Water-soluble soldering paste and preparation method thereof

Abstract

The invention provides a water-soluble soldering paste and a preparation method thereof, and belongs to the technical field of soldering pastes.The soldering paste is prepared from, by mass, 70-75% of alloy powder, 9-13% of conductive adhesive, 11-14% of scaling powder and 3-5% of other additives. The preparation method comprises the following steps: firstly, primarily mixing alloy powder and a conductive adhesive for 10 minutes, and then carrying out ball milling for 2.5 hours by using a ball mill at the rotating speed of 200rpm to obtain mixed powder; and the scaling powder and other additives are added into the mixed powder, stirring is conducted for 15 min through a stirring machine, then vacuum stirring is conducted for 20 min at the room temperature, bubbles in the materials are removed, and the water-soluble soldering paste is obtained. By adding the cobalt and the carbon nanodots, the wettability, the continuous printing stability, the welding spot mechanical strength and the thixotropic property of the water-soluble soldering paste are improved.

Description

Technical Field

[0001] This invention relates to the field of solder paste technology, specifically to a water-soluble solder paste and its preparation method. Background Technology

[0002] With the rapid development of electronic packaging technology, soldering materials are required to possess excellent electrical and thermal conductivity, high bonding strength, and long-term stability. Water-soluble solder paste, due to its environmentally friendly and easy-to-clean properties, avoids the ecotoxicity of traditional lead-containing solders and overcomes the long-term electrochemical migration risks that may arise from residual low-activity flux in no-clean solder pastes, making it an important choice for lead-free soldering. However, its practical application still faces key technological bottlenecks:

[0003] First, insufficient wettability: Compared with traditional lead-containing solders, Sn-Ag-Cu lead-free solders have problems such as slightly poor wettability, coarse microstructure of solder joints, and easy overgrowth of brittle intermetallic compound layers, which leads to increased voids in solder joints, insufficient mechanical strength, and affects long-term reliability.

[0004] Secondly, the stability of continuous printing is insufficient: In continuous printing operations, the viscosity of solder paste is prone to uncontrollable changes, resulting in inconsistent printing volume, pattern collapse or printing defects, which affects the consistency and yield of soldering.

[0005] Existing technologies are also attempting to improve these issues: Chinese invention patent CN118123318A proposes using cobalt-coated graphene nanosheets (CoGNSs), which improves the conductivity and wettability of the paste. Chinese invention patent CN119794650A improves the conductivity and thermal conductivity of solder paste by increasing the tin powder content and adding conductive fillers such as graphite and carbon nanotubes to construct conductive pathways.

[0006] While the introduction of conductive fillers such as graphene nanosheets and carbon nanotubes has improved the electrical and thermal conductivity of solder paste to some extent, conductive fillers still suffer from problems such as poor dispersibility and weak interfacial bonding. For example, two-dimensional graphene nanosheets have a large specific surface area and interfacial van der Waals forces, making them prone to agglomeration and stacking in aqueous systems. This can form a barrier layer inside the solder, hindering the escape of gases during soldering and exacerbating void defects. Carbon nanotubes, with their high surface chemical inertness, also face challenges in dispersibility.

[0007] Therefore, there is an urgent need to develop a water-soluble solder paste with welding reliability and rheological stability to meet the requirements of high-precision semiconductor manufacturing processes. Summary of the Invention

[0008] The purpose of this invention is to provide a water-soluble solder paste and its preparation method to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A water-soluble solder paste, by weight percentage, comprises the following raw material components: 70-75% alloy powder, 9-13% conductive binder, 11-14% flux, and 3-5% other additives.

[0011] The alloy powder is composed of Sn, Ag, Cu and Co;

[0012] The conductive adhesive is composed of polymer resin and carbon nanodots;

[0013] The flux, by weight percentage, consists of 25% organic acid chelating agent, 15% surfactant, 10% corrosion inhibitor and 50% film-forming agent;

[0014] The other additives consist of antioxidants, thixotropic agents, and leveling agents in a mass ratio of 1:1:1.

[0015] Preferably, the mass ratio of Sn:Ag:Cu:Co in the alloy powder is (96.4~96.45):3:0.5:(0.05~0.1), and the average particle size of the alloy powder is 12-25μm.

[0016] Preferably, the mass ratio of polymer resin to carbon nanodots in the conductive adhesive is 1:(0.4~0.5).

[0017] Preferably, the polymer resin is a phenolic resin, the organic acid chelating agent is selected from at least one of picric acid, phenyl succinic acid, succinic acid, glutaric acid, and pimelic acid, the surfactant is selected from at least one of OP-10 and 2-bromo-1-methyl-1H-imidazolium, the corrosion inhibitor is selected from at least one of 2-ethylimidazolium and benzotriazole, the film-forming agent is selected from at least one of polymerized rosin, hydrogenated rosin, and disproportionated rosin, the antioxidant is hydroquinone, the thixotropic agent is selected from at least one of hydrogenated castor oil and modified hydrogenated castor oil, and the leveling agent is selected from at least one of polyether-modified siloxane BYK-346 and polyether-modified siloxane BYK-348.

[0018] Preferably, the preparation of carbon nanodots includes the following steps:

[0019] S1. Dissolve 2 parts by weight of citric acid in a mixed solution containing 1 part by weight of ethylenediamine and 25 parts by weight of ultrapure water, and sonicate for 15 minutes to obtain solution A;

[0020] S2. Transfer solution A to a reaction vessel and react at 200°C for 6 hours, then allow it to cool naturally to room temperature to obtain a reddish-brown solution B;

[0021] S3. Collect solution B into a dialysis bag with a molecular weight cutoff of 100 Da, and place it in a beaker filled with ultrapure water for dialysis for 48 hours;

[0022] S4. Centrifuge the dialyzed sample at 10,000 rpm for 10 min, discard the aggregated particles, collect the carbon nanoparticle solution, then pre-dry the carbon nanoparticle solution at 60℃ and then freeze-dry it to obtain carbon nanoparticle powder.

[0023] This invention also discloses a method for preparing water-soluble solder paste, comprising the following steps:

[0024] A1. First, the alloy powder and conductive binder are initially mixed for 10 minutes, and then ball milled at 200 rpm for 2.5 hours to obtain the mixed powder.

[0025] A2. Add flux and other additives to the mixed powder from step A1, stir with a mixer for 15 minutes, and then vacuum stir at room temperature for 20 minutes to remove air bubbles from the material, thus obtaining water-soluble solder paste.

[0026] The beneficial effects of the above-described technical solution of the present invention are as follows:

[0027] 1. The carbon nanodots (CDs) prepared by the hydrothermal method in this invention contain abundant polar groups (such as hydroxyl and carboxyl groups). During the solder paste preparation and storage stages, CDs self-assemble into a stable network structure through hydrogen bonds and van der Waals forces. This structure can encapsulate solder powder and flux components, allowing the solder paste to maintain good viscosity for a long time, avoiding solder powder sedimentation or solvent stratification, and ensuring the uniformity of the solder paste. At the same time, this network structure can be destroyed under the action of printing shear force, reducing the viscosity of the paste and facilitating flow and filling. Once the shear force is removed, the network structure can be rebuilt, and the viscosity will rebound, preventing the printed pattern from collapsing and improving the stability and consistency of continuous printing. In addition, this invention uses hydrothermally synthesized CDs as conductive fillers, which can significantly reduce the cost of solder paste.

[0028] 2. Adding trace amounts of cobalt can improve the wetting properties of solder alloys; and cobalt atoms can form fine intermetallic compounds at weak points in the solder interface, increasing nucleation density and promoting grain refinement. At the same time, it can also inhibit the growth of Cu-Sn and Ag-Sn intermetallic compounds to a certain extent, thereby reducing solder joint brittleness and improving the mechanical strength of the solder joint.

[0029] 3. The synergistic system of CDs and trace cobalt played a key role:

[0030] (1) The addition of cobalt element improves the wettability of solder paste, and the addition of CDs further optimizes the rheological properties of solder paste, ensuring printing stability and filling. The two work together to reduce defects such as voids and poor solder joints caused by poor wetting or uneven printing.

[0031] (2) Cobalt forms a stable bond with CDs, "pinning" CDs into the solder matrix, further inhibiting CDs agglomeration and optimizing dispersion, while enhancing the mechanical properties of the solder joint. Detailed Implementation

[0032] The preferred embodiments of the present invention are described below. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0033] For experiments not specified in this protocol, the procedures and conditions described in the literature in this field should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0034] Example

[0035] The preparation of carbon nanodots (CDs) includes the following steps:

[0036] S1. Dissolve 2 parts by weight of citric acid in a mixed solution containing 1 part by weight of ethylenediamine and 25 parts by weight of ultrapure water, and sonicate for 15 minutes to obtain solution A;

[0037] S2. Transfer solution A to a reaction vessel and react at 200°C for 6 hours, then allow it to cool naturally to room temperature to obtain a reddish-brown solution B;

[0038] S3. Collect solution B into a dialysis bag with a molecular weight cutoff of 100 Da, and place it in a beaker filled with ultrapure water for dialysis for 48 hours;

[0039] S4. Centrifuge the dialyzed sample at 10,000 rpm for 10 min, discard the aggregated particles, collect the carbon nanoparticle solution, then pre-dry the carbon nanoparticle solution at 60℃ and then freeze-dry it to obtain carbon nanoparticle powder.

[0040] A method for preparing water-soluble solder paste includes the following steps:

[0041] A1. First, the alloy powder and conductive binder are initially mixed for 10 minutes, and then ball milled at 200 rpm for 2.5 hours to obtain the mixed powder.

[0042] A2. Add flux and other additives to the mixed powder from step A1, stir with a mixer for 15 minutes, and then vacuum stir at room temperature for 20 minutes to remove air bubbles from the material, thus obtaining water-soluble solder paste.

[0043] Example 1

[0044] The water-soluble solder paste prepared according to the above method comprises the following raw material components by mass percentage:

[0045] 73% alloy powder, 7.7% phenolic resin, 3.3% CDs, 1.75% adipic acid, 1.25% succinic acid, 1.8% OP-10, 1.2% 2-ethylimidazole, 3% hydrogenated rosin, 3% polymerized rosin, 4% other additives;

[0046] The mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96.43:3:0.5:0.07.

[0047] Other additives are hydroquinone, hydrogenated castor oil, and polyether-modified siloxane BYK-346 in a mass ratio of 1:1:1.

[0048] Example 2

[0049] The water-soluble solder paste prepared according to the above method comprises the following raw material components by mass percentage:

[0050] 70% alloy powder, 9.3% phenolic resin, 3.7% CDs, 1.75% adipic acid, 1.75% phenyl succinic acid, 2.1% 2-bromo-1-methyl-1H-imidazolium, 1.4% benzotriazole, 7% disproportionated rosin, 3% other additives;

[0051] The mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96.4:3:0.5:0.1.

[0052] Other additives are hydroquinone, hydrogenated castor oil, and polyether-modified siloxane BYK-348 in a mass ratio of 1:1:1.

[0053] Example 3

[0054] The water-soluble solder paste prepared according to the above method comprises the following raw material components by mass percentage:

[0055] 75% alloy powder, 6% phenolic resin, 3% CDs, 1.37% succinic acid, 0.92% glutaric acid, 0.46% pimecrolic acid, 1.65% OP-10, 1.1% 2-ethylimidazole, 5.5% hydrogenated rosin, 5% other additives;

[0056] The mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96.45:3:0.5:0.05.

[0057] Other additives are hydroquinone, modified hydrogenated castor oil, and polyether-modified siloxane BYK-346 in a mass ratio of 1:1:1.

[0058] Comparative Example 1

[0059] The difference between this and Example 1 is that CDs are not added.

[0060] Comparative Example 2

[0061] The difference between this and Example 1 is that Co is not added, and the mass ratio of Sn:Ag:Cu in the alloy powder is 96.5:3:0.5.

[0062] Comparative Example 3

[0063] The difference between this and Example 1 is that CDs and Co are not added, and the Sn:Ag:Cu mass ratio in the alloy powder is 96.5:3:0.5.

[0064] Comparative Example 4

[0065] The difference between this example and Example 1 is that industrial-grade nano-graphene sheets are used instead of CDs;

[0066] The industrial-grade nano-graphene sheets are commercially available products, purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences, product number TNIGNP.

[0067] Comparative Example 5

[0068] The difference between this and Example 1 is that industrial-grade multi-walled carbon nanotubes are used instead of CDs;

[0069] Among them, the industrial-grade multi-walled carbon nanotubes were commercially available products purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences, with product number TNIM4.

[0070] Comparative Example 6

[0071] The difference between this and Example 1 is that the mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96:3:0.5:0.5.

[0072] Comparative Example 7

[0073] The difference between this and Example 1 is that the mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96.49:3:0.5:0.01.

[0074] Comparative Example 8

[0075] The difference between this and Example 1 is that the mass ratio of Sn:Ag:Cu:Co in the alloy powder is 96.68:3:0.25:0.07.

[0076] Comparative Example 9

[0077] The difference between this and Example 1 is that the mass ratio of Sn:Ag:Cu:Co in the alloy powder is 97.93:1.5:0.5:0.07.

[0078] Experiment 1: Contact Angle Test

[0079] The wetting performance of solder paste is evaluated based on the contact angle of the solder paste as captured by a 3D surface profilometer. The smaller the contact angle, the stronger the wetting performance. Specific data are shown in Table 1 below.

[0080] Table 1 Contact Angle Data

[0081]

[0082] Examples 1-3 and Comparative Examples 1-3 show that cobalt and CDs can jointly improve the wetting properties of the paste; Comparative Examples 4 and 5 show that CDs are more effective than carbon nanosheets and carbon nanotubes; Comparative Examples 6 and 7 show that the wetting properties are reduced when cobalt is in excess or insufficient; Comparative Example 9 shows that when the silver ratio is reduced, the wetting properties of the paste are decreased.

[0083] Experiment 2: Continuous Printing Test

[0084] The state and viscosity changes were observed by continuously printing for different times using a printing press. The continuous printing test conditions were: Malcom PCU02V@10rpm 25℃. The plate life of each sample was characterized by the viscosity value of continuous printing.

[0085] Table 2 Viscosity values ​​for continuous printing (viscosity unit Pa·s)

[0086]

[0087] As shown in Table 2, the viscosity of Examples 1-3 increased slowly over time, while the viscosity of Comparative Examples 1 and 3 increased rapidly and showed poor stability, indicating that the network structure formed by CDs can effectively stabilize the viscosity. The viscosity change rate of Comparative Examples 4-5 was higher than that of the Examples, indicating that carbon nanosheets and carbon nanotubes were not as stable as CDs for the viscosity of the paste. The viscosity change rate of Comparative Examples 2 and 6-7 was lower, indicating that cobalt was not the main factor stabilizing the viscosity value of the paste, but the stability was lower than that of Example 1, indicating that the addition of cobalt is beneficial to increasing viscosity stability. The viscosity change rate of Comparative Example 8 was similar to that of Example 1, indicating that copper was not the factor stabilizing the viscosity value of the paste. Comparative Example 9 showed that a significant decrease in the proportion of Ag would also lead to a faster viscosity change.

[0088] Experiment 3: Shear Strength Test

[0089] (1) Preparation of shear test specimens: After pretreatment of T2 copper sheets with dimensions of 20×10×1.0 mm, different sample solder pastes are printed. Then another copper sheet is attached, and three stainless steel wires with a diameter of 0.3 mm are used to control the spacing at the edge of the copper sheet. The overlapped joint is placed in the reflow oven and soldered according to the set reflow process curve. After the reflow is completed, the shear test specimens are obtained by cleaning with anhydrous ethanol combined with ultrasonic vibration.

[0090] (2) Shear test: The above shear specimens were subjected to a shear test using an electronic universal testing machine. The shear rate was set to 1.0 mm / min, and the failure load value F of the shear specimens was recorded. S (N) and the overlap area A (mm) of the shear specimen 2 ), shear strength value τ (N / mm) 2 The shear strength of the final brazed joint is obtained by formula (1) and repeated three times.

[0091] τ= F S / A (1)

[0092] Table 3 Shear strength test results

[0093]

[0094] Note: 1N / mm 2 =1MPa

[0095] As shown in Table 3, the shear strength of the comparative examples decreased compared to the actual examples, with the largest decrease in shear strength observed in Comparative Example 3. This indicates that cobalt and carbon nanosheets (CDs) synergistically enhance shear strength. Adding carbon nanosheets and carbon nanotubes can improve the shear strength of the weld joint, but the effect is not as good as that of CDs. Excessive cobalt content leads to the formation of too many intermetallic compounds, increasing the brittleness of the weld joint and decreasing its shear strength. Insufficient cobalt content weakens its bonding force with CDs, also causing a decrease in the shear strength of the weld joint. A decrease in the ratio of copper and silver affects the formation of the interfacial reinforcement layer, thus also causing a decrease in the shear strength of the weld joint.

[0096] Experiment 4: Thixotropic Performance Test

[0097] Thixotropic coefficient T i Thixotropic coefficient is the ratio of the viscosity η of a material at two different shear rates (low shear viscosity / high shear viscosity). The larger the thixotropic coefficient, the stronger the ability of the fluid to recover its original structure after being destroyed under shear force. The specific test steps are as follows: The solder paste is tested using an NDJ-8S viscometer. The viscosity of the solder paste is measured at rotation speeds of 12 rpm and 30 rpm, respectively. The thixotropic coefficient T is calculated according to formula (2).i .

[0098] T i =(η 12rpm -η 30rpm ) / η 30rpm Equation (2)

[0099] Wherein: T i η is the thixotropic coefficient. 12rpm η is the viscosity of the solder paste at a rotation speed of 12 rpm. 30rpm The viscosity of the solder paste is measured at a rotation speed of 30 rpm.

[0100] Table 4. Thixotropic performance test results

[0101]

[0102] As shown in Table 4, the thixotropic coefficient of the embodiments is higher than that of the comparative examples without CDs, indicating that the addition of CDs enhances the structural recovery ability of the paste and is beneficial to the stability of the printed pattern. The thixotropic coefficients of comparative examples 2 and 6-9 are closer to those of the embodiments, indicating that CDs are the main factor affecting the thixotropic properties of the paste, while metal alloys, carbon nanosheets and carbon nanotubes have a weaker effect on the thixotropic properties.

[0103] In summary, based on the four alloy ratios selected, this invention improves the wettability, continuous printing stability, solder joint mechanical strength, and thixotropic properties of solder paste by introducing carbon nanoparticles (CDs).

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A water-soluble solder paste, characterized by, By weight percentage, it includes the following raw material components: 70-75% alloy powder, 9-13% conductive binder, 11-14% flux, and 3-5% other additives; The alloy powder is composed of Sn, Ag, Cu and Co; The conductive adhesive is composed of polymer resin and carbon nanodots; The flux, by weight percentage, consists of 25% organic acid chelating agent, 15% surfactant, 10% corrosion inhibitor and 50% film-forming agent; The other additives consist of antioxidants, thixotropic agents, and leveling agents in a mass ratio of 1:1:

1.

2. The water-soluble solder paste of claim 1, wherein, The mass ratio of Sn:Ag:Cu:Co in the alloy powder is (96.4~96.45):3:0.5:(0.05~0.1), and the average particle size of the alloy powder is 12-25μm.

3. The water-soluble solder paste of claim 1, wherein, The mass ratio of polymer resin to carbon nanodots in the conductive adhesive is 1:(0.4~0.5).

4. The water-soluble solder paste of claim 1, wherein, The polymer resin is a phenolic resin. The organic acid chelating agent is selected from at least one of picric acid, phenyl succinic acid, succinic acid, glutaric acid, and pimelic acid. The surfactant is selected from at least one of OP-10 and 2-bromo-1-methyl-1H-imidazolium. The corrosion inhibitor is selected from at least one of 2-ethylimidazolium and benzotriazole. The film-forming agent is selected from at least one of polymerized rosin, hydrogenated rosin, and disproportionated rosin. The antioxidant is hydroquinone. The thixotropic agent is selected from at least one of hydrogenated castor oil and modified hydrogenated castor oil. The leveling agent is selected from at least one of polyether-modified siloxane BYK-346 and polyether-modified siloxane BYK-348.

5. The water-soluble solder paste of claim 1, wherein, The preparation of carbon nanodots includes the following steps: S1. Dissolve 2 parts by weight of citric acid in a mixed solution containing 1 part by weight of ethylenediamine and 25 parts by weight of ultrapure water, and sonicate for 15 minutes to obtain solution A; S2. Transfer solution A to a reaction vessel and react at 200°C for 6 hours, then allow it to cool naturally to room temperature to obtain a reddish-brown solution B; S3. Collect solution B into a dialysis bag with a molecular weight cutoff of 100 Da, and place it in a beaker filled with ultrapure water for dialysis for 48 hours; S4. Centrifuge the dialyzed sample at 10,000 rpm for 10 min, discard the aggregated particles, collect the carbon nanoparticle solution, then pre-dry the carbon nanoparticle solution at 60℃ and then freeze-dry it to obtain carbon nanoparticle powder.

6. The method for preparing the water-soluble solder paste according to any one of claims 1 to 5, characterized in that, Includes the following steps: A1. First, the alloy powder and conductive binder are initially mixed for 10 minutes, and then ball milled at 200 rpm for 2.5 hours to obtain the mixed powder. A2. Add flux and other additives to the mixed powder from step A1, stir with a mixer for 15 minutes, and then vacuum stir at room temperature for 20 minutes to remove air bubbles from the material, thus obtaining water-soluble solder paste.

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