Low-stress tinning additive and preparation method and application thereof

Abstract

The invention relates to a low-stress tin plating additive, a preparation method and application thereof, and belongs to the technical field of tin plating additives, the additive comprises a biodegradable organic-inorganic hybrid microcapsule and a solvent, and the microcapsule comprises a core and a shell layer; the core is a substance which is in a liquid state at the tinning service temperature, is not co-soluble with tin and has the surface energy lower than 30mN / m, and the shell layer comprises silicon dioxide and a biodegradable polymer monomer; wherein the core-shell mass ratio of the microcapsule is (1-2): 1, the particle size of the core is 0.5-2 [mu] m, and the thickness of the shell layer is 0.1-0.3 [mu] m; biodegradable hybrid microcapsules and tin are co-deposited, a low-surface-energy liquid core material is released in subsequent heat treatment, and countless micro-stress buffer points are actively formed in a plating layer, so that tin whiskers are effectively removed, and the long-term reliability of electronic components is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of tin plating additive technology, specifically referring to a low-stress tin plating additive, its preparation method, and its application. Background Technology

[0002] Electroplated tin and its alloys are widely used in the manufacturing of electronic components, such as IC lead frames, precision connectors, and printed circuit boards, due to their low toxicity, excellent solderability, and conductivity. Among them, acidic tin plating systems (such as sulfate and methanesulfonate systems) have become the mainstream technology due to their high operating current density and fast deposition rate.

[0003] However, the internal stress of the plating layer is a key indicator affecting the quality and reliability of the tin plating layer, and it is also a long-standing technical problem. During the electrodeposition process, due to factors such as lattice defects, hydrogen penetration, impurity adsorption, and uneven crystal growth, significant internal stress will accumulate inside the plating layer. Excessive internal stress, whether tensile or compressive, will cause a series of serious quality problems: (1) it will cause the plating layer to blister, crack, or even peel off from the substrate, seriously affecting the bonding strength and long-term reliability of the product; (2) it is the main cause of tin whisker growth. Tin whiskers are the spontaneous growth of single-crystal tin structures, which may cause short circuits in micro-pitch circuits, posing a serious threat to modern electronic devices; (3) for ultra-precision or thin-walled electronic components, internal stress may cause micro-deformation of the workpiece, affecting the final assembly accuracy and product performance.

[0004] Currently, the industry commonly uses organic additives, including brighteners, leveling agents, and carrier surfactants, to improve coating quality. These additives primarily reduce internal stress to some extent by adsorbing onto the cathode surface, refining grains, and inhibiting dendrite growth. However, this adsorption-inhibition mechanism has limited effectiveness and significant drawbacks: First, the reduction in internal stress is often unsatisfactory, failing to meet the requirements of high-reliability electronic components; second, excessive use of additives in pursuit of low stress usually sacrifices other properties of the coating, such as deposition rate, hardness, and ductility; and third, the accumulation of additive decomposition products in the plating bath complicates process control.

[0005] Therefore, there is an urgent need in the existing technology for a new additive solution that can fundamentally and effectively reduce the internal stress of the tin plating layer without impairing other comprehensive properties of the plating layer, and is easy to integrate into existing production processes. Summary of the Invention

[0006] In order to overcome some of the problems mentioned in the background above, the present invention provides a low-stress tin plating additive and a method for preparing the same, so as to at least partially solve the above problems.

[0007] According to the technical solution of the present invention, a low-stress tin plating additive is provided, the additive comprising biodegradable organic-inorganic hybrid microcapsules and a solvent, wherein the microcapsules comprise a core and a shell; The core is a liquid material that is insoluble with tin and has a surface energy of less than 30 mN / m at the tin-plating service temperature. The shell consists of silica and biodegradable polymer monomers. The silica and biodegradable polymer monomers form a "ceramic-polymer" shell with silica as the framework and biodegradable polymer monomers as the filler. The shell is stable under electroplating conditions and biodegrades during subsequent heat treatment or long-term storage. The core-shell mass ratio of the microcapsule is (1-2):1, the particle size of the core is 0.5-2μm, and the thickness of the shell is 0.1-0.3μm.

[0008] Preferably, the core comprises one or more of silicone oil, perfluoropolyether oil, and gallium indium tin alloy; The biodegradable polymer monomers include lactic acid-hydroxyacetic acid copolymers.

[0009] Preferably, the microcapsules further include an auxiliary brightening agent and a carrier surfactant; The auxiliary brightener includes one or more of o-methoxybenzaldehyde, vanillin, and o-chlorobenzaldehyde; The carrier surfactant includes one or more of nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, and polyethylene glycol.

[0010] Preferably, the method for preparing the microcapsules includes the following steps: S1. Mix the core with the aqueous phase, add an emulsifier, and perform high-speed shear emulsification to form a stable oil-water primary emulsion; S2. Tetraethyl orthosilicate is added to the primary emulsion to form a silica inorganic framework on the surface of the core material droplets under the action of an alkaline catalyst; S3. Add an organic solvent containing dissolved lactic acid-glycolic acid copolymer and continue the reaction to allow the lactic acid-glycolic acid copolymer to be deposited and crosslinked on the silica framework to form a hybrid shell; S4. The product obtained in step S3 is centrifuged, washed and redispersed to obtain a dispersion of the organic-inorganic hybrid microcapsules.

[0011] Preferably, step S1 includes: adding Span-80 as an oil-soluble surfactant to the core and stirring it evenly at 40-50°C to obtain an oil phase; Tween-80 or sodium dodecyl sulfate is added to deionized water as a water-soluble emulsifier, and then concentrated ammonia solution with a concentration of 25-28% is added as a catalyst. The mixture is stirred until a homogeneous aqueous phase is obtained. The oil phase is slowly added to the aqueous phase to obtain a mixture, and the mixture is subjected to high-speed shearing for 5-20 minutes to obtain a stable oil-water primary emulsion.

[0012] Preferably, step S2 includes: mixing and diluting tetraethyl orthosilicate with an equal volume of anhydrous ethanol to obtain a precursor solution; The precursor solution was slowly added dropwise to the oil-water primary emulsion under continuous stirring at 300-500 rpm, and the reaction was carried out at 30-50℃ for 8-24 hours to form a dense inorganic silica network framework on the surface of the core droplet of the oil-water primary emulsion.

[0013] Preferably, step S3 includes: adding the lactic acid-glycolic acid copolymer to the organic solvent dichloromethane or ethyl acetate for complete dissolution to obtain an organic solvent containing the lactic acid-glycolic acid copolymer; The organic solvent containing the lactic acid-glycolic acid copolymer was slowly added to the product of step S2 under continuous stirring at 300-500 rpm. The mixture was reacted at 35-45°C for 4-12 hours and then cooled to room temperature to obtain a pre-dispersion.

[0014] Preferably, step S4 includes: centrifuging the pre-dispersion at 5000-10000 rpm for 10-20 min to precipitate the microcapsules in the pre-dispersion; Discard the supernatant and wash the precipitate 3-5 times alternately with deionized water and anhydrous ethanol to thoroughly remove unreacted raw materials, emulsifiers and solvents, and obtain purified wet microcapsules. The purified wet microcapsules were redispersed in deionized water to prepare a hybrid microcapsule stock dispersion with a solid content of 5%-15%.

[0015] Preferably, the additive comprises the following raw materials in the following amounts: Microcapsules at 1-10 g / L, auxiliary brightener at 5-20 ml / L, carrier surfactant at 10-30 ml / L, and the remainder water.

[0016] On the other hand, the present invention also provides an application of a low-stress tin plating additive, the application of the aforementioned low-stress tin plating additive in the field of tin plating. The additive is added to the tin plating solution at a rate of 10-100 ml / L. After electroplating, the plating layer is subjected to heat treatment at a temperature of 80-150℃ for a time of 30-120 min.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves effective removal of tin whiskers by co-depositing biodegradable hybrid microcapsules with tin and releasing a low surface energy liquid core material during subsequent heat treatment. This actively forms numerous micro-stress buffer points within the coating, greatly improving the long-term reliability of electronic components.

[0018] This invention utilizes microcapsules as functional components in synergy with traditional brighteners, carriers, and surfactants to achieve not only extremely low internal stress but also a fully glossy, mirror-like coating appearance, while maintaining a dense coating with excellent corrosion resistance.

[0019] This invention, by designing the type of core material of the microcapsule, such as silicone oil, perfluoropolyether oil, and the viscosity, wall thickness, and degradation rate of the core material, can actively control the degree and timing of stress release. It can be directly added to existing acidic or methanesulfonate tin plating systems without changing existing electroplating equipment and main processes, requiring only the addition of a standard heat treatment step. Detailed Implementation

[0020] The technical solutions in the embodiments will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection.

[0021] This invention provides a low-stress tin plating additive, the additive comprising biodegradable organic-inorganic hybrid microcapsules and a solvent, the microcapsules comprising a core and a shell; The core is a liquid material that is insoluble in tin and has a surface energy of less than 30 mN / m at the tin-plating service temperature, and the shell layer includes silicon dioxide and biodegradable polymer monomers. The core-shell mass ratio of the microcapsule is (1-2):1, the particle size of the core is 0.5-2μm, and the thickness of the shell is 0.1-0.3μm.

[0022] In a further embodiment of this invention, the core comprises one or more of silicone oil, perfluoropolyether oil, and gallium indium tin alloy; The biodegradable polymer monomers include lactic acid-hydroxyacetic acid copolymers.

[0023] In a further embodiment of this example, the microcapsule further includes an auxiliary brightening agent and a carrier surfactant; The auxiliary brightener includes one or more of o-methoxybenzaldehyde, vanillin, and o-chlorobenzaldehyde; The carrier surfactant includes one or more of nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, and polyethylene glycol.

[0024] In a further embodiment of this example, the method for preparing the microcapsules includes the following steps: S1. Mix the core with the aqueous phase, add an emulsifier, and perform high-speed shear emulsification to form a stable oil-water primary emulsion; S2. Tetraethyl orthosilicate is added to the primary emulsion to form a silica inorganic framework on the surface of the core material droplets under the action of an alkaline catalyst; S3. Add an organic solvent containing dissolved lactic acid-glycolic acid copolymer and continue the reaction to allow the lactic acid-glycolic acid copolymer to be deposited and crosslinked on the silica framework to form a hybrid shell; S4. The product obtained in step S3 is centrifuged, washed and redispersed to obtain a dispersion of the organic-inorganic hybrid microcapsules.

[0025] In a further embodiment of this example, step S1 includes: adding Span-80 as an oil-soluble surfactant into the core, stirring evenly at 40-50°C to obtain an oil phase; Tween-80 or sodium dodecyl sulfate is added to deionized water as a water-soluble emulsifier, and then concentrated ammonia solution with a concentration of 25-28% is added as a catalyst. The mixture is stirred until a homogeneous aqueous phase is obtained. The oil phase is slowly added to the aqueous phase to obtain a mixture, and the mixture is subjected to high-speed shearing for 5-20 minutes to obtain a stable oil-water primary emulsion.

[0026] In a further embodiment of this example, step S2 includes: mixing and diluting tetraethyl orthosilicate with an equal volume of anhydrous ethanol to obtain a precursor solution; The precursor solution was slowly added dropwise to the oil-water primary emulsion under continuous stirring at 300-500 rpm, and the reaction was carried out at 30-50℃ for 8-24 hours to form a dense inorganic silica network framework on the surface of the core droplet of the oil-water primary emulsion.

[0027] In a further embodiment of this example, step S3 includes: adding the lactic acid-glycolic acid copolymer to the organic solvent dichloromethane or ethyl acetate for complete dissolution, to obtain an organic solvent containing the lactic acid-glycolic acid copolymer; The organic solvent containing the lactic acid-glycolic acid copolymer was slowly added to the product of step S2 under continuous stirring at 300-500 rpm. The mixture was reacted at 35-45°C for 4-12 hours and then cooled to room temperature to obtain a pre-dispersion.

[0028] In a further embodiment of this example, step S4 includes: centrifuging the pre-dispersion liquid at a speed of 5000-10000 rpm for 10-20 min to precipitate the microcapsules in the pre-dispersion liquid. Discard the supernatant and wash the precipitate 3-5 times alternately with deionized water and anhydrous ethanol to thoroughly remove unreacted raw materials, emulsifiers and solvents, and obtain purified wet microcapsules. The purified wet microcapsules were redispersed in deionized water to prepare a hybrid microcapsule stock dispersion with a solid content of 5%-15%.

[0029] In a further embodiment of this example, the additive comprises the following raw materials in the following amounts: Microcapsules at 1-10 g / L, auxiliary brightener at 5-20 ml / L, carrier surfactant at 10-30 ml / L, and the remainder water.

[0030] On the other hand, embodiments of the present invention also provide an application of a low-stress tin plating additive, the application of the aforementioned low-stress tin plating additive in the field of tin plating. The additive is added to the tin plating solution at a rate of 10-100 ml / L. After electroplating, the plating layer is subjected to heat treatment at a temperature of 80-150℃ for a time of 30-120 min.

[0031] It should be noted that this invention is no longer limited to the traditional theory of organic molecule adsorption, but introduces the concept of "in-situ stress buffering." A biodegradable organic-inorganic hybrid microcapsule is designed as the core component of the additive. These microcapsules are co-deposited with tin ions during electroplating and embedded within the plating layer. Subsequently, during subsequent heat treatment or natural storage, the microcapsule shell degrades, releasing the low surface energy liquid substance inside, forming tiny "cavities" or "buffer layers" at the grain boundaries of the plating layer, thereby actively releasing and offsetting internal stress.

[0032] Because of their extremely small particle size and hydrophilic surface modification, microcapsules can be stably suspended in the plating solution. Under the action of the electroplating electric field, these negatively charged or neutral microcapsules are carried to the cathode (workpiece) surface and "encapsulated" by rapidly growing tin crystals, becoming part of the plating layer.

[0033] After electroplating, a large number of intact microcapsules are uniformly distributed inside the coating. During necessary heat treatments such as hydrogen removal, aging, or natural storage, the organic layer in the hybrid shell begins to slowly hydrolyze, and the shell strength decreases.

[0034] After the biodegradable polymer monomers in the shell degrade, the low surface energy liquid core material, such as silicone oil, is released. Since silicone oil does not wet tin, it forms a micron-scale "soft cavity" at the original microcapsule location. This cavity effectively absorbs and releases tensile or compressive stress in the surrounding tin lattice, similar to adding elastic particles to concrete. Simultaneously, the released silicone oil acts as a lubricant at grain boundaries, allowing for minute slippage between grains and further relaxing stress. This fundamentally alters the stress state of the coating, transforming it from a continuous, high-energy stress state into a low-stress state separated by numerous micro-buffer zones.

[0035] Preparation Example 1: S1. 1g of Span-80 as an oil-soluble surfactant was added to 100g of dimethyl silicone oil and stirred evenly at 45°C to obtain the oil phase; Add 5g of Tween-80 as a water-soluble emulsifier to 1000ml of deionized water, then add 15ml of concentrated ammonia solution with a concentration of 28% as a catalyst, and stir evenly to obtain an aqueous phase; The oil phase is slowly added to the aqueous phase to obtain a mixture, and the mixture is subjected to high-speed shearing at 12000 rpm for 10 min to obtain a stable oil-water primary emulsion.

[0036] S2. Mix and dilute 30g of tetraethyl orthosilicate with an equal volume of anhydrous ethanol to obtain a precursor solution; The precursor solution was slowly added dropwise to an oil-water primary emulsion under continuous stirring at 400 rpm, and the reaction was carried out at 40°C for 12 h to form a dense inorganic silica network framework on the surface of the core droplet of the oil-water primary emulsion.

[0037] S3. Add 15g of lactic acid-glycolic acid copolymer (where the molar ratio of lactic acid to glycolic acid is 1:1) to 150ml of organic solvent dichloromethane for complete dissolution to obtain an organic solvent containing lactic acid-glycolic acid copolymer; The organic solvent containing the lactic acid-glycolic acid copolymer was slowly added to the product of step S2 under continuous stirring at 400 rpm, and the reaction was carried out at 40°C for 6 h. The mixture was then cooled to room temperature to obtain a pre-dispersion.

[0038] S4. Centrifuge the pre-dispersion at 8000 rpm for 15 min to precipitate the microcapsules in the pre-dispersion. The supernatant was discarded, and the precipitate was washed four times alternately with deionized water and anhydrous ethanol to thoroughly remove unreacted raw materials, emulsifiers and solvents, and purified wet microcapsules were obtained. The purified wet microcapsules were redispersed in deionized water to prepare a hybrid microcapsule stock dispersion with a solid content of 10%.

[0039] Preparation Example 2: The difference from Preparation Example 1 is that the core material described in S1 is perfluoropolyether oil.

[0040] Preparation Example 3: The difference from Preparation Example 1 is that: S2. Mix 50g of tetraethyl orthosilicate with an equal volume of anhydrous ethanol to dilute and obtain a precursor solution; The precursor solution was slowly added dropwise to an oil-water primary emulsion under continuous stirring at 400 rpm, and the reaction was carried out at 40°C for 12 h to form a dense inorganic silica network framework on the surface of the core droplet of the oil-water primary emulsion.

[0041] S3. Add 25g of lactic acid-glycolic acid copolymer (where the molar ratio of lactic acid to glycolic acid is 1:1) to 150ml of organic solvent dichloromethane for complete dissolution to obtain an organic solvent containing lactic acid-glycolic acid copolymer; The organic solvent containing the lactic acid-glycolic acid copolymer was slowly added to the product of step S2 under continuous stirring at 400 rpm, and the reaction was carried out at 40°C for 6 h. The mixture was then cooled to room temperature to obtain a pre-dispersion.

[0042] The following experimental examples used the microcapsules obtained in Preparation Examples 1-3 to prepare tin plating solutions with different ratios of additive compositions and plating solutions. All plating solutions were prepared based on a base solution of 60 g / L stannous methanesulfonate and 120 mL / L methanesulfonic acid.

[0043] Experimental Example 1: The additive was obtained by mixing 0.1 parts of the dispersion obtained in Preparation Example 1, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0044] Add 50 ml of the additive to 1 L of the base plating solution, so that the actual concentration of microcapsules in the plating solution is 0.05 g / L.

[0045] Experimental Example 2: The additive was obtained by mixing 0.5 parts of the dispersion obtained in Preparation Example 1, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0046] Add 20 ml of the additive to 1 L of the base plating solution, so that the actual concentration of microcapsules in the plating solution is 0.1 g / L.

[0047] Experimental Example 3: The additive was obtained by mixing 0.5 parts of the dispersion obtained in Preparation Example 1, 2 parts of o-methoxybenzaldehyde, 1.5 parts of nonylphenol polyoxyethylene ether, and the balance water.

[0048] Add 15 ml of the additive to 1 L of the base plating solution, so that the actual concentration of microcapsules in the plating solution is 0.15 g / L.

[0049] Experimental Example 4: The additive was obtained by mixing 0.1 parts of the dispersion obtained in Preparation Example 1, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0050] Take 20ml of the additive and add it to 1L of the base plating solution, so that the actual concentration of microcapsules in the plating solution is 0.1g / L.

[0051] Experimental Example 5: The additive was obtained by mixing 0.5 parts of the dispersion obtained in Preparation Example 2, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0052] Take 20 ml of the additive and add it to 1 L of the base plating solution. This means that the actual concentration of microcapsules in the plating solution is 0.15 g / L.

[0053] Experimental Example 6: The additive was obtained by mixing 0.5 parts of the dispersion obtained in Preparation Example 3, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0054] Take 20 ml of the additive and add it to 1 L of the base plating solution. This means that the actual concentration of microcapsules in the plating solution is 0.15 g / L.

[0055] Comparative Example 1: The additive was prepared by uniformly mixing 1 part o-methoxybenzaldehyde, 2 parts nonylphenol polyoxyethylene ether, and the balance water. 20 ml of the additive was added to 1 L of the base plating solution.

[0056] Comparative Example 2: The additive was obtained by mixing 5 parts of the dispersion obtained in Preparation Example 1, 1 part of o-methoxybenzaldehyde, 2 parts of nonylphenol polyoxyethylene ether and the balance water.

[0057] Add 40 ml of the additive to 1 L of the base plating solution, so that the actual concentration of microcapsules in the plating solution is 2 g / L.

[0058] Electroplating was performed on the plating solutions of Experimental Examples 1-6 and Comparative Examples 1 and 2, respectively, with a cathode current density of 2 A / dm³. 2 The electroplating temperature was 25℃, the electroplating time was 30 minutes, and after electroplating, the plating was heated at 120℃ for 1 hour. Then, the plating layer was tested as follows: The internal stress of the coating was measured using X-ray diffraction stress analyzer. Accelerated aging was performed at 55°C and 85% relative humidity. After 1000 hours, the number of tin whiskers exceeding 10 μm in length per unit area was observed and counted using scanning electron microscopy. The brightness and uniformity of the coating were visually observed as coating performance. The time to the appearance of colored spots was recorded using a potassium ferricyanide drop test to indicate the porosity of the coating. The experimental data are shown in Table 1 below.

[0059] Table 1

[0060] The internal stress values ​​of all Experimental Examples 1-6 of this invention were significantly lower than those of Comparative Examples 1 and 2. Under the optimal ratio in Experimental Example 2, the internal stress decreased dramatically from -42.1 MPa in Comparative Example 1 to -5.8 MPa. Correspondingly, the number of tin whiskers also decreased sharply. The number of tin whiskers in Experimental Example 2 was only 2 / mm², far lower than the 90 / mm² in Comparative Example 1, thus strongly demonstrating that the "in-situ stress buffering" mechanism of the microcapsules is the fundamental reason for reducing stress and suppressing tin whiskers.

[0061] Experiments 1-3 show that the microcapsule content is optimal within the range defined by this invention. Too low a content weakens the effect, while too high a content, although lowering stress, affects the coating's gloss and may lead to uneven microstructure. Experiment 4 shows that, provided the core microcapsules are added reasonably within the range defined by this invention, appropriately adjusting the ratio of brightener and carrier can still achieve excellent low-stress effects. Comparative Example 2 shows that the amount of microcapsules added must be within the range defined by this invention; exceeding this range, while further reducing stress, severely sacrifices the coating's appearance quality and density.

[0062] Experiment 5 shows that, due to the lower surface tension of perfluoropolyether oil compared to silicone oil, it can spread more effectively at the tin grain boundaries after being released from the microcapsules, forming a superior stress-buffering interface. However, its slightly poorer compatibility with the plating solution system slightly affects the uniformity of the electrocrystallization process. Experiment 6 shows that excessively thick wall material results in higher internal stress and a greater number of tin whiskers, which cannot be fully degraded under the set heat treatment conditions of 120℃ / 60min. This leads to most microcapsules failing to effectively release the core material, causing the "in-situ stress buffering" mechanism to fail. The heat treatment time needs to be increased to achieve the same effect as the other experimental examples. However, the coating appearance remains bright, therefore, thickening the wall material has no negative impact on the electrodeposition process itself.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-stress tin plating additive, characterized in that, The additive includes biodegradable organic-inorganic hybrid microcapsules and a solvent, wherein the microcapsules include a core and a shell; The core is a liquid material that is insoluble with tin and has a surface energy of less than 30 mN / m at the tin-plating service temperature. The shell consists of silica and biodegradable polymer monomers. The silica and biodegradable polymer monomers form a "ceramic-polymer" shell with silica as the framework and biodegradable polymer monomers as the filler. The shell is stable under electroplating conditions and biodegrades during subsequent heat treatment or long-term storage. The core-shell mass ratio of the microcapsule is (1-2):1, the particle size of the core is 0.5-2μm, and the thickness of the shell is 0.1-0.3μm.

2. The low-stress tin plating additive according to claim 1, characterized in that, The core comprises one or more of silicone oil, perfluoropolyether oil, and gallium indium tin alloy; The biodegradable polymer monomers include lactic acid-hydroxyacetic acid copolymers.

3. The low-stress tin plating additive according to claim 1, characterized in that, The microcapsules also include auxiliary brighteners and carrier surfactants; The auxiliary brightener includes one or more of o-methoxybenzaldehyde, vanillin, and o-chlorobenzaldehyde; The carrier surfactant includes one or more of nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, and polyethylene glycol.

4. A method for preparing a low-stress tin plating additive according to any one of claims 1-3, characterized in that, The method for preparing the microcapsules includes the following steps: S1. Mix the core with the aqueous phase, add an emulsifier, and perform high-speed shear emulsification to form a stable oil-water primary emulsion; S2. Tetraethyl orthosilicate is added to the primary emulsion to form a silica inorganic framework on the surface of the core material droplets under the action of an alkaline catalyst; S3. Add an organic solvent containing dissolved lactic acid-glycolic acid copolymer and continue the reaction to allow the lactic acid-glycolic acid copolymer to be deposited and crosslinked on the silica framework to form a hybrid shell; S4. The product obtained in step S3 is centrifuged, washed and redispersed to obtain a dispersion of the organic-inorganic hybrid microcapsules.

5. The method for preparing the low-stress tin plating additive according to claim 4, characterized in that, Step S1 includes: adding Span-80 as an oil-soluble surfactant into the core, stirring evenly at 40-50°C to obtain an oil phase; Tween-80 or sodium dodecyl sulfate is added to deionized water as a water-soluble emulsifier, and then concentrated ammonia solution with a concentration of 25-28% is added as a catalyst. The mixture is stirred until a homogeneous aqueous phase is obtained. The oil phase is slowly added to the aqueous phase to obtain a mixture, and the mixture is subjected to high-speed shearing for 5-20 minutes to obtain a stable oil-water primary emulsion.

6. The method for preparing the low-stress tin plating additive according to claim 4, characterized in that, Step S2 includes: mixing and diluting tetraethyl orthosilicate with an equal volume of anhydrous ethanol to obtain a precursor solution; The precursor solution was slowly added dropwise to the oil-water primary emulsion under continuous stirring at 300-500 rpm, and the reaction was carried out at 30-50℃ for 8-24 hours to form a dense inorganic silica network framework on the surface of the core droplet of the oil-water primary emulsion.

7. The method for preparing the low-stress tin plating additive according to claim 4, characterized in that, Step S3 includes: adding the lactic acid-glycolic acid copolymer to the organic solvent dichloromethane or ethyl acetate for complete dissolution, to obtain an organic solvent containing the lactic acid-glycolic acid copolymer; The organic solvent containing the lactic acid-glycolic acid copolymer was slowly added to the product of step S2 under continuous stirring at 300-500 rpm. The mixture was reacted at 35-45°C for 4-12 hours and then cooled to room temperature to obtain a pre-dispersion.

8. The method for preparing the low-stress tin plating additive according to claim 4, characterized in that, Step S4 includes: centrifuging the pre-dispersion liquid at a speed of 5000-10000 rpm for 10-20 min to precipitate the microcapsules in the pre-dispersion liquid; Discard the supernatant and wash the precipitate 3-5 times alternately with deionized water and anhydrous ethanol to thoroughly remove unreacted raw materials, emulsifiers and solvents, and obtain purified wet microcapsules. The purified wet microcapsules were redispersed in deionized water to prepare a hybrid microcapsule stock dispersion with a solid content of 5%-15%.

9. The method for preparing the low-stress tin plating additive according to claim 4, characterized in that, The additive comprises the following raw materials in the following proportions: Microcapsules at 1-10 g / L, auxiliary brightener at 5-20 ml / L, carrier surfactant at 10-30 ml / L, and the remainder water.

10. The application of a low-stress tin plating additive, characterized in that, Application of the low-stress tin plating additive according to any one of claims 1-3 in the field of tin plating; The additive is added to the tin plating solution at a rate of 10-100 ml / L. After electroplating, the plating layer is subjected to heat treatment at a temperature of 80-150℃ for a time of 30-120 min.

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