Novel automobile back view mirror flexible printed circuit board prepared based on weldable silver paste

Abstract

The invention discloses a novel automobile back view mirror flexible printed circuit board prepared based on weldable silver paste, and belongs to the technical field of electronic materials and devices, conductor patterns in the flexible printed circuit board comprise 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 125-135 DEG C, and the weldable low-temperature silver paste comprises conductive filler, a polymer resin system, a high-boiling-point ester solvent, a curing agent, a silane coupling agent, a polyamide wax thixotropic agent, a polysiloxane defoaming agent, modified carbon nanotubes, modified aluminum oxide powder, modified barium sulfate and a low-stress modifier. The polymer resin system comprises novolac epoxy resin, biphenyl epoxy resin and a polyacrylate-CTBN copolymer; through the synergistic effect of all the components and an innovative process, comprehensive improvement of mechanical reliability, environmental durability, conductive stability and power load capacity is achieved, and the strict requirement for high reliability of the automobile rearview mirror in an extreme environment is perfectly met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic materials and devices, in particular to a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste. BACKGROUND

[0002] The flexible printed circuit board has become an important carrier for realizing function integration and structure optimization of modern automobile electronic systems due to its excellent three-dimensional spatial layout adaptability and lightweight characteristics. The automobile rearview mirror assembly, as a key module integrating multiple functions such as heating defrosting, steering indication, blind spot monitoring, and even lateral camera, puts forward more stringent performance and reliability requirements for the flexible printed circuit board inside it than in the consumer electronics field. Specifically, it includes: 1. Extreme environmental durability challenge, the rearview mirror is exposed to the outdoors for a long time and needs to withstand severe temperature alternation from -40℃ to 125℃, high humidity above 85%RH, and chemical corrosion such as rainwater, car washing liquid, and salt spray, requiring the circuit conductor to have extremely high environmental stability and corrosion resistance to avoid functional failure caused by oxidation or electrochemical migration; 2. High vibration and mechanical impact reliability challenge, the continuous vibration and possible collision impact during vehicle driving require the flexible circuit conductor to have excellent fatigue resistance, high adhesion strength, and bending resistance to avoid micro-cracks or even breakage of the circuit caused by vibration stress; 3. Power load and thermal management challenge, the rearview mirror heating function requires the circuit to carry high current (usually above 1A), generating significant Joule heat, requiring the silver paste not only to have low resistance to control heating, but also the resin matrix to have high heat resistance (high Tg) to prevent softening and deformation at high temperatures, while overcoming the accumulation of internal stress and interface peeling caused by power cycle thermal stress.

[0003] Therefore, for the manufacture of flexible printed circuit boards for automobile rearview mirrors, the conductive silver paste used must have good environmental durability, excellent mechanical reliability, high power carrying capacity, and outstanding long-term thermal stability, while the existing low-temperature silver paste system suitable for consumer electronics cannot meet the vehicle-grade requirements. SUMMARY

[0004] The purpose of the present application is to provide a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, to solve the contradiction that the existing flexible circuit technology cannot simultaneously meet the prominent problems of automobile rearview mirrors in terms of extreme environmental tolerance, high vibration reliability, and power load capacity, overcome the risk of circuit corrosion and failure caused by the insufficient weather resistance of traditional silver paste, eliminate the problem of circuit breakage caused by the poor vibration fatigue resistance of silver paste or insufficient adhesion to the substrate, and avoid reliability defects such as heat aging or even delamination caused by high resistivity and poor heat resistance of silver paste when working under high current.

[0005] To achieve the above-mentioned purpose, the present application provides the following solutions:

[0006] The application provides a novel automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, which comprises a flexible insulating substrate and a conductor pattern arranged on the substrate, and the key point is that the conductor pattern comprises a main circuit pattern formed by calendered copper and an auxiliary functional structure formed by a solderable low-temperature curing silver paste in a local area of the main circuit pattern; the auxiliary functional structure comprises at least one of a heating circuit, a turn signal driving circuit area and a blind spot monitoring sensor connecting area; the curing temperature of the solderable low-temperature silver paste is 125-135 DEG C; the solderable low-temperature silver paste comprises, in mass fraction, 60-72 parts of conductive filler, 15-22 parts of a polymer resin system, 15-25 parts of a high-boiling ester solvent, 4-7 parts of a curing agent, 1-5 parts of a silane coupling agent, 0.8-1.5 parts of a polyamide wax thixotropic agent, 0.1-0.3 parts of a polysiloxane antifoaming agent, 0.3-1 part of modified carbon nanotube, 1-2 parts of modified alumina powder, 1-2 parts of modified barium sulfate and 0.8-1.5 parts of a low-stress modifier.

[0007] The polymer resin system comprises phenolic epoxy resin, biphenyl type epoxy resin and polyacrylate-carboxyl-terminated liquid butyl nitrile rubber copolymer (polyacrylate-CTBN copolymer) at a mass ratio of 1:(0.9-1.1):(0.3-0.5); the conductive filler comprises silver-coated copper flaky powder, nano silver wire and micron silver powder, the D50 of the silver-coated copper flaky powder is 1.0-3.0 μm, the diameter-thickness ratio is >15, the length of the nano silver wire is 25-45 μm and the diameter is 30-60 nm, and the micron silver powder is spherical silver powder with a D50 of 3-8 μm.

[0008] Specifically, the mass ratio of the silver-coated copper flaky powder, the nano silver wire and the micron silver powder in the conductive filler is (1.2-1.8):1:(0.5-1.0); the curing agent is a mixture of phenolic resin and 2-ethyl-4-methyl imidazole at a mass ratio of (4-5):1; the silane coupling agent is a mixture of KH-560 and octyl triethoxysilane at a mass ratio of 1:(1-2), and KH-560 is γ-glycidyl ether propyl trimethoxysilane; the polysiloxane antifoaming agent is a non-silicon antifoaming agent containing a bubble-breaking polysiloxane; and the high-boiling 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 / 100 g to 0.65 eq / 100 g, and the viscosity at 25℃ is 5000 mPa·s to 12000 mPa·s; the preparation method of the above-mentioned polyacrylate-CTBN copolymer is: methyl methacrylate, butyl acrylate and carboxyl-terminated liquid butadiene-acrylonitrile rubber are prepared by active radical polymerization reaction at 75℃ to 85℃ for 3 h to 5 h in the presence of tert-butyl peroxy-2-ethylhexanoate initiator, with the mass ratio of (4 to 5):(4 to 5):1.

[0010] More specifically, the mass fraction of carboxyl in the above-mentioned carboxyl-terminated liquid butadiene-acrylonitrile rubber is 0.5% to 0.7%, and the number average molecular weight is 3000 Da to 5000 Da; the mass of tert-butyl peroxy-2-ethylhexanoate initiator is 1.0% to 2.0% of the total mass of methyl methacrylate, butyl acrylate and carboxyl-terminated liquid butadiene-acrylonitrile rubber.

[0011] Further, the preparation method of the above-mentioned modified alumina powder is: alumina powder with a particle size of 0.05 μm to 0.15 μm is dispersed in anhydrous ethanol, a mixed modifier composed of silane coupling agent KH-560 and titanate coupling agent NDZ-201 with a mass ratio of 1:(1 to 2) is added, and ultrasonic stirring is carried out at 60℃ to 70℃ and 300W to 500W for 2 h to 3 h, followed by calcination at 90℃ to 105℃ for 1 h to 2 h after filtration; wherein the total mass of the mixed modifier is 4% to 6% of the mass of the alumina powder.

[0012] Further, the preparation process of the above-mentioned modified barium sulfate is: precipitated barium sulfate powder with a particle size of 0.2 μm to 0.8 μm is dispersed in deionized water, sodium stearate with a mass of 2% to 4% of the mass of barium sulfate is added, and mechanical stirring is carried out at 80℃ to 90℃ and 600 r / min to 800 r / min for 1.5 h to 2.5 h; after the reaction is completed, filtration is carried out, washing is carried out with ethanol, vacuum drying is carried out at 100℃ to 120℃ for 12 h, and grinding is carried out through a 600 mesh to 800 mesh screen.

[0013] Preferably, the above-mentioned polyamide wax thixotropic agent is a polyamide wax obtained by condensation of castor oil acid and dimer fatty acid with ethylenediamine; the above-mentioned polyamide wax thixotropic agent needs to be activated before use, and the specific operation of the activation treatment is: the polyamide wax is added to a mixed solvent of isopropyl alcohol and diethylene glycol butyl ether acetate with a volume ratio of 1:1.5 to 2.0, amino propanol is added, the activation temperature is controlled at 45℃ to 55℃, the stirring speed is 800 r / min to 1200 r / min, and high-speed dispersion activation is carried out for 15 min to 30 min; wherein the mass ratio of the polyamide wax, the mixed solvent and the amino propanol is 1:(4 to 6):(0.1 to 0.2).

[0014] Further, the preparation process of the modified carbon nanotube is:

[0015] A1, electrolyte preparation: 15 parts of carbon nanotubes are dispersed in 350-450 parts of 1-ethyl-3-methyl imidazole bis-trifluoromethyl sulfonimide salt ionic liquid, 3-5 parts of 3-aminopropyl triethoxysilane is added as a functionalizing agent, and ultrasonic treatment is performed at a power of 500-700 W for 40-60 min to form a suspension electrolyte;

[0016] A2, electrochemical treatment: the electrolyte is placed in an electrolytic cell, a graphite rod is used as an anode, and a platinum sheet is used as a cathode, electrolysis is carried out at a voltage of 2.5-3.5 V and a stirring speed of 500-700 r / min for 1.5-2.5 h;

[0017] A3, post-reaction treatment: after the reaction is completed, the solid is obtained by filtration, and the solid is washed by centrifugation with ethanol and deionized water alternately until the conductivity of the supernatant is less than 5 μS / cm; vacuum drying is performed at 70-80 °C for 8-12 h to obtain the modified carbon nanotube.

[0018] Further, the low-stress modifier is carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticles with a poly(methyl methacrylate) shell and an average particle size range of 100-200 nm, and the preparation method is:

[0019] B1, preparation of core emulsion:

[0020] 80-100 parts of carboxyl-terminated liquid butyronitrile rubber, 5-10 parts of acrylonitrile, 15-25 parts of deionized water, 1.0-2.5 parts of sodium allyloxy hydroxypropyl sulfonate, and 0.5-1.5 parts of pH buffer are placed in a reaction kettle, emulsified at 65-75 °C and a stirring speed of 200-400 r / min for 30-60 min; then the temperature is increased to 125-135 °C, the pressure is maintained at 0.35-0.45 MPa, and the reaction is carried out for 3-5 h; after the reaction is completed, the product is 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 vinyl triethoxysilane, 1.0-2.0 parts of sodium allyloxy hydroxypropyl sulfonate, and 50-70 parts of deionized water are mixed and emulsified at a high speed of 1000-3000 r / min for 20-40 min to obtain a shell pre-emulsion;

[0023] B3, preparation of seed emulsion:

[0024] Take 20 parts to 30 parts of the core emulsion prepared in B1 step, add to the reaction kettle, dilute to solid content of 5% to 10%, heat to 75°C to 80°C, protect by nitrogen gas, add 0.1 parts to 0.3 parts of 2% to 5% mass concentration potassium persulfate aqueous solution and 1 part to 3 parts of the shell pre-emulsion prepared in B2 step under stirring, react for 20 min to 40 min, get seed emulsion;

[0025] B4, core-shell emulsion polymerization:

[0026] Add the remaining shell pre-emulsion prepared in B2 step and 0.5 parts to 1.2 parts of 2% to 5% mass concentration potassium persulfate aqueous solution to the seed emulsion above dropwise, control the dropwise temperature at 78°C to 82°C, after dropwise addition, keep warm for 1.5 h to 2.5 h;

[0027] B5, post-treatment:

[0028] Cool to room temperature, filter out the material with 100 mesh to 200 mesh screen, after demulsification, washing, vacuum drying, grind through 500 mesh to 800 mesh screen, get the carboxyl-terminated polybutadiene acrylonitrile core-shell rubber microparticles above.

[0029] Finally, the preparation method of the above solderable low-temperature silver paste is:

[0030] S1, add high-boiling ester solvent to the reaction kettle, then add high molecular resin system, stir at 80°C to 90°C until the resin is dissolved, add silane coupling agent and polysiloxane defoaming agent, cool to room temperature, add curing agent, keep the solution temperature below 30°C, stir until all phases are completely dissolved and mixed, get carrier solution;

[0031] S2, add polyamide wax thixotropic agent to the carrier solution, stir and mix at room temperature, get mixed solution A;

[0032] S3, add micron silver powder, modified alumina and modified barium sulfate to mixed solution A, stir and mix at room temperature, get mixed solution B;

[0033] S4, add silver-coated copper flaky powder and nanosilver wire to mixed solution B, stir and mix at room temperature, get mixed solution C;

[0034] S5, add modified carbon nanotube and low-stress modifier to mixed solution C, stir and mix, and control the stirring temperature not to exceed 30°C, get mixed solution D;

[0035] S6, transfer the mixed solution D to a three-roll mill, first, adjust the gap between the fast roller and the middle roller to 70-80 mu m, and the gap between the middle roller and the slow roller to 30-40 mu m, and grind for 1-2 times; then, adjust the gap between the fast roller and the middle roller to 40-50 mu m, and the gap between the middle roller and the slow roller to 10-20 mu m, and grind for 1-2 times again; a total of 2-4 times of grinding, to obtain the ground silver paste;

[0036] S7, transfer the ground silver paste to a vacuum stirred tank, and stir and mix under a vacuum degree of less than or equal to-0.095 MPa, and after the stirring is completed, package the silver paste, to obtain the above-mentioned solderable low-temperature silver paste.

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

[0038] The novel automobile rearview mirror flexible printed circuit board prepared from the solderable medium-temperature curing silver paste provided by the present application realizes the comprehensive improvement of mechanical reliability, environmental durability, conductive stability and power load capacity under the medium-temperature curing condition of 125-135 DEG C through the synergistic effect of the components and the innovative process, and perfectly meets the harsh demand of high reliability of the automobile rearview mirror under extreme environment. It is particularly pointed out that the present application proves that for automobile electronic application, the curing temperature is not the lower the better, but the best balance between the curing temperature and the comprehensive performance of the final product needs to be achieved through the comprehensive optimization of the resin system, filler modification and formula structure. The present application has the following beneficial effects:

[0039] Firstly, the present application breaks through the technical prejudice of simply pursuing low-temperature curing, and sets the curing temperature in a reasonable range of 125-135 DEG C through the use of a phenolic epoxy resin system and the optimized curing agent ratio. Although this design is higher than the resin system used in the comparative example one, the improvement of the crosslinking density, glass transition temperature and thermal mechanical property brought by this design is significant. The experimental results show that although the comparative example one has a lower curing temperature, the adhesion after the damp heat aging is completely lost, and the resistance change rate after the vibration test is as high as 25.6%, which cannot meet the vehicle level requirement. This fully proves that appropriately increasing the curing temperature to obtain a more stable three-dimensional network structure is crucial for the long-term reliability of automobile electronic products.

[0040] Secondly, the three-dimensional network structure formed after the curing of the silver paste of the present application endows the silver paste circuit with excellent mechanical strength and environmental stability, and the adhesion after 85 DEG C / 85%RH, 240h damp heat aging is maintained at 0 level, and the resistance change rate after the vibration test is less than 5%, which fully meets the severe requirements of the continuous vibration and harsh environment of the automobile driving on the circuit reliability.

[0041] Thirdly, the silver paste of the present application builds a high-efficiency and stable three-dimensional conductive network by multi-scale compounding of micron silver powder, silver-coated copper flaky powder and nano silver wire, and innovative application of electrochemically modified carbon nanotubes and specially modified alumina / barium sulfate, with a sheet resistance as low as 3.5 mΩ. After 200 severe thermal cycles from -40℃ to 125℃, the resistance change rate is less than 6.5%, showing excellent environmental stability, which perfectly matches the use requirements of the automobile rearview mirror in extreme temperature environment.

[0042] Fourthly, the special modified filler and optimized resin system introduced in the present application enable the silver paste to have excellent environmental aging resistance. After 240h salt spray test, the resistance change rate is less than 8.5%, the acid and alkali resistance is excellent, the yellowing index Δb is less than 3.8 after 500h UV aging, and the adhesion is well maintained, fully meeting the durability requirements of the automobile rearview mirror exposed to outdoor harsh environment for a long time, and effectively overcoming the corrosion and failure risks of the traditional silver paste due to insufficient weather resistance.

[0043] Fifthly, the silver paste of the present application has excellent power load capacity and long-term use stability. After 1000h continuous work under 3A large current, the resistance change rate is less than 12.8%, and the steady-state temperature rise is less than 55℃, fully meeting the harsh requirements of the automobile rearview mirror heating function on the long-term reliability of the circuit. This is due to its low resistivity, good thermal conductivity and stable conductive network structure, effectively solving the reliability problems such as heat aging and even delamination caused by high resistivity and poor heat resistance of the silver paste under large current work.

[0044] Sixthly, the present application integrates medium temperature curing characteristics, excellent mechanical reliability, excellent environmental durability and strong power load capacity, providing a comprehensive solution suitable for automobile rearview mirror heating circuit, turn signal driving circuit and blind spot monitoring sensor connection, effectively solving the reliability problems caused by insufficient material weather resistance, poor vibration fatigue resistance and insufficient power load capacity in the prior art, and providing high-reliability hardware support for advanced driver assistance systems. DETAILED DESCRIPTION

[0045] The various exemplary embodiments of the present application will now be described in detail, which should not be considered as limiting the present application, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present application.

[0046] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of, for example, concentrations, solvent amounts, and the like, there are intended to be included in the present application each and every intermediate value and sub-range thereof. For example, a range of 1 to 10 includes each and every intermediate value and sub-range thereof, i.e., 1-9.5, 1-4.5, 1-3.75, 1-2, 2-10, 3-10, 4-10, 5-10, 6-10, 7-10, 8-10, 9-10, etc. These smaller ranges are intended to be individually and collectively

[0047] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as those of an ordinarily skilled artisan in the field of the application. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, preferred methods and materials are described. All publications mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. In case of conflict, the content of the present specification will control.

[0048] Various modifications and changes can be made to the specific embodiments of the present application described herein without departing from the scope or spirit of the application. Other embodiments of the application will be apparent to those of ordinary skill in the art from the description and examples herein. The description and examples are illustrative of the application and are not intended to limit the scope of the application.

[0049] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended terms that mean including, but not limited to.

[0050] Example 1

[0051] The present embodiment provides a new type of flexible printed circuit board for automobile rearview mirror prepared based on a solderable silver paste, which comprises a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by a calendered copper, and an auxiliary functional structure formed by a solderable low-temperature silver paste sample 1 in a local area of the main circuit pattern. The auxiliary functional structure in the present embodiment comprises a heating circuit, a steering lamp driving circuit area, and a blind spot monitoring sensor connection area.

[0052] The solderable low-temperature silver paste sample 1 of the present embodiment comprises, in mass parts: 64 parts of an electrically conductive filler sample 1, 18 parts of a high polymer resin system sample 1, 20 parts of γ-butyrolactone, 5 parts of a curing agent sample 1, 3 parts of a silane coupling agent sample 1, 1 part of a polyamide wax thixotropic agent sample 1, 0.2 parts of a polysiloxane antifoaming agent BYK-055, 0.6 parts of a modified carbon nanotube sample 1, 1.5 parts of a modified alumina powder sample 1, 1.5 parts of a modified barium sulfate sample 1, and 1.0 parts of a low-stress modifier sample 1.

[0053] The conductive filler sample 1 comprises: silver-coated copper flaky powder sample 1 with D50 of 2.0 μm, diameter-thickness ratio of 18, length of 35 μm, silver nanowire sample 1 with diameter of 45 nm, and micron silver powder sample 1 with D50 of 5 μm; wherein the mass ratio of the silver-coated copper flaky powder sample 1, the silver nanowire sample 1 and the micron silver powder sample 1 is 1.5:1:0.7.

[0054] The high-molecular resin system sample 1 comprises phenolic epoxy resin, biphenyl type epoxy resin sample 1 and polyacrylate-CTBN copolymer sample 1 with mass ratio of 1:1:0.4; wherein the epoxy value of the biphenyl type epoxy resin sample 1 is 0.58 eq / 100 g, and the viscosity at 25℃ is 8000 mPa·s; the polyacrylate-CTBN copolymer sample 1 is prepared by the following method: methyl methacrylate, butyl acrylate and carboxyl-terminated liquid butadiene-acrylonitrile rubber are mixed with mass ratio of 4.5:4.7:1, and then subjected to active radical polymerization reaction at 80℃ for 4 h in the presence of tert-butyl peroxy-2-ethylhexanoate initiator; wherein the mass fraction of carboxyl in the carboxyl-terminated liquid butadiene-acrylonitrile rubber is 0.6%, and the number average molecular weight is 4000 Da; the mass of the tert-butyl peroxy-2-ethylhexanoate initiator is 1.5% of the total mass of the methyl methacrylate, the butyl acrylate and the carboxyl-terminated liquid butadiene-acrylonitrile rubber.

[0055] The curing agent sample 1 is phenolic resin and 2-ethyl-4-methyl imidazole with mass ratio of 4.5:1.

[0056] The polyamide wax thixotropic agent sample 1 is a polyamide wax sample 1 prepared by condensation of castor oil acid and dimer fatty acid with ethylenediamine, specifically BYK-410; it needs to be activated before use, and the specific operation of the activation treatment is: BYK-410 is added into a mixed solvent of isopropyl alcohol and diethylene glycol butyl ether acetate with volume ratio of 1:1.8, and then amino propanol is added, the activation temperature is controlled at 50℃, the stirring speed is 1000 r / min, and high-speed dispersion is activated for 20 min; wherein the mass ratio of BYK-410, the mixed solvent and the amino propanol is 1:5:0.15.

[0057] The silane coupling agent sample 1 is a mixture of KH-560 and octyl triethoxysilane with mass ratio of 1:1.5.

[0058] The modified alumina sample 1 is prepared by dispersing 1 part of alumina powder with particle size of 0.1 μm in 8 parts of anhydrous ethanol, adding a mixed modifier composed of silane coupling agent KH-560 and titanate coupling agent NDZ-201 with mass ratio of 1:1.5, and then performing ultrasonic stirring reaction at 65℃ for 2.5 h; after filtration, calcination is performed at 100℃ for 1.5 h; wherein the total mass of the mixed modifier is 5% of the mass of the alumina powder.

[0059] The modified carbon nanotube sample 1 is prepared by the following process:

[0060] A1, electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 400 parts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ionic liquid, 4 parts of 3-aminopropyl triethoxysilane were added as a functionalizing agent, and ultrasonic treatment was performed at a power of 600 W for 50 min to form a suspension electrolyte 1;

[0061] A2, electrochemical treatment: the suspension electrolyte 1 was placed in an electrolytic cell, a graphite rod was used as the anode, and a platinum plate was used as the cathode, and electrolysis was carried out at a voltage of 3.0 V and a stirring speed of 600 r / min for 2 h;

[0062] A3, post-reaction treatment: after the reaction was completed, the solid was obtained by filtration, and the solid was washed with ethanol and deionized water by centrifugation for 5 times until the conductivity of the supernatant was less than 5 μS / cm; vacuum drying at 75°C for 10h, to obtain modified carbon nanotube sample 1.

[0063] The preparation process of the low-stress modifier sample 1 is as follows in terms of mass fraction:

[0064] B1, preparation of core emulsion:

[0065] 90 parts of carboxyl-terminated liquid butadiene-acrylonitrile rubber, 8 parts of acrylonitrile, 20 parts of deionized water, 2 parts of sodium allyloxy hydroxypropyl sulfonate and 1 part of pH buffer were placed in a reaction kettle and emulsified at 70°C and 300 r / min for 45 min; then the temperature was raised to 130°C, the pressure was maintained at 0.40 MPa, and the reaction was carried out for 4 h; after the reaction was completed, the reaction kettle was cooled and discharged to obtain the core emulsion 1;

[0066] B2, pre-emulsification of shell monomers:

[0067] 50 parts of methyl methacrylate, 3 parts of β-hydroxyethyl methacrylate, 2 parts of vinyl triethoxysilane, 1.5 parts of sodium allyloxy hydroxypropyl sulfonate and 60 parts of deionized water were mixed and emulsified at a high speed of 2000 r / min for 30 min to obtain the shell pre-emulsion 1;

[0068] B3, preparation of seed emulsion:

[0069] 25 parts of the core emulsion 1 prepared in step B1 was added to the reaction kettle and diluted to a solid content of 8%, then the temperature was raised to 78°C, nitrogen was introduced for protection, and 0.2 parts of 3.5% potassium persulfate aqueous solution and 2 parts of the shell pre-emulsion 1 prepared in step B2 were added under stirring, and the reaction was carried out for 30 min to obtain the seed emulsion 1;

[0070] B4, core-shell emulsion polymerization:

[0071] The remaining shell pre-emulsion 1 prepared in step B2 was added dropwise into the seed emulsion 1 with 0.8 parts of a 2.5% mass concentration potassium persulfate aqueous solution, the dropwise temperature was controlled at 80°C, and after the dropwise addition was completed, the solution was kept for 2.0 hours for maturation;

[0072] B5, post-treatment:

[0073] After cooling to room temperature, the product was filtered through a 150-mesh screen, and after demulsification, washing, and vacuum drying, it was ground through a 600-mesh screen to obtain carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 1; the average particle size of the carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 1 was 156 nm.

[0074] Using the above raw materials, the preparation of the solderable low-temperature silver paste sample 1 was carried out, specifically as follows:

[0075] S1, 20 parts of γ-butyrolactone were added to a reaction kettle, and then 18 parts of the high molecular resin system sample 1 were added. The solution was stirred at 85°C and a rotation speed of 400 r / min for 30 min until the resin was completely dissolved. Then 3 parts of the silane coupling agent sample 1 and 0.2 parts of the polysiloxane defoaming agent BYK-055 were added. After cooling to room temperature, 5 parts of the curing agent sample 1 were added, and the solution temperature was kept at 25°C. The solution was stirred until all the phases were completely dissolved and mixed to obtain the carrier solution 1.

[0076] S2, 1 part of the activated polyamide wax thixotropic agent sample 1 was added to the carrier solution 1, and the mixture was stirred at room temperature and a rotation speed of 650 r / min for 38 min to obtain the mixed solution A1.

[0077] S3, 14 parts of the micron silver powder sample 1, 1.5 parts of the modified alumina sample 1, and 1.5 parts of the modified barium sulfate sample 1 were added to the mixed solution A1. The mixture was stirred at room temperature and a rotation speed of 1000 r / min for 75 min to obtain the mixed solution B1.

[0078] S4, 30 parts of the silver-coated copper flaky powder sample 1 and 20 parts of the nanosilver wire sample 1 were added to the mixed solution B1. The mixture was stirred at room temperature and a rotation speed of 1200 r / min for 80 min to obtain the mixed solution C1.

[0079] S5, 0.6 parts of the modified carbon nanotube sample 1 and 1 part of the low-stress modifier sample 1 were added to the mixed solution C1. The mixture was stirred at a rotation speed of 1200 r / min for 110 min, and the stirring temperature was controlled at 25°C to obtain the mixed solution D1.

[0080] S6, the mixed solution D1 was transferred to a three-roll mill. The gap between the fast roller and the middle roller was adjusted to 75 μm, and the gap between the middle roller and the slow roller was adjusted to 35 μm. The mixture was ground for 2 passes. Then the gap between the fast roller and the middle roller was adjusted to 45 μm, and the gap between the middle roller and the slow roller was adjusted to 15 μm. The mixture was ground for another pass. In total, the mixture was ground for 3 passes to obtain the ground silver paste sample 1.

[0081] S7, the ground silver paste sample 1 is transferred to a vacuum stirred tank, and stirred and mixed at a rotation speed of 30 r / min under a vacuum degree of-0.100 MPa for 45 min. After stirring, the silver paste is packaged to obtain a solderable low-temperature silver paste sample 1. The curing temperature of the solderable low-temperature silver paste sample 1 is 125°C.

[0082] Example Two

[0083] A new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste is provided in the present example, which comprises a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by a calendered copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste sample 2 in a local area of the main circuit pattern. The auxiliary functional structure in the present example comprises a heating circuit and a steering lamp driving circuit area.

[0084] The solderable low-temperature silver paste sample 2 of the present example comprises, in mass parts: 72 parts of a conductive filler sample 2, 15 parts of a high polymer resin system sample 2, 15 parts of dipropylene glycol butyl ether acetate, 4 parts of a curing agent sample 2, 1 part of a silane coupling agent sample 2, 0.8 parts of a polyamide wax thixotropic agent sample 2, 0.3 parts of a polysiloxane antifoaming agent BYK-088, 0.3 parts of a modified carbon nanotube sample 2, 2 parts of a modified alumina powder sample 2, 1 part of a modified barium sulfate sample 2, and 1.5 parts of a low-stress modifier sample 2.

[0085] The conductive filler sample 2 comprises a silver-coated copper flaky powder sample 2 with a D50 of 3.0 μm, a diameter-thickness ratio of 16, a length of 45 μm, a nanosilver wire sample 2 with a diameter of 30 nm, and a micron silver powder sample 2 with a D50 of 8 μm. The mass ratio of the silver-coated copper flaky powder sample 2, the nanosilver wire sample 2, and the micron silver powder sample 2 is 1.2:1:1.

[0086] The high polymer resin system sample 2 comprises a phenolic epoxy resin, a biphenyl type epoxy resin sample 2, and a polyacrylate-CTBN copolymer sample 2 in a mass ratio of 1:1.1:0.3. The biphenyl type epoxy resin sample 2 has an epoxy value of 0.65 eq / 100 g and a viscosity of 5000 mPa·s at 25°C. The polyacrylate-CTBN copolymer sample 2 is prepared by active radical polymerization reaction at 75°C for 5 h in the presence of a tert-butyl peroxy-2-ethylhexanoate initiator, with methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber in a mass ratio of 4:5:1. The carboxyl-terminated liquid nitrile rubber has a carboxyl mass fraction of 0.7% and a number average molecular weight of 3000 Da. The mass of the tert-butyl peroxy-2-ethylhexanoate 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 a mixture of phenolic resin and 2-ethyl-4-methyl imidazole in a mass ratio of 5:1.

[0088] Polyamide wax thixotropic agent sample 2 is a polyamide wax sample 2 prepared by condensation of castor oil acid and dimer fatty acid with ethylenediamine, specifically DISPARLON 6650; it needs to be activated before use, and the specific operation of the activation treatment is: DISPARLON 6650 is added into a mixed solvent of isopropyl alcohol and diethylene glycol butyl ether acetate in a volume ratio of 1:2.0, amino propanol is added, the activation temperature is controlled at 45℃, the stirring speed is 1200r / min, and high-speed dispersion activation is performed for 30min; wherein the mass ratio of DISPARLON 6650, the mixed solvent and amino propanol is 1:4:0.2.

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

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

[0091] Modified carbon nanotube sample 2 is prepared by mixing 1 part of carbon nanotube with 3 parts of 3-aminopropyl triethoxysilane as a functionalizing agent in 450 parts of 1-ethyl-3-methyl imidazole bistrifluoromethanesulfonylimide ionic liquid, and ultrasonic treatment at 500W for 60min.

[0092] A1, electrolyte preparation: 15 parts of carbon nanotube are dispersed in 450 parts of 1-ethyl-3-methyl imidazole bistrifluoromethanesulfonylimide ionic liquid, 3 parts of 3-aminopropyl triethoxysilane are added as a functionalizing agent, and ultrasonic treatment is performed at 500W for 60min to form a suspension electrolyte 2;

[0093] A2, electrochemical treatment: the suspension electrolyte 2 is placed in an electrolytic cell, a graphite rod is used as an anode, and a platinum sheet is used as a cathode, and electrolysis is performed at a voltage of 3.5V and a stirring speed of 500r / min for 1.5h;

[0094] A3, post-reaction treatment: after the reaction is completed, the solid is obtained by filtration, and the solid is washed with ethanol and deionized water by alternating centrifugation for 4 times until the conductivity of the supernatant is <5μS / cm; vacuum drying at 80℃ for 8h to obtain modified carbon nanotube sample 2.

[0095] Low-stress modifier sample 2 is prepared by mixing 1 part of carbon nanotube with 3 parts of 3-aminopropyl triethoxysilane as a functionalizing agent in 450 parts of 1-ethyl-3-methyl imidazole bistrifluoromethanesulfonylimide ionic liquid, and ultrasonic treatment at 500W for 60min.

[0096] B1, preparation of core emulsion:

[0097] Put 100 parts of carboxyl-terminated liquid nitrile rubber, 5 parts of acrylonitrile, 15 parts of deionized water, 2.5 parts of allyloxy hydroxypropyl sodium sulfonate and 0.5 parts of pH buffer into a reaction kettle, emulsify at 75℃, 200r / min stirring for 30min; then increase the temperature to 135℃, the pressure is maintained at 0.45MPa, reaction for 3h; after the reaction is completed, cooling, discharge, get core emulsion 2;

[0098] B2, pre-emulsification of shell monomer:

[0099] Mix 40 parts of methyl methacrylate, 5 parts of beta-hydroxyethyl methacrylate, 1 part of vinyl triethoxysilane, 2 parts of allyloxy hydroxypropyl sodium sulfonate and 70 parts of deionized water, high-speed shearing emulsification for 40min, get shell pre-emulsion 2;

[0100] B3, seed emulsion preparation:

[0101] Take 30 parts of core emulsion 2 prepared in step B1, add to the reaction kettle, dilute to solid content of 5%, increase the temperature to 75℃, nitrogen protection, stirring, add 0.1 parts of 5% mass concentration of potassium persulfate aqueous solution and 3 parts of shell pre-emulsion 2 prepared in step B2, reaction for 40min, get seed emulsion 2;

[0102] B4, core-shell emulsion polymerization:

[0103] Add the remaining shell pre-emulsion 2 prepared in step B2 and 1.2 parts of 5% mass concentration of potassium persulfate aqueous solution to the seed emulsion 2, control the dropping temperature to 78℃, after the dropping is completed, keep warm for 2.5h;

[0104] B5, post-treatment:

[0105] Cool to room temperature, filter the discharge with 100 mesh screen, after demulsification, washing, vacuum drying, grinding through 800 mesh screen, get carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 2; the average particle size of carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 2 is 100nm.

[0106] Use the above raw materials and proportion, carry out the preparation of weldable low temperature silver paste sample 2, specifically:

[0107] S1, add 15 parts of dipropylene glycol butyl ether acetate to the reaction kettle, then add 15 parts of high molecular resin system sample 2, stir at 80℃, 300r / min for 40min 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℃, stir until each phase is completely dissolved and mixed, get carrier solution 2;

[0108] S2, 0.8 parts of the activated polyamide wax thixotropic agent sample 2 was added into the carrier solution 2, mixed at 800 r / min for 30 min at room temperature to obtain a mixed solution A2;

[0109] S3, 22.5 parts of the micron silver powder sample 2, 2 parts of the modified alumina sample 2 and 1 part of the modified barium sulfate sample 2 were added into the mixed solution A2, mixed at 1200 r / min for 60 min at room temperature to obtain a mixed solution B2;

[0110] S4, 27 parts of the silver-coated copper flaky powder sample 2 and 22.5 parts of the nanosilver wire sample 2 were added into the mixed solution B2, mixed at 1500 r / min for 90 min at room temperature to obtain a mixed solution C2;

[0111] S5, 0.3 parts of the modified carbon nanotube sample 2 and 1.5 parts of the low-stress modifier sample 2 were added into the mixed solution C2, mixed at 1800 r / min for 60 min at room temperature, and the stirring temperature was controlled at 30℃ to obtain a mixed solution D2;

[0112] S6, the mixed solution D2 was transferred to a three-roll mill, the gap between the fast roller and the middle roller was adjusted to 80μm, and the gap between the middle roller and the slow roller was adjusted to 30μm for grinding for 1 time; then the gap between the fast roller and the middle roller was adjusted to 40μm, and the gap between the middle roller and the slow roller was adjusted to 10μm for grinding for 1 time; a total of 2 times of grinding were performed to obtain a ground silver paste sample 2;

[0113] S7, the ground silver paste sample 2 was transferred to a vacuum stirring kettle, and stirred at a speed of 40 r / min for 30 min under a vacuum degree of-0.095 MPa, and then the silver paste was packaged to obtain a solderable low-temperature silver paste sample 2; the curing temperature of the solderable low-temperature silver paste sample 2 was 135℃.

[0114] Example Three

[0115] In this example, a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste is provided, which comprises a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by a calendered copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste sample 3 in a local area of the main circuit pattern; the auxiliary functional structure in this example is a steering lamp driving circuit area.

[0116] The solderable low-temperature silver paste sample 3 of the embodiment contains, in mass fraction: 60 parts of the conductive filler sample 3, 22 parts of the polymer resin system sample 3, 25 parts of gamma-butyrolactone, 7 parts of the curing agent sample 3, 5 parts of the silane coupling agent sample 3, 1.5 parts of the polyamide wax thixotropic agent sample 3, 0.1 part of the polysiloxane antifoaming agent TEGO Flow 300, 1 part of the modified carbon nanotube, 1 part of the modified alumina powder sample 3, 2 parts of the modified barium sulfate sample 3, and 1.5 parts of the low-stress modifier sample 3.

[0117] The conductive filler sample 3 contains: silver-coated copper flaky powder sample 3 with a D50 of 1.0 μm, a diameter-thickness ratio of 20, a length of 25 μm, and a diameter of 60 nm nanosilver wire sample 3, and micron silver powder sample 3 with a D50 of 3 μm; wherein the mass ratio of the silver-coated copper flaky powder sample 3, the nanosilver wire sample 3, and the micron silver powder sample 3 is 1.8:1:0.5.

[0118] The polymer resin system sample 3 includes phenolic epoxy resin, biphenyl type epoxy resin sample 3, and polyacrylate-CTBN copolymer sample 3 in a mass ratio of 1:0.9:0.5; wherein the biphenyl type epoxy resin sample 3 has an epoxy value of 0.50 eq / 100 g and a viscosity of 12000 mPa·s at 25℃; the specific preparation method of the polyacrylate-CTBN copolymer sample 3 is: methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber are mixed in a mass ratio of 5:4:1 in the presence of a tert-butyl peroxy-2-ethylhexanoate initiator, and active radical polymerization is carried out at 85℃ for 3 h to obtain the product; wherein the carboxyl-terminated liquid nitrile rubber has a carboxyl mass fraction of 0.5% and a number average molecular weight of 5000 Da; the mass of the tert-butyl peroxy-2-ethylhexanoate initiator is 1.0% of the total mass of methyl methacrylate, butyl acrylate, and carboxyl-terminated liquid nitrile rubber.

[0119] The curing agent sample 3 is phenolic resin and 2-ethyl-4-methyl imidazole in a mass ratio of 4:1.

[0120] The polyamide wax thixotropic agent sample 3 is a polyamide wax sample 3 obtained by condensation of castor oil acid and dimer fatty acid with ethylenediamine, specifically CRAYVALLAC SUPER; it needs to be activated before use; the specific operation of the activation treatment is: CRAYVALLAC SUPER is added to a mixed solvent of isopropyl alcohol and diethylene glycol butyl ether acetate in a volume ratio of 1:1.5, amino propanol is added, the activation temperature is controlled at 55℃, the stirring speed is 800 r / min, and high-speed dispersion activation is carried out for 15 min; wherein the mass ratio of CRAYVALLAC SUPER, the mixed solvent, and amino propanol is 1:6:0.1.

[0121] The silane coupling agent sample 3 is a mixture of KH-560 and octyl triethoxysilane in a mass ratio of 1:2.

[0122] The modified alumina sample 3 is 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 composed of silane coupling agent KH-560 and titanate coupling agent NDZ-201 at a mass ratio of 1:2, and ultrasonically stirring at 60°C for 3 h at a power of 500 W. After filtration, the mixture is calcined at 95°C for 2 h. The total mass of the mixed modifier is 6% of the mass of the alumina powder.

[0123] The modified carbon nanotube sample 3 is prepared by mixing, in parts by mass:

[0124] A1, electrolyte preparation: 15 parts of carbon nanotubes are dispersed in 350 parts of 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonimide ionic liquid, 5 parts of 3-aminopropyl triethoxysilane are added as a functionalizing agent, and the mixture is ultrasonically treated at a power of 700 W for 40 min to form a suspension electrolyte 3;

[0125] A2, electrochemical treatment: the suspension electrolyte 3 is placed in an electrolytic cell, a graphite rod is used as the anode, and a platinum sheet is used as the cathode. Electrolysis is carried out at a voltage of 2.5 V and a stirring speed of 700 r / min for 2.5 h;

[0126] A3, post-reaction treatment: after the reaction is completed, the solid is obtained by filtration, and the solid is washed with ethanol and deionized water by centrifugation for 5 times until the conductivity of the supernatant is less than 5 μS / cm. The solid is dried under vacuum at 70°C for 12 h to obtain the modified carbon nanotube sample 3.

[0127] The low-stress modifier sample 3 is prepared by mixing, in parts by mass:

[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 parts of sodium allyloxy hydroxypropyl sulfonate, and 1.5 parts of pH buffer are placed in a reaction kettle, emulsified at 65°C and a stirring speed of 400 r / min for 60 min, then heated to 125°C, and the pressure is maintained at 0.35 MPa. The reaction is carried out for 5 h. After the reaction is completed, the mixture is cooled and discharged to obtain the 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 vinyl triethoxysilane, 1 part of sodium allyloxy hydroxypropyl sulfonate, and 50 parts of deionized water are mixed and emulsified at a high speed of 3000 r / min for 20 min to obtain the shell pre-emulsion 3.

[0132] B3, preparation of seed emulsion:

[0133] Take 20 parts of the core emulsion 3 prepared in step B1, dilute to solid content of 10%, heat to 80℃, protect with nitrogen, add 0.3 parts of 2% mass concentration potassium persulfate aqueous solution and 1 part of the 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] Add the remaining shell pre-emulsion 3 prepared in step B2 and 0.5 parts of 2% mass concentration potassium persulfate aqueous solution to the seed emulsion 3 dropwise, control the dropwise temperature at 82℃, and keep the temperature for 1.5 h after the dropwise addition is completed;

[0136] B5, post-treatment:

[0137] Cool to room temperature, filter out the material with a 200 mesh screen, and after demulsification, washing, and vacuum drying, grind through a 500 mesh screen to obtain carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 3. The average particle size of the carboxyl-terminated polybutadiene acrylonitrile core-shell rubber particle sample 3 is 200 nm.

[0138] Use the above raw materials in the proportions to prepare a solderable low-temperature silver paste sample 3, specifically:

[0139] S1, add 25 parts of γ-butyrolactone to the reaction kettle, then add 22 parts of the high molecular resin system sample 3, stir at 90℃ and 500 r / min for 20 min until the resin is completely dissolved, add 5 parts of the silane coupling agent sample 3 and 0.1 parts of the polysiloxane defoaming agent TEGO Flow 300, cool to room temperature, add 7 parts of the curing agent sample 3, keep the solution temperature at 22℃, and stir until all phases are completely dissolved and mixed to obtain a carrier solution 3;

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

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

[0142] S4, add 32.7 parts of the silver-coated copper flaky powder sample 3 and 18.2 parts of the nanosilver wire sample 3 to the mixed solution B3, stir at room temperature and 1000 r / min for 120 min to mix, to obtain a mixed solution C3;

[0143] S5, 1 part of the modified carbon nanotube sample 3 and 1.5 parts of the low stress modifier sample 3 were added into the mixed solution C3, stirred at a speed of 1200 r / min for 90 min, and the stirring temperature was controlled at 22℃, to obtain a mixed solution D3;

[0144] S6, the mixed solution D3 was transferred to a three-roll mill, the gap between the fast roller and the middle roller was adjusted to 70μm, and the gap between the middle roller and the slow roller was adjusted to 40μm, and the mixed solution D3 was ground for 2 times; then the gap between the fast roller and the middle roller was adjusted to 50μm, and the gap between the middle roller and the slow roller was adjusted to 20μm, and the mixed solution D3 was ground for another 2 times; in total, the mixed solution D3 was ground for 4 times, to obtain a ground silver paste sample 3.

[0145] S7, the ground silver paste sample 3 was transferred to a vacuum stirred tank, and stirred at a speed of 20 r / min for 60 min under a vacuum degree of-0.102 MPa, and then the silver paste was packaged after the stirring was completed, to obtain a solderable low-temperature silver paste sample 3; the curing temperature of the solderable low-temperature silver paste sample 3 was 130℃.

[0146] Comparative Example One

[0147] The present comparative example provides a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, which comprises a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by a calendered copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste comparative sample 1 in a local area of the main circuit pattern; the auxiliary functional structure in the present comparative example comprises a heating circuit, a turn signal driving circuit area, and a blind spot monitoring sensor connection area.

[0148] The solderable low-temperature silver paste comparative sample 1 comprises, in mass parts: 64 parts of the conductive filler sample 1, 18 parts of the high molecular resin system comparative sample 1, 20 parts of γ-butyrolactone, 5 parts of the curing agent sample 1, 3 parts of the silane coupling agent sample 1, 1 part of the polyamide wax thixotropic agent sample 1, 0.2 parts of the polysiloxane defoaming agent BYK-055, 0.6 parts of the modified carbon nanotube sample 1, 1.5 parts of the modified alumina powder sample 1, 1.5 parts of the modified barium sulfate sample 1, and 1.0 parts of the low stress modifier sample 1.

[0149] The high molecular resin system comparative sample 1 comprises polyester resin, acrylic resin, and polyvinyl butyral at a mass ratio of 1:1:0.4.

[0150] The solderable low-temperature silver paste comparative sample 1 was prepared using the above raw materials, and the specific process was the same as that of Example One, except that the high molecular resin system sample 1 was replaced by the high molecular resin system comparative sample 1. The curing temperature of the prepared solderable low-temperature silver paste comparative sample 1 was 110℃.

[0151] Comparative Example Two

[0152] The present comparative example provides a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, comprising a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by rolled copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste comparative example 2 in the local area of the main circuit pattern. The auxiliary functional structure in the present comparative example comprises a heating circuit, a turn signal driving circuit area, and a blind spot monitoring sensor connection area.

[0153] The solderable low-temperature silver paste comparative example 2 comprises, in mass fraction: 64 parts of the conductive filler sample 1, 18 parts of the high polymer resin system sample 1, 20 parts of γ-butyrolactone, 5 parts of the curing agent sample 1, 3 parts of the silane coupling agent sample 1, 1 part of the polyamide wax thixotropic agent sample 1, 0.2 parts of the polysiloxane antifoaming agent BYK-055, 0.6 parts of the modified carbon nanotube sample 1, 1.5 parts of the modified alumina powder sample 1, 1.5 parts of the modified barium sulfate sample 1, and 1.0 parts of the low-stress modifier sample 1.

[0154] The solderable low-temperature silver paste comparative example 2 is prepared using the above raw materials, and the specific process is the same as that of Example 1, except that no low-stress modifier is added in the S5 step. The curing temperature of the prepared solderable low-temperature silver paste comparative example 2 is 135°C.

[0155] Comparative Example Three

[0156] The present comparative example provides a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, comprising a flexible insulating substrate and a conductor pattern arranged on the substrate. The conductor pattern comprises a main circuit pattern formed by rolled copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste comparative example 2 in the local area of the main circuit pattern. The auxiliary functional structure in the present comparative example comprises a heating circuit, a turn signal driving circuit area, and a blind spot monitoring sensor connection area.

[0157] The solderable low-temperature silver paste comparative example 2 comprises, in mass fraction: 64 parts of the conductive filler sample 1, 18 parts of the high polymer resin system sample 1, 20 parts of γ-butyrolactone, 5 parts of the curing agent sample 1, 3 parts of the silane coupling agent sample 1, 1 part of the polyamide wax thixotropic agent sample 1, 0.2 parts of the polysiloxane antifoaming agent BYK-055, 0.6 parts of the modified carbon nanotube sample 1, 1.5 parts of the modified alumina powder sample 1, 1.5 parts of the modified barium sulfate sample 1, and 1.0 parts of the low-stress modifier sample 1.

[0158] The preparation method of the modified carbon nanotube comparative example 1 is as follows:

[0159] A1, acid treatment: 15 parts of carbon nanotubes were added to a mixed acid composed of 300 parts of concentrated nitric acid and 120 parts of concentrated sulfuric acid, 3% of iron nitrate nonahydrate by mass of the carbon nanotubes was added, and reflux reaction was performed at 110°C for 6h. After cooling, filtration was performed, washing with deionized water was performed until neutral, and vacuum drying was performed at 90°C for 18h to obtain a carboxylated carbon nanotube sample to be cleaned;

[0160] A2, cleaning of the carboxylated carbon nanotubes: the carboxylated carbon nanotube sample to be cleaned was dispersed in a nitric acid solution having a concentration of 0.3 mol / L, and ultrasonic treatment was performed at 50°C for 30 min at a power of 300W. Filtration was performed, washing with deionized water was performed until the conductivity of the filtrate decreased by <10 μS / cm, and drying was performed to obtain a carboxylated carbon nanotube sample;

[0161] A3, silanization modification: 10 parts of the carboxylated carbon nanotube sample were dispersed in a mixed solution of 160 parts of ethanol and water, the volume ratio of ethanol to water was 1:1.0, 2 parts of KH-560 were added, and reaction was performed at 70°C for 6h under ultrasonic treatment at a power of 300W and stirring at a rotation speed of 400r / min. After the reaction was completed, filtration was performed, washing with anhydrous ethanol was performed 3 times, and drying was performed to obtain a silanized modified carbon nanotube sample;

[0162] A4, supercritical treatment: 5 parts of the silanized modified carbon nanotube sample, 150 parts of deionized water, and 15 parts of cocamidopropyl dimethyl amine lactone were added to a supercritical reaction device, carbon dioxide was introduced until the pressure reached 30MPa, reaction was performed at a temperature of 75°C for 6h, 5 parts of dodecyl dimethyl benzyl ammonium chloride were added after the pressure was released, carbon dioxide was introduced until the pressure reached 50MPa, and stirring was performed at a rotation speed of 300r / min for 4h;

[0163] A5, after the pressure was released, separation was performed, washing with deionized water was performed, and vacuum drying was performed at 90°C for 8h to obtain a modified carbon nanotube reference 1.

[0164] Using the above raw materials, preparation of a solderable low-temperature silver paste reference 3 was performed, and the specific process was the same as in Example One, except that the modified carbon nanotube reference 1 was used instead of the modified carbon nanotube sample 1. The curing temperature of the prepared solderable low-temperature silver paste reference 3 was 140°C.

[0165] Comparative Example Four

[0166] This comparative example provides a new type of automobile rearview mirror flexible printed circuit board prepared based on a solderable silver paste, which includes a flexible insulating substrate and a conductor pattern provided on the substrate. The conductor pattern includes a main circuit pattern formed by rolled copper, and an auxiliary functional structure formed by the solderable low-temperature silver paste reference 4 in a local area of the main circuit pattern. The auxiliary functional structure in this comparative example includes a heating circuit, a turn signal driving circuit area, and a blind spot monitoring sensor connection area.

[0167] The weldable low-temperature silver paste control sample 4 contains, in parts by mass: 64 parts of the conductive filler sample 1, 18 parts of the high polymer resin system sample 1, 20 parts of γ-butyrolactone, 5 parts of the curing agent sample 1, 3 parts of the silane coupling agent sample 1, 1 part of the polyamide wax thixotropic agent sample 1, 0.2 parts of the polysiloxane antifoaming agent BYK-055, 0.6 parts of the modified carbon nanotube sample 1, 1.5 parts of the modified alumina powder sample 1, 1.5 parts of barium sulfate with a D50 of 0.5 μm, and 1.0 part of the low-stress modifier sample 1.

[0168] Using the above raw materials, the weldable low-temperature silver paste control sample 4 was prepared, and the specific process was the same as in Example One, except that 1.5 parts of barium sulfate was used instead of 1.5 parts of the modified barium sulfate sample 1, i.e., no modification of barium sulfate was performed during the preparation process. The curing temperature of the prepared weldable low-temperature silver paste control sample 4 was 132 °C.

[0169] Analysis and testing

[0170] The weldable low-temperature cured silver paste samples 1-3 and the weldable low-temperature cured silver paste control samples 1-4 were respectively used to form flexible circuits for automobile rearview mirrors on a polyimide (PI) substrate using a screen printing device, and after curing at the curing temperature of each sample for 30 min, performance testing was performed. The line width of the flexible circuit was 50 μm, the thickness of the PI substrate was 75 μm, and the thickness of the silver paste was 20 μm ± 3 μm.

[0171] I. Mechanical property testing

[0172] The mechanical properties included adhesion testing, bending resistance testing, and hardness, and the test results are shown in Table 1.

[0173] The adhesion testing was performed by cross-hatch testing, and after pulling using 3M 610 tape, the peeling grade was evaluated (0 grade is the best, and 5 grade is the worst). The sample to be tested was placed in a damp heat environment of 85 °C / 85% RH for 240 h for damp heat aging, and the peeling grade after aging was evaluated (0 grade is the best, and 5 grade is the worst).

[0174] The vibration resistance testing was performed by mounting the sample on a vibration table and performing a 10 Hz-2000 Hz sine sweep vibration test with an acceleration of 10 G and a duration of 4 h. After the test, the circuit was checked for whether it was broken, and whether the resistance change rate exceeded 5%.

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

[0176] Table 1: Mechanical property test results

[0177] Test sample Adhesion (grade) Adhesion after aging (grade) Resistance change rate after vibration test (%) Hardness Weldable low temperature silver paste sample 1 0 0 1.2 5H Weldable low temperature silver paste sample 2 0 0 2.5 4H Weldable low temperature silver paste sample 3 0 1 3.8 6H Weldable low temperature silver paste control 1 3 5 25.6 3H Weldable low temperature silver paste control 2 1 3 12.4 5H Weldable low temperature silver paste control 3 2 4 34.4 4H Weldable low temperature silver paste control 4 1 3 18.7 4H

[0178] As can be seen from the results of Table 1, the solderable low-temperature silver paste samples of the present application all exhibit excellent comprehensive performance, among which the solderable low-temperature silver paste sample 1 is the most outstanding, with a resistance change rate of only 1.2% after a rigorous vibration test, far exceeding the automotive electronics industry standard, fully meeting the reliability requirements of the automobile rearview mirror against vibration impact during driving; the solderable low-temperature silver paste sample 1 still maintains a 0-level adhesion after 85℃ / 85%RH, 240h damp-heat aging, proving that the phenolic epoxy resin system used has excellent aging resistance and interface stability. The solderable low-temperature silver paste control sample 1 uses a polyester resin / acrylic resin system with poor heat resistance, and its cured film hardness and adhesion are both poor, with an initial adhesion of only 3-level; after 85℃ / 85%RH aging, the resin is severely softened and degraded, and the adhesion is completely lost; the resistance change rate after vibration test is as high as 25.6%, indicating that the resin system cannot withstand the automobile vibration environment. The solderable low-temperature silver paste control sample 2 lacks a low-stress modifier, and has a large internal stress, although the initial adhesion is acceptable, but it decreases significantly after aging, and the resistance change rate after vibration test is much higher than that of the sample of the present application. The solderable low-temperature silver paste control sample 3 uses carbon nanotubes modified by a traditional method, and has poor dispersibility, which destroys the integrity of the resin system, resulting in the worst mechanical performance indicators among all samples, and the highest resistance change rate after vibration test. The solderable low-temperature silver paste control sample 4 uses unmodified barium sulfate, which has poor compatibility with the resin, weak interface bonding, and insufficient vibration stability, with a resistance change rate of 18.7%.

[0179] II. Conductive property test

[0180] The sheet resistance of the silver paste conductor after curing was measured using a four-probe tester, and the results are shown in Table 2.

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

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

[0183] Table 2: Conductive property test results

[0184]

[0185] As can be seen from the results of Table 2, the solderable low-temperature silver paste samples of the present application all exhibit low sheet resistance, excellent environmental stability and extremely high conductivity uniformity. The sheet resistance of the solderable low-temperature silver paste sample 1 is as low as 3.5 mΩ, and the resistance change rate after 200 thermal cycles from -40℃ to 125℃ is only 2.8%, fully meeting the stringent requirements of the circuit stability of the automobile rearview mirror under extreme temperature environment; the relative standard deviation of the conductivity uniformity is as low as 1.5%, proving the excellent dispersibility and film uniformity of the modified filler. The solderable low-temperature silver paste control sample 1 has insufficient heat resistance of the resin system, and is prone to degradation in high temperature and high humidity environment, resulting in high sheet resistance and a resistance change rate of 18.7% after thermal cycling. The solderable low-temperature silver paste control sample 2 lacks a low-stress modifier, and the conductivity path thereof has slightly poor stability under thermal stress, showing a high resistance change rate after thermal cycling. The solderable low-temperature silver paste control sample 3 uses carbon nanotubes modified by a traditional method, and the conductivity thereof is reduced and prone to agglomeration, resulting in a sheet resistance of 10.5 mΩ, a resistance change rate of 25.4% after thermal cycling, and the worst conductivity uniformity. The solderable low-temperature silver paste control sample 4 uses unmodified barium sulfate, and the dispersibility thereof is poor, resulting in high sheet resistance and insufficient thermal stability.

[0186] III. Weather resistance and chemical medium resistance test

[0187] The sample to be tested is immersed in a 5% NaCl salt water solution at 35℃ for 240h, and the resistance change rate and adhesion change are tested to evaluate the salt spray corrosion resistance;

[0188] The sample to be tested is immersed in a 5% NaCl salt water solution at 35℃ for 240h, and the resistance change rate and adhesion change are tested to evaluate the salt spray corrosion resistance;

[0189] The sample is subjected to 500h ultraviolet aging test using an ultraviolet aging box (UVA-340 lamp, 0.76W / m 2 @340nm), and the yellowing index (Δb) and adhesion change are tested to evaluate the ultraviolet aging resistance. The results are shown in Table 3.

[0190] Table 3: Soldering performance test results

[0191]

[0192] As shown in Table 3, the solderable low-temperature silver paste sample of the present application exhibits excellent weather resistance and chemical medium resistance. The resistance change rate of the solderable low-temperature silver paste sample 1 is only 3.2% after 240h salt spray test, and it also exhibits excellent stability in acid and alkali environment, the adhesion remains 0 level and the yellowing is minimal after 500h ultraviolet aging, fully meeting the durability requirements of the automobile rearview mirror exposed to outdoor harsh environment for a long time. The solderable low-temperature silver paste control sample 1 has insufficient hydrolysis resistance and poor chemical resistance due to the insufficient hydrolysis resistance of the lipid system used, and the performance deteriorates sharply in salt spray and acid and alkali tests, the resistance change rate exceeds 25%, the yellowing is serious after ultraviolet aging and the adhesion decreases significantly. The solderable low-temperature silver paste control sample 2 has slightly poor compactness and is weaker than the sample of the present application in medium penetration resistance. The solderable low-temperature silver paste control sample 3 has poor modified filler dispersibility, and the coating has defects, resulting in the worst chemical medium resistance and ultraviolet resistance among all samples. The solderable low-temperature silver paste control sample 4 has poor compatibility with the resin due to the use of unmodified barium sulfate, and the coating has insufficient protection performance, and the weather resistance test results are significantly worse than those of the sample of the present application.

[0193] IV. Power load test

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

[0195] Table 4: Power load test results

[0196] Test sample Initial resistance (mΩ) Resistance change rate after 1000 h (%) Steady state temperature rise (°C) Weldable low temperature silver paste sample 1 350 5.2 42 Weldable low temperature silver paste sample 2 420 8.5 48 Weldable low temperature silver paste sample 3 550 12.8 55 Weldable low temperature silver paste control 1 680 28.7 72 Weldable low temperature silver paste control 2 520 15.6 58 Weldable low temperature silver paste control 3 1050 45.2 85 Weldable low temperature silver paste control 4 890 22.5 68

[0197] As shown in Table 4, the solderable low-temperature silver paste sample of the present application exhibits excellent power load capacity and long-term stability. The resistance change rate of the solderable low-temperature silver paste sample 1 is only 5.2% after working under 3A current for 1000h, and the steady-state temperature rise is only 42℃, fully meeting the stringent requirements of the automobile rearview mirror heating function on the long-term reliability of the silver paste circuit. This is due to its low resistivity, good thermal conductivity and stable three-dimensional conductive network structure. The control samples have obvious performance decay under long-term high current work due to their respective structural defects, especially the solderable low-temperature silver paste control sample 1 and the solderable low-temperature silver paste control sample 3, which cannot meet the reliability requirements of automobile applications.

[0198] In summary, the prepared weldable low-temperature silver paste sample, especially sample 1, successfully integrates excellent mechanical properties, outstanding conductive stability, excellent weather resistance and strong power load capacity, and the comprehensive performance is far superior to various comparative examples. Through the innovative use of a phenolic epoxy resin system, an optimized conductive filler combination and a special modification process, the performance is significantly improved and optimized, providing a high-reliability solution for automobile rearview mirrors, especially high-end intelligent rearview mirror systems integrated with heating, steering indication and blind spot monitoring functions, and perfectly meeting the harsh requirements of durability and safety for vehicle-grade applications.

[0199] The above-described embodiments are merely preferred modes of the present application and are not intended to limit the scope of the present application, and various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application as defined by the claims.

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 solderable low-temperature silver paste is 125℃~135℃; by mass parts, the solderable low-temperature silver paste comprises 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. The D50 of the silver-coated copper flake powder ranges from 1.0μm to 3.0μm, and the aspect ratio is >15. The length of the silver nanowires ranges from 25μm to 45μm, and the diameter ranges from 30nm to 60nm. The micron-sized silver powder is spherical silver powder with a D50 range of 3μm to 8μm.

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 butadiene 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-2-ethylhexanoate initiator is 1.0% to 2.0% of the total mass of methyl methacrylate, butyl acrylate and the carboxyl-terminated liquid nitrile butadiene 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 0.05 μm to 0.15 μm 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 300 W to 500 W 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 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.

7. 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).

8. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, 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.

9. 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.

10. The flexible printed circuit board for an automotive rearview mirror according to claim 1, characterized in that, The method for preparing the solderable low-temperature 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 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.

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