circuit area, a sensor micro-pad area, and a lens mount connection area.

[0145] By mass, the solderable low-temperature silver paste control sample 1 of this comparative example comprises: 65 parts conductive filler sample 1, 15 parts polymer resin system control sample 1, 25 parts diethylene glycol butyl ether acetate, 4 parts curing agent sample 1, 3 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 0.6 parts modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, 1.5 parts modified ceramic filler sample 1, and 1.0 part low-stress modifier sample 1.

[0146] Reference standard 1 for polymeric resin system includes bisphenol A type epoxy resin, thermoplastic acrylic resin and polyvinyl butyral in a mass ratio of 1:1:0.4.

[0147] Using the above-mentioned raw materials, a solderable low-temperature silver paste reference standard 1 was prepared. The specific process was the same as in Example 1, except that the polymer resin system sample 1 was replaced with the polymer resin system reference standard 1. The curing temperature of the prepared solderable low-temperature silver paste reference standard 1 was 118°C.

[0148] Comparative Example 2

[0149] This comparative example provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed of rolled copper and auxiliary functional structures formed by solderable low-temperature silver paste reference 2 in a local area of ​​the main circuit pattern. The auxiliary functional structures in this comparative example include a VCM drive circuit area, a sensor micro-pad area, and a lens mount connection area.

[0150] By mass, the solderable low-temperature silver paste control sample 2 of this comparative example contains: 65 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 25 parts diethylene glycol butyl ether acetate, 4 parts curing agent sample 1, 3 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 0.6 parts modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, and 1.5 parts modified ceramic filler sample 1.

[0151] Using the above-mentioned raw materials, a solderable low-temperature silver paste reference standard 2 was prepared. The specific process was the same as in Example 1, except that a low-stress modifier was not added in step S4. The curing temperature of the prepared solderable low-temperature silver paste reference standard 2 was 101°C.

[0152] Comparative Example 3

[0153] This comparative example provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed of rolled copper and auxiliary functional structures formed by solderable low-temperature silver paste reference 3 in a local area of ​​the main circuit pattern. The auxiliary functional structures in this comparative example include a VCM drive circuit area, a sensor micro-pad area, and a lens mount connection area.

[0154] By mass, the solderable low-temperature silver paste reference standard 3 of this comparative example comprises: 65 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 25 parts diethylene glycol butyl ether acetate, 4 parts curing agent sample 1, 3 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 0.6 parts modified carbon nanotube reference standard 1, 1 part modified antimony tin oxide powder reference standard 1, 1.5 parts modified ceramic filler sample 1, and 1.0 part low-stress modifier sample 1.

[0155] The preparation method of modified carbon nanotube reference standard 1 is as follows:

[0156] A1. Acidification treatment: 15 parts of carbon nanotubes were added to a mixed acid consisting of 300 parts of concentrated nitric acid and 120 parts of concentrated sulfuric acid. 3% of the carbon nanotubes were added to ferric nitrate nonahydrate. The mixture was refluxed at 110°C for 6 hours. After cooling, the mixture was filtered, washed with deionized water until neutral, and vacuum dried at 90°C for 18 hours to obtain the carboxylated carbon nanotube sample to be cleaned.

[0157] A2. Cleaning carboxylated carbon nanotubes: Disperse the carboxylated carbon nanotube sample to be cleaned in a 0.3 mol / L nitric acid solution and sonicate at 300 W at 50 °C for 30 min; filter, wash with deionized water until the conductivity of the filtrate decreases by <10 μS / cm, and dry to obtain the carboxylated carbon nanotube sample.

[0158] A3. Silanization modification: Take 10 parts of carboxylated carbon nanotube sample and disperse them in a mixed solution of 160 parts of ethanol and water. The volume ratio of ethanol to water is 1:1.0. Add 2 parts of KH-560 and react with ultrasonication at 300W power and stirring at 400r / min at 70℃ for 6h. After the reaction is completed, filter, wash 3 times with anhydrous ethanol, and dry to obtain silanized modified carbon nanotube sample.

[0159] A4. Supercritical treatment: Take 5 parts of silanized modified carbon nanotube sample, 150 parts of deionized water, and 15 parts of cocamidopropyl dimethylamine acetone and add them to a supercritical reactor. After sealing, introduce carbon dioxide to a pressure of 30 MPa and react at 75°C for 6 hours. After depressurization, add 5 parts of dodecyl dimethyl benzyl ammonium chloride and inject carbon dioxide to a pressure of 50 MPa. Stir at 300 r / min for 4 hours.

[0160] A5. After depressurization and separation, the sample was washed with deionized water and vacuum dried at 90℃ for 8 hours to obtain modified carbon nanotube reference standard 1.

[0161] The preparation method of modified antimony tin oxide powder reference standard 1 is as follows:

[0162] B1. Disperse 10 parts of antimony tin oxide powder with an average particle size of 50 nm in 110 parts of anhydrous ethanol, and sonicate at 300 W for 30 min to obtain a suspension.

[0163] B2. Add 1.5 parts KH-560 and 0.5 parts acetic acid to the suspension as catalysts, and stir the mixture at 400 r / min at 60℃ for 4 h.

[0164] B3. After the reaction was completed, the mixture was centrifuged, washed four times with anhydrous ethanol, dried under vacuum at 80°C for 12 hours, ground, and passed through a 600-mesh sieve to obtain modified antimony tin oxide powder reference standard 1.

[0165] Using the above-mentioned raw materials, a solderable low-temperature silver paste reference standard 3 was prepared. The specific process was the same as in Example 1, except that modified carbon nanotube reference standard 1 was used instead of modified carbon nanotube sample 1, and modified antimony tin oxide powder reference standard 1 was used instead of modified antimony tin oxide powder sample 1. The curing temperature of the prepared solderable low-temperature silver paste reference standard 3 was 104℃.

[0166] Comparative Example 4

[0167] This comparative example provides a novel flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate. The conductor pattern includes a main circuit pattern formed from rolled copper, and auxiliary functional structures formed by the solderable low-temperature silver paste sample 4 in a local area of ​​the main circuit pattern. In this embodiment, the auxiliary functional structures include a VCM driving circuit area, a sensor micro-pad area, and a lens mount connection area.

[0168] By mass, the solderable low-temperature silver paste reference standard 4 of this comparative example comprises: 65 parts conductive filler reference standard 1, 15 parts polymer resin system sample 1, 25 parts diethylene glycol butyl ether acetate, 4 parts curing agent sample 1, 3 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 0.6 parts modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, 1.5 parts modified ceramic filler sample 1, and 1.0 part low-stress modifier sample 1.

[0169] The conductive filler reference standard 1 comprises: flake silver powder with a D50 of 2.0 μm and an aspect ratio of 18 and spherical silver powder with a D50 of 1.0 μm, without adding any silver nanowires or low-temperature alloy powder; wherein the mass ratio of flake silver powder to spherical silver powder is 12:1.

[0170] Using the above-mentioned raw materials, a solderable low-temperature silver paste reference standard 4 was prepared. The specific process was the same as in Example 1, except that the electro-filler reference standard 1 was used instead of the electro-filler sample 1. Specifically, in step S3, Sn57.6Bi40Ag2.4 low-temperature alloy powder sample 1 was not added, but only 1.5 parts of modified ceramic filler sample 1 were added; in step S4, 40 parts of silver-coated copper flake powder sample 1 and 25 parts of silver nanowire sample 1 were not added, but instead 60 parts of flake silver powder and 5 parts of spherical silver powder were added. The curing temperature of the prepared solderable low-temperature silver paste reference standard 4 was 97℃.

[0171] Analysis and Testing

[0172] Flexible circuits were formed on polyimide (PI) substrates using screen printing equipment for solderable low-temperature silver paste samples 1–3 and solderable low-temperature silver paste controls 1–4. After curing at their respective curing temperatures for 30 minutes, mechanical and electrical properties were tested. The linewidth of the flexible circuit was 15 μm, the thickness of the PI substrate was 50 μm, and the thickness of the silver paste was 15 ± 2 μm.

[0173] I. Mechanical Performance Testing

[0174] Mechanical properties include adhesion test, flexural strength test and hardness test. The test results are shown in Table 1.

[0175] Adhesion testing was conducted using a cross-cut adhesion test. After stretching with 3M 610 tape, the peeling level was evaluated (0 being the best and 5 the worst). The test sample was then placed in a humid heat environment at 85℃ / 85%RH for 72 hours for humid heat aging, and the peeling level after aging was evaluated (0 being the best and 5 the worst). Bending resistance testing simulated the actual installation and use environment of a camera module in a mobile phone. A dynamic bending tester was used at normal room temperature (26℃), high temperature (60℃), and low temperature (-20℃) to repeatedly bend the PI substrate with silver paste conductors at a bending radius of R=2mm at 90°. The resistance change was monitored in real time, and the number of bends when the resistance change exceeded 10% was recorded. Hardness was tested using a pencil hardness tester according to standard ASTM D3363.

[0176] Table 1: Mechanical Performance Test Results

[0177]

[0178] As shown in Table 1, the solderable low-temperature silver paste samples of this invention all exhibit excellent comprehensive performance. Among them, solderable low-temperature silver paste sample 1 shows the most outstanding performance, demonstrating a bending life of over 10,000 cycles under normal temperature, high temperature, and low temperature environments, far exceeding industry standards and fully meeting the flexibility requirements of camera module wiring in compact spaces. After 85%RH / 85℃ humid heat aging, the adhesion of solderable low-temperature silver paste sample 1 still remains at grade 0 or 1, proving the outstanding aging resistance and interface stability of the resin system used. Solderable low-temperature silver paste control sample 1, because it uses non-copolymerized CTBN as a toughening agent, has far inferior compatibility and synergistic effect with epoxy resin compared to the polyacrylate-CTBN copolymer of this invention. This results in a significant decrease in its adhesion, aging resistance, and high and low temperature bending resistance, especially at high temperatures where the resin softens and the performance deteriorates sharply. The solderable low-temperature silver paste control sample 2, lacking a low-stress modifier, exhibited slightly weaker curing density and interfacial bonding, resulting in a decrease in adhesion to grade 2 after hygrothermal aging. Its bending resistance was also slightly lower than the three samples of this invention. The solderable low-temperature silver paste control sample 3 used modified carbon nanotubes and modified antimony tin oxide powder prepared using an unmodified process. This resulted in poor dispersibility, potentially forming stress concentration points in the resin matrix, and weak interfacial bonding, leading to a significant decrease in adhesion, aging resistance, and bending resistance. Its mechanical properties were the worst among all samples. The solderable low-temperature silver paste control sample 4 used a single silver powder filler system without silver nanowires and low-temperature alloy powder. Due to the lack of bridging effect from silver nanowires and sintering-promoting effect from low-temperature alloy powder, the resulting conductive network lacked density, resulting in significantly lower bending resistance than the samples of this invention. Furthermore, its adhesion decreased to grade 3 after hygrothermal aging.

[0179] II. Conductivity Test

[0180] The sheet resistance of the cured silver paste conductors was measured using a four-probe tester, and the results are shown in Table 2. The sample was subjected to 50 temperature cycles at 5 GHz, from -20℃ to 60℃, and the shift in characteristic impedance (Z) was measured, expressed as ΔZ / Z, to evaluate high-frequency impedance stability. The results are shown in Table 2. Here, ΔZ represents the absolute value of the maximum deviation between the characteristic impedance value measured at the same frequency (5 GHz) after 50 temperature cycles and the initial value Z0; Z refers to the initial characteristic impedance value measured at room temperature and 5 GHz. This test simulates the temperature variation environment of the camera module when working in a mobile device, which is crucial for the stability of autofocus and optical image stabilization control signal transmission. The sheet resistance of 100 points (probe diameter 5 μm) on a 40 μm × 40 μm pad array was measured, and the relative standard deviation (RSD) was calculated to determine the conductivity uniformity of the micro-pads. The results are shown in Table 2.

[0181] Table 2: Conductivity Test Results

[0182] Sample to be tested Shear resistance (mΩ / □) <![CDATA[High-frequency impedance stability (ΔZ / Z0, %)]]> RSD (%) Solderable low-temperature silver paste sample 1 2.8 1.8 1.1 Sample 2 of solderable low-temperature silver paste 3.2 2.2 1.5 3 samples of solderable low-temperature silver paste 4.5 3.2 2.2 Solderable low-temperature silver paste reference standard 1 5.2 8.5 7.2 Solderable low-temperature silver paste reference standard 2 4.2 4.8 3.8 Weldable Low-Temperature Silver Paste Reference Standard 3 8.5 15.6 12.5 Solderable low-temperature silver paste reference standard 4 12.8 9.8 8.2

[0183] As shown in Table 2, the solderable low-temperature silver paste samples of this invention all exhibit low sheet resistance, excellent high-frequency stability, and extremely high micropad uniformity. Solderable low-temperature silver paste sample 1 has a sheet resistance as low as 2.8 mΩ / □, and its impedance change rate after rigorous temperature cycling at high frequencies is only 1.8%, perfectly matching the stringent requirements of modern multi-camera modules for high-speed data transmission and stable signal transmission. Its micropad sheet resistance has a relative standard deviation as low as 1.1%, demonstrating the excellent dispersibility and film uniformity of the modified filler. Solderable low-temperature silver paste control 1, due to poor resin system compatibility and high internal stress after curing, may generate microcracks, resulting in a higher sheet resistance and poorer high-frequency stability and uniformity. Solderable low-temperature silver paste control 2 lacks a low-stress modifier, resulting in slightly poorer conductivity, manifested as a higher sheet resistance than sample 1, and significantly inferior high-frequency stability and uniformity compared to the samples of this invention. The solderable low-temperature silver paste reference 3 uses fillers modified using traditional methods. The carbon nanotubes suffer structural damage after strong acid treatment and supercritical processes, resulting in decreased conductivity and easy agglomeration. The simple silanization modification of antimony tin oxide powder has limited effect and poor dispersibility, leading to a sheet resistance as high as 8.5 mΩ / □, a high-frequency impedance change rate of 15.6%, and the worst micropad uniformity (RSD = 12.5%). The solderable low-temperature silver paste reference 4 uses a single sheet / spherical silver powder filler system, completely lacking nano-silver wires and low-temperature alloy powder. It cannot form an efficient three-dimensional conductive network, resulting in poor conductivity and a sheet resistance as high as 12.8 mΩ / □, failing to meet the high conductivity requirements of camera modules.

[0184] III. Welding Performance Testing

[0185] On the silver-soldered pads, Sn42Bi58 low-temperature solder paste and Mini LED chips were mounted using a standard SMT reflow soldering process (peak temperature 110±3℃, time 60±10s). 200 solder joints were observed, and the soldering yield (i.e., the percentage of solder joints without cold solder joints, false solder joints, or tombstoning) was calculated. A high-precision push-pull force tester (resolution 0.001N) was used to select 20 solder joints, raising the lower limit to 5.0N / mm. 2 The shear strength of the solder joint was tested, and the failure modes were recorded. The failure modes were categorized into three types: internal solder joint fracture, silver paste-pad interface peeling, and a mixed mode involving both. The results are shown in Table 3. Artificial sweat prepared according to ISO 3160-2 was applied to the solder pad surface and sprayed continuously at 35°C for 24 hours according to ASTM B117. Using an untreated sample as a comparison, the wetting and spreading area of ​​the solder on the pad was tested, and the wetting area decay rate was calculated. The results are shown in Table 3.

[0186] Table 3: Welding Performance Test Results

[0187] Sample to be tested Welding yield (%) Shear strength (N) Failure Mode Wetting area attenuation rate (%) Solderable low-temperature silver paste sample 1 99.8 7.8 Internal fracture of solder joint 2.1 Sample 2 of solderable low-temperature silver paste 99.4 7.2 Internal fracture of solder joint 3.5 3 samples of solderable low-temperature silver paste 98.5 6.8 Hybrid mode 4.8 Solderable low-temperature silver paste reference standard 1 92.8 4.5 Silver paste interface stripping 16.7 Solderable low-temperature silver paste reference standard 2 95.6 5.8 Hybrid mode 9.4 Weldable Low-Temperature Silver Paste Reference Standard 3 88.5 3.8 Silver paste interface stripping 22.5 Solderable low-temperature silver paste reference standard 4 90.2 4.2 Silver paste interface stripping 18.6

[0188] As shown in Table 3, the solderable low-temperature silver paste samples of this invention all exhibit extremely high soldering yield, soldering strength, and excellent environmental durability. The soldering yield of solderable low-temperature silver paste samples 1 and 2 is close to 100%, and the failure mode for both is internal fracture at the solder joint, proving that the strength of the weld joint is far higher than the bonding strength between the silver paste and the substrate, achieving top-level reliability. After salt spray and sweat corrosion tests, the solder wetting area attenuation rate of solder samples 1 and 2 is extremely low, indicating that their solder pad surfaces have good resistance to environmental corrosion, which is crucial for the long-term reliability of the camera module in humid environments. Solderable low-temperature silver paste control 1 is prone to silver paste interface peeling under soldering thermal stress, exhibiting poor soldering strength and corrosion resistance. Solderable low-temperature silver paste control 2 has insufficient silver paste layer density and weak resistance to corrosive media penetration, resulting in a higher wetting area attenuation rate. The solderable low-temperature silver paste control sample 3, due to the poor dispersibility of the traditional modified filler used, forms an uneven microstructure on the silver paste surface, resulting in poor solderability of the pads, leading to the lowest solder yield, the weakest shear strength, and all failure modes being silver paste interface debonding, with a wetting area attenuation rate as high as 22.5%. The solderable low-temperature silver paste control sample 4, due to the use of a single silver powder filler and the lack of an active surface from the low-temperature alloy powder, has poor pad wettability, resulting in low solder yield and strength, as well as poor corrosion resistance.

[0189] IV. Application Performance Testing of Camera Module

[0190] The camera module samples prepared in Examples 4 to 6 were tested as follows:

[0191] 1. Optical alignment stability test: The module was mounted on a vibration test platform and subjected to a sinusoidal sweep vibration test at a frequency of 1000Hz and an acceleration of 10Gn for 2 hours. After the test, the optical center offset of the camera was measured. The optical center offset of the camera module samples prepared in Examples 4 to 6 was less than 2μm, which is far below the industry standard of 5μm, proving the dimensional stability of the silver paste circuit of the present invention under mechanical vibration.

[0192] 2. Autofocus accuracy test: Under temperature cycling conditions of 25℃→60℃→25℃, the rate of change of the linear relationship between VCM drive current and lens displacement was tested. The linearity change rate of the camera module samples prepared in Examples 4 to 6 was <1.5%, ensuring the autofocus accuracy of the camera throughout the entire operating temperature range.

[0193] 3. High Temperature and High Humidity Test: The camera module was tested for 500 hours at 85℃ / 85%RH. During and after the test, the signal-to-noise ratio of the image sensor output signal of the camera module samples prepared in Examples 4 to 6 decreased by <1.5dB, and no bright spots or dark spots were generated, proving the long-term stability and reliability of the silver paste circuit of the present invention under high temperature and high humidity conditions.

[0194] 4. High-temperature thermal shock test: 500 cycles of thermal shock were conducted at temperatures ranging from -40℃ to 85℃. After the test, no micro-cracks were observed at the solder joint interface of the camera module samples prepared in Examples 4 to 6, and the chip shear strength showed no attenuation, meeting the stringent requirements of smartphone cameras to withstand complex usage environments.

[0195] In summary, the solderable low-temperature silver paste samples prepared by this invention, especially sample 1, successfully combine excellent mechanical properties, superior conductivity and high-frequency characteristics, and ultra-high solderability, exhibiting overall performance far exceeding that of various comparative examples. Building upon previous technologies, this invention innovatively employs a specific resin system ratio, an optimized combination of conductive fillers, and a special modification process, achieving significant performance improvements and comprehensive optimization. This provides a more reliable solution for camera modules, particularly multi-camera systems in high-end smartphones.

[0196] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A flexible printed circuit board based on solderable low-temperature silver paste, comprising a flexible insulating substrate and a conductor pattern disposed on the substrate, characterized in that, The conductor pattern includes a main circuit pattern formed of rolled copper and an auxiliary functional structure formed in a local area of ​​the main circuit pattern by solderable low-temperature silver paste; the auxiliary functional structure includes at least one of a VCM drive line area, a sensor micro pad area, and a lens mount connection area. The curing temperature of the solderable low-temperature silver paste is 87℃~103℃; by mass parts, the solderable low-temperature silver paste comprises 60 parts~72 parts conductive filler, 12 parts~18 parts polymer resin system, 18 parts~30 parts ester solvent, 3 parts~6 parts curing agent, 1 part~5 parts silane coupling agent, 0.8 parts~1.5 parts polyamide wax thixotropic agent, 0.1 parts~0.3 parts polysiloxane defoamer, 0.3 parts~1 part modified carbon nanotubes, 0.5 parts~1.5 parts modified antimony tin oxide powder, 1 part~2 parts modified ceramic filler and 0.8 parts~1.5 parts low-stress modifier; The polymer resin system comprises hydrogenated bisphenol A type epoxy resin, biphenyl type epoxy resin, and polyacrylate-carboxyl-terminated liquid nitrile rubber copolymer in a mass ratio of 1:(0.9~1.1):(0.3~0.5); the conductive filler comprises silver-coated copper sheet powder, silver nanowires, and low-temperature alloy powder. The D50 of the silver-coated copper sheet powder ranges from 1.0μm to 3.0μm, and the aspect ratio is >15. The length of the silver nanowires is 25μm to 45μm, and the diameter is 30nm to 60nm. The low-temperature alloy powder has a D50 range of 3μm to 8μm.

2. The flexible printed circuit board according to claim 1, characterized in that, The conductive filler comprises silver-coated copper flake powder, silver nanowires, and low-temperature alloy powder in a mass ratio of (1.2–1.8):1:(0.1–0.3); the low-temperature alloy powder is Sn57.6Bi40Ag2.4 or Sn58Bi42; the curing agent is a linear phenolic resin and an imidazole accelerator in a mass ratio of (4–5):1, wherein the imidazole accelerator is 2-phenylimidazoline or 2-heptadecylimidazolium; the silane coupling agent is a mixture of KH-560 and a silane coupling agent with anti-migration function in a mass ratio of 1:(1–2), wherein the silane coupling agent with anti-migration function is KH-590 or aniline methyltriethoxysilane; the polysiloxane defoamer is a non-silicone defoamer containing defoaming polysiloxane; and the ester solvent is any one of diethylene glycol butyl ether acetate, ethylene glycol ethyl ether acetate, or butyl butyrate.

3. The flexible printed circuit board according to claim 1, characterized in that, The biphenyl-type epoxy resin has an epoxy value of 0.50 eq / 100g to 0.65 eq / 100g and a viscosity of 5000 mPa·s to 12000 mPa·s at 25℃. The preparation method of the polyacrylate-carboxyl-terminated liquid nitrile rubber copolymer is as follows: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are prepared by carrying out a living free radical polymerization reaction at 75℃ to 85℃ for 3h to 5h in the presence of tert-butyl peroxide initiator at a mass ratio of (4-5):(4-5):

1. The carboxyl-terminated liquid nitrile rubber has a carboxyl group mass fraction of 0.5% to 0.7% and a number average molecular weight of 3000 Da to 5000 Da. The mass of tert-butyl peroxide initiator is 1.0% to 2.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

4. The flexible printed circuit board according to claim 1, characterized in that, The modified ceramic filler is prepared by dispersing aluminum nitride powder with a particle size of 0.05μm to 0.15μm in anhydrous ethanol, adding a mixed modifier of silane coupling agent KH-560 and polyethyleneimine in a mass ratio of 1:(2 to 3), and reacting with ultrasonic stirring at 300W to 500W at 60℃ to 70℃ for 2 to 3 hours. After filtration, it is calcined at 90℃ to 105℃ for 1 to 2 hours. The total mass of the mixed modifier is 4% to 6% of the aluminum nitride powder.

5. The flexible printed circuit board according to claim 1, characterized in that, The polyamide wax thixotropic agent is a polyamide wax formed by the condensation of ricinoleic acid and dimer fatty acids with ethylenediamine. Before use, the polyamide wax thixotropic agent needs to be activated. The specific operation of the activation treatment is as follows: add the polyamide wax to a mixed solvent of isopropanol and propylene glycol methyl ether acetate with a volume ratio of 1:1.5 to 2.0, add aminopropanol, control the activation temperature at 45℃ to 55℃, and the stirring speed at 800 r / min to 1200 r / min for high-speed dispersion activation for 15 min to 30 min; wherein, the mass ratio of polyamide wax, mixed solvent and aminopropanol is 1:(4 to 6):(0.1 to 0.2).

6. The flexible printed circuit board according to claim 1, characterized in that, The preparation process of the modified carbon nanotubes, by mass parts, is as follows: A1. Electrolyte preparation: Disperse 15 parts of carbon nanotubes in 350 to 450 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid, add 3 to 5 parts of 3-aminopropyltriethoxysilane as a functionalizing agent, and sonicate at 500 W to 700 W for 40 to 60 minutes to form a suspension electrolyte; A2. Electrochemical treatment: The suspended electrolyte is placed in an electrolytic cell, with a graphite rod as the anode and a platinum sheet as the cathode, and the electrolysis reaction is carried out for 1.5h to 2.5h under a voltage of 2.5V to 3.5V and a stirring speed of 500r / min to 700r / min. A3. Post-reaction treatment: After the reaction is completed, the solid is obtained by filtration. The solid is washed by alternating centrifugation with ethanol and deionized water until the conductivity of the supernatant is <5μS / cm. The modified carbon nanotubes are obtained by vacuum drying at 70℃~80℃ for 8h~12h.

7. The flexible printed circuit board according to claim 1, characterized in that, The preparation process of the modified antimony tin oxide powder, by mass parts, is as follows: B1. Preparation of suspension: Disperse 10 parts of antimony tin oxide powder with an average particle size of 20nm to 40nm in 200 to 250 parts of isopropanol, add 0.5 to 1 part of polyvinylpyrrolidone, and sonicate at 600W to 800W for 50 to 70 minutes to prepare a suspension. B2. In-situ polymerization: Add 0.8 to 1.2 parts of 3,4-ethylenedioxythiophene monomer to the suspension and stir at 15°C to 20°C for 40 to 60 minutes; add 3 parts of ethanol solution of potassium persulfate with a mass concentration of 8% to 12% and control the dropping temperature at 20°C to 24°C, and react for 8 to 10 hours. B3. Post-reaction treatment: After the reaction is completed, filter the mixture and wash the filter cake repeatedly with ethanol and acetone until the filtrate is colorless. Dry the filter cake under vacuum at 60℃~70℃ for 24h~36h, grind it, and pass it through a 1000-1250 mesh sieve to obtain the modified antimony tin oxide powder.

8. The flexible printed circuit board according to claim 1, characterized in that, By weight, the low-stress modifier is a core-shell rubber microparticle of polybutadiene acrylonitrile with a polymethyl methacrylate shell and an average particle size range of 100 nm to 200 nm. The preparation method is as follows: C1. Preparation of core emulsion: 80-100 parts of carboxyl-terminated liquid nitrile rubber, 5-10 parts of acrylonitrile, 15-25 parts of deionized water, 1.0-2.5 parts of sodium allyloxyhydroxypropyl sulfonate, and 0.5-1.5 parts of pH buffer were placed in a reactor and emulsified at 65-75°C and 200-400 rpm for 30-60 minutes with stirring. Then, the temperature was raised to 125-135°C, and the pressure was maintained at 0.35-0.45 MPa for 3-5 hours. After the reaction was completed, the mixture was cooled and discharged to obtain the core emulsion. C2. Pre-emulsification of shell monomers: 40-60 parts of methyl methacrylate, 2-5 parts of β-hydroxyethyl methacrylate, 1-3 parts of vinyltriethoxysilane, 1.0-2.0 parts of sodium allyl hydroxypropyl sulfonate and 50-70 parts of deionized water are mixed and emulsified by high-speed shearing at 1000-3000 r / min for 20-40 min to obtain a shell pre-emulsion. C3. Seed emulsion preparation: Take 20 to 30 parts of the core emulsion prepared in step C1, add it to the reaction vessel, dilute it to a solid content of 5% to 10%, heat it to 75℃ to 80℃, purge it with nitrogen for protection, add 0.1 to 0.3 parts of potassium persulfate aqueous solution with a mass concentration of 2% to 5% and 1 to 3 parts of the shell pre-emulsion prepared in step C2 under stirring, and react for 20 to 40 minutes to obtain the seed emulsion; C4, Core-shell emulsion polymerization: The remaining shell pre-emulsion prepared in step C2 and 0.5 to 1.2 parts of a 2% to 5% potassium persulfate aqueous solution were added dropwise to the seed emulsion, with the addition temperature controlled at 78°C to 82°C. After the addition was completed, the mixture was kept warm and matured for 1.5 to 2.5 hours. C5. Post-processing: After cooling to room temperature, the material is filtered through a 100-200 mesh sieve, demulsified, washed, and vacuum dried, then ground through a 500-800 mesh sieve to obtain the aforementioned carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticles.

9. The flexible printed circuit board according to claim 1, characterized in that, The method for preparing the solderable low-temperature silver paste is as follows: S1. Add ester solvent to the reactor, then add polymer resin system, stir at 60℃~80℃ until the resin dissolves, add silane coupling agent and polysiloxane defoamer, cool to room temperature, add curing agent, keep the solution temperature below 30℃, stir until all phases are completely dissolved and mixed to obtain carrier solution. S2. Add polyamide wax thixotropic agent to the carrier solution and stir at room temperature to obtain mixed solution A; S3. Add low-temperature alloy powder and modified ceramic filler to mixed solution A, and stir and mix at room temperature to obtain mixed solution B; S4. Add the silver-coated copper sheet powder and silver nanowires to the mixed solution B, then add the low-stress modifier, and stir and mix at room temperature to obtain the mixed solution C. S5. Add the modified carbon nanotubes and modified antimony tin oxide powder to the mixed solution C, stir and mix, and control the stirring temperature to not exceed 30℃ to obtain the mixed solution D; S6. Transfer the mixed solution D to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 65μm-75μm and the gap between the middle roller and the slow roller to 25μm-35μm, and grind for 1-2 passes. Then, adjust the gap between the fast roller and the middle roller to 35μm-45μm and the gap between the middle roller and the slow roller to 5-15μm, and grind for another 1-2 passes. Grind for a total of 2-4 passes to obtain the ground silver paste. S7. Transfer the ground silver paste to a vacuum stirring vessel and stir and mix it under a vacuum of ≤-0.095MPa. After stirring, encapsulate the silver paste to obtain the solderable low-temperature silver paste.

10. A camera module based on solderable low-temperature silver paste, comprising a flexible printed circuit board as described in any one of claims 1 to 9, and an image sensor chip and / or VCM driver chip soldered onto a conductor pattern on the circuit board via a chip-level packaging process, characterized in that, The image sensor chip is soldered to the sensor micro pad area of ​​the conductor pattern. The solder used for soldering is Sn42Bi58 low-temperature solder, and the soldering parameters are: peak temperature 110℃±3℃ and time 60s±10s. The VCM driver chip is bonded to the VCM driver circuit area of ​​the conductor pattern using anisotropic conductive adhesive, and the curing conditions are maintained at 90°C for 20 minutes.

Images