Low-temperature curing high-reliability lead-free solder paste and preparation method thereof

Abstract

The invention discloses a low-temperature curing high-reliability lead-free solder paste and a preparation method thereof, and the low-temperature curing high-reliability lead-free solder paste comprises the following components in percentage by weight: 85.0%-92.0% of lead-free solder alloy powder and 8.0%-15.0% of paste flux. The paste flux comprises rosin resin, a thixotropic agent, a solvent, a corrosion inhibitor and an active agent. The active agent is a composite microsphere with an inner core-middle layer-outer shell layer triple structure, the inner core comprises an eutectic mixture of organic halide and a thermal initiator, the middle layer is made of a wax material, and the outer shell layer is a porous coating layer formed by compounding organic acid powder and thermal degradable polymer powder through an adhesive. In the heating process, when the temperature reaches T3, the thermally degradable polymer is subjected to chain segment fracture to form holes, so that the organic acid powder gradually escapes; when the temperature rises to T2, the wax middle layer is molten to form a diffusion channel; and when the temperature reaches T1, the thermal initiator is decomposed to generate gas, so that the core is broken, and the organic halide is released. The time sequence release characteristic is matched with the temperature of the reflow soldering process, and it is ensured that the most suitable active ingredients efficiently work in each temperature zone.

Description

TECHNICAL FIELD

[0001] The present application relates to a low-temperature curing high-reliability lead-free solder paste and a preparation method thereof. BACKGROUND

[0002] A kind of lead-free solder paste active agent and its preparation method and a kind of lead-free solder paste disclosed in the authorized announcement No.CN104191109B, active agent is wrapped by microcapsule technology to improve stability and realize segmented release. However, this technical scheme has the following problems: The patent only uses single capsule wall (rosin) to wrap mixed active ingredients, and its release trigger mechanism depends on the overall melting or softening of the capsule wall. This is a kind of "all or nothing" on-off release, which cannot control the release timing and release kinetics of different active ingredients. Organic acid and organic halide will be released at approximately the same temperature interval, which cannot truly match the differentiated needs of different stages of reflow soldering curve; In addition, organic acid and organic halide are simply mixed and wrapped together. At the moment of microcapsule rupture, the two active ingredients are exposed to the welding environment at the same time, which may cause mutual reaction or interference between active ingredients, so-called "segmented release" effect is greatly discounted, which is closer to a kind of "delayed one-time release". SUMMARY

[0003] The present application aims to solve at least one of the technical problems existing in the prior art. To this end, the present application provides a low-temperature curing high-reliability lead-free solder paste and a preparation method thereof.

[0004] A kind of low-temperature curing high-reliability lead-free solder paste, by weight percentage, including 85.0%~92.0% lead-free solder alloy powder and 8.0%~15.0% soldering paste;The soldering paste contains rosin, thixotropic agent, solvent, corrosion inhibitor and active agent;The active agent comprises core-shell layer from inside to outside, and the active agent is a composite microsphere structure, the core contains a eutectic mixture of organic halide and thermal initiator, the thermal initiator decomposes and produces gas when reaching the first set temperature T1;The intermediate layer is wrapped around the core, which is composed of waxy material with a melting point of second set temperature T2, and T2 is lower than T1;The shell layer is a porous coating layer formed by compounding organic acid powder and thermally degradable polymer powder with adhesive, and the thermal degradation onset temperature T3 of the thermally degradable polymer is lower than T2;During the heating process from room temperature to the peak temperature of reflow soldering, the composite microspheres release active substances in the following order: when the temperature reaches T3, the thermally degradable polymer undergoes chain segment rupture and pore formation, allowing the organic acid powder to gradually escape;When the temperature reaches T2, the waxy intermediate layer melts and forms a diffusion channel connecting the core and the shell;When the temperature reaches T1, the thermal initiator decomposes to produce gas to break the core and release the organic halide.

[0005] Preferably, the first set temperature T1 is 200-230℃, the second set temperature T2 is 150-180℃, and the thermal degradation onset temperature T3 is 100-130℃.

[0006] Preferably, the thermal initiator is azobisisobutyronitrile (AIBN); the waxy material is Fischer-Tropsch wax; and the thermally degradable polymer is polylactic acid (PLA).

[0007] Preferably, the organic acid powder is a mixture of succinic acid and octadecanedioic acid at a molar ratio of 1:1 to 1:5; and the organic halide is 3-chloro-1,2-propanediol.

[0008] Preferably, the adhesive in the shell layer is rosin resin, and the amount of the adhesive accounts for 5-15% of the total weight of the shell layer.

[0009] Preferably, the lead-free solder alloy powder comprises, by weight percentage: Sn 88.0%-92.0%, Bi 5.0%-8.0%, Ag 1.5%-3.0%, Cu 0.3%-1.0%, Ni 0.1%-0.5%, and Ge 0.01%-0.1%.

[0010] The preparation method of the low-curing high-reliability lead-free solder paste comprises the following steps: S1: Preparation of composite microsphere active agent Preparation of inner core: melt and blend the organic halide and the thermal initiator at 60-80℃, and form solid inner core microspheres by spray condensation or liquid dropping method; Coating of intermediate layer: fluidize the inner core microspheres obtained above, and spray the molten waxy material in the fluidized bed to form a core-shell structure precursor coated with an intermediate layer after cooling; Formation of shell layer: mix the organic acid powder, the thermally degradable polymer powder, and the adhesive solution to prepare a slurry, uniformly coat the slurry on the surface of the core-shell structure precursor obtained above by using fluidized bed coating process, and obtain the composite microsphere active agent with inner core-intermediate layer-shell layer triple structure after drying and sieving; The thermal cracking inner core comprises an organic halide as a second active component, and a thermal initiator for decomposing and generating gas when the first critical temperature T1 is reached, sufficient to cause the thermal cracking inner core to rupture; S2: Preparation of soldering paste The rosin resin is heated to melt and mixed with the solvent, stirred at 60-80 DEG C until completely dissolved to form a uniform solution; after the solution is cooled to 40-50 DEG C, the thixotropic agent is added, dispersed and fully gelled; then the composite microspheres active agent prepared in step S1 and the corrosion inhibitor are added, and mixed uniformly under vacuum condition at low speed to obtain the soldering paste; S3: mixing of tin paste The lead-free solder alloy powder and the soldering paste prepared in step S2 are put into a vacuum stirrer in proportion, mixed at a revolution speed of 10-20 rpm for 10-15 minutes and then at a revolution speed of 30-50 rpm for 20-30 minutes under the condition of vacuum degree ≤-0.095 MPa, to ensure that the powder and the soldering paste are mixed uniformly and air bubbles are removed, thereby obtaining the low-temperature curing high-reliability lead-free tin paste.

[0011] The present application has the following advantages: The present application successfully realizes the step-by-step and sequential release of active ingredients at three different temperature nodes (T3, T2, T1) by designing a triple structure of core-intermediate layer-outer shell layer and using three different physical and chemical mechanisms of thermal degradation, phase change melting and thermal initiation gas production rupture as release triggers. This release mode matches the "preheating-holding-reflow" temperature curve of the reflow soldering process, ensuring that the active ingredients work efficiently in each temperature zone, and fundamentally overcoming the drawbacks of the prior art of one-time release of active ingredients. DETAILED DESCRIPTION

[0012] The orientation words such as up, down, left, right, top, bottom and the like in the specification can directly contact or contact through other features between them; for example, above can be directly above and obliquely above, or it only means higher than others; other orientations can be similarly understood.

[0013] The manufacturing materials of the components with solid shapes represented in the specification can be metal materials or non-metal materials or other synthetic materials; the machining processes used for the components with solid shapes can be stamping, forging, casting, wire cutting, laser cutting, injection molding, numerical milling, three-dimensional printing, machining and the like; the ordinary skilled in the art can adaptively select or combine according to different processing conditions, costs and precision, but are not limited to the above materials and manufacturing processes.

[0014] The low-temperature curing high-reliability lead-free solder paste comprises, by weight percentage, 85.0%~92.0% lead-free solder alloy powder and 8.0%~15.0% soldering paste; the soldering paste comprises rosin resin, thixotropic agent, solvent, corrosion inhibitor and active agent; the active agent comprises, from inside to outside, core-middle layer-shell layer; the active agent is a composite microsphere structure; the core comprises a eutectic mixture of organic halide and thermal initiator; the thermal initiator decomposes and generates gas when reaching a first set temperature T1; the middle layer is a wrapping layer of the core, which is composed of waxy material with a melting point of a second set temperature T2, and T2 is lower than T1; the shell layer is a porous coating layer formed by compounding organic acid powder and thermally degradable polymer powder through adhesive; the thermally degradable polymer has a thermal degradation onset temperature T3 lower than T2; during the heating process from room temperature to reflow solder peak temperature, the composite microspheres release active substances in the following order: when the temperature reaches T3, the thermally degradable polymer undergoes chain segment rupture and pore formation, causing the organic acid powder to gradually escape; when the temperature reaches T2, the waxy middle layer melts and forms diffusion channels connecting the core and the shell; when the temperature reaches T1, the thermal initiator decomposes to generate gas, causing the core to rupture and release the organic halide.

[0015] Further, the first set temperature T1 is 200℃~230℃, the second set temperature T2 is 150℃~180℃, and the thermal degradation onset temperature T3 is 100℃~130℃.

[0016] Further, the thermal initiator is azobisisobutyronitrile (AIBN); the waxy material is Fischer-Tropsch wax; and the thermally degradable polymer is polylactic acid (PLA).

[0017] Further, the organic acid powder is a mixture of succinic acid and octadecanedioic acid in a molar ratio of 1:1 to 1:5; and the organic halide is 3-chloro-1,2-propanediol.

[0018] Further, the adhesive in the shell layer is rosin resin, and the amount of the adhesive accounts for 5%~15% of the total weight of the shell layer.

[0019] Further, the composition of the lead-free solder alloy powder, by weight percentage, is Sn 88.0%~92.0%, Bi 5.0%~8.0%, Ag 1.5%~3.0%, Cu 0.3%~1.0%, Ni 0.1%~0.5%, and Ge 0.01%~0.1%.

[0020] The preparation method of the above-mentioned low-temperature curing high-reliability lead-free solder paste comprises the following steps: S1: Preparation of composite microsphere active agent Preparation of inner core: melt-blending organic halide and thermal initiator at 60-80℃, granulating by spray condensation or dropping in liquid, forming solid inner core microspheres; Coating intermediate layer: fluidizing the inner core microspheres obtained above, spraying molten waxy material in fluidized bed, forming core-shell structure precursor coated with intermediate layer after cooling; Forming outer shell layer: mixing organic acid powder, thermally degradable polymer powder and adhesive solution to form slurry, uniformly coating the slurry on the surface of the core-shell structure precursor obtained above by fluidized bed coating process, after drying and sieving, obtaining the composite microspheres active agent with inner core-intermediate layer-outer shell triple structure; The thermal cracking inner core contains an organic halide as a second active component, and a thermal initiator for decomposing and generating gas when reaching the first critical temperature T1, sufficient to make the thermal cracking inner core break; S2: Preparation of soldering flux After heating the rosin resin to melt and mixing with solvent, stirring at 60-80℃ until completely dissolved to form a uniform solution, adding thixotropic agent after the solution is cooled to 40-50℃, dispersing and fully gelling; then adding the composite microspheres active agent prepared in step S1 and corrosion inhibitor, mixing uniformly under vacuum condition at low speed, obtaining the soldering flux; S3: Mixing of tin paste Put the lead-free solder alloy powder and the soldering flux prepared in step S2 into a vacuum stirrer in proportion, first mix at 10-20 rpm of revolution speed for 10-15 minutes, then mix at 30-50 rpm of revolution speed for 20-30 minutes under the condition of vacuum degree ≤-0.095 MPa, ensuring that the powder and the soldering flux are mixed uniformly and air bubbles are excluded, obtaining the low-temperature curing high-reliability lead-free tin paste.

[0021] Example 1 Preparation of composite microspheres active agent: Preparation of inner core: melt-blending 3-chloro-1,2-propanediol and azobisisobutyronitrile (AIBN) at 70℃ in a weight ratio of 95:5, granulating by spray condensation method, forming solid inner core microspheres with an average particle size of 30μm.

[0022] Coating intermediate layer: placing the above inner core microspheres in a fluidized bed, maintaining fluidized state, spraying molten Fischer-Tropsch wax (melting point 160℃) at 65℃, controlling the thickness of the wax layer, forming core-shell structure precursor after cooling.

[0023] Forming the shell layer: mix succinic acid and octadecanedioic acid (molar ratio 1:3) to be crushed into powder below 5 μm, mix with polylactic acid (PLA) powder of similar particle size at a weight ratio of 7:3, and then add 10% of rosin resin (dissolved in a small amount of ethanol as an adhesive) based on the total weight of the mixture to make a slurry. The slurry is uniformly coated on the surface of the core-shell structure precursor by using fluidized bed coating process, dried and solidified at 40°C, and then sieved to obtain composite microspheres active agents with an average particle size of 45 μm.

[0024] Preparation of the soldering paste: According to the total weight of the soldering paste being 100%, take hydrogenated rosin (70%), propylene glycol methyl ether acetate (25%), hydrogenated castor oil (2%), benzotriazole (0.5%), and the composite microspheres active agent (2.5%) prepared above.

[0025] Heat the hydrogenated rosin to 75°C to melt, add propylene glycol methyl ether acetate, and stir until completely dissolved. After cooling to 45°C, add hydrogenated castor oil, and high-speed disperse at a speed of 2000 rpm for 10 minutes to make it fully gel. Finally, add the composite microspheres active agent and benzotriazole, and stir at a speed of 50 rpm under a vacuum degree of -0.095 MPa for 20 minutes to mix uniformly, to obtain the soldering paste.

[0026] Preparation of the lead-free solder paste: The lead-free solder alloy powder uses SAC307 alloy (Sn90.5%, Bi6.0%, Ag2.2%, Cu0.7%, Ni0.3%, Ge0.03%), and the particle size is Type 4 (20-38 μm). Put 88.5% of the alloy powder and 11.5% of the above soldering paste into a vacuum stirrer. First, mix at a revolution speed of 15 rpm for 12 minutes, and then mix at a revolution speed of 40 rpm for 25 minutes under a vacuum degree of -0.098 MPa, to obtain a low-temperature solidified lead-free solder paste with uniform texture and good gloss.

[0027] Comparative Example 1 Use the same alloy powder and soldering paste formula as in Example 1, but use a simple physical mixture of 3-chloro-1,2-propanediol, succinic acid, and octadecanedioic acid with the same weight as the inner core, shell layer, etc. of the active agent in Example 1, instead of preparing composite microspheres.

[0028] Table 1 Performance Test

[0029] The test results show that the solder paste of the present application is significantly superior to the traditional physical mixed active agent solder paste in terms of wettability, tin bead resistance, and storage stability.

[0030] In the prior art, low-temperature lead-free solder paste (such as Sn-Bi system) generally faces the common problems of poor solder wettability and insufficient interface bonding strength while reducing the welding temperature, which seriously restricts its application in high-end reliability products. The timing release active agent system of the present application releases organic halides at the welding peak temperature zone (T1, 200-230 DEG C), providing a wetting driving force for low-temperature melting solder, so that it can achieve an expansion rate of more than 85% at a lower process temperature. At the same time, the mild release of the active agent in the preheating zone (T3) effectively protects the pads and avoids pre-oxidation, and the addition of Ni and Ge elements in the alloy refines the microstructure of the solder joints at low temperature. These synergies specially address and solve the two major bottlenecks of "insufficient wetting force" and "poor IMC growth" inherent in low-temperature solder joints, making it possible to obtain solder joints with high mechanical strength and excellent thermal fatigue resistance in a low-temperature process window of 220-235 DEG C.

[0031] The lead-free solder paste prepared by the present application has the organic halides firmly encapsulated in the microsphere inner core and physically isolated from the alloy powder during room temperature storage and printing, avoiding the slow reaction between the active agent and the tin powder in the traditional formula. In addition, the present application ensures the cleanliness of the interface through the active agent, laying the foundation for the formation of a dense IMC layer necessary for high-reliability solder joints. At the same time, the trace amount of Ni element in the alloy can refine the IMC grains, and the Ge element effectively inhibits oxidation, both of which synergistically promote the formation of a thin and uniform IMC layer with better toughness.

[0032] The present application successfully realizes the step-by-step and sequential release of active ingredients at three different temperature nodes (T3, T2, T1) by designing a triple structure of core-intermediate layer-outer shell layer and using three different physical and chemical mechanisms of thermal degradation, phase change melting and heat-induced gas rupture as release triggers. This release mode precisely matches the "preheating-holding-reflow" temperature curve of the reflow soldering process, ensuring that the active ingredients work efficiently in each temperature zone, and fundamentally overcoming the drawbacks of the prior art active "one-time release".

[0033] Although the present application has been described in detail with reference to the above examples, it will be apparent to those skilled in the art, through this disclosure, that various changes or modifications can be made to the present application without departing from the principles and spirit of the present application defined by the claims. Therefore, the detailed description of the present disclosure is only used to explain, not to limit the present application, and the scope of protection is defined by the content of the claims.

Claims

1. A low-temperature curing high-reliability lead-free solder paste, comprising, by weight percentage, 85.0% to 92.0% of lead-free solder alloy powder, and 8.0% to 15.0% of soldering flux; the soldering flux comprises rosin resin, thixotropic agent, solvent, corrosion inhibitor and active agent; characterized in that, The active agent comprises, from inside to outside, an inner core, an intermediate layer, and an outer shell layer, the active agent is a composite microsphere structure, the inner core comprises a eutectic mixture of an organic halide and a thermal initiator, the thermal initiator decomposes to produce gas when reaching a first set temperature T1; the intermediate layer is wrapped around the inner core and is composed of a waxy material with a melting point of a second set temperature T2, and T2 is lower than T1; the outer shell layer is a porous coating layer formed by compounding organic acid powder and thermally degradable polymer powder with an adhesive, the thermally degradable polymer has a thermal degradation onset temperature T3 lower than T2; during the heating process from room temperature to the peak temperature of reflow soldering, the composite microspheres release active substances in the following order: when the temperature reaches T3, the thermally degradable polymer undergoes chain scission and pore formation, causing the organic acid powder to gradually escape; when the temperature reaches T2, the waxy intermediate layer melts and forms a diffusion channel connecting the inner core and the outer shell; when the temperature reaches T1, the thermal initiator decomposes to produce gas, causing the inner core to rupture and release the organic halide.

2. The low-temperature solidified high-reliability lead-free solder paste according to claim 1, wherein, The first set temperature T1 is 200-230 DEG C, the second set temperature T2 is 150-180 DEG C, and the thermal degradation onset temperature T3 is 100-130 DEG C.

3. The low-temperature solidified high-reliability lead-free solder paste of claim 1, wherein the solder paste is characterized by: The thermal initiator is azobisisobutyronitrile (AIBN), the waxy material is Fischer-Tropsch wax, and the thermally degradable polymer is polylactic acid (PLA).

4. The low-temperature solidified high-reliability lead-free solder paste of claim 1, wherein, The organic acid powder is a mixture of succinic acid and octadecanedioic acid in a molar ratio of 1:1 to 1:5, and the organic halide is 3-chloro-1,2-propanediol.

5. The low-temperature solidified high-reliability lead-free solder paste of claim 1, wherein the solder paste is characterized by: The adhesive in the outer shell layer is rosin resin, and the amount of the adhesive accounts for 5-15% of the total weight of the outer shell layer.

6. The low-temperature solidified high-reliability lead-free solder paste of claim 1, wherein, The composition of the lead-free solder alloy powder is, by weight percentage: Sn 88.0%-92.0%, Bi 5.0%-8.0%, Ag 1.5%-3.0%, Cu 0.3%-1.0%, Ni 0.1%-0.5%, and Ge 0.01%-0.1%.

7. A method of preparing the low solidification high reliability lead-free solder paste according to any one of claims 1 to 6, characterized by, The method comprises the following steps: S1: preparation of a composite microsphere active agent Preparation of the inner core: melt and blend the organic halide and the thermal initiator at 60-80 DEG C, and form solid inner core microspheres by spray condensation or liquid dropping method; Wrap the intermediate layer: fluidize the obtained inner core microspheres, spray the molten waxy material in the fluidized bed, and form the core-shell structure precursor wrapped with the intermediate layer after cooling; Form the outer shell layer: mix the organic acid powder, the thermally degradable polymer powder, and the adhesive solution to prepare a slurry, uniformly coat the slurry on the surface of the core-shell structure precursor obtained in the above step by using the fluidized bed coating process, and obtain the composite microsphere active agent with the triple structure of inner core-intermediate layer-outer shell layer after drying and sieving; The thermal cracking inner core contains an organic halide as a second active component, and a thermal initiator for decomposing and producing gas when reaching a first critical temperature T1, sufficient to make the thermal cracking inner core rupture; S2: preparation of a soldering paste The rosin resin is heated to melt and mixed with the solvent, stirred at 60-80°C until completely dissolved to form a uniform solution; after the solution is cooled to 40-50°C, the thixotropic agent is added, dispersed and fully gelled; then the composite microsphere active agent and corrosion inhibitor prepared in step S1 are added, and the mixture is uniformly stirred under vacuum to obtain the soldering paste; S3: mixing of the tin paste The lead-free solder alloy powder and the soldering paste prepared in step S2 are proportionally put into a vacuum stirrer, mixed at a revolution speed of 10-20 rpm for 10-15 minutes, and then mixed at a revolution speed of 30-50 rpm for 20-30 minutes under the condition of a vacuum degree of ≤-0.095 MPa, so as to ensure that the powder and the soldering paste are uniformly mixed and air bubbles are removed, thereby obtaining the low-temperature curing high-reliability lead-free tin paste.

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