A new type of flexible printed circuit board and module for smart phones based on a solderable silver paste

Abstract

The application discloses a novel smart phone flexible printed circuit board and module prepared based on weldable silver paste, and belongs to the technical field of electronic materials and devices. The conductor pattern in the flexible printed circuit board comprises a main circuit pattern and an auxiliary function structure formed by weldable low-temperature silver paste. The solidification temperature of the weldable low-temperature silver paste is 90 DEG C to 110 DEG C. The weldable low-temperature silver paste comprises conductive fillers, a polymer resin system, ester solvents, a curing agent, a silane coupling agent, a polyamide wax thixotropic agent, a polysiloxane defoaming agent, modified carbon nanotubes, modified antimony tin oxide powder, modified ceramic fillers and nano yttrium oxide. The polymer resin system comprises cashew phenol-based epoxy resin, hydrogenated bisphenol A epoxy resin and polyacrylate-CTBN copolymer. The application provides a solution suitable for smart phone high-density interconnection, micro jumper, dynamic bending compensation and micro pad direct packaging, and solves the problems caused by high-temperature process damage to elements, welding pre-plating and high-density integration.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic materials and devices, in particular to a novel flexible printed circuit board and module for smart phones prepared based on a solderable silver paste. BACKGROUND

[0002] The flexible printed circuit board has the remarkable characteristics of light weight, ultra-thin thickness and three-dimensional bending capability, and has become the core component for smart phones to realize full-screen, folding screen and high-density module integration. The flexible printed circuit board of the traditional smart phone meets the triple challenge: first, form adaptability: the folding screen hinge area needs to withstand dynamic bending (bending radius ≤2mm, bending life >100,000 times), and the full-screen narrow frame (≤0.3mm) requires the wiring density of the flexible printed circuit board to be improved to line width / line spacing ≤0.05mm; second, functional integration: Mini LED backlight, ultrasonic fingerprint, 5G millimeter wave antenna and other modules need to be directly soldered with micro components on the flexible printed circuit board, which requires the solder pad size to be ≤30μm and has high soldering strength; third, thermal management limitation: OLED screen, lithium battery and other heat-sensitive components (temperature resistance <150℃) require strict limitation of processing temperature.

[0003] Therefore, when applying the silver paste to the fine manufacturing of the flexible printed circuit board of the smart phone, it is necessary to simultaneously meet the ultra-low temperature curing (<110℃) to protect the heat-sensitive components, the nanoscale printing precision to adapt to high-density integration, the direct solderability to realize Mini LED direct connection and the dynamic bending stability to support the folding form. SUMMARY

[0004] The purpose of the present application is to provide a novel flexible printed circuit board and module for smart phones prepared based on a solderable silver paste, in order to break through the three technical boundaries of high-density, high-reliability and low-temperature manufacturing of smart phones, to solve the problems existing in the prior art, to overcome the defects of damage to flexible substrates and heat-sensitive components caused by high-temperature silver paste curing temperature, to eliminate the increase in pre-gold cost and the risk of micro-solder pad soldering failure caused by the non-direct solderability of general silver paste, and to crack the dynamic bending failure and printing failure caused by the aggregation of nanofiller.

[0005] To achieve the above purpose, the present application provides the following scheme:

[0006] The application provides a novel smart phone 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 silver paste in a local area of the main circuit pattern; the auxiliary functional structure comprises at least one of a micro-jumper area, a high-density pad area and a dynamic bending compensation area; the solidification temperature of the solderable low-temperature silver paste is 90-110 DEG C; the solderable low-temperature silver paste comprises 55-70 parts by mass of conductive fillers, 10-20 parts by mass of a polymer resin system, 20-35 parts by mass of an ester solvent, 3-8 parts by mass of a curing agent, 1-3 parts by mass of a silane coupling agent, 0.5-2 parts by mass of a polyamide wax thixotropic agent, 0.2-0.5 parts by mass of a polysiloxane defoaming agent, 0.5-2 parts by mass of modified carbon nanotubes, 0.5-2 parts by mass of modified antimony tin oxide powder, 0.5-2 parts by mass of modified ceramic fillers and 0.05-0.1 parts by mass of nano yttrium oxide;

[0007] The polymer resin system comprises cashew phenolic epoxy resin, hydrogenated bisphenol A epoxy resin and polyacrylate-carboxyl-terminated liquid butyl nitrile rubber copolymer in a mass ratio of 1:(0.8-1.2):(0.3-0.6), and the polyacrylate-carboxyl-terminated liquid butyl nitrile rubber copolymer is abbreviated as polyacrylate-CTBN copolymer; the conductive fillers comprise nano silver wire, spherical silver powder with a D50 range of 0.5-0.8 mu m and low-temperature alloy powder, and the low-temperature alloy powder is low-temperature alloy powder with a D50 range of 5-10 mu m.

[0008] Specifically, the mass ratio of the nano silver wire, the spherical silver powder and the low-temperature alloy powder in the conductive fillers is (1.0-2.0):1:(0.2-0.5); the specific surface area of the nano silver wire is 30 m 2 / g-60 m 2 / g; the low-temperature alloy powder is Sn42Bi58; and the specific surface area of the spherical silver powder is 1.0 m 2 / g-2.5 m 2 / g.

[0009] Specifically, the cashew phenol-based epoxy resin has an epoxy value of 0.45 eq / 100 g to 0.55 eq / 100 g and a viscosity of 10,000 mPa·s to 20,000 mPa·s at 25°C; the polyacrylate-CTBN copolymer is prepared by free radical polymerization of acrylate monomers and carboxyl-terminated liquid nitrile rubber at a mass ratio of (8-12):1 in the presence of benzoyl peroxide initiator at 60°C to 80°C for 4 h to 6 h; the carboxyl-terminated liquid nitrile rubber has a carboxyl mass fraction of 0.4% to 0.6% and a number average molecular weight of 3,000 Da to 5,000 Da; and the mass of the benzoyl peroxide initiator is 1.5% to 3.0% of the total mass of the acrylate monomers and the carboxyl-terminated liquid nitrile rubber.

[0010] More specifically, the curing agent is a polyether polyamine curing agent and an imidazole-based accelerator at a mass ratio of (5-7):1, the imidazole-based accelerator is selected from any one of 2-methylimidazole, 2-ethyl-4-methylimidazole, or 1-cyanoethyl-2-phenylimidazole, the polyether polyamine curing agent is a linear polyether diamine, the silane coupling agent is a mixed coupling agent of KH-550 and KH-560 at a mass ratio of 1:(1.5-3), KH-550 is γ-aminopropyltriethoxysilane, and KH-560 is γ-glycidyl ether propyltrimethoxysilane, the polysiloxane-based defoaming agent is a non-silicon defoaming agent containing a defoaming polysiloxane, and the ester-based solvent is selected from any one of diethylene glycol butyl ether acetate, ethylene glycol ethyl ether acetate, or butyl butyrate.

[0011] More specifically, the modified ceramic filler is prepared by dispersing boron nitride powder with a particle size of 0.1 μm to 0.3 μm in anhydrous ethanol, adding a mixed modifier composed of silane coupling agent KH-560 and polydopamine at a mass ratio of 1:(1.5-2.5), ultrasonically stirring and reacting at 200 W to 400 W and 50°C to 60°C for 4 h to 6 h, filtering, and calcining at 80°C to 100°C for 2 h to 4 h; and the total mass of the mixed modifier is 3% to 5% of the mass of the boron nitride powder.

[0012] Further, the polyamide wax thixotropic agent is a polyamide wax obtained by condensation of plant fatty acid and dimer fatty acid with ethylenediamine; the polyamide wax thixotropic agent needs to be activated before use, and the specific operation of the activation treatment is: adding the polyamide wax into a mixed solvent of isopropyl alcohol and N-methyl pyrrolidone at a volume ratio of 1:1.5-2.0, adding aminopropanol, controlling the activation temperature to be 40°C-50°C, the stirring speed to be 100 r / min-150 r / min, and the activation time to be 15 min-30 min; and the mass ratio of the polyamide wax, the mixed solvent, and the aminopropanol is 1:(5-8):(0.05-0.1).

[0013] Further, the preparation process of the modified carbon nanotube is as follows in mass fraction:

[0014] A1, electrolyte preparation: 15 parts of carbon nanotubes are dispersed in 280-320 parts of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, 5-10 parts of 4-aminobenzene diazonium salt is added as a functionalizing agent, and ultrasonic treatment is performed at a power of 200-400 W for 30-60 min to form a suspension electrolyte;

[0015] 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 3-5 V and a stirring speed of 200-400 r / min for 2-4 h;

[0016] A3, reaction post-treatment: after the reaction is completed, black solids are obtained by filtration, and the black solids are washed by centrifugation with N,N-dimethylformamide and deionized water alternately until the supernatant is colorless and transparent; vacuum drying is performed at 60-80 °C for 10-14 h to obtain the modified carbon nanotube.

[0017] Further, the preparation process of the modified antimony tin oxide powder is as follows in mass fraction:

[0018] B1, suspension preparation: 10 parts of antimony tin oxide powder with an average particle size of 30-70 nm are dispersed in 150-200 parts of deionized water, 1-2 parts of sodium dodecylbenzenesulfonate is added, and ultrasonic treatment is performed at a power of 400-600 W for 30-60 min to obtain a suspension;

[0019] B2, in-situ polymerization: 1-3 parts of pyrrole monomer is added to the suspension, and stirring is performed under ice water bath conditions for 25-35 min; 5 parts of ammonium persulfate aqueous solution with a mass concentration of 5-10% is added dropwise, the dropwise addition temperature is controlled to be 2-4 °C, and the reaction is performed for 6-8 h;

[0020] B3, reaction post-treatment: after the reaction is completed, the filter cake is filtered and washed with deionized water and ethanol alternately until the filtrate is colorless; the filter cake is vacuum dried at 60-80 °C for 20-28 h, and then ground and sieved through a 600-800 mesh screen to obtain the modified antimony tin oxide powder.

[0021] Further, the preparation method of the weldable low-temperature silver paste is as follows:

[0022] S1, an ester solvent is added to a reaction kettle, a high molecular resin system is then added, stirring is performed at 60-80 °C until the resin is dissolved, a silane coupling agent and a polysiloxane defoaming agent are added, the solution is cooled to room temperature, a curing agent is added, the solution temperature is kept below 30 °C, and stirring is performed until all phases are completely dissolved and mixed to obtain a carrier solution;

[0023] S2, polyamide wax thixotropic agent is added to the carrier solution, and the mixture is stirred at room temperature to obtain a mixed solution A;

[0024] S3, low-temperature alloy powder and modified ceramic filler are added to the mixed solution A, and the mixture is stirred at room temperature to obtain a mixed solution B;

[0025] S4, nano-silver wire and spherical silver powder are added to the mixed solution B, and nano-yttrium oxide is added, and the mixture is stirred at room temperature to obtain a mixed solution C;

[0026] S5, modified carbon nanotubes and modified antimony tin oxide powder are added to the mixed solution C, and the mixture is stirred, and the stirring temperature is controlled not to exceed 30 DEG C, to obtain a mixed solution D;

[0027] S6, the mixed solution D is transferred to a three-roll mill, the gap between the fast roller and the middle roller is adjusted to 65-75 mu m, and the gap between the middle roller and the slow roller is adjusted to 25-35 mu m, and the mixture is ground for 1-2 times; then the gap between the fast roller and the middle roller is adjusted to 35-45 mu m, and the gap between the middle roller and the slow roller is adjusted to 5-15 mu m, and the mixture is ground for 1-2 times; a total of 2-4 times of grinding is carried out, and a ground silver paste is obtained;

[0028] S7, the ground silver paste is transferred to a vacuum stirred tank, and is stirred and mixed under a vacuum degree of less than or equal to -0.095 MPa, and after the stirring is completed, the silver paste is packaged, and the above-mentioned solderable low-temperature silver paste is obtained.

[0029] The application discloses a smart phone backlight module, and the key lies in comprising the novel smart phone flexible printed circuit board and the Mini LED chip welded on the conductor pattern of the circuit board through low-temperature solder; the Mini LED chip is welded on the high-density pad area formed by the solderable low-temperature silver paste in the conductor pattern; the size of the Mini LED chip is 20 mu m x 40 mu m-50 mu m x 100 mu m, the low-temperature solder used for welding is Sn42Bi58, and the welding process parameters are as follows: peak temperature 125 DEG C plus or minus 2 DEG C, and reflow time 45 s plus or minus 5 s.

[0030] The application discloses the following technical effects:

[0031] The application provides the solderable low-temperature silver paste, and the application of the smart phone flexible printed circuit board and the module thereof, and through the synergistic effect of the components, the comprehensive improvement of mechanical properties, conductive properties, high-frequency characteristics and welding reliability is realized under the premise of ultralow-temperature solidification, and the harsh requirements of super-high integration and high reliability of smart phones are perfectly met.

[0032] Firstly, the silver paste of the present application can be quickly cured at ultra-low temperature, effectively avoiding the damage of high-temperature process to the heat-sensitive elements such as OLED screen and lithium battery in the internal part of the smart phone, and meanwhile, the three-dimensional network structure of polyacrylate-CTBN copolymer formed after curing of the silver paste gives the silver paste line excellent dynamic bending resistance, fully meeting the reliability requirements of the folding screen phone hinge area for extremely small bending radius and ultra-long service life.

[0033] Secondly, the silver paste of the present application, through multi-scale compounding of nano-silver wire, micro-sphere silver powder and low-temperature alloy powder, and innovative application of electrochemically modified carbon nanotube and polypyrrole coated antimony tin oxide, builds a high-efficiency and stable three-dimensional conductive network with low sheet resistance, and after rigorous temperature cycling at 10GHz high frequency, the impedance change rate is less than 5%, perfectly matching the extreme requirement of 5G mobile phone millimeter wave antenna on transmission loss, and ensuring the integrity of high-speed signals.

[0034] Thirdly, the filler treated by nano yttrium oxide and specific modification process introduced in the present application realizes the ultra-uniform dispersion of nano phase in the system without using traditional dispersants, and makes the silver paste curing surface form a high-activity solderable layer directly. The solderable layer can realize firm metallurgical bonding with Sn42Bi58 solder, the welding shear strength is as high as 8.5N, and the failure mode is mostly internal fracture of the solder joint, the welding yield is close to 100%, and the pre-plating gold / tin process is successfully saved, providing a high-reliability and low-cost solution for direct chip mounting of smart phone camera, Mini LED display and other modules.

[0035] Fourthly, the present application integrates ultra-low temperature curing, ultra-high flexibility, excellent high-frequency conductivity and direct solderability, and provides a comprehensive solution suitable for high-density interconnection, micro-jumper, dynamic bending compensation and micro-pad direct packaging of smart phones, effectively solving the problems of conductive and reliability caused by element damage due to high-temperature process, pre-plating layer for welding and high-density integration in the prior art. DETAILED DESCRIPTION

[0036] 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.

[0037] It should be understood that the terms described in the present application are only for describing the particular embodiments, and are not used to limit the present application. In addition, for the numerical range in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any stated value or intermediate value within the stated range, and any other stated value or intermediate value within the stated range, is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0038] Unless otherwise indicated, all technical and scientific terms have the same meaning as those one of ordinary skill in the art of the disclosure would understand. Although preferred methods and materials are described, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict between the content of the specification and that of any document incorporated by reference, the content of the specification prevails.

[0039] Various modifications and changes can be made to the specific embodiments of the disclosure described herein without departing from the scope or spirit of the disclosure. Other embodiments of the disclosure will be apparent to those of ordinary skill in the art from the disclosure. The specification and examples given are exemplary only.

[0040] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", and the like are open-ended terms that are intended to permit but not limit the inclusion of elements or the number of elements, as well as the possibility that one or more other elements can be added or otherwise included.

[0041] Example 1

[0042] The present embodiment provides a new type of flexible printed circuit board for smart phones 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 rolled 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 micro-jumper area, a high-density pad area, and a dynamic bending compensation area.

[0043] The solderable low-temperature silver paste sample 1 of the present embodiment comprises, in mass parts: 63 parts of a conductive filler sample 1, 15 parts of a high polymer resin system sample 1, 28 parts of diethylene glycol butyl ether acetate, 5 parts of a curing agent sample 1, 2 parts of a silane coupling agent sample 1, 1 part of a polyamide wax thixotropic agent sample 1, 0.3 parts of a polysiloxane antifoaming agent BYK-055, 1 part of a modified carbon nanotube sample 1, 1 part of a modified antimony tin oxide powder sample 1, 1 part of a modified ceramic filler sample 1, and 0.08 parts of nano yttrium oxide.

[0044] The conductive filler sample 1 comprises: a nano silver wire sample 1 with a specific surface area of 45 m 2 / g, a spherical silver powder sample 1 with a D50 of 0.6 μm, and a Sn42Bi58 (indicating 42% tin and 58% bismuth) low-temperature alloy powder sample 1 with a D50 of 8 μm; wherein the mass ratio of the nano silver wire sample 1, the spherical silver powder sample 1, and the low-temperature alloy powder sample 1 is 1.5:1:0.3; and the specific surface area of the spherical silver powder sample 1 is 2.1 m 2 / g.

[0045] Sample 1 of the polymer resin system includes cashew nut shell epoxy resin sample 1, hydrogenated bisphenol A epoxy resin, and polyacrylate-CTBN copolymer sample 1 in a mass ratio of 1:1:0.4. The cashew nut shell epoxy resin sample 1 has an epoxy value of 0.50 eq / 100g and a viscosity of 15000 mPa·s at 25℃. The polyacrylate-CTBN copolymer sample 1 is prepared by reacting acrylate monomers and carboxyl-terminated liquid nitrile rubber in a mass ratio of 10:1 at 70℃ for 5 hours using a benzoyl peroxide initiator. The mass of the benzoyl peroxide initiator is 2.5% of the total mass of the acrylate monomers and the carboxyl-terminated liquid nitrile rubber. The carboxyl group mass fraction in the carboxyl-terminated liquid nitrile rubber sample 1 is 0.51%, and the number average molecular weight is 4000 Da.

[0046] Curing agent sample 1 is a polyether polyamine curing agent and an imidazole accelerator in a mass ratio of 6:1. The imidazole accelerator is a latent imidazole accelerator, specifically 2-methylimidazole; the polyether polyamine curing agent is a linear polyether diamine, specifically EDR-148.

[0047] Silane coupling agent sample 1 is a mixed coupling agent of KH-550 and KH-560 with a mass ratio of 1:2.

[0048] Polyamide wax thixotropic agent sample 1 is a polyamide wax sample 1, specifically PA-8800, formed by the condensation of plant fatty acids and dimer fatty acids with ethylenediamine. Before use, PA-8800 needs to be activated. The specific activation procedure is as follows: add PA-8800 to a mixed solvent of isopropanol and N-methylpyrrolidone with a volume ratio of 1:1.8, add aminopropanol, control the activation temperature at 45℃, the stirring speed at 120 r / min, and activate for 20 min; wherein, the mass ratio of PA-8800, mixed solvent and aminopropanol is 1:6:0.08.

[0049] The modified ceramic filler sample 1 was prepared by dispersing 1 part of boron nitride powder with a particle size of 0.2 μm in 8 parts of anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and polydopamine in a mass ratio of 1:2, reacting with ultrasonic stirring at 300 W at 55℃ for 5 h, filtering, and then calcining at 90℃ for 3 h; wherein, the total mass of the mixed modifier is 4% of the boron nitride powder.

[0050] The preparation process of modified carbon nanotube sample 1, by mass parts, is as follows:

[0051] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 300 parts of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, and 8 parts of 4-aminophenyl diazonium salt were added as functionalizing reagent. The mixture was ultrasonically treated with 300W power for 45 minutes to form suspension electrolyte 1.

[0052] A2. Electrochemical treatment: 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 3 hours under a voltage of 4V and a stirring speed of 300r / min.

[0053] A3. Post-reaction treatment: After the reaction was completed, the black solid was obtained by filtration. The black solid was washed 5 times by alternating centrifugation with N,N-dimethylformamide and deionized water until the supernatant was colorless and transparent. The supernatant was dried under vacuum at 70℃ for 12h to obtain modified carbon nanotube sample 1.

[0054] The preparation process of modified antimony tin oxide powder sample 1, by mass parts, is as follows:

[0055] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 50 nm were dispersed in 180 parts of deionized water, and 1.5 parts of sodium dodecylbenzene sulfonate were added. The mixture was ultrasonically treated with 500 W power for 45 min to prepare suspension 1.

[0056] B2. In-situ polymerization: Add 2 parts of pyrrole monomer to the suspension 1 and stir for 30 min under ice-water bath conditions; add 5 parts of 8% ammonium persulfate aqueous solution dropwise, controlling the dropwise addition temperature at 3℃, and react for 7 h.

[0057] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with deionized water and ethanol until the filtrate is colorless. The filter cake is vacuum dried at 70°C for 24 hours, ground and passed through a 700-mesh sieve to obtain modified antimony tin oxide powder sample 1.

[0058] Using the above-mentioned raw materials, a solderable low-temperature silver paste sample 1 was prepared, specifically as follows:

[0059] S1. Add 28 parts of diethylene glycol butyl ether acetate to the reactor, then add 15 parts of polymer resin system sample 1. Stir at 70°C and 400 r / min for 30 min until the resin is completely dissolved. Add 2 parts of silane coupling agent sample 1 and 0.3 parts of polysiloxane defoamer BYK-055. Cool to room temperature, add 5 parts of curing agent sample 1, keep the solution temperature at 25°C, and stir until all phases are completely dissolved and mixed to obtain carrier solution 1.

[0060] S2. Add 1 part of activated polyamide wax thixotropic agent sample 1 to carrier solution 1, and stir at 500 r / min for 45 min at room temperature to obtain mixed solution A1.

[0061] S3. Add 6.75 parts of Sn42Bi58 low-temperature alloy powder sample 1 and 1 part of modified ceramic filler sample 1 to the mixed solution A1, and stir at 500 r / min for 45 min at room temperature to obtain mixed solution B1.

[0062] S4. Add 33.75 parts of nano-silver wire sample 1 and 22.5 parts of spherical silver powder sample 1 to the mixed solution B1, then add 0.08 parts of nano-yttrium oxide, and mix at room temperature at a speed of 1000 r / min for 80 min to obtain mixed solution C1.

[0063] S5. Add 1 part of modified carbon nanotube sample 1 and 1 part of modified antimony tin oxide powder sample 1 to the mixed solution C1, stir at 1200 r / min for 110 min, and control the stirring temperature at 25℃ to obtain mixed solution D1.

[0064] S6. Transfer the mixed solution D1 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 70 μm and the gap between the middle roller and the slow roller to 30 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 40 μm and the gap between the middle roller and the slow roller to 10 μm and grind once more. Grind a total of 3 times to obtain the ground silver paste sample 1.

[0065] S7. Transfer the ground silver paste sample 1 to a vacuum mixing vessel and stir at a speed of 40 r / min for 30 min under a vacuum of -0.100 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 1. The curing temperature of solderable low-temperature silver paste sample 1 is 105℃.

[0066] Example 2

[0067] This embodiment provides a novel flexible printed circuit board for smartphones 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 micro-jump wire areas and high-density pad areas.

[0068] By mass, the solderable low-temperature silver paste sample 2 of this embodiment contains: 70 parts conductive filler sample 2, 10 parts polymer resin system sample 2, 20 parts ethylene glycol ethyl ether acetate, 8 parts curing agent sample 2, 1 part silane coupling agent sample 2, 0.5 parts polyamide wax thixotropic agent sample 2, 0.2 parts polysiloxane defoamer BYK-088, 0.5 parts modified carbon nanotube sample 2, 2 parts modified antimony tin oxide powder sample 2, 2 parts modified ceramic filler sample 2, and 0.05 parts nano-yttrium oxide.

[0069] Conductive filler sample 2 contains: a specific surface area of ​​30 m² 2 Sample 2 consists of three materials: silver nanowires (with a density of / g), spherical silver powder (with a D50 of 0.5μm), and Sn42Bi58 cryogenic alloy powder (with a D50 of 5μm). The mass ratio of these materials is 2.0:1:0.5. The specific surface area of ​​the spherical silver powder sample is 1.0 m². 2 / g.

[0070] Polymer resin system sample 2 includes cashew nut shell phenolic epoxy resin sample 2, hydrogenated bisphenol A epoxy resin, and polyacrylate-CTBN copolymer sample 2 with a mass ratio of 1:0.8:0.6. Among them, the cashew nut shell phenolic epoxy resin sample 2 has an epoxy value of 0.45 eq / 100g and a viscosity of 20000 mPa·s at 25℃. The specific preparation method of polyacrylate-CTBN copolymer sample 2 is as follows: acrylate monomer and carboxyl-terminated liquid nitrile rubber are subjected to free radical polymerization at 60℃ for 6h in the presence of benzoyl peroxide initiator at a mass ratio of 12:1 to obtain polyacrylate-CTBN copolymer sample 2. Among them, the mass of benzoyl peroxide initiator is 1.5% of the total mass of acrylate monomer and carboxyl-terminated liquid nitrile rubber. The carboxyl mass fraction of carboxyl-terminated liquid nitrile rubber sample 2 is 0.6%, and the number average molecular weight is 5000 Da.

[0071] Curing agent sample 2 is a polyether polyamine curing agent and an imidazole accelerator in a mass ratio of 7:1. The imidazole accelerator is 2-methylimidazole; the polyether polyamine curing agent is EDR-148.

[0072] Silane coupling agent sample 2 is a mixed coupling agent of KH-550 and KH-560 with a mass ratio of 1:3.

[0073] Polyamide wax thixotropic agent sample 2 is a polyamide wax sample 2 composed of plant fatty acids and dimer fatty acids condensed with ethylenediamine, specifically MT 6650. Before use, MT 6650 needs to be activated. The specific activation procedure is as follows: add MT 6650 to a mixed solvent of isopropanol and N-methylpyrrolidone with a volume ratio of 1:1.5, add aminopropanol, control the activation temperature at 50℃, the stirring speed at 100 r / min, and activate for 30 min; wherein, the mass ratio of MT 6650, mixed solvent and aminopropanol is 1:8:0.05.

[0074] The modified ceramic filler sample 2 was prepared by dispersing 1 part of boron nitride powder with a particle size of 0.1 μm in 9 parts of anhydrous ethanol, adding a mixed modifier composed of silane coupling agent KH-560 and polydopamine in a mass ratio of 1:1.5, reacting with ultrasonic stirring at 200 W at 60℃ for 4 h, filtering, and calcining at 100℃ for 2 h; wherein, the total mass of the mixed modifier is 3% of the boron nitride powder.

[0075] The preparation process of modified carbon nanotube sample 2, by mass parts, is as follows:

[0076] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 280 parts of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, and 5 parts of 4-aminophenyl diazonium salt were added as functionalizing reagent. The mixture was ultrasonically treated with 400W power for 30 minutes to form suspension electrolyte 2.

[0077] A2. Electrochemical treatment: 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 4 hours under a voltage of 3V and a stirring speed of 400r / min.

[0078] A3. Post-reaction treatment: After the reaction, the black solid was obtained by filtration. The black solid was washed 6 times by alternating centrifugation with N,N-dimethylformamide and deionized water until the supernatant was colorless and transparent. The supernatant was dried under vacuum at 80℃ for 10h to obtain modified carbon nanotube sample 2.

[0079] The preparation process of modified antimony tin oxide powder sample 2, by mass parts, is as follows:

[0080] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 70 nm were dispersed in 200 parts of deionized water, 2 parts of sodium dodecylbenzene sulfonate were added, and the mixture was ultrasonically treated with 400 W power for 30 min to prepare suspension 2.

[0081] B2. In-situ polymerization: Add 3 parts of pyrrole monomer to the suspension 1 and stir for 25 min under ice-water bath conditions; add 5 parts of 10% ammonium persulfate aqueous solution dropwise, controlling the dropwise addition temperature at 4°C, and react for 6 h.

[0082] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with deionized water and ethanol until the filtrate is colorless; the filter cake is vacuum dried at 80℃ for 20h, ground and passed through a 600-mesh sieve to obtain modified antimony tin oxide powder sample 2.

[0083] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 2 was prepared, specifically as follows:

[0084] S1. Add 20 parts of ethylene glycol ethyl ether acetate to the reactor, then add 10 parts of polymer resin system sample 2. Stir at 60°C and 300 r / min for 40 min until the resin is completely dissolved. Add 1 part of silane coupling agent sample 2 and 0.2 parts of polysiloxane defoamer BYK-088. Cool to room temperature, add 8 parts of curing agent sample 2, keep the solution temperature at 30°C, and stir until all phases are completely dissolved and mixed to obtain carrier solution 2.

[0085] S2. Add 0.5 parts of activated polyamide wax thixotropic agent sample 2 to carrier solution 2, and stir at 600 r / min for 30 min at room temperature to obtain mixed solution A2.

[0086] S3. Add 10 parts of Sn42Bi58 low-temperature alloy powder sample 2 and 2 parts of modified ceramic filler sample 2 to the mixed solution A2, and stir at 600 r / min for 30 min at room temperature to obtain mixed solution B2.

[0087] S4. Add 40 parts of nano-silver wire sample 2 and 20 parts of spherical silver powder sample 2 to the mixed solution B2, then add 0.05 parts of nano-yttrium oxide, and mix at room temperature at a speed of 1500 r / min for 90 min to obtain mixed solution C2.

[0088] S5. Add 0.5 parts of modified carbon nanotube sample 2 and 2 parts of modified antimony tin oxide powder sample 2 to the mixed solution C2, stir at 1500 r / min for 90 min, and control the stirring temperature at 30℃ to obtain mixed solution D2.

[0089] S6. Transfer the mixed solution D2 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 75 μm and the gap between the middle roller and the slow roller to 25 μm and grind once. Then, adjust the gap between the fast roller and the middle roller to 35 μm and the gap between the middle roller and the slow roller to 5 μm and grind once more. Grind a total of 2 times to obtain the ground silver paste sample 2.

[0090] S7. Transfer the ground silver paste sample 2 to a vacuum mixing vessel and stir at 50 r / min for 20 min under a vacuum of -0.095 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 2. The curing temperature of solderable low-temperature silver paste sample 2 is 110℃.

[0091] Example 3

[0092] This embodiment provides a novel flexible printed circuit board for smartphones 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; the auxiliary functional structure in this embodiment includes a dynamic bending compensation region.

[0093] By mass, the solderable low-temperature silver paste sample 3 of this embodiment contains: 55 parts conductive filler sample 3, 20 parts polymer resin system sample 3, 35 parts butyl butyrate, 3 parts curing agent sample 3, 3 parts silane coupling agent sample 3, 2 parts polyamide wax thixotropic agent sample 3, 0.5 parts polysiloxane defoamer TEGO Flow300, 2 parts modified carbon nanotube sample 3, 0.5 parts modified antimony tin oxide powder sample 3, 0.5 parts modified ceramic filler sample 3, and 0.1 parts nano-yttrium oxide.

[0094] Conductive filler sample 3 contains: a specific surface area of ​​60 m² 2 Sample 3 consists of silver nanowires (with a density of / g), spherical silver powder (with a D50 of 0.8μm), and Sn42Bi58 cryogenic alloy powder (with a D50 of 10μm); the mass ratio of the three samples is 1:1:0.2; the specific surface area of ​​the spherical silver powder sample is 2.5m². 2 / g.

[0095] Polymer resin system sample 3 includes cashew nut shell phenolic epoxy resin sample 3, hydrogenated bisphenol A epoxy resin and polyacrylate-CTBN copolymer sample 3 with a mass ratio of 1:1.2:0.3; wherein, the cashew nut shell phenolic epoxy resin sample 3 has an epoxy value of 0.55 eq / 100g and a viscosity of 10000 mPa·s at 25℃; the specific preparation method of polyacrylate-CTBN copolymer sample 3 is as follows: acrylate monomer and carboxyl-terminated liquid nitrile rubber are subjected to free radical polymerization at 80℃ for 4h in the presence of benzoyl peroxide initiator to obtain polyacrylate-CTBN copolymer sample 3; wherein, the mass of benzoyl peroxide initiator is 3.0% of the total mass of acrylate monomer and carboxyl-terminated liquid nitrile rubber; the carboxyl mass fraction of carboxyl-terminated liquid nitrile rubber sample 3 is 0.4%, and the number average molecular weight is 3000 Da.

[0096] Curing agent sample 3 is a polyether polyamine curing agent and an imidazole accelerator in a mass ratio of 5:1. The imidazole accelerator is 2-methylimidazole, and the polyether polyamine curing agent is EDR-148.

[0097] Silane coupling agent sample 3 is a mixed coupling agent of KH-550 and KH-560 with a mass ratio of 1:1.5.

[0098] Polyamide wax thixotropic agent sample 3 is a polyamide wax sample 3 formed by the condensation of plant fatty acids and dimer fatty acids with ethylenediamine, specifically thixotropic agent 7500. Before use, thixotropic agent 7500 needs to be activated. The specific operation of the activation treatment is as follows: add thixotropic agent 7500 to a mixed solvent of isopropanol and N-methylpyrrolidone with a volume ratio of 1:2.0, add aminopropanol, control the activation temperature at 40℃, the stirring speed at 150r / min, and activate for 15min; wherein, the mass ratio of thixotropic agent 7500, mixed solvent and aminopropanol is 1:5:0.1.

[0099] The modified ceramic filler sample 3 was prepared by dispersing 1 part of boron nitride powder with a particle size of 0.3 μm in 7 parts of anhydrous ethanol, adding a mixed modifier consisting of silane coupling agent KH-560 and polydopamine in a mass ratio of 1:2.5, reacting with ultrasonic stirring at 400W at 50℃ for 6 hours, filtering, and then calcining at 80℃ for 4 hours; wherein, the total mass of the mixed modifier is 5% of the boron nitride powder.

[0100] The preparation process of modified carbon nanotube sample 3, by mass fraction, is as follows:

[0101] A1. Electrolyte preparation: 15 parts of carbon nanotubes were dispersed in 320 parts of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, and 10 parts of 4-aminophenyl diazonium salt were added as functionalizing reagent. The mixture was ultrasonically treated with 200W power for 60 minutes to form suspension electrolyte 3.

[0102] A2. Electrochemical treatment: Electrolyte 3 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 2 hours under a voltage of 5V and a stirring speed of 200r / min.

[0103] A3. Post-reaction treatment: After the reaction was completed, the black solid was obtained by filtration. The black solid was washed four times by alternating centrifugation with N,N-dimethylformamide and deionized water until the supernatant was colorless and transparent. The supernatant was dried under vacuum at 60°C for 14 hours to obtain modified carbon nanotube sample 3.

[0104] The preparation process of modified antimony tin oxide powder sample 3, by mass parts, is as follows:

[0105] B1. Preparation of suspension: 10 parts of antimony tin oxide powder with an average particle size of 30 nm were dispersed in 150 parts of deionized water, 1 part of sodium dodecylbenzene sulfonate was added, and the mixture was ultrasonically treated with 600 W power for 60 min to prepare suspension 3.

[0106] B2. In-situ polymerization: Add 1 part of pyrrole monomer to the suspension 3 and stir for 35 min under ice-water bath conditions; add 5 parts of 5% ammonium persulfate aqueous solution dropwise, controlling the dropwise addition temperature at 2°C, and react for 8 h.

[0107] B3. Post-reaction treatment: After the reaction is completed, the filter cake is filtered and repeatedly washed with deionized water and ethanol until the filtrate is colorless. The filter cake is vacuum dried at 60°C for 28 hours, ground and passed through an 800-mesh sieve to obtain modified antimony tin oxide powder sample 3.

[0108] Using the above-mentioned raw materials and in the specified proportions, solderable low-temperature silver paste sample 3 was prepared, specifically as follows:

[0109] S1. Add 35 parts of butyl butyrate to the reactor, then add 20 parts of polymer resin system sample 3. Stir at 80℃ and 500r / min for 20min until the resin is completely dissolved. Add 3 parts of silane coupling agent sample 3 and 0.5 parts of polysiloxane defoamer TEGO Flow 300. Cool to room temperature, add 3 parts of curing agent sample 3, keep the solution temperature at 22℃, and stir until all phases are completely dissolved and mixed to obtain carrier solution 3.

[0110] S2. Add 2 parts of activated polyamide wax thixotropic agent sample 3 to carrier solution 3, and mix at room temperature and 400 r / min for 60 min to obtain mixed solution A3.

[0111] S3. Add 5 parts of Sn42Bi58 low-temperature alloy powder sample 3 and 0.5 parts of modified ceramic filler sample 3 to mixed solution A3, and stir at 400 r / min for 60 min at room temperature to obtain mixed solution B3.

[0112] S4. Add 25 parts of nano-silver wire sample 3 and 25 parts of spherical silver powder sample 3 to the mixed solution B3, then add 0.1 parts of nano-yttrium oxide, and stir at 1000 r / min for 150 min at room temperature to obtain mixed solution C3.

[0113] S5. Add 2 parts of modified carbon nanotube sample 3 and 0.5 parts of modified antimony tin oxide powder sample 3 to the mixed solution C3, stir at 1000 r / min for 150 min, and control the stirring temperature at 22℃ to obtain mixed solution D3.

[0114] S6. Transfer the mixed solution D3 to a three-roll mill. First, adjust the gap between the fast roller and the middle roller to 65 μm and the gap between the middle roller and the slow roller to 35 μm and grind twice. Then, adjust the gap between the fast roller and the middle roller to 25 μm and the gap between the middle roller and the slow roller to 15 μm and grind twice more. Grind a total of 4 times to obtain the ground silver paste sample 3.

[0115] S7. Transfer the ground silver paste sample 3 to a vacuum mixing vessel and stir at 30 r / min for 40 min under a vacuum of -0.102 MPa. After stirring, encapsulate the silver paste to obtain solderable low-temperature silver paste sample 3. The curing temperature of solderable low-temperature silver paste sample 3 is 90℃.

[0116] Example 4

[0117] This embodiment provides a smartphone backlight module, including a novel flexible printed circuit board for smartphones prepared according to Embodiment 1, and a Mini LED chip soldered onto a conductor pattern of the circuit board using a low-temperature solder. The Mini LED chip is soldered onto a high-density pad area formed by the solderable low-temperature silver paste in the conductor pattern. The Mini LED chip has a size of 20μm×40μm~50μm×100μm, and the low-temperature solder used for soldering is Sn42Bi58. The soldering process parameters are: peak temperature 125℃±2℃, reflow time 45s±5s.

[0118] Example 5

[0119] This embodiment provides a smartphone backlight module, including a novel flexible printed circuit board for smartphones prepared according to Embodiment 2, and a Mini LED chip soldered onto a conductor pattern on the circuit board using low-temperature solder. The Mini LED chip is soldered onto a high-density pad area formed by the solderable low-temperature silver paste in the conductor pattern. The Mini LED chip has a size of 20μm×40μm~50μm×100μm, and the low-temperature solder used for soldering is Sn42Bi58. The soldering process parameters are: peak temperature 125℃±2℃, reflow time 45s±5s.

[0120] Example 6

[0121] This embodiment provides a smartphone backlight module, including a novel flexible printed circuit board for smartphones prepared according to Embodiment 3, and a Mini LED chip soldered onto a conductor pattern on the circuit board using a low-temperature solder. The Mini LED chip is soldered onto a high-density pad area formed by the solderable low-temperature silver paste in the conductor pattern. The Mini LED chip has a size of 20μm×40μm~50μm×100μm, and the low-temperature solder used for soldering is Sn42Bi58. The soldering process parameters are: peak temperature 125℃±2℃, reflow time 45s±5s.

[0122] Comparative Example 1

[0123] This comparative example provides a novel flexible printed circuit board for smartphones 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 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 micro-jump wire areas, high-density pad areas, and dynamic bending compensation areas.

[0124] By mass, the solderable low-temperature silver paste control 1 of this comparative example contains: 63 parts conductive filler sample 1, 15 parts polymer resin system control 1, 28 parts diethylene glycol butyl ether acetate, 5 parts curing agent sample 1, 2 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 1 part modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, 1 part modified ceramic filler sample 1, and 0.08 parts nano-yttrium oxide.

[0125] The polymer resin system reference standard 1 includes cashew phenolic epoxy resin sample 1, hydrogenated bisphenol A epoxy resin, and carboxyl-terminated liquid nitrile rubber used in Example 1, with a mass ratio of 1:1:0.4.

[0126] 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 123°C.

[0127] Comparative Example 2

[0128] This comparative example provides a novel flexible printed circuit board for smartphones 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 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 micro-jump wire areas, high-density pad areas, and dynamic bending compensation areas.

[0129] By mass, the solderable low-temperature silver paste control sample 1 of this comparative example comprises: 63 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 28 parts diethylene glycol butyl ether acetate, 5 parts curing agent sample 1, 2 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 1 part modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder sample 1, and 1 part modified ceramic filler sample 1.

[0130] 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 nano-yttrium oxide was not added in step S4. The curing temperature of the prepared solderable low-temperature silver paste reference standard 2 was 105°C.

[0131] Comparative Example 3

[0132] This comparative example provides a novel flexible printed circuit board for smartphones 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 in local areas of the main circuit pattern by a solderable low-temperature silver paste reference sample 3. The auxiliary functional structures in this comparative example include micro-jump wire areas, high-density pad areas, and dynamic bending compensation areas.

[0133] By mass, the solderable low-temperature silver paste reference standard 3 of this comparative example contains: 63 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 28 parts diethylene glycol butyl ether acetate, 5 parts curing agent sample 1, 2 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 1 part modified carbon nanotube reference standard 1, 1 part modified antimony tin oxide powder sample 1, 1 part modified ceramic filler sample 1, and 0.08 parts nano-yttrium oxide.

[0134] The preparation method of modified carbon nanotube reference standard 1 is as follows:

[0135] 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.

[0136] 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.

[0137] 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 with a volume ratio of ethanol to water of 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.

[0138] 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.

[0139] A5. After depressurization and separation, the sample was washed with deionized water and vacuum dried at 90°C for 8 hours to obtain modified carbon nanotube reference standard 1.

[0140] 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 112℃.

[0141] Comparative Example 4

[0142] This comparative example provides a novel flexible printed circuit board for smartphones 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 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 micro-jump wire areas, high-density pad areas, and dynamic bending compensation areas. By mass parts, the solderable low-temperature silver paste reference 4 of this comparative example comprises: 63 parts conductive filler sample 1, 15 parts polymer resin system sample 1, 28 parts diethylene glycol butyl ether acetate, 5 parts curing agent sample 1, 2 parts silane coupling agent sample 1, 1 part polyamide wax thixotropic agent sample 1, 0.3 parts polysiloxane defoamer BYK-055, 1 part modified carbon nanotube sample 1, 1 part modified antimony tin oxide powder reference 1, 1 part modified ceramic filler sample 1, and 0.08 parts nano-yttrium oxide.

[0143] The preparation method of modified antimony tin oxide powder reference standard 1 is as follows:

[0144] B1. Disperse 10 parts of antimony tin oxide powder with an average particle size of 50 nm in 110 parts of anhydrous ethanol, and sonicate at 300 W for 30 min to obtain a suspension.

[0145] B2. Add 1.5 parts of KH-560 and 0.5 parts of acetic acid to the suspension as catalysts, and stir the mixture at 400 r / min at 60°C for 4 h.

[0146] B3. After the reaction was completed, the mixture was centrifuged, washed four times with anhydrous ethanol, dried under vacuum at 80°C for 12 hours, ground, and passed through a 600-mesh sieve to obtain modified antimony tin oxide powder reference standard 1.

[0147] 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 modified antimony tin oxide powder reference standard 1 was used instead of modified antimony tin oxide powder sample 1. The curing temperature of the prepared solderable low-temperature silver paste reference standard 4 was 105℃.

[0148] Analysis and Testing

[0149] Flexible circuits were formed on polyimide (PI) substrates using screen printing equipment for solderable low-temperature silver paste samples 1–3 and solderable low-temperature silver paste controls 1–4. After curing at their respective curing temperatures for 30 minutes, mechanical and electrical properties were tested. The linewidth of the flexible circuit was 15 μm, the thickness of the PI substrate was 50 μm, and the thickness of the silver paste was 15 μm ± 2 μm.

[0150] I. Mechanical Performance Testing

[0151] Mechanical properties include adhesion test, flexural strength test and hardness test. The test results are shown in Table 1.

[0152] 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 72 hours for humid heat aging, and the peeling level after aging was evaluated (0 being the best and 5 being the worst).

[0153] Bending resistance test: The PI substrate with silver paste wires was bent 180° repeatedly at a bending radius of R=3mm using a dynamic bending tester at normal room temperature (26℃), high temperature of 80℃ and low temperature of -40℃. The resistance change was monitored in real time and the number of bends when the resistance change exceeded 10% was recorded.

[0154] Hardness was tested using a pencil hardness tester according to standard number ASTM D3363.

[0155] Table 1: Mechanical Performance Test Results

[0156]

[0157]

[0158] As shown in Table 1, the weldable low-temperature silver paste samples of this invention all exhibit excellent comprehensive performance. Among them, weldable low-temperature silver paste sample 1 shows the most outstanding performance, demonstrating a bending life of over 10,000 cycles under normal temperature, high temperature, and low temperature environments, far exceeding industry standards and fully meeting the needs of extreme dynamic bending scenarios such as the hinge area of ​​foldable screen mobile phones. After undergoing 85%RH / 85℃ humid heat aging, the adhesion of weldable low-temperature silver paste sample 1 still remained at grade 0 or 1, proving that the resin system used has outstanding aging resistance and interface stability. Weldable low-temperature silver paste control sample 1, because it uses non-copolymerized CTBN as a toughening agent, has far inferior compatibility and synergistic effect with epoxy resin compared to the polyacrylate-CTBN copolymer of this invention. This results in a significant decrease in its adhesion, aging resistance, and high and low temperature bending resistance, especially at high temperatures where the resin softens and the performance deteriorates sharply. The solderable low-temperature silver paste reference 2, lacking nano-yttrium oxide, exhibits slightly weaker curing density and interfacial bonding, resulting in a decrease in adhesion to grade 2 after hygrothermal aging, and slightly lower bending resistance compared to samples 1 and 2 of this invention. The solderable low-temperature silver paste reference 3 uses modified carbon nanotubes prepared using an unimproved process, which have poorer dispersibility and aspect ratio retention than the electrochemically modified carbon nanotubes of this invention. This leads to stress concentration points forming in the resin matrix, resulting in lower bending resistance and interfacial bonding compared to the samples of this invention. The solderable low-temperature silver paste reference 4 uses modified antimony tin oxide prepared using an unimproved process, whose nanoparticles are more prone to agglomeration and have weaker interfacial bonding with the resin than those of this invention. While its mechanical properties are superior to references 1 and 3, they are still inferior to the samples of this invention.

[0159] II. Conductivity Test

[0160] The sheet resistance of the cured silver paste conductors was measured using a four-probe tester. The results are shown in Table 2.

[0161] The sample under test was subjected to 50 temperature cycles at a frequency of 10 GHz, from -40 ℃ to 85 ℃, and the shift of the characteristic impedance (Z) was measured and expressed as ΔZ / Z to evaluate the high-frequency impedance stability. The results are shown in Table 2. ΔZ represents the absolute value of the maximum deviation between the characteristic impedance value measured at the same frequency (10 GHz) and the initial value Z0 after 50 temperature cycles; Z refers to the initial characteristic impedance value measured at room temperature and a frequency of 10 GHz.

[0162] Shear resistance (probe diameter 5μm) was measured at 100 points on a 30μm×30μm pad array. The relative standard deviation (RSD) was calculated to determine the conductivity uniformity of the micro pads. The results are shown in Table 2.

[0163] Table 2: Conductivity Test Results

[0164] Test sample Sheet resistance (mΩ / □) High frequency impedance stability (ΔZ / Z0, %) RSD (%) Weldable low temperature silver paste sample 1 3.5 3.8 1.2 Weldable low temperature silver paste sample 2 4.2 4.5 1.8 Weldable low temperature silver paste sample 3 5.8 5.2 2.5 Weldable low temperature silver paste control 1 5.5 12.7 8.5 Weldable low temperature silver paste control 2 4.5 7.2 4.0 Weldable low temperature silver paste control 3 7.9 15.3 7.8 Weldable low temperature silver paste control 4 6.3 9.6 6.4

[0165] As shown in Table 2, the solderable low-temperature silver paste samples of the present invention all exhibit low sheet resistance, excellent high-frequency stability, and extremely high micropad uniformity. Solderable low-temperature silver paste sample 1 has a sheet resistance as low as 3.5 mΩ / □, and its impedance change rate after rigorous temperature cycling at high frequencies is only 3.8%, perfectly matching the extreme requirements of 5G millimeter-wave mobile phone antennas for transmission loss; its micropad sheet resistance has a relative standard deviation as low as 1.2%, demonstrating the excellent dispersibility and film uniformity of the modified filler. Solderable low-temperature silver paste control 1, due to poor resin system compatibility and high internal stress after curing, may generate microcracks, resulting in a higher sheet resistance and poorer high-frequency stability and uniformity. Solderable low-temperature silver paste control 2 lacks the promoting effect of nano-yttrium oxide on sintering densification, resulting in slightly poorer conductivity, manifested as a higher sheet resistance than sample 1, and significantly inferior high-frequency stability and uniformity compared to the samples of the present invention. The solderable low-temperature silver paste control 3 used modified carbon nanotubes prepared with an unmodified process. Its dispersibility was slightly poor, and its conductivity was impaired, preventing it from forming a more efficient conductive network within the system. Instead, it became an insulating point, resulting in the worst sheet resistance and high-frequency stability among all samples. The solderable low-temperature silver paste control 4 used modified antimony tin oxide powder prepared with an unmodified process. Agglomeration was possible, preventing it from effectively reducing contact resistance, leading to a higher sheet resistance, and poorer high-frequency stability and uniformity.

[0166] III. Welding Performance Testing

[0167] On the silver-soldered pads, Sn42Bi58 low-temperature solder paste and Mini LED chips were mounted using a standard SMT reflow soldering process (peak temperature 125℃). 200 solder joints were observed, and the soldering yield (i.e., the percentage of solder joints without cold solder joints, false solder joints, or tombstoning) was calculated. A high-precision push-pull force tester (resolution 0.001N) was used to select 20 solder joints, raising the lower limit to 5.0N / mm. 2 The shear strength was tested, and the failure modes were recorded. The failure modes were divided into three categories: internal fracture of the solder joint, delamination of the silver paste-pad interface, and a mixed mode of both. The results are shown in Table 3.

[0168] Artificial sweat prepared according to ISO 3160-2 standard was applied to the surface of the solder pads and sprayed continuously at 35°C for 24 hours. The untreated sample was used as a comparison. The wetting and spreading area of ​​the solder on the solder pads was tested, and the wetting area decay rate was calculated. The results are shown in Table 3.

[0169] Table 3: Welding Performance Test Results

[0170] Test sample Welding yield (%) Shear strength (N) Failure mode Wetted area decay rate (%) Weldable low temperature silver paste sample 1 99.9 8.5 Intra-joint fracture 2.1 Weldable low temperature silver paste sample 2 99.5 7.8 Intra-joint fracture 3.8 Weldable low temperature silver paste sample 3 98.2 6.5 Mixed mode 5.2 Weldable low temperature silver paste control 1 92.4 4.8 Silver paste interface peeling 18.7 Weldable low temperature silver paste control 2 95.1 5.5 Mixed mode 10.4 Weldable low temperature silver paste control 3 97.6 6.0 Mixed mode 6.9 Weldable low temperature silver paste control 4 94.2 5.2 Silver paste interface peeling 12.6

[0171] As shown in Table 3, the solderable low-temperature silver paste samples of this invention all exhibit extremely high soldering yield, soldering strength, and excellent environmental durability. The soldering yield of solderable low-temperature silver paste samples 1 and 2 is close to 100%, and the failure mode for both is internal fracture of the solder joint, proving that the strength of the weld joint is far higher than the bonding strength between the silver paste and the substrate, achieving top-level reliability. After salt spray and sweat corrosion tests, the solder wetting area attenuation rate of solder samples 1 and 2 is extremely low, indicating that their solder pad surfaces have good resistance to environmental corrosion. Solderable low-temperature silver paste control 1 is prone to silver paste interface peeling under soldering thermal stress, resulting in poor soldering strength and corrosion resistance. Solderable low-temperature silver paste control 2 has insufficient silver paste layer density and weak resistance to corrosive media penetration, leading to a high wetting area attenuation rate. In solderable low-temperature silver paste control 3, the modified carbon nanotubes have slightly poor dispersion, which can cause certain defects on the silver paste surface. Some solder joints peel off from the interface, while others fracture internally, exhibiting a mixed-mode failure, resulting in decreased soldering strength and consistency. The modified antimony tin oxide powder in the solderable low-temperature silver paste reference 4 failed to effectively protect the solder pad surface, had only average corrosion resistance, and may have affected the flatness due to agglomerates, leading to a decrease in soldering yield and strength.

[0172] IV. Application Performance of New Smartphone Modules

[0173] The novel smartphone module samples prepared in Examples 4 to 6 were tested as follows:

[0174] 1. Dynamic Bending Reliability Test: The module was installed in a foldable screen testing machine and subjected to more than 200,000 reciprocating bends with a bending radius of R=1.5mm in environments of -20℃, room temperature, and 60℃. After the test, the circuit conductivity of all new smartphone module samples remained at 100% yield, the silver paste circuit showed no cracks or peeling, and the Mini LED display function was normal.

[0175] 2. Dual 85°C High Temperature and Humidity Electrical Test: The display module was powered on for 1000 hours under 85°C / 85%RH conditions. During and after the test, the brightness decay rate of all new smartphone module samples was <3%, the color coordinate offset Δxy was <0.005, and there were no LED flickering or dead LED phenomena, proving the long-term stability and reliability of the silver paste circuit under high temperature and humidity conditions.

[0176] 3. High-temperature thermal shock test: 1000 cycles of thermal shock were conducted at temperatures ranging from -40℃ to 125℃. After the test, no micro-cracks were observed at the solder joint interfaces of all new smartphone module samples, and the chip shear strength showed no decrease, meeting the stringent requirements of smartphones to withstand complex usage environments.

[0177] In summary, the solderable low-temperature silver paste samples prepared by this invention, especially Sample 1, successfully combine excellent mechanical properties, outstanding conductivity, and ultra-high solderability, exhibiting comprehensive performance far superior to various comparative examples. Building upon previous technologies, this invention innovatively employs a polyacrylate-CTBN copolymer resin system, introduces nano-yttrium oxide, and develops a novel process for electrochemically modified carbon nanotubes and conductive polymer-coated modified antimony-tin oxide powder. This results in significant performance improvements and comprehensive optimization, providing a more reliable solution for the application of core components in smartphones, particularly high-end foldable screen phones.

[0178] 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 new type of flexible printed circuit board for smart phone based on solderable silver paste preparation comprising a flexible insulating substrate and a conductor pattern provided on the substrate, characterized in that, The conductor pattern comprises a main circuit pattern formed by calendered copper, and an auxiliary functional structure formed by a solderable low-temperature silver paste in a local area of the main circuit pattern; the auxiliary functional structure comprises at least one of a micro-jumper area, a high-density pad area and a dynamic bending compensation area; The curing temperature of the solderable low-temperature silver paste is 90-110 DEG C; the solderable low-temperature silver paste comprises, in mass fraction, 55-70 parts of conductive filler, 10-20 parts of a polymer resin system, 20-35 parts of an ester solvent, 3-8 parts of a curing agent, 1-3 parts of a silane coupling agent, 0.5-2 parts of a polyamide wax thixotropic agent, 0.2-0.5 parts of a polysiloxane antifoaming agent, 0.5-2 parts of modified carbon nanotubes, 0.5-2 parts of modified antimony tin oxide powder, 0.5-2 parts of modified ceramic filler and 0.05-0.1 parts of nano yttrium oxide. The polymer resin system comprises cashew phenolic epoxy resin, hydrogenated bisphenol A epoxy resin and polyacrylate-carboxyl-terminated liquid butadiene-acrylonitrile rubber copolymer in a mass ratio of 1:(0.8-1.2):(0.3-0.6); the conductive filler comprises nano silver wire, spherical silver powder with a D50 range of 0.5-0.8 μm and low-temperature alloy powder with a D50 range of 5-10 μm.

2. The new smart phone flexible printed circuit board according to claim 1, wherein, The mass ratio of nanosilver wire, spherical silver powder and low-temperature alloy powder in the conductive filler is (1.0-2.0):1:(0.2-0.5); the specific surface area of the nanosilver wire is 30m 2 / g-60m 2 / g; the low-temperature alloy powder is Sn42Bi58; the specific surface area of the spherical silver powder is 1.0m 2 / g-2.5m 2 / g.

3. The new smart phone flexible printed circuit board according to claim 1, wherein, The cashew phenolic epoxy resin has an epoxy value of 0.45-0.55 eq / 100 g and a viscosity of 10,000-20,000 mPa·s at 25 DEG C; the polyacrylate-carboxyl-terminated liquid butadiene-acrylonitrile rubber copolymer is prepared by free radical polymerization of acrylate monomer and carboxyl-terminated liquid butadiene-acrylonitrile rubber in a mass ratio of (8-12):1 in the presence of benzoyl peroxide initiator at 60-80 DEG C for 4-6 h; the carboxyl-terminated liquid butadiene-acrylonitrile rubber has a carboxyl mass fraction of 0.4-0.6% and a number average molecular weight of 3,000-5,000 Da; the mass of the benzoyl peroxide initiator is 1.5-3.0% of the total mass of the acrylate monomer and the carboxyl-terminated liquid butadiene-acrylonitrile rubber.

4. The new smart phone flexible printed circuit board of claim 1, wherein, The curing agent is polyether polyamine curing agent and imidazole accelerator in a mass ratio of (5-7):1; the imidazole accelerator is selected from any one of 2-methylimidazole, 2-ethyl-4-methylimidazole and 1-cyanoethyl-2-phenylimidazole; the polyether polyamine curing agent is linear polyether diamine; the silane coupling agent is a mixed coupling agent of KH-550 and KH-560 in a mass ratio of 1:(1.5-3); the polysiloxane antifoaming agent is a non-silicon antifoaming agent containing defoaming polysiloxane; the ester solvent is selected from any one of diethylene glycol butyl ether acetate, ethylene glycol ethyl ether acetate and butyl butyrate.

5. The new smart phone flexible printed circuit board of claim 1, wherein, The preparation method of the modified ceramic filler is as follows: boron nitride powder with a particle size of 0.1-0.3 microns is dispersed in anhydrous ethanol, a mixed modifier composed of silane coupling agent KH-560 and polydopamine with a mass ratio of 1:(1.5-2.5) is added, ultrasonic stirring is carried out at 50-60 DEG C and 200-400 W for 4-6 hours, and then the mixture is calcined at 80-100 DEG C for 2-4 hours after filtration; wherein the total mass of the mixed modifier is 3-5% of the boron nitride powder.

6. The new smart phone flexible printed circuit board of claim 1, wherein, The polyamide wax thixotropic agent is polyamide wax obtained by condensation of plant fatty acid and dimer fatty acid with ethylenediamine; the polyamide wax thixotropic agent needs to be activated before use, and the specific operation of the activation treatment is as follows: the polyamide wax is added into a mixed solvent of isopropyl alcohol and N-methyl pyrrolidone with a volume ratio of 1:1.5-2.0, amino propanol is added, the activation temperature is controlled to be 40-50 DEG C, the stirring speed is controlled to be 100-150 r / min, and the activation is carried out for 15-30 minutes; wherein the mass ratio of the polyamide wax, the mixed solvent and the amino propanol is 1:(5-8):(0.05-0.1).

7. The new smart phone flexible printed circuit board of claim 1, wherein, The preparation process of the modified carbon nanotube is as follows in terms of mass fraction: A1, electrolyte preparation: 15 parts of carbon nanotubes are dispersed in 280-320 parts of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, 5-10 parts of 4-aminobenzene diazonium salt is added as a functionalization reagent, and ultrasonic treatment is carried out at a power of 200-400 W for 30-60 minutes to form a suspension electrolyte; 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, and electrolysis is carried out at a voltage of 3-5 V and a stirring speed of 200-400 r / min for 2-4 hours; A3, reaction post-treatment: after the reaction, black solids are obtained by filtration, and the black solids are washed by centrifugation with N,N-dimethylformamide and deionized water alternately until the supernatant is colorless and transparent; vacuum drying is carried out at 60-80 DEG C for 10-14 hours to obtain the modified carbon nanotube.

8. The new smart phone flexible printed circuit board of claim 1, wherein, The preparation process of the modified antimony tin oxide powder is as follows in terms of mass fraction: B1, suspension preparation: 10 parts of antimony tin oxide powder with an average particle size of 30-70 nm are dispersed in 150-200 parts of deionized water, 1-2 parts of sodium dodecyl benzene sulfonate is added, and ultrasonic treatment is carried out at a power of 400-600 W for 30-60 minutes to prepare a suspension; B2, in-situ polymerization: 1-3 parts of pyrrole monomer is added to the suspension under ice water bath condition, and stirring is carried out for 25-35 minutes; 5 parts of ammonium persulfate aqueous solution with a mass concentration of 5-10% is added dropwise, the dropwise adding temperature is controlled to be 2-4 DEG C, and the reaction is carried out for 6-8 hours; B3, reaction post-treatment: after the reaction, the filter cake is filtered and washed with deionized water and ethanol alternately until the filtrate is colorless; the filter cake is vacuum dried at 60-80 DEG C for 20-28 hours, and then ground and sieved through a 600-800 mesh screen to obtain the modified antimony tin oxide powder.

9. The new smart phone flexible printed circuit board of claim 1, wherein, The preparation method of the weldable low-temperature silver paste is: S1, adding an ester solvent into a reaction kettle, then adding a polymer resin system, stirring at 60-80°C until the resin is dissolved, adding a silane coupling agent and a polysiloxane defoaming agent, cooling to room temperature, adding a curing agent, keeping the solution temperature below 30°C, stirring until each phase is completely dissolved and mixed to obtain a carrier solution; S2, adding a polyamide wax thixotropic agent to the carrier solution, stirring and mixing at room temperature to obtain a mixed solution A; S3, adding a low-temperature alloy powder and a modified ceramic filler to the mixed solution A, stirring and mixing at room temperature to obtain a mixed solution B; S4, adding nano silver wires and spherical silver powder to the mixed solution B, then adding nano yttrium oxide, stirring and mixing at room temperature to obtain a mixed solution C; S5, adding modified carbon nanotubes and modified antimony tin oxide powder to the mixed solution C, stirring and mixing, and controlling the stirring temperature not to exceed 30°C to obtain a mixed solution D; S6, transferring the mixed solution D to a three-roll mill, first adjusting the gap between the fast roller and the middle roller to 65-75μm, and the gap between the middle roller and the slow roller to 25-35μm, grinding for 1-2 times; then adjusting the gap between the fast roller and the middle roller to 35-45μm, and the gap between the middle roller and the slow roller to 5-15μm, grinding for another 1-2 times; a total of 2-4 times of grinding, to obtain a ground silver paste; S7, transferring the ground silver paste to a vacuum stirred tank, stirring and mixing under a vacuum degree of ≤-0.095MPa, packaging the silver paste after stirring is completed, to obtain the weldable low-temperature silver paste. 10.A back light module of a smart phone, characterized in that, The novel intelligent mobile phone flexible printed circuit board and the Mini LED chip welded on the conductor pattern of the circuit board by low-temperature solder according to any one of claims 1-9; the Mini LED chip is welded on the high-density pad area formed by the weldable low-temperature silver paste in the conductor pattern; the size of the Mini LED chip is 20μm×40μm-50μm×100μm, the low-temperature solder used for welding is Sn42Bi58, and the welding process parameters are: peak temperature 125℃±2℃, reflow time 45s±5s.

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