Epoxy resin composite sn-bi-sb lead-free solder paste and preparation method thereof

Abstract

The application discloses an epoxy resin composite Sn-Bi-Sb lead-free soldering paste and a preparation method thereof, and belongs to the field of soldering materials of metal materials and metallurgy. The epoxy resin composite Sn-Bi-Sb lead-free soldering paste is composed of 3-6% of a mixture of an epoxy resin, a curing agent and an accelerator, 1.5-5.0% of nano silver paste, and the balance of a commercial Sn-Bi-Sb soldering paste. In the mixture of the epoxy resin, the curing agent and the accelerator, the mass percentage of each component in the composition is epoxy resin: curing agent: accelerator = 100: 70-85: 4-9. The epoxy resin composite Sn-Bi-Sb lead-free soldering paste has good wetting and spreading performance, can significantly improve the shear strength and drop resistance of a soldering joint, and can be used for reflow soldering of some products in the electronic industry which have specific requirements for temperature and drop resistance.

Description

Technical Field

[0001] This invention relates to an epoxy resin composite Sn-Bi-Sb lead-free solder paste and its preparation method, belonging to the category of brazing materials in the metal materials and metallurgical fields. It is mainly used for the assembly and packaging of electronic components and is a lead-free solder with excellent brazing performance and superior solder joint mechanical properties. Background Technology

[0002] With the increasing demand for low-temperature soldering in the electronics industry, SnBi-based low-temperature lead-free solders are being used more and more widely in the electronics field. Sn-58Bi eutectic solder is the preferred low-temperature solder, as it can reduce warpage of printed circuit boards and electronic components during reflow soldering, reduce energy emissions, and lower production costs. It has been widely used in some products with specific temperature requirements. However, due to the high Bi content in Sn-58Bi eutectic solder, the ductility, strength, temperature cycling reliability, and mechanical shock reliability of the solder joints are degraded, limiting its application areas.

[0003] Currently, there are two main methods for improving the performance of lead-free solders both domestically and internationally. One method is alloying, which uses existing Sn-based lead-free solders as the matrix and improves the solder's performance by adding one or more trace alloying elements. The other method is particle reinforcement, which involves generating (e.g., in-situ synthesis) or directly adding second-phase particles (e.g., adding various nanoparticles, carbon nanotubes, graphene, etc.) into Sn-based solders to prepare composite lead-free solders, thereby improving the solder's performance.

[0004] Through literature search, the applicant found that patent application number 201310002577.1, entitled "A Low-Temperature Composite Solder Paste and Its Preparation and Application Methods," reports a low-temperature composite solder paste and its preparation and application methods. The low-temperature composite solder paste contains a low-temperature solder matrix, flux, and metal particles, which plays a stabilizing role in the soldering process of electronic devices, thereby ensuring the high precision requirements of the weld. Patent application number 201811229313.9, entitled "Epoxy Resin Composite Sn-Bi Lead-Free Solder Paste," reports an epoxy resin composite Sn-Bi lead-free solder paste, characterized by its composition consisting of 3%–8% epoxy resin, curing agent, and accelerator (epoxy resin: curing agent: accelerator = 100:8–30:1–10) by mass percentage, with the remainder being commercially available Sn-Bi solder paste. The aforementioned composite Sn-Bi lead-free solder paste exhibits excellent wetting and spreading properties, significantly improving the shear strength of brazed joints, and can be used for reflow soldering of electronic components. Application No. 201811230768.2, entitled "A Composite Sn-Bi Lead-Free Solder Paste Containing Epoxy Resin," reports a composite Sn-Bi lead-free solder paste containing epoxy resin. This composite Sn-Bi lead-free solder paste improves the wetting and spreading properties of the solder and significantly enhances the shear strength of brazed joints.

[0005] Application No. 202111234886.2, entitled "Epoxy Resin Composite Sn-Ag-Cu Lead-Free Solder Paste," reported that the solder joint strength of Sn-Ag-Cu lead-free solder paste with added epoxy resin was increased by 14% to 26% compared to Sn-Ag-Cu solder paste without epoxy resin (21.36 N). At the optimal addition amount of 5%, it increased to 27.19 N (when the total addition amount of epoxy resin + curing agent + accelerator was 5%), an increase of 26%. This fully demonstrates that the addition of epoxy resin can significantly improve the shear strength of Sn-Ag-Cu lead-free solder paste solder joints without any negative effects. However, existing solder literature has not reported any improvement in the "drop resistance" of solder joints or electronic devices.

[0006] In 2022, domestic smartphone shipments reached 264 million units, making mobile phones almost a necessity in people's lives. However, they are also considered consumables and easily damaged goods, and drops are the most common cause of damage. Therefore, one of the main assessment targets of drop tests for mobile phones is the drop resistance of solder joints. To improve the drop resistance and impact resistance of electronic devices (components), thereby enhancing their reliability, potting compounds are used in many products both domestically and internationally to protect electronic components on PCB boards. However, the application of potting compounds also has limitations. Through experiments, the applicant discovered that using epoxy resin composite lead-free solder joints can significantly improve the drop resistance of electronic devices (components), thus overcoming the limitations of potting compound applications. This invention was completed under this technological background. Summary of the Invention

[0007] The objective of this invention is to provide a low-temperature lead-free brazing filler metal with good wetting and spreading properties, which can significantly improve the shear strength of brazed joints, especially the drop resistance of the solder joints.

[0008] The objective of this invention is accomplished as follows.

[0009] An epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by its composition comprising a mixture of epoxy resin, curing agent, and accelerator in a mass percentage ratio of 3% to 6%, nano silver paste in a mass percentage ratio of 1.5% to 5.0%, and the balance being commercially available Sn-Bi-Sb solder paste. The mass percentage ratio of each component in the mixture of epoxy resin, curing agent, and accelerator is epoxy resin:curing agent:accelerator = 100:70-85:4-9. The epoxy resin is a combination of 50% E44 type bisphenol A and 50% E51 type bisphenol A; the curing agent is a combination of 40% maleic anhydride and 60% phthalic anhydride; and the accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol, and 40% resorcinol.

[0010] The preparation method of the epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by the following steps: Step 1: Using commercially available tin ingots, bismuth blocks, and antimony ingots, various elemental raw materials are proportioned as needed. During smelting, an optimized and screened "covering agent" is added or "inert gas" protection is used for smelting and casting to obtain Sn-Bi-Sb alloy rods; the smelted Sn-Bi-Sb alloy is then melted and, under argon protection, powdered using an "air atomization" powder-making device. Prepare alloy powder with a particle diameter of 20μm to 75μm; Pb element is used as an "impurity element" in the raw materials, and its total amount (mass percentage) is controlled within the range of Pb ≤ 0.07wt.%; Add an appropriate amount of ethanol or ethylene glycol to Sn-Bi-Sb alloy powder with a mass percentage of 85% to 95% and the balance being commercially available flux, and stir for 10 to 30 minutes using a high-speed mixer at a speed of 300 r / min to 1200 r / min to obtain Sn-Bi-Sb lead-free solder paste; then A mixture of 50% E44 bisphenol A and 50% E51 bisphenol A epoxy resin, a mixture of 40% maleic anhydride and 60% phthalic anhydride curing agent, and a mixture of 30% 2,4,6-tris(dimethylaminomethyl)phenol, 30% catechol, and 40% resorcinol accelerators are pre-mixed and thoroughly mixed according to an epoxy resin:curing agent:accelerator ratio of 100:70-85:4-9. Then, 3%-6% by mass of... A mixture of epoxy resin, curing agent, and accelerator, 1.5%–5.0% of commercially available nano-silver paste, and the balance being the Sn-Bi-Sb lead-free solder paste prepared above, are mixed with an appropriate amount of ethanol or ethylene glycol. The mixture is stirred for 10–30 minutes using a high-speed mixer at a speed of 300–1200 r / min. This yields the epoxy resin composite Sn-Bi-Sb lead-free solder paste. The commercially available nano-silver paste contains nano-silver particles with a particle size of 150 nm–250 nm, and the pressureless sintering temperature is not higher than 220 °C.

[0011] The commercially available Sn-Bi-Sb solder paste comprises Sn-Bi-Sb alloy powder accounting for 85%–95% of its mass, with the remainder being commercially available flux. Bi element comprises 57%–59% of the alloy powder, Sb comprises 1.2%–2.0%, and the remainder is Sn. The alloy powder particle diameter is 20μm–75μm. The newly invented epoxy resin composite Sn-Bi-Sb lead-free solder paste exhibits excellent wetting and spreading properties (comparable to commercially available Sn-Bi-Sb solder paste, with no negative impact), significantly improving the shear strength and drop resistance of brazed joints. It can be used for reflow soldering of certain products in the electronics industry with specific requirements for temperature and drop resistance.

[0012] The technical solution provided by this invention uses epoxy resin, curing agent, and accelerator, all of which are inexpensive (the market price per kilogram is lower than that of Sn, Bi, Sb, and Ag metals), non-toxic (extremely low toxicity), and environmentally friendly commercially available chemical materials. The total amount used accounts for only 3% to 6% of the paste-like brazing filler metal, and the amount of nano-silver paste added accounts for only 1.5% to 5.0% of the paste-like brazing filler metal. Moreover, it can improve the wetting and spreading properties of the brazing filler metal and significantly improve the shear strength and drop resistance of the brazed joint. Attached Figure Description

[0013] Figure 1 The figure shows the average shear force of the solder joints of commercially available Sn-58Bi-1.6Sb solder paste compared with those of Examples 1 to 3 and the comparative example solder pastes.

[0014] Figure 2 The data shown is a comparison of the number of drops when the specimen fails in the plate drop test. Detailed Implementation

[0015] Sn-Bi-Sb lead-free solder is prepared using conventional methods. Commercially available tin ingots, bismuth blocks, and antimony ingots are used, with various elements proportioned as needed. During smelting, an optimized and screened "covering agent" is added, or an "inert gas" protective atmosphere is employed for smelting and casting to obtain Sn-Bi-Sb alloy rods. The smelted Sn-Bi-Sb alloy is then remelted and, under argon protection, processed into alloy powder with a particle diameter of 20μm to 75μm using a "gas atomization" powder-making device. Lead (Pb) is used as an "impurity element" in the raw materials, with its total amount (mass percentage) controlled within the range of Pb≤0.07wt.%, to meet the requirements of the People's Republic of China National Standard GB / T 20422-2018 "Lead-Free Solder" (the standard specifies Pb≤0.07wt.%).

[0016] Add 85%–95% Sn-Bi-Sb alloy powder by mass, the balance being commercially available flux (such as ECO Flux 823), and an appropriate amount of ethanol or ethylene glycol. Stir using a high-speed mixer (300 rpm–1200 rpm) for 10–30 minutes to obtain Sn-Bi-Sb lead-free solder paste.

[0017] The epoxy resin mixture of 50% E44 type bisphenol A and 50% E51 type bisphenol A, the curing agent mixture of 40% maleic anhydride and 60% phthalic anhydride, and the accelerator mixture of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol and 40% resorcinol are pre-mixed and thoroughly mixed according to the ratio of epoxy resin:curing agent:accelerator = 100:70~85:4~9 and then set aside for later use (this application refers to it as a mixture composed of epoxy resin, curing agent and accelerator). Then, a mixture of epoxy resin, curing agent and accelerator (3%–6% by mass), commercially available nano silver paste (1.5%–5.0% by mass), and the balance being the Sn-Bi-Sb lead-free solder paste prepared above are added, along with an appropriate amount of ethanol or ethylene glycol. The mixture is stirred for 10–30 minutes using a high-speed mixer (300–1200 r / min) to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0018] Please see Figure 1 The figure shows a comparison of the average solder joint shear force of commercially available Sn-58Bi-1.6Sb solder paste and the solder pastes from the four examples. Figure 1 The results showed that, when comparing tests using three sizes of chip resistors (R0603, R0805, and R1210), the shear strength (i.e., shear force) of the solder joints obtained in the example was more than 80% higher than that of commercially available Sn-58Bi-1.6Sb solder paste. This is because the cured epoxy resin forms a thin and uniform coating layer on the solder joint surface, providing physical (or mechanical) reinforcement. Furthermore, during the soldering process, the nano-silver reacts with the Sn-58Bi-1.6Sb alloy in the Sn-58Bi-1.6Sb solder paste, forming a Sn-Bi-Sb-Ag quaternary alloy with the nano-Ag particles, further enhancing the shear strength of the solder joint. Although the shear force of the "comparative example" without added nano silver paste was higher than that of commercially available Sn-58Bi-1.6Sb solder joints, it was significantly lower than that of Examples 1 to 3, indicating that the addition of nano silver paste does indeed help improve the shear strength of epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0019] Board-level drop impact testing is commonly used in the early development stages of a product prototype, or for a specific packaging form or a specific connection material. By monitoring the performance during the test, it provides data support for the product's lifespan. This invention, based on the internationally accepted JEDEC standard JESD22-B111A-2016 "Board-level drop test method for handheld electronic product components," conducts drop impact reliability tests on solder joints. The test parameters are: peak acceleration of 1500g, pulse duration of 0.5ms, and equivalent drop height of 1120mm (1.12 meters). The solder joint failure criterion is based on the change in resistance between the positive and negative terminals of the PCB detected by a digital multimeter. The device undergoing the drop test is considered to have a broken circuit (no continuity) as the final failure criterion. Since the "board-level drop test method" is time-consuming and labor-intensive, if the number of drop tests in Examples 1, 2, and 3 reaches 3500 (more than twice that of Sn-58Bi solder balls / Sn-58Bi-1.6Sb solder paste) and the product still does not fail, the test will not be continued according to industry practice.

[0020] Please see Figure 2 The figure shows a comparison of the number of drops required for specimen failure in a plate-level drop test. Figure 2 The results show that Sn-58Bi solder balls / Sn-58Bi-1.6Sb solder paste, Sn-58Bi solder balls / epoxy resin Sn-Bi-1.6Sb composite solder paste (i.e., Sn-58Bi solder balls / Example 1 solder paste, Sn-58Bi solder balls / Example 2 solder paste, Sn-58Bi solder balls / Example 3 solder paste), and Sn-58Bi solder balls / comparative example solder paste (i.e., without added nano-silver paste) were used to prepare Sn-58Bi solder balls on a BGA structure with a double-sided Cu substrate. The drop impact resistance of solder joints made with 8Bi solder balls / Sn-58Bi-1.6Sb solder paste and composite solder joints made with Sn-58Bi solder balls / Example 1 solder paste, Sn-58Bi solder balls / Example 2 solder paste, Sn-58Bi solder balls / Example 3 solder paste, and Sn-58Bi solder balls / Comparative solder paste was investigated. The experimental results show that the drop impact resistance of the epoxy resin composite Sn-Bi-Sb lead-free solder paste (Examples 1-3) is significantly better than that of the Sn-58Bi-1.6Sb solder paste. Although the drop impact resistance of the comparative sample without nano-silver paste was worse than that of Examples 1 to 3, it was still better than that of Sn-58Bi-1.6Sb solder paste. This fully demonstrates that the addition of epoxy resin and its curing agent and accelerator after optimization and proportioning significantly improved the drop impact resistance of the solder joint.

[0021] The inventiveness of this invention compared to previous studies lies in:

[0022] 1) The mechanism by which epoxy resin and nano-silver paste enhance the shear strength of solder joints in composite Sn-58Bi-1.6Sb solder paste was discovered and elucidated. Specifically, the cured epoxy resin forms a thin and uniform coating layer on the solder joint surface, providing physical (or mechanical) reinforcement. Comparative brazing tests on three sizes of chip resistors (R0603, R0805, and R1210) revealed that the larger the size of the chip resistor, the larger the area of ​​the epoxy resin coating layer on the solder joint surface, and the greater the absolute value of the shear force of the brazed joint. However, a comparison of the shear strength (i.e., shear force) of the solder joints obtained from the examples with the shear force of solder joints from commercially available Sn-58Bi-1.6Sb solder paste showed that the shear force improvement rate was approximately 80%. This indicates that the increase in shear force is not solely due to the mechanical reinforcement of the cured epoxy resin, but is also related to the addition of nano-silver. During the brazing process, nano-silver underwent a "metallurgical reaction" with the Sn-Bi-Sb alloy in the Sn-58Bi-1.6Sb solder paste. The nano-Ag particles formed a Sn-Bi-Sb-Ag quaternary alloy with Sn-58Bi-1.6Sb, thereby further improving the shear strength of the solder joint. This argument can be explained by the data from the "comparative example"—although the shear force of the "comparative example" without nano-silver paste was higher than that of commercially available Sn-58Bi-1.6Sb solder joints, it was significantly lower than that of Examples 1 to 3. This indicates that the addition of nano-silver paste did indeed cause a "metallurgical reaction" with the Sn-58Bi-1.6Sb alloy, and the addition of nano-silver paste helps to further improve the shear strength of the epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0023] 2) It was discovered that the addition of an appropriate amount of nano-silver paste can significantly improve the drop resistance of solder joints made with epoxy resin Sn-Bi-1.6Sb composite solder paste. The drop resistance reliability of the solder joints was tested according to the internationally accepted JEDEC standard JESD22-B111A-2016 "Board-level drop test method for handheld electronic components". The test parameters were: peak acceleration of 1500g, pulse duration of 0.5ms, and equivalent drop height of 1120mm (i.e., 1.12 meters). The test results showed that the drop impact resistance of solder joints made with epoxy resin composite Sn-Bi-Sb lead-free solder paste (Examples 1-3) was far superior to that of solder joints made with Sn-58Bi-1.6Sb solder paste.

[0024] Although the comparative brazed joints without nano-silver paste showed poorer drop impact resistance than those in Examples 1-3, they were still superior to those brazed joints with Sn-58Bi-1.6Sb solder paste. This is because the cured epoxy resin forms a thin and uniform coating layer on the surface of the solder joint, which acts as a "physical reinforcement" (or mechanical reinforcement) and thus improves the drop impact resistance. However, compared with the test data of Examples 1-3, the Sn-58Bi-1.6Sb-Ag quaternary alloy formed by the addition of nano-silver paste exhibits even better drop impact resistance.

[0025] Microstructural analysis of drop-failed solder joints revealed that under drop impact loads, the solder joints were affected not only by the normal stress (tensile and compressive stresses) from the bending deformation of the PCB substrate, but also by shear stress, which impacted the grain boundary bonding force within the solder joint, leading to cracks between grains. Although the fracture morphology of the epoxy resin composite solder joint showed that the solder joint was encapsulated by the cured epoxy resin, the mechanical resistance of the epoxy resin reinforcement layer and its own plastic deformation capacity provided mechanical resistance to drop impact vibrations and PCB substrate flexural deformation, thus improving the solder joint's drop resistance reliability through mechanical reinforcement (or physical reinforcement). Based solely on the above analysis, a thicker or more abundant epoxy resin reinforcement layer should result in better drop resistance. However, actual results show that the drop impact resistance mechanism is similar to that of brazed solder joints, meaning it is not solely due to the reinforcement of the cured epoxy resin, but also related to the addition of nano-silver.

[0026] As mentioned earlier, the nano-silver paste added in this technical solution has a nano-silver particle size of 150nm to 250nm, and the "pressureless sintering" temperature does not exceed 220℃. In this example, the peak reflow soldering temperature is 250℃, with a holding time of 5 minutes. Under these conditions, the nano-silver paste can completely react with Sn-58Bi-1.6Sb to form a Sn-Bi-Sb-Ag quaternary alloy. During the "cooling stage" of reflow soldering, the nano-Ag particles, acting as "nucleation sites," accelerate the crystallization of the Sn-Bi-Sb-Ag alloy and play a role in "refining the grains." Grain refinement generally leads to fine-grain strengthening and improved "plasticity." Under the conditions of a peak reflow soldering temperature of 250℃ and a holding time of 5 minutes, the added epoxy resin, under the action of the accelerator, completely cures with the curing agent and achieves the best reinforcing effect. Therefore, the "epoxy resin composite Sn-Bi-Sb lead-free solder paste" (Examples 1 to 3) with added nano silver paste showed further improvements in the shear strength and drop resistance of the solder joints compared to the comparative examples.

[0027] Based on the mass ratio of the "epoxy resin composite Sn-Bi-Sb lead-free solder paste" according to the present invention, the specific embodiments of the present invention are described below.

[0028] Example 1

[0029] An epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by its composition comprising a mixture of 3% epoxy resin, curing agent, and accelerator by mass percentage, 5.0% nano silver paste, and the balance being commercially available Sn-Bi-Sb solder paste. The mixture of epoxy resin, curing agent, and accelerator is formulated in the following mass percentage ratio: epoxy resin: curing agent: accelerator = 100:70:9. The epoxy resin is a combination of 50% E44 type bisphenol A and 50% E51 type bisphenol A; the curing agent is a combination of 40% maleic anhydride and 60% phthalic anhydride; and the accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol, and 40% resorcinol. The nano-silver paste contains nano-silver particles with a diameter of 150nm to 250nm, and the pressureless sintering temperature does not exceed 220℃. The commercially available Sn-Bi-Sb solder paste contains Sn-Bi-Sb alloy powder comprising 85% by mass of the paste, with the remainder being commercially available flux. Bi comprises 57% of the alloy powder, Sb comprises 2.0%, and the remainder is Sn; the alloy powder particle diameter is 20μm to 75μm.

[0030] According to the above ratio, add an appropriate amount of ethanol or ethylene glycol, and stir with a high-speed mixer (300r / min to 1200r / min) for 10 to 30 minutes to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0031] The prepared epoxy resin composite Sn-Bi-Sb lead-free solder paste showed improved wetting and spreading properties (without negative impacts). Using an FR-4 type PCB board as the test board and R0603, R0805, and R1210 chip resistors as test elements, reflow soldering was performed at a peak temperature of 250℃ for 5 minutes. The shear strength of the solder joints was more than 80% higher than that of commercially available Sn-58Bi-1.6Sb solder paste. The number of drops before failure in the board-level drop test was more than three times that of the solder joints of commercially available Sn-58Bi-1.6Sb solder paste.

[0032] The solder paste still meets the performance requirements after being stored in a refrigerator at 0°C for more than 1 year.

[0033] Example 2

[0034] An epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by its composition being a mixture of 6% epoxy resin, curing agent, and accelerator by mass percentage, 1.5% nano silver paste, and the balance being commercially available Sn-Bi-Sb solder paste. The mixture of epoxy resin, curing agent, and accelerator is formulated with the following mass percentage ratio: epoxy resin: curing agent: accelerator = 100:85:4. The epoxy resin is a combination of 50% E44 bisphenol A and 50% E51 bisphenol A; the curing agent is a combination of 40% maleic anhydride and 60% phthalic anhydride; and the accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol, and 40% resorcinol. The nano-silver paste contains nano-silver particles with a diameter of 150nm to 250nm, and the pressureless sintering temperature does not exceed 220℃. The commercially available Sn-Bi-Sb solder paste contains Sn-Bi-Sb alloy powder comprising 85% by mass of the paste, with the remainder being commercially available flux. Bi comprises 59% of the alloy powder, Sb comprises 1.2%, and the remainder is Sn; the alloy powder particle diameter is 20μm to 75μm.

[0035] According to the above ratio, add an appropriate amount of ethanol or ethylene glycol, and stir with a high-speed mixer (300r / min to 1200r / min) for 10 to 30 minutes to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0036] The prepared epoxy resin composite Sn-Bi-Sb lead-free solder paste showed improved wetting and spreading properties (without negative impacts). Using an FR-4 type PCB board as the test board and R0603, R0805, and R1210 chip resistors as test elements, reflow soldering was performed at a peak temperature of 250℃ for 5 minutes. The shear strength of the solder joints was more than 80% higher than that of commercially available Sn-58Bi-1.6Sb solder paste. The number of drops before failure in the board-level drop test was more than three times that of the solder joints of commercially available Sn-58Bi-1.6Sb solder paste.

[0037] The solder paste still meets the performance requirements after being stored in a refrigerator at 0°C for more than 1 year.

[0038] Example 3

[0039] An epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by its composition comprising a mixture of epoxy resin, curing agent, and accelerator in a mass percentage ratio of 4.5%, nano silver paste in a mass percentage ratio of 3.2%, and the balance being commercially available Sn-Bi-Sb solder paste. The mass percentage ratio of each component in the epoxy resin, curing agent, and accelerator mixture is epoxy resin:curing agent:accelerator = 100:80:6. The epoxy resin is a combination of 50% E44 type bisphenol A and 50% E51 type bisphenol A; the curing agent is a combination of 40% maleic anhydride and 60% phthalic anhydride; and the accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol, and 40% resorcinol. The nano-silver paste contains nano-silver particles with a diameter of 150nm to 250nm, and the pressureless sintering temperature does not exceed 220℃. The commercially available Sn-Bi-Sb solder paste contains Sn-Bi-Sb alloy powder comprising 90% by mass of the paste, with the remainder being commercially available flux. Bi comprises 58% of the alloy powder, Sb comprises 1.6%, and the remainder is Sn; the alloy powder particle diameter is 20μm to 75μm.

[0040] According to the above ratio, add an appropriate amount of ethanol or ethylene glycol, and stir with a high-speed mixer (300r / min to 1200r / min) for 10 to 30 minutes to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste.

[0041] The prepared epoxy resin composite Sn-Bi-Sb lead-free solder paste showed improved wetting and spreading properties (without negative impacts). Using an FR-4 type PCB board as the test board and R0603, R0805, and R1210 chip resistors as test elements, reflow soldering was performed at a peak temperature of 250℃ for 5 minutes. The shear strength of the solder joints was more than 80% higher than that of commercially available Sn-58Bi-1.6Sb solder paste. The number of drops before failure in the board-level drop test was more than three times that of the solder joints of commercially available Sn-58Bi-1.6Sb solder paste.

[0042] The solder paste still meets the performance requirements after being stored in a refrigerator at 0°C for more than 1 year.

[0043] Comparative Example

[0044] An epoxy resin composite Sn-Bi-Sb lead-free solder paste is characterized by its composition comprising a mixture of epoxy resin, curing agent, and accelerator in a mass percentage ratio of 4.5%, with the remainder being commercially available Sn-Bi-Sb solder paste. The mass percentage ratio of each component in the mixture of epoxy resin, curing agent, and accelerator is epoxy resin:curing agent:accelerator = 100:80:6. The epoxy resin is a combination of 50% E44 type bisphenol A and 50% E51 type bisphenol A; the curing agent is a combination of 40% maleic anhydride and 60% phthalic anhydride; and the accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol (i.e., DMP-30), 30% catechol, and 40% resorcinol. The commercially available Sn-Bi-Sb solder paste comprises Sn-Bi-Sb alloy powder accounting for 90% by mass percentage, with the remainder being commercially available flux. Bi element accounts for 58% of the alloy powder, Sb accounts for 1.6%, and the remainder is Sn. The alloy powder particle diameter is 20μm to 75μm.

[0045] According to the above ratio, add an appropriate amount of ethanol or ethylene glycol, and stir with a high-speed mixer (300r / min to 1200r / min) for 10 to 30 minutes to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste without the addition of nano silver paste.

[0046] The prepared epoxy resin composite Sn-Bi-Sb lead-free solder paste showed improved wetting and spreading properties (without negative impact). Using an FR-4 type PCB board as the test board and R0603, R0805, and R1210 chip resistors as test elements, reflow soldering was performed at a peak temperature of 250℃ and a holding time of 5 min. The shear strength of the solder joints was about 68% higher than that of commercially available Sn-58Bi-1.6Sb solder paste (lower than Examples 1 to 3). The number of drops before failure in the board-level drop test was about twice that of the solder joints of commercially available Sn-58Bi-1.6Sb solder paste, and significantly lower than that of Examples 1 to 3 with added nano silver paste.

[0047] The solder paste still meets the performance requirements after being stored in a refrigerator at 0°C for more than 1 year.

Claims

1. An epoxy resin composite Sn-Bi-Sb lead-free solder paste, characterized in that: Its composition is a mixture of epoxy resin, curing agent and accelerator in a mass percentage ratio of 3% to 6%, nano silver paste in a mass ratio of 1.5% to 5.0%, and the balance is Sn-Bi-Sb solder paste. In the mixture of epoxy resin, curing agent and accelerator, the mass percentage of each component in the mixture is epoxy resin: curing agent: accelerator = 100: 70~85: 4~9; the epoxy resin is a combination of 50% E44 type bisphenol A and 50% E51 type bisphenol A. The curing agent mentioned above is a combination of 40% maleic anhydride and 60% phthalic anhydride; The aforementioned accelerator is a combination of 30% 2,4,6-tris(dimethylaminomethyl)phenol, 30% catechol, and 40% resorcinol; The Sn-Bi-Sb solder paste contains Sn-Bi-Sb alloy powder accounting for 85% to 95% of the Sn-Bi-Sb solder paste by mass percentage, with the remainder being flux. In the Sn-Bi-Sb alloy powder, by mass percentage, Bi element accounts for 57% to 59% of the alloy powder, Sb accounts for 1.2% to 2.0%, and the balance is Sn; the particle diameter of the alloy powder is 20μm to 75μm.

2. The method for preparing epoxy resin composite Sn-Bi-Sb lead-free solder paste according to claim 1, characterized in that... Includes the following processes: Step 1: Using tin ingots, bismuth metal blocks, and antimony ingots, various elemental raw materials are proportioned as needed. During smelting, an optimized and screened "covering agent" is added or "inert gas" protection is used for smelting and casting to obtain Sn-Bi-Sb alloy rods. The smelted Sn-Bi-Sb alloy is remelted and then prepared into alloy powder with a particle diameter of 20μm to 75μm using an "air atomization" powder preparation equipment under argon protection. The total amount (mass percentage) of Pb element, as an "impurity element" in the raw material, is controlled within the range of Pb≤0.07wt.%. Sn-Bi-Sb lead-free solder paste is obtained by adding an appropriate amount of ethanol or ethylene glycol to Sn-Bi-Sb alloy powder with a mass percentage of 85% to 95% and the balance as flux, and stirring with a high-speed mixer at a speed of 300 r / min to 1200 r / min for 10 min to 30 min. Then, the epoxy resin mixture of 50% E44 type bisphenol A and 50% E51 type bisphenol A, the curing agent mixture of 40% maleic anhydride and 60% phthalic anhydride, and the accelerator mixture of 30% 2,4,6-tris(dimethylaminomethyl)phenol, 30% catechol and 40% resorcinol are pre-mixed and thoroughly mixed according to the epoxy resin:curing agent:accelerator ratio of 100:70~85:4~9 and set aside for later use. Then, a mixture of epoxy resin, curing agent and accelerator (3%–6% by mass), nano silver paste (1.5%–5.0% by mass), and the balance being the Sn-Bi-Sb lead-free solder paste prepared above are added to an appropriate amount of ethanol or ethylene glycol. The mixture is stirred for 10–30 minutes using a high-speed mixer at a speed of 300–1200 r / min to obtain epoxy resin composite Sn-Bi-Sb lead-free solder paste. The nano silver paste contains nano silver particles with a particle size of 150 nm–250 nm, and the pressureless sintering temperature is not higher than 220 °C.

Images