A new type of automobile rearview mirror flexible printed circuit board based on solderable silver paste

Abstract

The application discloses a novel automobile rearview mirror flexible printed circuit board prepared based on weldable silver paste, and belongs to the technical field of electronic materials and devices. A conductor pattern in the flexible printed circuit board comprises a main circuit pattern and an auxiliary function structure formed by weldable low-temperature curing silver paste. The curing temperature of the weldable low-temperature silver paste is 125 DEG C to 135 DEG C, and the silver paste comprises conductive fillers, a high polymer resin system, high-boiling ester solvents, a curing agent, a silane coupling agent, a polyamide wax thixotropic agent, a polysiloxane defoaming agent, modified carbon nanotubes, modified alumina powder, modified barium sulfate and a low-stress modifier. The high polymer resin system comprises phenolic epoxy resin, diphenyl epoxy resin and polyacrylate-CTBN copolymer. Through the synergistic effect of the components and the innovative process, the mechanical reliability, environmental durability, conductive stability and power load capacity are comprehensively improved, and the harsh requirements of the automobile rearview mirror under extreme environments are perfectly met.

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 for automotive rearview mirrors based on solderable silver paste. Background Technology

[0002] Flexible printed circuit boards (PCBs), with their excellent adaptability to three-dimensional spatial layout and lightweight characteristics, have become an important carrier for functional integration and structural optimization in modern automotive electronic systems. The automotive rearview mirror assembly, as a key module integrating multiple functions such as heating and defrosting, turn signals, blind spot monitoring, and even side cameras, places even more stringent performance and reliability requirements on its internal flexible printed circuit boards than those in consumer electronics. Specifically, these challenges include: 1. Extreme environmental durability challenges: Rearview mirrors are exposed to the outdoors for extended periods, needing to withstand drastic temperature fluctuations from -40°C to 125°C, high humidity above 85% RH, and chemical corrosion from rainwater, car wash liquid, and salt spray. This requires circuit conductors to possess extremely high environmental stability and corrosion resistance to prevent functional failure due to oxidation or electrochemical migration. 2. High vibration and mechanical shock reliability challenges: Continuous vibration during vehicle operation and potential collision impacts require flexible circuit conductors to possess excellent fatigue resistance, high adhesion strength, and bending resistance to prevent micro-cracks or even breakage caused by vibration stress. 3. Power load and thermal management challenges: The rearview mirror heating function requires the circuit to carry a high current (typically above 1A), generating significant Joule heating. This requires the silver paste to not only have low resistance to control heat generation, but also that its resin matrix possess high heat resistance (high Tg) to prevent softening and deformation at high temperatures. Simultaneously, it must overcome the accumulation of internal stress and interface delamination caused by power cycle thermal stress.

[0003] Therefore, the conductive silver paste used in the manufacturing of flexible printed circuit boards for automotive rearview mirrors must have good environmental durability, excellent mechanical reliability, high power handling capacity, and excellent long-term thermal stability. However, the existing low-temperature silver paste systems suitable for consumer electronics cannot meet automotive-grade requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a novel flexible printed circuit board for automotive rearview mirrors based on solderable silver paste, in order to solve the contradiction that existing flexible circuit technology cannot simultaneously meet the outstanding problems of automotive rearview mirrors in terms of extreme environmental tolerance, high vibration reliability and power load capacity. It overcomes the risk of circuit corrosion and failure caused by insufficient weather resistance of traditional silver paste, eliminates the problem of circuit breakage caused by poor vibration fatigue resistance or insufficient adhesion to the substrate of silver paste, and avoids the reliability defects such as easy heating and aging, or even delamination caused by high current operation of silver paste due to its high resistivity and poor heat resistance.

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

[0006] This invention provides a novel flexible printed circuit board for automotive rearview mirrors based on solderable 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 from rolled copper, and auxiliary functional structures formed by solderable low-temperature curing silver paste in local areas of the main circuit pattern. The auxiliary functional structures include at least one of a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area. The curing temperature of the solderable low-temperature silver paste is 125°C. The temperature range is ℃~135℃; by mass parts, the above-mentioned solderable low-temperature silver paste contains 60 parts~72 parts conductive filler, 15 parts~22 parts polymer resin system, 15 parts~25 parts high-boiling-point ester solvent, 4 parts~7 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, 1 part~2 parts modified alumina powder, 1 part~2 parts modified barium sulfate and 0.8 parts~1.5 parts low-stress modifier;

[0007] The aforementioned polymer resin system includes phenolic epoxy resin, biphenyl 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 polyacrylate-carboxyl-terminated liquid nitrile rubber copolymer is abbreviated as polyacrylate-CTBN copolymer. The aforementioned conductive filler includes silver-coated copper flake powder, silver nanowires, and micron-sized silver powder. The D50 of the aforementioned silver-coated copper flake powder ranges from 1.0μm to 3.0μm, and the aspect ratio is >15. The length of the aforementioned silver nanowires is 25μm to 45μm, and the diameter is 30nm to 60nm. The aforementioned micron-sized silver powder is spherical silver powder with a D50 range of 3μm to 8μm.

[0008] Specifically, the mass ratio of silver-coated copper flake powder, silver nanowires, and micron-sized silver powder in the above-mentioned conductive filler is (1.2-1.8):1:(0.5-1.0); the above-mentioned curing agent is phenolic resin and 2-ethyl-4-methylimidazolium in a mass ratio of (4-5):1; the above-mentioned silane coupling agent is a mixture of KH-560 and octyltriethoxysilane in a mass ratio of 1:(1-2), where KH-560 is γ-glycidoxypropyltrimethoxysilane; the above-mentioned polysiloxane defoamer is a non-silicone defoamer containing defoaming polysiloxane; and the above-mentioned high-boiling-point ester solvent is γ-butyrolactone or dipropylene glycol butyl ether acetate.

[0009] 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 to 5):(4 to 5):1.

[0010] More specifically, the mass fraction of carboxyl groups in the above-mentioned 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] Furthermore, the preparation method of the above-mentioned modified alumina powder, by mass parts, is as follows: alumina powder with a particle size of 0.05μm to 0.15μm is dispersed in anhydrous ethanol, and a mixed modifier consisting of silane coupling agent KH-560 and titanate coupling agent NDZ-201 in a mass ratio of 1:(1 to 2) 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 alumina powder.

[0012] Furthermore, the preparation process of the modified barium sulfate, by mass fraction, is as follows: precipitated barium sulfate powder with a particle size of 0.2 μm to 0.8 μm is dispersed in deionized water, and 2% to 4% sodium stearate by mass of barium sulfate is added. The mixture is mechanically stirred at 600 r / min to 800 r / min at 80℃ to 90℃ for 1.5 h to 2.5 h. After the reaction is completed, the mixture is filtered, washed with ethanol, vacuum dried at 100℃ to 120℃ for 12 h, and then ground through a 600-800 mesh sieve.

[0013] Preferably, the above-mentioned polyamide wax thixotropic agent is a polyamide wax formed by the condensation of castor oil acid and dimer fatty acid 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 diethylene glycol butyl 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).

[0014] Furthermore, the preparation process of the above-mentioned modified carbon nanotubes is as follows:

[0015] 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;

[0016] A2. Electrochemical treatment: The above 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.

[0017] 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.

[0018] Furthermore, 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:

[0019] B1. Preparation of core emulsion:

[0020] 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.

[0021] B2. Pre-emulsification of shell monomers:

[0022] 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.

[0023] B3. Seed emulsion preparation:

[0024] Take 20 to 30 parts of the core emulsion prepared in step B1, 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 B2 while stirring, and react for 20 to 40 minutes to obtain the seed emulsion;

[0025] B4. Core-shell emulsion polymerization:

[0026] The remaining shell pre-emulsion prepared in step B2 and 0.5 to 1.2 parts of potassium persulfate aqueous solution with a mass concentration of 2% to 5% were added dropwise to the above seed emulsion, and the dropping temperature was controlled at 78℃ to 82℃. After the dropping was completed, the mixture was kept warm and matured for 1.5 to 2.5 hours.

[0027] B5. Post-processing:

[0028] 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 above-mentioned carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticles.

[0029] Finally, the above-mentioned method for preparing solderable low-temperature silver paste is as follows:

[0030] S1. Add a high-boiling-point ester solvent to the reactor, then add a polymer resin system. Stir at 80℃~90℃ until the resin dissolves. Add a silane coupling agent and a polysiloxane defoamer. Cool to room temperature. Add a curing agent. Keep the solution temperature below 30℃. Stir until all phases are completely dissolved and mixed to obtain a carrier solution.

[0031] S2. Add polyamide wax thixotropic agent to the carrier solution and stir at room temperature to obtain mixed solution A;

[0032] S3. Add micron-sized silver powder, modified alumina and modified barium sulfate to mixed solution A, and stir and mix at room temperature to obtain mixed solution B;

[0033] S4. Add the silver-coated copper sheet powder and silver nanowires to the mixed solution B, and stir and mix at room temperature to obtain mixed solution C;

[0034] S5. Add the modified carbon nanotubes and low-stress modifier to the mixed solution C, stir and mix, and control the stirring temperature to not exceed 30℃ to obtain the mixed solution D;

[0035] S6. Transfer the mixed solution D to a three-roll mill. First, adjust the gap between the fast and medium rollers to 70μm-80μm and the gap between the medium and slow rollers to 30μm-40μm, and grind for 1-2 passes. Then, adjust the gap between the fast and medium rollers to 40μm-50μm and the gap between the medium and slow rollers to 10μm-20μm, and grind for another 1-2 passes. Grind for a total of 2-4 passes to obtain the ground silver paste.

[0036] 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.

[0037] The present invention discloses the following technical effects:

[0038] The novel flexible printed circuit board for automotive rearview mirrors, prepared using a solderable, medium-temperature curable silver paste, achieves a comprehensive improvement in mechanical reliability, environmental durability, conductivity stability, and power load capacity under medium-temperature curing conditions of 125℃ to 135℃ through the synergistic effect of its components and innovative processes. This perfectly meets the stringent high reliability requirements of automotive rearview mirrors in extreme environments. It is particularly noteworthy that this invention, through comprehensive optimization of the resin system, filler modification, and formulation structure, demonstrates that for automotive electronic applications, a lower curing temperature is not necessarily better; rather, an optimal balance must be achieved between the curing temperature and the overall performance of the final product. This invention offers the following beneficial effects:

[0039] First, this invention overcomes the technical bias of simply pursuing low-temperature curing. By employing a phenolic epoxy resin system and an optimized curing agent ratio, the curing temperature is set within a reasonable range of 125℃ to 135℃. Although this design is more demanding than the resin system used in Comparative Example 1, the resulting improvements in crosslinking density, glass transition temperature, and thermomechanical properties are significant. Experimental results show that although Comparative Example 1 has a lower curing temperature, its adhesion is completely lost after hygrothermal aging, and its resistivity change rate after vibration testing is as high as 25.6%, which is far from meeting automotive-grade requirements. This fully demonstrates that appropriately increasing the curing temperature to obtain a more stable three-dimensional network structure is crucial for the long-term reliability of automotive electronic products.

[0040] Secondly, the three-dimensional network structure formed after the silver paste of the present invention is cured gives the silver paste circuit excellent mechanical strength and environmental stability. After 240h of humid heat aging at 85℃ / 85%RH, the adhesion remains at level 0, and the resistance change rate is less than 5% after vibration test, which fully meets the stringent requirements of continuous vibration and harsh environment during automobile driving for circuit reliability.

[0041] Third, the silver paste of this invention constructs a highly efficient and stable three-dimensional conductive network through multi-scale compounding of micron-sized silver powder, silver-coated copper flake powder, and silver nanowires, as well as the innovative application of electrochemically modified carbon nanotubes and specially modified alumina / barium sulfate, achieving a sheet resistance as low as 3.5mΩ. After undergoing 200 rigorous thermal cycles from -40℃ to 125℃, the resistivity change rate is <6.5%, demonstrating excellent environmental stability and perfectly matching the usage requirements of automotive rearview mirrors in extreme temperature environments.

[0042] Fourth, the filler treated with a special modification process and the optimized resin system introduced in this invention give the silver paste excellent resistance to environmental aging. After 240 hours of salt spray testing, the resistivity change rate is <8.5%, and it exhibits excellent acid and alkali resistance. After 500 hours of UV aging, the yellowing index Δb is <3.8, and the adhesion remains good. This fully meets the durability requirements of automotive rearview mirrors exposed to harsh outdoor environments for extended periods, effectively overcoming the risk of circuit corrosion and failure caused by insufficient weather resistance in traditional silver pastes.

[0043] Fifth, the silver paste of this invention possesses excellent power load capacity and long-term stability. After continuous operation at a high current of 3A for 1000 hours, the resistance change rate is <12.8%, and the steady-state temperature rise is <55℃, fully meeting the stringent requirements for long-term reliability of automotive rearview mirror heating functions. This is attributed to its low resistivity, good thermal conductivity, and stable conductive network structure, effectively solving reliability problems such as easy overheating and aging, and even delamination, caused by the high resistivity and poor heat resistance of silver paste under high current operation.

[0044] Sixth, this invention integrates medium-temperature curing characteristics, excellent mechanical reliability, outstanding environmental durability, and strong power load capacity into one, providing a comprehensive solution suitable for connecting automotive rearview mirror heating circuits, turn signal drive circuits, and blind spot monitoring sensors. It effectively solves the reliability problems caused by insufficient material weather resistance, poor vibration fatigue resistance, and insufficient power load capacity in existing technologies, providing highly reliable hardware support for advanced driver assistance systems. Detailed Implementation

[0045] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0046] 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, with respect to 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 in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0047] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0048] 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, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0049] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0050] Example 1

[0051] This embodiment provides a novel flexible printed circuit board for automotive rearview mirrors prepared using solderable 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 heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area.

[0052] By mass, the solderable low-temperature silver paste sample 1 of this embodiment comprises: 64 parts conductive filler sample 1, 18 parts polymer resin system sample 1, 20 parts γ-butyrolactone, 5 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, 1.5 parts modified alumina powder sample 1, 1.5 parts modified barium sulfate sample 1, and 1.0 part low-stress modifier sample 1.

[0053] 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 micron-sized silver powder sample 1 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 micron-sized silver powder sample 1 is 1.5:1:0.7.

[0054] Sample 1 of the polymer resin system includes phenolic epoxy resin, biphenyl epoxy resin, and polyacrylate-CTBN copolymer in a mass ratio of 1:1:0.4. The biphenyl epoxy resin sample 1 has an epoxy value of 0.58 eq / 100g and a viscosity of 8000 mPa·s at 25℃. The polyacrylate-CTBN copolymer sample 1 is prepared by reacting methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber in a mass ratio of 4.5:4.7:1 at 80℃ with a living radical polymerization reaction for 4 hours. The carboxyl-terminated liquid nitrile rubber has a carboxyl group mass fraction of 0.6% and a number-average molecular weight of 4000 Da. The mass of the carboxyl-terminated liquid nitrile rubber initiator is 1.5% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0055] Curing agent sample 1 is a mixture of phenolic resin and 2-ethyl-4-methylimidazole in a mass ratio of 4.5:1.

[0056] 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 diethylene glycol butyl 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.

[0057] Silane coupling agent sample 1 is a mixture of KH-560 and octyltriethoxysilane in a mass ratio of 1:1.5.

[0058] Modified alumina sample 1 was prepared by dispersing 1 part of alumina powder with a particle size of 0.1 μm in 8 parts of anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and titanate coupling agent NDZ-201 in a mass ratio of 1:1.5, reacting with ultrasonic stirring at 400W 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 alumina powder.

[0059] The preparation process of modified carbon nanotube sample 1, by mass parts, is as follows:

[0060] 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.

[0061] 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.

[0062] 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.

[0063] The preparation process of low-stress modifier sample 1, by mass parts, is as follows:

[0064] B1. Preparation of core emulsion:

[0065] 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.

[0066] B2. Pre-emulsification of shell monomers:

[0067] Mix 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, and emulsify by high-speed shearing at 2000 r / min for 30 min to obtain shell pre-emulsion 1.

[0068] B3. Seed emulsion preparation:

[0069] Take 25 parts of the core emulsion 1 prepared in step B1, add it to the reaction vessel, dilute it to a solid content of 8%, heat it to 78°C, purge it with nitrogen for 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 B2 under stirring, react for 30 min to obtain seed emulsion 1;

[0070] B4. Core-shell emulsion polymerization:

[0071] The remaining shell pre-emulsion 1 prepared in step B2 and 0.8 parts of 2.5% potassium persulfate aqueous solution were added dropwise to seed emulsion 1, with the addition temperature controlled at 80℃. After the addition was completed, the mixture was kept warm and matured for 2.0h.

[0072] B5. Post-processing:

[0073] After cooling to room temperature, the material was filtered through a 150-mesh sieve, demulsified, washed, and vacuum dried, and then ground 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 was 156 nm.

[0074] Using the above-mentioned raw materials, a solderable low-temperature silver paste sample 1 was prepared, specifically as follows:

[0075] S1. Add 20 parts of γ-butyrolactone to the reactor, then add 18 parts of polymer resin system sample 1. Stir at 85℃ and 400r / min for 30min 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 5 parts of curing agent sample 1, keep the solution temperature at 25℃, and stir until all phases are completely dissolved and mixed to obtain carrier solution 1.

[0076] S2. Add 1 part of activated polyamide wax thixotropic agent sample 1 to carrier solution 1, and stir at 650 r / min for 38 min at room temperature to obtain mixed solution A1.

[0077] S3. Add 14 parts of micron silver powder sample 1, 1.5 parts of modified alumina sample 1 and 1.5 parts of modified barium sulfate sample 1 to mixed solution A1, and stir at 1000 r / min for 75 min at room temperature to obtain mixed solution B1.

[0078] S4. Add 30 parts of silver-coated copper sheet powder sample 1 and 20 parts of silver nanowire sample 1 to the mixed solution B1, and stir at 1200 r / min for 80 min at room temperature to obtain mixed solution C1.

[0079] S5. Add 0.6 parts of modified carbon nanotube sample 1 and 1 part of low stress modifier 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.

[0080] S6. Transfer the mixed solution D1 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 35 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 45 μm and the gap between the middle roller and the slow roller to 15 μm and grind once more. Grind a total of 3 times to obtain the ground silver paste sample 1.

[0081] S7. Transfer the ground silver paste sample 1 to a vacuum mixing vessel and stir at a speed of 30 r / min for 45 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 125℃.

[0082] Example 2

[0083] This embodiment provides a novel flexible printed circuit board for automotive rearview mirrors based on solderable silver paste, comprising a flexible insulating substrate and conductor patterns disposed on the substrate. The conductor patterns include a main circuit pattern formed from rolled copper, and auxiliary functional structures formed by a 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 heating circuit and a turn signal drive circuit area.

[0084] By mass, the solderable low-temperature silver paste sample 2 of this embodiment contains: 72 parts conductive filler sample 2, 15 parts polymer resin system sample 2, 15 parts dipropylene glycol butyl ether acetate, 4 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, 0.3 parts modified carbon nanotube sample 2, 2 parts modified alumina powder sample 2, 1 part modified barium sulfate sample 2, and 1.5 parts low-stress modifier sample 2.

[0085] 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 16, a length of 45 μm, and a diameter of 30 nm, and a silver micron sample 2 with a D50 of 8 μm; wherein the mass ratio of the silver-coated copper flake powder sample 2, the silver nanowire sample 2, and the silver micron sample 2 is 1.2:1:1.

[0086] Sample 2 of the polymer resin system includes phenolic epoxy resin, biphenyl epoxy resin, and polyacrylate-CTBN copolymer in a mass ratio of 1:1.1:0.3. The biphenyl epoxy resin sample 2 has an epoxy value of 0.65 eq / 100g and a viscosity of 5000 mPa·s at 25℃. The polyacrylate-CTBN copolymer sample 2 is prepared by reacting methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber in a mass ratio of 4:5:1 at 75℃ with a living radical polymerization reaction for 5 h. The carboxyl-terminated liquid nitrile rubber has a carboxyl group mass fraction of 0.7% and a number average molecular weight of 3000 Da. The mass of the carboxyl-terminated liquid nitrile rubber initiator is 2.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0087] Curing agent sample 2 is phenolic resin and 2-ethyl-4-methylimidazole in a mass ratio of 5:1.

[0088] 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 diethylene glycol butyl 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.

[0089] Silane coupling agent sample 2 is a mixture of KH-560 and octyltriethoxysilane in a mass ratio of 1:1.

[0090] Modified alumina sample 2 was prepared by dispersing 1 part of alumina powder with a particle size of 0.05 μm in 7 parts of anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and titanate coupling agent NDZ-201 in a mass ratio of 1:1, reacting with ultrasonic stirring at 300W at 70℃ for 2 hours, filtering, and then calcining at 105℃ for 1 hour; wherein, the total mass of the mixed modifier was 4% of the alumina powder.

[0091] The preparation process of modified carbon nanotube sample 2, by mass parts, is as follows:

[0092] 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.

[0093] 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.

[0094] 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.

[0095] The preparation process of low-stress modifier sample 2, by mass parts, is as follows:

[0096] B1. Preparation of core emulsion:

[0097] 100 parts of carboxyl-terminated liquid nitrile rubber, 5 parts of acrylonitrile, 15 parts of deionized water, 2.5 parts of sodium allyl hydroxypropyl sulfonate and 0.5 parts of pH buffer were placed in a reaction vessel and emulsified at 75°C and 200 r / min for 30 min with stirring. Then the temperature was raised to 135°C 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.

[0098] B2. Pre-emulsification of shell monomers:

[0099] 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.

[0100] B3. Seed emulsion preparation:

[0101] Take 30 parts of the core emulsion 2 prepared in step B1, add it to the reaction vessel, dilute it to a solid content of 5%, heat it to 75°C, introduce nitrogen gas for 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 B2 under stirring, react for 40 min to obtain seed emulsion 2.

[0102] B4. Core-shell emulsion polymerization:

[0103] The remaining shell pre-emulsion 2 prepared in step B2 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.

[0104] B5. Post-processing:

[0105] After cooling to room temperature, the material was filtered through a 100-mesh sieve, demulsified, washed, and vacuum dried, and then ground 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 was 100 nm.

[0106] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 2 was prepared, specifically as follows:

[0107] S1. Add 15 parts of dipropylene glycol butyl ether acetate to the reactor, then add 15 parts of polymer resin system sample 2. Stir at 80°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 4 parts of curing agent sample 2, keep the solution temperature at 28°C, and stir until all phases are completely dissolved and mixed to obtain carrier solution 2.

[0108] S2. Add 0.8 parts of activated polyamide wax thixotropic agent sample 2 to carrier solution 2, and stir at 800 r / min for 30 min at room temperature to obtain mixed solution A2.

[0109] S3. Add 22.5 parts of micron silver powder sample 2, 2 parts of modified alumina sample 2 and 1 part of modified barium sulfate sample 2 to mixed solution A2, and stir at 1200 r / min for 60 min at room temperature to obtain mixed solution B2.

[0110] S4. Add 27 parts of silver-coated copper sheet powder sample 2 and 22.5 parts of silver nanowire sample 2 to mixed solution B2, and stir at 1500 r / min for 90 min at room temperature to obtain mixed solution C2.

[0111] S5. Add 0.3 parts of modified carbon nanotube sample 2 and 1.5 parts of low stress modifier sample 2 to the mixed solution C2, stir at 1800 r / min for 60 min, and control the stirring temperature at 30℃ to obtain mixed solution D2.

[0112] S6. Transfer the mixed solution D2 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 80 μm and the gap between the middle roller and the slow roller to 30 μm and grind once. 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 a total of 2 times to obtain the ground silver paste sample 2.

[0113] S7. Transfer the ground silver paste sample 2 to a vacuum mixing vessel and stir at a speed of 40 r / min for 30 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 135℃.

[0114] Example 3

[0115] This embodiment provides a novel flexible printed circuit board for automotive rearview mirrors based on solderable 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 functional structure is the turn signal drive circuit area.

[0116] By mass, the solderable low-temperature silver paste sample 3 of this embodiment contains: 60 parts conductive filler sample 3, 22 parts polymer resin system sample 3, 25 parts γ-butyrolactone, 7 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, 1 part modified carbon nanotube, 1 part modified alumina powder sample 3, 2 parts modified barium sulfate sample 3 and 1.5 parts low-stress modifier sample 3.

[0117] 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 20, a length of 25 μm and a diameter of 60 nm, and a micron-sized silver powder sample 3 with a D50 of 3 μm; wherein the mass ratio of the silver-coated copper flake powder sample 3, the silver nanowire sample 3 and the micron-sized silver powder sample 3 is 1.8:1:0.5.

[0118] Sample 3 of the polymer resin system includes phenolic epoxy resin, biphenyl epoxy resin, and polyacrylate-CTBN copolymer in a mass ratio of 1:0.9:0.5. The biphenyl epoxy resin sample 3 has an epoxy value of 0.50 eq / 100g and a viscosity of 12000 mPa·s at 25℃. The polyacrylate-CTBN copolymer sample 3 is prepared by reacting methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber in a mass ratio of 5:4:1 at 85℃ with a living radical polymerization reaction for 3 hours. The carboxyl-terminated liquid nitrile rubber has a carboxyl group mass fraction of 0.5% and a number average molecular weight of 5000 Da. The mass of the carboxyl-terminated liquid nitrile rubber initiator is 1.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0119] Curing agent sample 3 is phenolic resin and 2-ethyl-4-methylimidazole in a mass ratio of 4:1.

[0120] The polyamide wax thixotropic agent sample 3 is a polyamide wax sample 3 formed by the condensation of castor oil acid and dimer fatty acids with ethylenediamine, specifically CRAYVALLAC SUPER. It requires activation before use. The specific activation procedure is as follows: CRAYVALLAC SUPER is added to a mixed solvent of isopropanol and diethylene glycol butyl ether acetate at a volume ratio of 1:1.5, aminopropanol is added, the activation temperature is controlled at 55℃, the stirring speed is 800 r / min, and high-speed dispersion activation is performed for 15 min; wherein, the mass ratio of CRAYVALLAC SUPER, the mixed solvent, and aminopropanol is 1:6:0.1.

[0121] Silane coupling agent sample 3 is a mixture of KH-560 and octyltriethoxysilane in a mass ratio of 1:2.

[0122] Modified alumina sample 3 was prepared by dispersing 1 part of alumina powder with a particle size of 0.15 μm in 9 parts of anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and titanate coupling agent NDZ-201 in a mass ratio of 1:2, reacting with ultrasonic stirring at 500W at 60℃ for 3 h, filtering, and then calcining at 95℃ for 2 h; wherein, the total mass of the mixed modifier was 6% of the alumina powder.

[0123] The preparation process of modified carbon nanotube sample 3, by mass parts, is as follows:

[0124] 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.

[0125] 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.

[0126] 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.

[0127] The preparation process of low-stress modifier sample 3, by mass parts, is as follows:

[0128] B1. Preparation of core emulsion:

[0129] 80 parts of carboxyl-terminated liquid nitrile rubber, 10 parts of acrylonitrile, 25 parts of deionized water, 1.0 part of sodium allyl hydroxypropyl sulfonate, and 1.5 parts of pH buffer were placed in a reactor and emulsified at 65°C and 400 r / min for 60 min with stirring. Then the temperature was raised to 125°C 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.

[0130] B2. Pre-emulsification of shell monomers:

[0131] 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.

[0132] B3. Seed emulsion preparation:

[0133] Take 20 parts of the core emulsion 3 prepared in step B1, add it to the reaction vessel, dilute it to a solid content of 10%, heat it to 80°C, introduce nitrogen 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 B2 under stirring, react for 20 min to obtain seed emulsion 3.

[0134] B4. Core-shell emulsion polymerization:

[0135] The remaining shell pre-emulsion 3 prepared in step B2 and 0.5 parts of 2% potassium persulfate aqueous solution were added dropwise to the seed emulsion 3, and the dropping temperature was controlled at 82℃. After the dropping was completed, the mixture was kept warm and matured for 1.5h.

[0136] B5. Post-processing:

[0137] After cooling to room temperature, the material was filtered through a 200-mesh sieve, demulsified, washed, and vacuum dried, and then ground 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 was 200 nm.

[0138] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 3 was prepared, specifically as follows:

[0139] S1. Add 25 parts of γ-butyrolactone to the reactor, then add 22 parts of polymer resin system sample 3. Stir at 90℃ 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 7 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.

[0140] S2. Add 1.5 parts of activated polyamide wax thixotropic agent sample 3 to carrier solution 3, and stir at 500 r / min for 45 min at room temperature to obtain mixed solution A3.

[0141] S3. Add 9.1 parts of micron silver powder sample 3, 1 part of modified alumina sample 3 and 2 parts of modified barium sulfate sample 3 to mixed solution A3, and stir at 800 r / min for 90 min at room temperature to obtain mixed solution B3.

[0142] S4. Add 32.7 parts of silver-coated copper sheet powder sample 3 and 18.2 parts of silver nanowire sample 3 to mixed solution B3, and stir at 1000 r / min for 120 min at room temperature to obtain mixed solution C3.

[0143] S5. Add 1 part of modified carbon nanotube sample 3 and 1.5 parts of low stress modifier sample 3 to the mixed solution C3, stir at 1200 r / min for 90 min, and control the stirring temperature at 22℃ to obtain mixed solution D3.

[0144] S6. Transfer the mixed solution D3 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 40 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 50 μm and the gap between the middle roller and the slow roller to 20 μm and grind twice more. Grind a total of 4 times to obtain the ground silver paste sample 3.

[0145] S7. Transfer the ground silver paste sample 3 to a vacuum stirring vessel and stir at a speed of 20 r / min for 60 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 130℃.

[0146] Comparative Example 1

[0147] This comparative example provides a novel flexible printed circuit board for automotive rearview mirrors prepared using solderable 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 in local areas of the main circuit pattern by solderable low-temperature silver paste reference 1. The auxiliary functional structures in this comparative example include a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area.

[0148] By mass, the weldable low-temperature silver paste control sample 1 of this comparative example comprises: 64 parts conductive filler sample 1, 18 parts polymer resin system control sample 1, 20 parts γ-butyrolactone, 5 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, 1.5 parts modified alumina powder sample 1, 1.5 parts modified barium sulfate sample 1, and 1.0 part low-stress modifier sample 1.

[0149] The polymer resin system reference standard 1 includes polyester resin, acrylic resin and polyvinyl butyral in a mass ratio of 1:1:0.4.

[0150] 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 110°C.

[0151] Comparative Example 2

[0152] This comparative example provides a novel flexible printed circuit board for automotive rearview mirrors prepared using solderable 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 in local areas of the main circuit pattern by solderable low-temperature silver paste reference 2. The auxiliary functional structures in this comparative example include a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area.

[0153] By mass, the solderable low-temperature silver paste control sample 2 of this comparative example contains: 64 parts conductive filler sample 1, 18 parts polymer resin system sample 1, 20 parts γ-butyrolactone, 5 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, 1.5 parts modified alumina powder sample 1 and 1.5 parts modified barium sulfate sample 1.

[0154] 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 a low-stress modifier was not added in step S5. The curing temperature of the prepared solderable low-temperature silver paste reference standard 2 was 135°C.

[0155] Comparative Example 3

[0156] This comparative example provides a novel flexible printed circuit board for automotive rearview mirrors prepared using solderable 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 in local areas of the main circuit pattern by a solderable low-temperature silver paste reference standard 3. The auxiliary functional structures in this comparative example include a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area.

[0157] By mass, the weldable low-temperature silver paste reference standard 3 of this comparative example comprises: 64 parts conductive filler sample 1, 18 parts polymer resin system sample 1, 20 parts γ-butyrolactone, 5 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 reference standard 1, 1.5 parts modified alumina powder sample 1, 1.5 parts modified barium sulfate sample 1, and 1.0 part low-stress modifier sample 1.

[0158] The preparation method of modified carbon nanotube reference standard 1 is as follows:

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] A5. After depressurization and separation, the sample was washed with deionized water and dried under vacuum at 90°C for 8 hours to obtain modified carbon nanotube reference standard 1.

[0164] 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. The curing temperature of the prepared solderable low-temperature silver paste reference standard 3 was 140℃.

[0165] Comparative Example 4

[0166] This comparative example provides a novel flexible printed circuit board for automotive rearview mirrors prepared using solderable 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 in local areas of the main circuit pattern by solderable low-temperature silver paste reference 4. The auxiliary functional structures in this comparative example include a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area.

[0167] By mass, the solderable low-temperature silver paste control sample 4 of this comparative example comprises: 64 parts conductive filler sample 1, 18 parts polymer resin system sample 1, 20 parts γ-butyrolactone, 5 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, 1.5 parts modified alumina powder sample 1, 1.5 parts barium sulfate with a D50 of 0.5 μm, and 1.0 part low-stress modifier sample 1.

[0168] 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 1.5 parts of barium sulfate were used instead of 1.5 parts of modified barium sulfate sample 1, meaning that no modification of barium sulfate was performed during the preparation process. The curing temperature of the prepared solderable low-temperature silver paste reference standard 4 was 132℃.

[0169] Analysis and Testing

[0170] Flexible circuits for automotive rearview mirrors were formed on polyimide (PI) substrates using screen printing equipment on solderable low-temperature curable silver paste samples 1-3 and solderable low-temperature curable silver paste controls 1-4. After curing at their respective curing temperatures for 30 minutes, performance tests were performed. The linewidth of the flexible circuit was 50 μm, the PI substrate thickness was 75 μm, and the silver paste thickness was 20 μm ± 3 μm.

[0171] I. Mechanical Performance Testing

[0172] Mechanical properties include adhesion test, flexural strength test and hardness test. The test results are shown in Table 1.

[0173] Adhesion was tested using a cross-cut adhesion test. After pulling with 3M 610 tape, the peeling level was evaluated (0 being the best and 5 being the worst). The test sample was placed in a humid heat environment of 85℃ / 85%RH for 240 hours for humid heat aging, and the peeling level after aging was evaluated (0 being the best and 5 being the worst).

[0174] For vibration resistance testing, the sample is mounted on a vibration table and subjected to a sinusoidal sweep vibration test from 10Hz to 2000Hz with an acceleration of 10Gn for 4 hours. After the test, the circuit is checked for breakage and the resistance change rate is checked for exceeding 5%.

[0175] Hardness was tested using a pencil hardness tester according to standard number ASTM D3363.

[0176] Table 1: Mechanical Performance Test Results

[0177] Sample to be tested Adhesion (Grade) Adhesion after aging (grade) Rate of change in resistance (%) after vibration test hardness Solderable low-temperature silver paste sample 1 0 0 1.2 5H Sample 2 of solderable low-temperature silver paste 0 0 2.5 4H 3 samples of solderable low-temperature silver paste 0 1 3.8 6H Solderable low-temperature silver paste reference standard 1 3 5 25.6 3H Solderable low-temperature silver paste reference standard 2 1 3 12.4 5H Weldable Low-Temperature Silver Paste Reference Standard 3 2 4 34.4 4H Solderable low-temperature silver paste reference standard 4 1 3 18.7 4H

[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, with a resistance change rate of only 1.2% after rigorous vibration testing, far exceeding the automotive electronics industry standard, fully meeting the reliability requirements of automotive rearview mirrors for vibration and shock resistance during driving. After 240 hours of damp heat aging at 85℃ / 85%RH, the adhesion of solderable low-temperature silver paste sample 1 remained at grade 0, proving that the phenolic epoxy resin system used has excellent aging resistance and interface stability. Solderable low-temperature silver paste control sample 1, due to the use of a polyester resin / acrylic resin system with poor heat resistance, has poor hardness and adhesion of its cured film, with an initial adhesion of only grade 3. After aging at 85℃ / 85%RH, the resin severely softened and degraded, and the adhesion was completely lost. Its resistance change rate after vibration testing was as high as 25.6%, indicating that this resin system cannot withstand the vibration environment of automobiles. The solderable low-temperature silver paste reference sample 2, lacking a low-stress modifier, exhibited high internal stress. Although its initial adhesion was acceptable, it significantly decreased after aging, and its resistance change rate after vibration testing was much higher than that of the sample of this invention. The solderable low-temperature silver paste reference sample 3 used carbon nanotubes modified by traditional methods, resulting in poor dispersibility and disrupting the integrity of the resin system. Consequently, its mechanical properties were the worst among all samples, and it had the highest resistance change rate after vibration testing. The solderable low-temperature silver paste reference sample 4 used unmodified barium sulfate, which had poor compatibility with the resin, weak interfacial bonding, and insufficient vibration stability, resulting in a resistance change rate of 18.7%.

[0179] II. Conductivity Test

[0180] The sheet resistance of the cured silver paste conductors was measured using a four-probe tester. The results are shown in Table 2.

[0181] The sample to be tested was subjected to a temperature cycle of -40℃ to 125℃ for 200 cycles, and the rate of change of resistance was measured to evaluate the stability of the circuit under extreme temperature conditions. The results are shown in Table 2.

[0182] Sheet resistance (probe diameter 5μm) was measured at 100 points on a 50μm×50μm pad array. The relative standard deviation (RSD) was calculated, and the conductivity uniformity was determined. The results are shown in Table 2.

[0183] Table 2: Conductivity Test Results

[0184]

[0185] As shown in Table 2, the solderable low-temperature silver paste samples of this invention all exhibit low sheet resistance, excellent environmental stability, and extremely high conductivity uniformity. Solderable low-temperature silver paste sample 1 has a sheet resistance as low as 3.5 mΩ, and after 200 thermal cycles from -40℃ to 125℃, the resistance change rate is only 2.8%, fully meeting the stringent requirements for circuit stability in automotive rearview mirrors under extreme temperature environments. Its relative standard deviation for conductivity uniformity is as low as 1.5%, demonstrating the excellent dispersibility and film uniformity of the modified filler. Solderable low-temperature silver paste control 1, due to insufficient heat resistance of the resin system, is easily degraded in high-temperature and high-humidity environments, resulting in a higher sheet resistance and a resistance change rate of 18.7% after thermal cycling. Solderable low-temperature silver paste control 2 lacks a low-stress modifier, and its conductive pathway exhibits slightly poorer stability under thermal stress, manifested as a higher resistance change rate after thermal cycling. The solderable low-temperature silver paste reference 3 uses carbon nanotubes modified by conventional methods, which reduces conductivity and easily aggregates, resulting in a sheet resistance as high as 10.5 mΩ. After thermal cycling, the resistivity change rate reaches 25.4%, and the conductivity uniformity is also the worst. The solderable low-temperature silver paste reference 4 uses unmodified barium sulfate, which has poor dispersibility, resulting in a high sheet resistance and insufficient thermal stability.

[0186] III. Weather resistance and chemical resistance tests

[0187] The sample to be tested was immersed in a 5% NaCl aqueous solution at 35℃ for 240 hours. The change rate of resistance and the change of adhesion were tested to evaluate the salt spray corrosion resistance.

[0188] The samples to be tested were immersed in dilute sulfuric acid solution at 25℃ and pH=3 and NaOH solution at 25℃ and pH=10 for 72 hours, respectively. The resistance change rate was tested to evaluate the chemical resistance performance.

[0189] Using a UV aging chamber (UVA-340 lamp, 0.76W / m) 2 The samples were subjected to a 500-hour UV aging test at 340 nm to measure changes in yellowing index (Δb) and adhesion, and to evaluate their UV aging resistance. The results are shown in Table 3.

[0190] Table 3: Welding Performance Test Results

[0191]

[0192] As shown in Table 3, the solderable low-temperature silver paste samples of this invention exhibit excellent weather resistance and chemical resistance. Solderable low-temperature silver paste sample 1 showed a resistivity change rate of only 3.2% after 240 hours of salt spray testing, and also demonstrated excellent stability in acidic and alkaline environments. After 500 hours of UV aging, its adhesion remained at grade 0 with minimal yellowing, fully meeting the durability requirements of automotive rearview mirrors exposed to harsh outdoor environments for extended periods. Solderable low-temperature silver paste control sample 1, due to the insufficient hydrolysis resistance and poor chemical resistance of its ester system, showed a sharp deterioration in performance during salt spray and acid / alkali tests, with a resistivity change rate exceeding 25%. After UV aging, it exhibited severe yellowing and a significant decrease in adhesion. Solderable low-temperature silver paste control sample 2, due to slightly lower density, had weaker resistance to media penetration than the samples of this invention. Solderable low-temperature silver paste control sample 3, due to poor dispersion of the modified filler and defects in the coating, had the worst chemical resistance and UV resistance among all samples. The weldable low-temperature silver paste reference sample 4 uses unmodified barium sulfate, which has poor compatibility with the resin and insufficient coating protection performance. The results of various weather resistance tests are significantly worse than those of the sample of this invention.

[0193] IV. Power Load Test

[0194] A 3A current (equivalent to the working current of the rearview mirror heating film) was passed through the silver paste wire and operated continuously for 1000 hours. The resistance change rate and temperature rise were tested to evaluate the long-term power load stability. The results are shown in Table 4.

[0195] Table 4: Power Load Test Results

[0196] Sample to be tested Initial resistance (mΩ) Rate of change in resistance (%) after 1000 hours Steady-state temperature rise (°C) Solderable low-temperature silver paste sample 1 350 5.2 42 Sample 2 of solderable low-temperature silver paste 420 8.5 48 3 samples of solderable low-temperature silver paste 550 12.8 55 Solderable low-temperature silver paste reference standard 1 680 28.7 72 Solderable low-temperature silver paste reference standard 2 520 15.6 58 Weldable Low-Temperature Silver Paste Reference Standard 3 1050 45.2 85 Solderable low-temperature silver paste reference standard 4 890 22.5 68

[0197] As shown in Table 4, the solderable low-temperature silver paste samples of this invention exhibit excellent power load capacity and long-term stability. After operating at 3A for 1000 hours, the resistivity change rate of solderable low-temperature silver paste sample 1 was only 5.2%, and the steady-state temperature rise was only 42℃, fully meeting the stringent long-term reliability requirements of the silver paste circuit for automotive rearview mirror heating functions. This is attributed to its low resistivity, good thermal conductivity, and stable three-dimensional conductive network structure. Due to their respective structural defects, the control samples showed significant performance degradation under long-term high-current operation, especially solderable low-temperature silver paste control samples 1 and 3, which could not meet the reliability requirements for automotive applications.

[0198] In summary, the solderable low-temperature silver paste samples prepared by this invention, especially Sample 1, successfully combine excellent mechanical properties, outstanding electrical conductivity stability, excellent weather resistance, and strong power load capacity, exhibiting comprehensive performance far superior to various comparative examples. This invention, through the innovative use of a phenolic epoxy resin system, optimized conductive filler combinations, and a special modification process, achieves significant performance improvements and comprehensive optimization. It provides a highly reliable solution for automotive rearview mirrors, particularly high-end intelligent rearview mirror systems integrating heating, turn signals, and blind spot monitoring functions, perfectly meeting the stringent durability and safety requirements of automotive-grade applications.

[0199] 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 novel flexible printed circuit board for automotive rearview mirrors, prepared using solderable 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 curing silver paste; the auxiliary functional structure includes at least one of a heating circuit, a turn signal drive circuit area, and a blind spot monitoring sensor connection area. The curing temperature of the weldable low-temperature curing silver paste is 125℃~135℃; by mass parts, the weldable low-temperature curing silver paste contains 60 parts~72 parts conductive filler, 15 parts~22 parts polymer resin system, 15 parts~25 parts high-boiling-point ester solvent, 4 parts~7 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, 1 part~2 parts modified alumina powder, 1 part~2 parts modified barium sulfate, and 0.8 parts~1.5 parts low-stress modifier; The polymer resin system comprises phenolic epoxy resin, biphenyl 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 flake powder, silver nanowires, and micron-sized silver powder, wherein the D50 of the silver-coated copper flake powder is in the range of 1.0μm-3.0μm and the aspect ratio is >15; the length of the silver nanowires is 25μm-45μm and the diameter is 30nm-60nm; and the micron-sized silver powder is spherical silver powder with a D50 range of 3μm-8μm. The preparation process of the modified barium sulfate is as follows: precipitated barium sulfate powder with a particle size of 0.2 μm to 0.8 μm is dispersed in deionized water, and sodium stearate of 2% to 4% by mass of barium sulfate is added. The mixture is mechanically stirred at 600 r / min to 800 r / min at 80℃ to 90℃ for 1.5 h to 2.5 h. After the reaction is completed, the mixture is filtered, washed with ethanol, vacuum dried at 100℃ to 120℃ for 12 h, and then ground through a 600-800 mesh sieve. The preparation process of the modified carbon nanotubes 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 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.

2. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, The conductive filler has a mass ratio of (1.2-1.8):1:(0.5-1.0) of silver-coated copper flake powder, silver nanowires, and micron-sized silver powder; the curing agent is a mixture of phenolic resin and 2-ethyl-4-methylimidazolium with a mass ratio of (4-5):1; the silane coupling agent is a mixture of KH-560 and octyltriethoxysilane with a mass ratio of 1:(1-2); the polysiloxane defoamer is a non-silicone defoamer containing defoaming polysiloxane; and the high-boiling-point ester solvent is γ-butyrolactone or dipropylene glycol butyl ether acetate.

3. The flexible printed circuit board for an automotive rearview mirror 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°C. The polyacrylate-CTBN copolymer is prepared by: carrying out a living free radical polymerization reaction of methyl methacrylate, butyl acrylate and carboxyl-terminated liquid nitrile rubber at a mass ratio of (4-5):(4-5):1 in the presence of tert-butyl peroxide-2-ethylhexanoate initiator at 75°C to 85°C for 3-5 hours.

4. The flexible printed circuit board for an automotive rearview mirror according to claim 3, characterized in that, 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 the tert-butyl peroxide initiator is 1.0% to 2.0% of the total mass of methyl methacrylate, butyl acrylate and the carboxyl-terminated liquid nitrile rubber.

5. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, The modified alumina powder is prepared by dispersing alumina powder with a particle size of 0.05μm to 0.15μm in anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and titanate coupling agent NDZ-201 in a mass ratio of 1:(1 to 2), 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 alumina powder.

6. The flexible printed circuit board for an automotive rearview mirror 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 diethylene glycol butyl 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).

7. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, 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: B1. 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. B2. 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. B3. Seed emulsion preparation: Take 20 to 30 parts of the core emulsion prepared in step B1, 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 B2 while stirring, and react for 20 to 40 minutes to obtain the seed emulsion; B4. Core-shell emulsion polymerization: The remaining shell pre-emulsion prepared in step B2 and 0.5 to 1.2 parts of potassium persulfate aqueous solution with a mass concentration of 2% to 5% were added dropwise to the seed emulsion, and the dropping temperature was controlled at 78℃ to 82℃. After the dropping was completed, the mixture was kept warm and matured for 1.5 to 2.5 hours. B5. 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.

8. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, The preparation method of the solderable low-temperature curing silver paste is as follows: S1. Add a high-boiling-point ester solvent to the reactor, then add a polymer resin system. Stir at 80℃~90℃ until the resin dissolves. Add a silane coupling agent and a polysiloxane defoamer. Cool to room temperature. Add a curing agent. Keep the solution temperature below 30℃. Stir until all phases are completely dissolved and mixed to obtain a carrier solution. S2. Add polyamide wax thixotropic agent to the carrier solution and stir at room temperature to obtain mixed solution A; S3. Add micron-sized silver powder, modified alumina and modified barium sulfate 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, and stir and mix at room temperature to obtain mixed solution C; S5. Add the modified carbon nanotubes and low-stress modifier 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 and medium rollers to 70μm-80μm and the gap between the medium and slow rollers to 30μm-40μm, and grind for 1-2 passes. Then, adjust the gap between the fast and medium rollers to 40μm-50μm and the gap between the medium and slow rollers to 10μm-20μm, 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 mixing vessel and mix it under a vacuum of ≤-0.095MPa. After mixing, encapsulate the silver paste to obtain the solderable low-temperature curing silver paste.

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