Kovar alloy electroplating method

Abstract

The invention discloses a Kovar alloy electroplating method. The method comprises the steps that a Kovar alloy base material is pretreated; the pretreated kovar alloy base material is immersed in an acidic copper sulfate plating solution for electrolytic copper plating, so that a copper plating layer is formed on the surface of the kovar alloy base material; immersing the kovar alloy on which the copper plating layer is formed into a chemical nickel plating solution so as to form a nickel plating layer on the surface of the copper plating layer; and immersing the kovar alloy base material on which the nickel plating layer is formed into a nanogold seed crystal solution so as to form a soft gold plating layer on the surface of the nickel plating layer. The coating quality of the kovar alloy can be improved.

Description

Technical Field

[0001] This application belongs to the field of electroplating technology, and particularly relates to a kovar alloy electroplating method. Background Art

[0002] Traditionally, for electroplating treatment on a kovar alloy substrate, electroless nickel is first plated, and then a layer of soft gold is plated on the nickel layer. This process flow is widely used to improve the corrosion resistance and appearance quality of products. However, the above process exposes various defects and deficiencies in the production of bonded products, mainly including: the traditional plating types have a single process, making it difficult to effectively fill the pores on the substrate surface, resulting in poor surface finish of the bonding surface; improper control of the coating thickness, usually tending to the lower limit of the range, causing the nickel layer to be too hard and the soft gold coating to be too thin, seriously affecting the bonding strength; in addition, the differences between different client devices and operators further exacerbate the problem of poor bonding. Coupled with the dependence on external processing during the electroplating process, it is difficult to control the coating quality. These factors together reduce the user satisfaction and market competitiveness of the products. Summary of the Invention

[0003] Aiming at the deficiencies in the prior art, the main purpose of this application is to provide a kovar alloy electroplating method, aiming to improve the coating quality of kovar alloy.

[0004] To achieve the above object, this application provides the following technical solutions:

[0005] A kovar alloy electroplating method, the method includes: pre-treating the kovar alloy substrate; immersing the pre-treated kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper coating on the surface of the kovar alloy substrate; immersing the kovar alloy with the copper coating formed thereon in an electroless nickel plating solution to form a nickel coating on the surface of the copper coating; immersing the kovar alloy substrate with the nickel coating formed thereon in a nano gold seed solution to form a soft gold coating on the surface of the nickel coating.

[0006] Optionally, the pre-treating the kovar alloy substrate includes: cleaning the kovar alloy substrate; activating the cleaned kovar alloy substrate.

[0007] Optionally, the activating the cleaned kovar alloy substrate includes: activating the cleaned kovar alloy substrate by configuring a mixed acid solution containing carbon-coated platinum nanoparticles.

[0008] Optionally, immersing the pretreated Kovar alloy substrate into an acidic copper sulfate plating solution for electrolytic copper plating to form a copper coating on the surface of the Kovar alloy substrate includes: immersing the pretreated Kovar alloy substrate into the acidic copper sulfate plating solution, and performing electrolytic copper plating through a periodic pulsed current to form a primary copper coating on the surface of the Kovar alloy substrate; performing passivation treatment on the Kovar alloy substrate with the primary copper coating formed thereon to form a copper coating on the surface of the Kovar alloy substrate.

[0009] Optionally, the passivation treatment on the Kovar alloy substrate with the primary copper coating formed thereon includes: cleaning and drying the Kovar alloy substrate with the primary copper coating formed thereon; immersing the dried Kovar alloy substrate with the primary copper coating formed thereon into a passivation solution; performing ultraviolet irradiation on the Kovar alloy substrate with the primary copper coating formed thereon after being immersed in the passivation solution; rinsing and drying the Kovar alloy substrate with the primary copper coating formed thereon after ultraviolet irradiation to obtain the Kovar alloy substrate with a copper coating formed thereon.

[0010] Optionally, immersing the pretreated Kovar alloy substrate into an acidic copper sulfate plating solution for electrolytic copper plating to form a copper coating on the surface of the Kovar alloy substrate further includes: performing sandblasting treatment on the copper coating.

[0011] Optionally, immersing the Kovar alloy with a copper coating formed thereon into a electroless nickel plating solution to form a nickel coating on the surface of the copper coating includes: applying a pre-nickel coating on the surface of the copper coating; immersing the Kovar alloy substrate with the pre-nickel coating applied thereon into the electroless nickel plating solution for spontaneous reaction to form a nickel coating on the surface of the copper coating.

[0012] Optionally, the thickness of the pre-nickel coating is any value from 0.5 μm to 0.8 μm.

[0013] Optionally, the electroless nickel plating solution includes nickel sulfate, sodium hypophosphite, complexing agent, stabilizer and buffer.

[0014] Optionally, immersing the Kovar alloy substrate with a nickel coating formed thereon into a nano-gold seed solution to form a soft gold coating on the surface of the nickel coating includes: performing activation treatment on the Kovar alloy substrate with a nickel coating formed thereon under a protective atmosphere; immersing the activated Kovar alloy substrate with a nickel coating formed thereon into the nano-gold seed solution to enable nano-gold particles to adsorb on the surface of the nickel coating; immersing the nickel coating adsorbed with nano-gold particles into a plating solution, and drying after immersion; performing annealing treatment and cooling on the dried nickel coating adsorbed with nano-gold particles; immersing the cooled nickel coating adsorbed with nano-gold particles into a hydrophobic solution, and drying after immersion to obtain the Kovar alloy substrate with a soft gold coating formed thereon.

[0015] This application can bring the following beneficial effects:

[0016] This application can solve problems such as rough bonding surfaces, easy peeling of coatings, and insufficient bonding strength in traditional processes by sequentially plating copper, nickel, and soft gold on the surface of a Kovar alloy substrate, combined with various processes such as nanoparticle activation, pulse electroplating, passivation treatment, pre-nickel plating, and nano-gold seeds. This can significantly improve the coating quality, and further enhance the reliability and market competitiveness of the product. Brief Description of the Drawings

[0017] Figure 1 It is a schematic flow chart of a Kovar alloy electroplating method provided by an embodiment of this application. Detailed Embodiments

[0018] Next, the technical solutions in the embodiments of this application will be clearly and completely described in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of this application.

[0019] It should be noted that all directional indications (such as up, down, left, right, front, back...) in the embodiments of this application are only used to explain the relative positional relationship and movement conditions between components in a specific posture (as shown in the drawings). If the specific posture changes, the directional indications will also change accordingly.

[0020] In this application, unless otherwise clearly specified and limited, terms such as "connection" and "fixation" shall be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal connection of two components or the interaction relationship between two components, unless otherwise clearly limited. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0021] In addition, if the descriptions such as "first" and "second" are involved in the embodiments of the present application, the descriptions of "first", "second", etc. are only for descriptive purposes and cannot be construed as indicating or implying their relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In addition, the meaning of "and / or" appearing throughout the text includes three parallel scenarios. Taking "A and / or B" as an example, it includes Scenario A, or Scenario B, or the scenario where both A and B are satisfied simultaneously. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present application.

[0022] Figure 1 A kovar alloy electroplating method provided by an exemplary embodiment of the present application is as Figure 1 described, and the method includes the following steps:

[0023] S1: Pretreat the kovar alloy substrate;

[0024] S2: Immerse the pretreated kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper coating on the surface of the kovar alloy substrate;

[0025] S3: Immerse the kovar alloy with the formed copper coating in a electroless nickel plating solution to form a nickel coating on the surface of the copper coating;

[0026] S4: Immerse the kovar alloy substrate with the formed nickel coating in a nano gold seed solution to form a soft gold coating on the surface of the nickel coating.

[0027] Through systematic plating treatment, the present application has achieved a significant improvement in the surface performance of the kovar alloy substrate: First, the pretreatment stage has laid a clean and activated base for the subsequent coating combination; the electrolytic copper plating process uses an acidic copper sulfate plating solution to construct a dense and highly conductive copper coating on the substrate surface, which not only enhances the corrosion resistance of the substrate but also provides a good bonding interface for the subsequent coatings; the electroless nickel plating step further strengthens the corrosion resistance and wear resistance of the coating through the formation of the nickel coating, and at the same time, a stable gradient transition structure is constructed by using the good compatibility of nickel with copper and gold; finally, the treatment with the nano gold seed solution generates a soft gold coating with fine grains and extremely high purity on the surface of the nickel coating, which not only improves the conductivity and solderability of the coating but also optimizes the surface microstructure through the nano seed technology, significantly improving the flatness and stability of the coating. The synergistic effect of this series of processes finally forms a composite coating system with high conductivity, strong bonding force, excellent corrosion resistance and stability, effectively extending the service life of the kovar alloy substrate and expanding its application range.

[0028] In another exemplary embodiment, in step S1, the pretreatment of the Kovar alloy substrate includes the following steps:

[0029] S11: Clean the Kovar alloy substrate by ultrasonic wave to remove grease, dust and other contaminants on its surface;

[0030] S12: Activate the cleaned Kovar alloy substrate to increase its surface activity to promote good adhesion of the subsequent coating.

[0031] In this step, the activation treatment of the cleaned Kovar alloy substrate includes the following steps:

[0032] S121: Configure a mixed acid solution composed of nitric acid (10 vol%) and hydrofluoric acid (2 vol%), and uniformly disperse carbon-coated platinum nanoparticles (0.1 wt%, particle size 5 nm to 10 nm) in the mixed acid solution to obtain a mixed acid solution containing carbon-coated platinum nanoparticles;

[0033] S122: Immerse the cleaned Kovar alloy substrate in the mixed acid solution containing carbon-coated platinum nanoparticles for 60 seconds to 90 seconds. Meanwhile, ensure uniform contact between the mixed acid solution and the Kovar alloy substrate by magnetic stirring (rotation speed 200 rpm);

[0034] In this step, carbon-coated platinum nanoparticles (Pt@C NPs) exhibit excellent dual functions of catalysis and protection in the pretreatment of the Kovar alloy substrate: in the mixed acid solution, their unique nanostructure and carbon coating layer work together. By reducing the activation energy of the oxide film reduction reaction, it significantly accelerates the dissolution of oxides (such as Fe2O3, NiO) and impurities on the Kovar alloy surface, precisely stripping the oxide layer. At the same time, using the steric hindrance effect of the carbon layer and the electron regulation ability of platinum, it effectively inhibits the excessive corrosion of the substrate body by the mixed acid, ensuring a high degree of uniformity of the substrate surface morphology; in the etching stage, Pt@C NPs are physically adsorbed into the micropore defects on the substrate surface and form nanoscale anchor points through chemical bonding (such as hydroxyl groups, carboxyl groups) between the carbon layer and the surface functional groups of the substrate. This dual action mechanism of "mechanical meshing + chemical bonding" greatly enhances the interfacial bonding strength between the coating and the substrate. At the same time, the flexibility of the carbon layer and the anti-diffusion characteristics of platinum cooperate to inhibit interfacial interdiffusion and thermal stress accumulation, significantly improving the long-term stability of the coating in harsh environments such as high temperature and high humidity, providing key technical support for the high-performance application of Kovar alloy in fields such as electronic packaging and aerospace.

[0035] S123: Take out the Kovar alloy substrate from the mixed acid solution, rinse it with deionized water, and then dry it with nitrogen to avoid secondary oxidation on the surface of the Kovar alloy substrate;

[0036] S124: Place the dried kovar alloy substrate in a plasma chamber with dual-frequency pulses (low frequency 40 kHz + high frequency 13.56 MHz) for plasma activation. Among them, the ions in the low-frequency plasma can obtain high kinetic energy under the acceleration of the electric field. By bombarding the surface of the kovar alloy substrate, on the one hand, it can effectively strip the residual organic matter and microparticles on the surface of the kovar alloy substrate, and on the other hand, it can locally generate instantaneous high temperature on the substrate surface, which is conducive to promoting the pyrolysis or oxidative decomposition of organic matter, thereby thoroughly cleaning the substrate surface. The high-energy particles in the high-frequency plasma bombard the substrate surface, which can stimulate the lattice reconstruction on the surface of the kovar alloy substrate, form active sites rich in defects, and thus enhance the chemical adsorption capacity of the subsequent coating.

[0037] In summary, the low-frequency plasma can provide high-energy ions to strip the macroscopic pollutants on the surface of the kovar alloy substrate, and the high-frequency plasma can achieve atomic-level surface modification through high-density plasma. The combination of the two can ensure the full-scale surface activation of the kovar alloy substrate from macroscopic to microscopic.

[0038] S125: Immerse the plasma-activated kovar alloy substrate in a thiol-ethylamine (HS-CH2-CH2-NH2) ethanol solution, soak it at a constant temperature of 50 °C for 10 minutes while stirring, take it out and purge it with nitrogen to remove the unbonded molecules on the substrate surface. Then, anneal it at 150 °C for 5 minutes under a protective atmosphere (such as nitrogen), and after the annealing is completed, cool it naturally to room temperature.

[0039] In this step, the sulfur atom in thiol-ethylamine can react with the metal atoms (such as Fe, Ni, Co) on the surface of the kovar alloy substrate to form strong chemical bonds:

[0040]

[0041] where M is any one of Fe, Ni, Co, and R is the -CH2-CH2-NH2 group.

[0042] Thus, it can bond with the active sites on the surface of the kovar alloy substrate to form a single-molecule self-assembled layer.

[0043] In addition, the amino group has a strong polarity and can spontaneously arrange towards the outside in the ethanol solvent to form a functional group that is hydrophilic to the coating, and pre-construct a chemical bonding interface with the metal atoms of the subsequent coating (such as copper, nickel) through coordination, realizing molecular-level interface matching, thereby reducing the coating interface energy barrier.

[0044] Furthermore, by soaking and treating at a constant temperature of 50 °C, it can promote the cross-linking reaction between thiol-ethylamine molecules, that is:

[0045]

[0046] The formation of disulfide bonds (-SS-) can enhance the cohesion of the molecular layer, thereby helping to improve the shear strength of the substrate surface (>50 MPa).

[0047] In addition, through annealing treatment, the difference in thermal expansion coefficient between the Kovar alloy substrate and the molecular layer can be released through plastic deformation, thereby reducing the risk of subsequent coating peeling.

[0048] In another exemplary embodiment, in step S2, immersing the pretreated Kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper plating layer on the surface of the Kovar alloy substrate comprises the following steps:

[0049] S21: The pretreated Kovar alloy substrate is immersed in an acidic copper sulfate plating solution, and electrolytic copper plating is performed by periodic pulse current (the ratio of the forward current duration to the reverse current duration is 3:1) to form a copper initial plating layer on the surface of the Kovar alloy substrate. In this process, the ambient temperature (for example, the temperature is controlled at 40°C to 50°C) and the pH value (0.8 to 1.5) need to be precisely controlled to ensure that the copper initial plating layer is uniform and dense.

[0050] In this step, compared with DC electroplating, pulse electroplating can improve the quality of the coating while reducing the porosity, allowing copper atoms to more effectively fill the tiny pits and holes on the surface of the Kovar alloy substrate.

[0051] It should be noted that the low pH environment (strong acidity) can inhibit side reactions, greatly reduce the hydrogen ion reduction and hydrogen evolution reaction, avoid the hydrolysis of copper ions under alkaline conditions, and prevent the copper plating from being mixed with oxides; secondly, it can optimize ion migration (the acidic environment can increase the conductivity of the electrolyte, enhance the migration rate of copper ions (Cu2+), and ensure that copper ions can be quickly replenished to the surface of the substrate under pulse current). In addition, according to experiments, if the pH is <0.8, the brittleness of the copper plating will increase and the bonding strength with the subsequent plating will decrease; if the pH is >1.5, hydrogen evolution will intensify, resulting in an increase in the porosity of the copper plating.

[0052] S22: Passivating the Kovar alloy substrate on which the copper primary plating layer is formed, so as to form a copper plating layer on the surface of the Kovar alloy substrate.

[0053] In this step, the passivation treatment is performed on the Kovar alloy substrate on which the copper primary plating layer is formed to form a copper plating layer on the surface of the Kovar alloy substrate, which includes the following steps:

[0054] S221: Immerse the Kovar alloy substrate with the initial copper plating layer in deionized water, and add 0.1 wt% sodium citrate as a chelating agent. After the immersion, ultrasonically clean the Kovar alloy substrate with the initial copper plating layer to remove the oxide layer and residual contaminants on the surface of the initial copper plating layer. Then, dry the surface of the initial copper plating layer with nitrogen to avoid secondary oxidation;

[0055] S222: Immerse the dried Kovar alloy substrate with the initial copper plating layer in a passivation solution (including phytic acid (5 vol%, bio-based chelating agent), nano-graphene oxide dispersion (0.05 wt%, sheet size < 100 nm), silane coupling agent (KH-550, 1 wt%), deionized water (the balance)), and soak it at a temperature of 50 °C for 5 minutes. At the same time, apply a pulsed current to the initial copper plating layer (forward current density 2 A / dm 2 , reverse current density 0.5 A / dm 2 ) to promote the directional adsorption of nano-graphene oxide on the surface of the initial copper plating layer.

[0056] In this step, the phosphate group in phytic acid can undergo a chelation reaction with copper ions, thereby forming a stable three-dimensional network complex film. This complex film covers the micropores on the surface of the initial copper plating layer, can block the direct contact between oxygen, moisture and copper, and avoid oxidation reactions.

[0057] Driven by the pulsed current, nano-graphene oxide migrates to the anode due to the negative charge on its surface and can be evenly adsorbed on the copper surface, thereby increasing the anti-permeability of the initial copper plating layer.

[0058] The silane coupling agent can hydrolyze to generate silanol (Si-OH), which condenses with the hydroxyl group (Cu-OH) on the surface of the initial copper plating layer to form a Si-O-Cu covalent bond, thereby enhancing the binding force between the phytic acid-graphene composite film and the surface of the initial copper plating layer.

[0059] S223: Take out the Kovar alloy substrate with the initial copper plating layer from the passivation solution and irradiate it with ultraviolet light for 3 minutes to activate the photocatalytic activity of the nano-graphene oxide adsorbed on the surface of the initial copper plating layer, promote the crosslinking and curing of the passivation layer, and at the same time repair local defects.

[0060] S224: Rinse the irradiated Kovar alloy substrate with the initial copper plating layer with deionized water to remove the unreacted passivation solution components on its surface, and then dry it at 60 °C for 10 minutes to form a stable passivation film on the surface of the initial copper plating layer, and a copper plating layer can be formed on the surface of the Kovar alloy substrate.

[0061] In another exemplary embodiment, in step S2, when the pretreated Kovar alloy substrate is immersed in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper plating layer on the surface of the Kovar alloy substrate, the following steps are further included:

[0062] S23: Perform sandblasting on the copper plating layer, specifically including the following steps:

[0063] S231: Mix nano-aluminum oxide and bio-based biodegradable polymer particles (particle size 50 μm to 100 μm, polylactic acid PLA substrate) at a mass ratio of 1:100, place them in a fluidized bed, and uniformly coat the alumina on the surface of the polymer through chemical vapor deposition to obtain the sandblasting medium;

[0064] S232: Clamp the kovar alloy substrate with the copper plating layer formed on it on a rotating platform, and ensure that the surface of the copper plating layer is flat and free of warping. Use supercritical CO2 cleaning (temperature 31°C, pressure 7.4 MPa) to clean the copper plating layer to remove residual passivation solution or contaminants;

[0065] S233: Turn on the sandblaster and ultrasonic generator simultaneously, adjust the distance between the spray gun and the copper plating layer to 10 cm to 15 cm to ensure the uniformity of impact, and synchronously start the rotating platform to perform sandblasting on the copper plating layer.

[0066] In this step, when ultrasonic waves propagate in a liquid medium, they can generate periodic compression and expansion, forming local high-pressure and low-pressure regions. When the sound pressure exceeds the tensile strength of the liquid, cavitation bubbles are generated in the low-pressure region and collapse instantly, releasing huge energy. The microjets and shock waves generated by the collapse of cavitation bubbles can apply shear force to the sandblasting medium, and can destroy the van der Waals force and electrostatic adsorption between the sandblasting medium particles, thereby preventing medium agglomeration and facilitating the uniform distribution of medium particles.

[0067] It should be noted that the surface of the passivated copper plating layer is too smooth, which is not conducive to the mechanical interlocking of the subsequent plating layer. Through sandblasting treatment, a micro-nano level concave-convex structure can be formed on the surface of the copper plating layer, thereby enhancing the surface roughness of the copper plating layer, enabling the subsequent plating layer to improve the mechanical bonding force with the copper plating layer through the "anchoring effect". If sandblasting treatment is not performed, the smooth surface of the copper plating layer cannot provide sufficient mechanical anchor points for the subsequent plating layer, resulting in insufficient adhesion of the nickel layer and soft gold plating layer, and easy peeling or blistering.

[0068] In another exemplary embodiment, in step S3, the step of immersing the kovar alloy with the copper plating layer formed on it in an electroless nickel plating solution to form a nickel plating layer on the surface of the copper plating layer includes the following steps:

[0069] S31: Before formal nickel plating, first apply a pre-nickel plating layer (the pre-nickel plating layer is an extremely thin transition nickel layer applied between the copper plating layer and the main nickel plating layer, formed through an electroless plating process) on the surface of the copper plating layer;

[0070] In this step, the pre-nickel plating layer can serve as a buffer layer to help adjust the difference in thermal expansion coefficients between the copper plating layer and the main nickel plating layer, which is beneficial to reducing stress concentration caused by temperature changes.

[0071] After testing, the optimal thickness of the pre-nickel plating layer is 0.5 μm to 0.8 μm. If the thickness is less than 0.5 μm, the pre-nickel plating layer will be too thin to effectively cover the copper plating layer, resulting in direct contact between the subsequent main nickel plating layer and the copper plating layer, thereby forming galvanic corrosion (oxidation caused by the Ni-Cu potential difference). If the thickness is greater than 0.8 μm, the pre-nickel plating layer will be too thick, which will inhibit the formation of the amorphous Ni-P structure of the main nickel plating layer (when electroless nickel plating, too thick a pre-plating layer leads to a reduction in phosphorus eutectoid and an increase in hardness). Controlling the thickness within 0.5 μm to 0.8 μm is mainly based on the following considerations: First, this thickness can ensure complete coverage of the copper layer (coverage efficiency > 99.9%, confirmed by AFM observation); Second, this thickness can maintain the amorphous structure of the main nickel plating layer (phosphorus content stabilized at 8 wt% - 10 wt%).

[0072] S32: Immerse the kovar alloy substrate with the pre-nickel plating layer in the electroless nickel plating solution and carry out a spontaneous reaction under no-current conditions until a nickel plating layer with a thickness of 3.5 μm to 5.5 μm is formed.

[0073] In this step, the electroless nickel plating solution includes nickel sulfate as the nickel source (concentration 20 g / L to 30 g / L), sodium hypophosphite as the reducing agent (concentration 20 g / L to 40 g / L), complexing agents (such as sodium citrate, concentration 10 g / L to 20 g / L), stabilizers (such as thiourea), and buffering agents (such as boric acid). After testing, the electroless nickel plating solution with the above components can provide a stable nickel ion source through nickel sulfate and achieve the reduction deposition of nickel and the eutectoid phosphorus element through sodium hypophosphite, and can form a dense amorphous Ni-P alloy plating layer. This plating layer has both high corrosion resistance (salt spray test > 1000 hours), excellent adhesion (no peeling after 1000 thermal shock cycles), and a uniform plating layer structure (porosity < 0.1%). The complexing agent can prevent nickel ion hydrolysis, the stabilizer inhibits the spontaneous decomposition of the plating solution, and the buffering agent maintains the pH within the optimal reaction window of 4.5 to 5.0, which can jointly ensure the long-term stability of the plating solution (service life > 8 cycles) and the consistency of the plating layer performance.

[0074] In another exemplary embodiment, in step S4, the step of immersing the kovar alloy substrate with the formed nickel plating layer in the nano-gold seed solution to form a soft gold plating layer on the surface of the nickel plating layer includes the following steps:

[0075] S41: Activate the kovar alloy substrate with the formed nickel plating layer by using dual-frequency plasma (low frequency 40 kHz + high frequency 13.56 MHz) in a mixed protective atmosphere (argon (70%) + hydrogen (30%));

[0076] In this step, during the formation of the nickel coating, due to the easy formation of a nickel oxide (NiO) passivation film and the residue of organic pollutants on its surface, the bonding strength of the soft gold coating will be significantly reduced (which can lead to a decrease in the bonding strength by more than 50%), and traditional chemical cleaning methods are difficult to completely remove nanoscale adsorbates. However, plasma activation can achieve atomic-level cleaning of the nickel coating.

[0077] Hydrogen ions and oxygen ions can react with nickel oxide (NiO) on the surface of the nickel coating to cause a reduction reaction, exposing fresh nickel atoms on the surface of the nickel coating, thereby significantly increasing the surface energy of the nickel coating (detected that the surface energy increases by 30% to 40%). In addition, high-frequency plasma enables hydrogen ions to be injected into the nickel lattice with an energy of 50 eV to 100 eV, generating dislocation and vacancy defects. These defects can become preferential nucleation sites during the subsequent deposition process of gold atoms, thereby refining the grain size of the soft gold coating. Experiments show that the grain size of the soft gold coating formed on the nickel coating treated by plasma activation can be refined to 20 nm to 30 nm, while the grain size of the soft gold coating under traditional processes is 50 nm to 100 nm.

[0078] S42: Immerse the kovar alloy substrate with a nickel coating formed after activation treatment into a nano gold seed solution (including gold nanoparticles with a concentration of 50 ppm, polyvinylpyrrolidone (PVP, 0.1 wt%), and ammonia water with a pH value of 8.5), and perform ultrasonic-assisted (40 kHz) treatment at room temperature for 5 minutes to make the gold nanoparticles evenly adsorbed on the nickel surface.

[0079] In this step, during the process of immersing the activated nickel coating into the nano gold seed solution, gold nanoparticles with a concentration of 50 ppm provide a high-density nucleation site of up to 108 cm 2 to 109 / cm 2 . The particle size of these gold nanoparticles is precisely controlled between 5 nm and 10 nm to perfectly match the defect size on the surface of the nickel coating, promoting the uniform deposition of gold atoms. At the same time, polyvinylpyrrolidone (PVP, with a content of 0.1 wt%) is used as a steric hindrance agent. Through the coordination bond between its pyrrolidone group and the crystal plane of the gold nanoparticles, the agglomeration of gold nanoparticles is effectively prevented, ensuring the stability and dispersibility of the nano gold seed solution. In addition, ammonia water with a pH value of 8.5 is used to maintain the weakly alkaline environment of the plating solution. This environment makes the surface of the nickel coating positively charged, thus generating a strong electrostatic adsorption effect with the negatively charged gold nanoparticles, further enhancing the adhesion and distribution uniformity of the gold nanoparticles on the surface of the nickel coating.

[0080] S43: Connect the nickel coating adsorbed with nano-gold particles to the cathode of a pulsed power supply and immerse it in the plating solution (including sodium gold sulfite (gold content 3 g / L), ethylenediaminetetraacetic acid (EDTA, 10 g / L), potassium dihydrogen phosphate (KHPO, 20 g / L), polyethylene glycol (PEG-600, 0.5 g / L), thiourea (0.1 g / L)). Set the pulse parameters as the forward current density of 1.5 A / dm 2 , reverse 0.3 A / dm 2 , duty cycle 4:1, frequency 100 Hz; Start electroplating and continue for 15 minutes. After the coating evenly covers, take out the nickel coating adsorbed with nano-gold particles, rinse it with deionized water and dry it with nitrogen.

[0081] S44: Anneal the dried nickel coating adsorbed with nano-gold particles. Immediately after annealing is completed, introduce nitrogen (flow rate 10 L / min) and quickly cool it to room temperature.

[0082] In this step, during electroplating, due to the deposition of gold nanoparticles and the growth of crystals, certain stresses will be generated inside the nickel coating. Through annealing treatment, the metal atoms can be rearranged by heating, thereby eliminating or reducing these internal stresses and improving the stability and reliability of the soft gold coating. In addition, the annealing treatment can also enhance the bonding force between the soft gold coating and the nickel coating. By promoting the diffusion and rearrangement of metal atoms, annealing can form a stronger chemical bonding between the soft gold coating and the nickel coating, thereby reducing the risk of coating peeling.

[0083] Specifically, annealing the dried nickel coating adsorbed with nano-gold particles specifically includes the following steps:

[0084] First, set the annealing temperature 50 °C to 100 °C above the recrystallization temperature of the material (such as 700 °C to 800 °C for nickel-based materials) to ensure uniform nucleation of grains and avoid abnormal grain growth.

[0085] Second, use stepwise heating. In the initial stage, heat at a rate of 5 °C to 10 °C / min to 500 °C, and then reduce to 2 °C to 3 °C / min until the target temperature. Stepwise heating can avoid local overheating, thereby reducing thermal stress concentration.

[0086] Third, cool the nickel coating adsorbed with nano-gold particles that has reached the target temperature. After cooling to room temperature, perform tempering treatment at 200 °C to 300 °C for 1 hour to 2 hours to further eliminate residual stresses.

[0087] In addition, through rapid cooling, firstly, it can quickly fix the fine grain structure formed during the annealing process, prevent the grains from growing further at high temperatures, and thus contribute to maintaining the high density and uniformity of the soft gold coating; secondly, it can reduce the exposure time of the metal at high temperatures, thereby reducing the risk of oxidation, and thus facilitating the maintenance of the brightness and purity of the soft gold coating.

[0088] S45: Immerse the annealed nickel coating adsorbed with nano gold particles into a hydrophobic solution (for example, an ethanol solution dissolved with perfluorooctyl mercaptan (HS-C8F17)), let it stand for 10 minutes, take it out and blow it with nitrogen, and then place it in an oven and heat it at 60°C for 10 minutes, thus completing the soft gold electroplating operation on the surface of the nickel coating.

[0089] In this step, after immersing the annealed nickel coating adsorbed with nano gold particles into the hydrophobic solution, the mercapto group (-SH) of perfluorooctyl mercaptan forms a strong Au-S bond (bond energy is about 180 kJ / mol) with the surface of the gold nanoparticles, constructing a dense self-assembled monolayer. This self-assembled film not only enhances the attachment stability of the gold nanoparticles on the nickel coating, but also effectively isolates the nickel substrate through its hydrophobicity, keeping the nickel substrate hydrophilic, thereby preventing the adverse interaction between the nickel and the soft gold coating. In addition, the perfluorinated chain part of perfluorooctyl mercaptan exhibits significant steric repulsion, which can effectively block the deposition of the subsequent electroplating solution in non-target areas (such as the sidewalls or defects of the coating), significantly improving the pattern accuracy of the soft gold coating to ±2 μm (compared with ±5 μm of the traditional process). More notably, the high electronegativity of the fluorine atoms in the perfluorinated chain forms an electron cloud shielding effect, effectively resisting the oxidation reaction that may occur during the subsequent processing or storage of the soft gold coating. Through X-ray photoelectron spectroscopy (XPS) detection, the content of gold oxide is controlled below a very low 1 at%, further ensuring the high purity and long-term stability of the soft gold coating.

[0090] Next, the present application compares the coating quality of this solution with the traditional solution, and the comparison results are shown in Table 1:

[0091] Table 1

[0092]

[0093]

[0094] As can be seen from Table 1, compared with the traditional solution, the present application has improvements in various performance indicators. Specifically:

[0095] Bonding surface finish: This solution reduces the Ra value from the traditional 0.8 - 1.2 μm to 0.2 - 0.4 μm, which means that the coating surface is smoother and the pores and surface defects are significantly reduced. This is crucial for improving the bonding strength and the appearance quality of the product.

[0096] Porosity of the coating: In this solution, the porosity is reduced from the traditional 5%-10% to less than 1%, indicating that the coating is denser, which can effectively prevent the intrusion of corrosive media and improve the corrosion resistance and service life of the coating.

[0097] Adhesion of the coating: In this solution, through techniques such as pre-treatment activation, pre-nickel coating, and annealing treatment, the adhesion between the coating and the substrate is significantly improved, and the number of thermal shock cycles is increased from the traditional 300-500 times to more than 1000 times. This means that the coating is more firmly adhered and is not easily peeled or fallen off.

[0098] Hardness of the coating: Although the hardness of the coating in the traditional solution is moderate, in this solution, by controlling the thickness of the pre-nickel coating, the adjustability of the coating hardness (HV value 100-300) is achieved, and it can be customized according to different application requirements.

[0099] Coating thickness control: In this solution, by precisely controlling the electroplating parameters and the composition of the plating solution, the deviation of the coating thickness is reduced from the traditional ±10% to less than ±5%, significantly improving the consistency and reliability of the product.

[0100] Corrosion resistance of the coating: In this solution, the composition of the electroless nickel plating solution is optimized to form a dense amorphous Ni-P alloy coating, and the salt spray test time is increased from the traditional 300-500 hours to more than 1000 hours, significantly improving the corrosion resistance of the coating.

[0101] Grain size of the coating: In this solution, a nano-gold seed solution is used to provide a high density of nucleation sites, and the grain size of the soft gold coating is refined from the traditional 50-100 nm to 20-30 nm, improving the performance and surface finish of the coating.

[0102] Purity of the coating: In this solution, through hydrophobic solution treatment, the nickel substrate is effectively isolated to prevent adverse interactions, and the content of oxidized gold is reduced from the traditional 2-5 at% to less than 1 at%, significantly improving the purity and long-term stability of the coating.

[0103] In summary, this application has achieved an overall improvement in the coating quality compared with the traditional solution. It not only improves the surface smoothness, density, adhesion, and corrosion resistance of the coating, but also realizes the adjustability of the coating hardness, precise control of the thickness, and significant improvement of the purity. These performance improvements will help improve the reliability and market competitiveness of the product and meet a wider range of application requirements.

[0104] The above are only the preferred embodiments of this application, and do not limit the patent scope of this application accordingly. Any equivalent structural or equivalent process transformation made using the content of the specification and drawings of this application, or directly or indirectly applied in other related technical fields, shall be included in the patent protection scope of this application by the same token.

Claims

1. A Kovar alloy electroplating method, characterized in that, The method includes: Pre-treating a Kovar alloy substrate; Immersing the pre-treated Kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper plating layer on the surface of the Kovar alloy substrate; Immersing the Kovar alloy with the formed copper plating layer in a electroless nickel plating solution to form a nickel plating layer on the surface of the copper plating layer; Immersing the Kovar alloy substrate with the formed nickel plating layer in a nano gold seed solution to form a soft gold plating layer on the surface of the nickel plating layer.

2. The Kovar alloy electroplating method according to claim 1, wherein The pre-treating of the Kovar alloy substrate includes: Cleaning the Kovar alloy substrate; Activating the cleaned Kovar alloy substrate.

3. The kovar alloy electroplating method according to claim 2, characterized in that, The activating of the cleaned Kovar alloy substrate includes: Activating the cleaned Kovar alloy substrate by configuring a mixed acid solution containing carbon-coated platinum nanoparticles.

4. The kovar alloy electroplating method according to claim 1, characterized in that, The immersing of the pre-treated Kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper plating layer on the surface of the Kovar alloy substrate includes: Immersing the pre-treated Kovar alloy substrate in an acidic copper sulfate plating solution and performing electrolytic copper plating with a periodic pulsed current to form a primary copper plating layer on the surface of the Kovar alloy substrate; Passivating the Kovar alloy substrate with the formed primary copper plating layer to form a copper plating layer on the surface of the Kovar alloy substrate.

5. The kovar alloy electroplating method according to claim 4, characterized in that, The passivating of the Kovar alloy substrate with the formed primary copper plating layer includes: Cleaning and drying the Kovar alloy substrate with the formed primary copper plating layer; Immersing the dried Kovar alloy substrate with the formed primary copper plating layer in a passivating solution; Performing ultraviolet irradiation on the Kovar alloy substrate with the formed primary copper plating layer after being immersed in the passivating solution; Rinsing and drying the Kovar alloy substrate with the formed primary copper plating layer after ultraviolet irradiation to obtain a Kovar alloy substrate with a formed copper plating layer.

6. The kovar alloy electroplating method according to claim 4, characterized in that, The immersing of the pre-treated Kovar alloy substrate in an acidic copper sulfate plating solution for electrolytic copper plating to form a copper plating layer on the surface of the Kovar alloy substrate further includes: Performing sandblasting on the copper plating layer.

7. The kovar alloy electroplating method according to claim 1, characterized in that, The immersing of the Kovar alloy with the formed copper plating layer in a electroless nickel plating solution to form a nickel plating layer on the surface of the copper plating layer includes: Applying a pre-nickel plating layer on the surface of the copper plating layer; Immersing the Kovar alloy substrate with the applied pre-nickel plating layer in a electroless nickel plating solution for spontaneous reaction to form a nickel plating layer on the surface of the copper plating layer.

8. The kovar alloy electroplating method according to claim 7, characterized in that, The thickness of the pre-nickel plating layer is any value from 0.5 μm to 0.8 μm.

9. The kovar alloy electroplating method according to claim 7, characterized in that The electroless nickel plating solution includes nickel sulfate, sodium hypophosphite, complexing agent, stabilizer and buffer.

10. The kovar alloy electroplating method according to claim 1, characterized in that, The immersing of the Kovar alloy substrate with the formed nickel plating layer in a nano gold seed solution to form a soft gold plating layer on the surface of the nickel plating layer includes: Activating the Kovar alloy substrate with the formed nickel plating layer under a protective atmosphere; Immersing the activated Kovar alloy substrate with the formed nickel plating layer in a nano gold seed solution to enable nano gold particles to adsorb on the surface of the nickel plating layer; Immersing the nickel plating layer adsorbed with nano gold particles in a plating solution, and drying after immersion; Performing annealing treatment and cooling on the dried nickel plating layer adsorbed with nano gold particles; Immersing the cooled nickel plating layer adsorbed with nano gold particles in a hydrophobic solution, and drying after immersion to obtain a Kovar alloy substrate with a formed soft gold plating layer.

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