Production method of tin-whisker-proof tinned copper-clad steel wire

Abstract

The invention discloses a production method of an anti-tin-whisker tinned copper-clad steel wire. The method comprises the steps of steel wire base material pretreatment, copper layer coating, intermediate annealing, hot-dip tinning, tin layer stress relief treatment, surface passivation treatment and film coating. Wherein in the tin plating process, 0.1%-0.5% of bismuth or zinc element is added into tin liquid, low-temperature heat treatment at the temperature of 90-130 DEG C is subsequently adopted to relieve the stress of a tin layer, and the surface is coated with a high-molecular polymer film with the thickness of 1-5 microns to further improve the corrosion resistance and the mechanical stability. The method can effectively inhibit the growth of tin whiskers, improves the adhesive force and stability of a tin coating, is environment-friendly in process and is suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of copper-clad steel wire technology, specifically relating to a method for producing tin-plated copper-clad steel wire that prevents tin whiskers. Background Technology

[0002] Tinned copper-clad steel wire is a composite metal wire with a steel core and an outer copper and tin coating. It is widely used in electronics, power, communications, and automotive industries, and has significant advantages, especially in high-frequency signal transmission and electromagnetic interference suppression. While maintaining good conductivity, copper-clad steel wire also possesses high strength, while the outer tin coating improves its corrosion resistance and solderability, meeting the higher requirements of modern electronic devices for welding reliability and service life.

[0003] However, traditional tin-plated copper-clad steel wire is prone to tin whisker growth during long-term use, especially in high-temperature and high-humidity environments. Tin whiskers are fibrous metallic crystals that spontaneously grow from the surface of the tin plating layer. They are less than a few micrometers in diameter but can reach hundreds of micrometers in length, or even exceed 1 millimeter. Tin whisker formation is usually caused by residual stress, crystal defects, electrochemical instability, and other factors. It can lead to serious faults such as short circuits, signal interference, and poor contact, posing a significant hidden danger, especially in high-precision electronic equipment (such as aerospace, military, and medical devices).

[0004] Currently, to suppress tin whisker growth, the industry has tried annealing and adding a barrier layer, such as a nickel layer, between tin and copper. The best method found so far is to add lead to tin, but since lead is not environmentally friendly, the suppression of tin whiskers needs to be addressed from other angles.

[0005] Therefore, there is an urgent need to propose a production method for tin-plated copper-clad steel wire that features optimized process flow, significant tin whisker suppression effect, excellent comprehensive performance, and environmental compliance, in order to meet the pressing needs of the modern high-performance cable industry. Summary of the Invention

[0006] To address the above problems, this invention provides a method for producing tin-plated copper-clad steel wire that prevents tin whiskers.

[0007] The technical solution provided by this invention is as follows: A method for producing tin-plated copper-clad steel wire with anti-tin whisker properties includes the following steps: S1: Steel wire substrate pretreatment: The steel wire is degreased, pickled, rinsed and dried to remove surface oil and oxides; S2: Copper coating: A copper layer with a thickness of 5-30µm is formed on the surface of the steel wire by electroplating to obtain copper-clad steel wire; S3: Intermediate Annealing: Place the copper-clad steel wire in a protective atmosphere and anneal it at a temperature range of 450℃-600℃ for 10-30 minutes. S4: Hot-dip tin plating: The annealed copper-clad steel wire is hot-dip tin-plated through molten tin. The temperature of the tin liquid is controlled at 250℃-290℃, and 0.1%-0.5% of bismuth or zinc is added to the tin liquid. S5: Tin layer stress relief treatment: The tin-plated wire is subjected to low-temperature heat treatment at 90℃-130℃ for 2-6 hours in an inert atmosphere. S6: Surface passivation treatment: The heat-treated wire is passivated in a passivation solution; S7: Film coating: A 1-5 μm thick polymer protective film is coated on the passivated wire surface.

[0008] In some implementations, the copper layer is coated using an acidic copper sulfate electroplating process, with the current density controlled at 1-5 A / dm².

[0009] In some implementations, 0.3% bismuth is added to the molten tin.

[0010] In some embodiments, the passivation solution is a chromium-free organophosphate solution, and the passivation time is 1-5 minutes.

[0011] In some embodiments, the polymer is selected from polyethylene terephthalate, modified polyimide, or polyamide-imide copolymer.

[0012] In some implementations, the tin layer thickness of the wire after tin plating is 2-10 μm.

[0013] In some implementations, the linear speed during hot-dip galvanizing is 20-60 m / min.

[0014] In some implementations, an automatic cleaning and surface activation step is provided between step S3 and step S4.

[0015] In some embodiments, the automatic cleaning step includes sequentially passing through an ultrasonic cleaning tank, an alkaline spray tank, a weak acid activation tank, a deionized water cleaning tank, and a hot air drying section.

[0016] In some implementations, the surface activation step includes treatment with 1-3% citric acid for 10-30 seconds.

[0017] In summary, the beneficial effects of this invention are: (1) This invention effectively releases residual stress in the tin layer and significantly inhibits tin whisker growth through the synergistic effect of multiple processes such as intermediate annealing, adding trace amounts of bismuth or zinc to the tin melt, low-temperature heat treatment, surface passivation, and protective film coating. At the same time, it makes the tin layer have strong adhesion, uniform thickness, and excellent corrosion resistance. The wire exhibits good oxidation resistance in salt spray testing, meets the usage requirements in complex environments, and greatly improves the welding performance and long-term reliability of the product.

[0018] (2) The present invention uses chromium-free environmentally friendly passivation liquid and polymer protective film to avoid environmental pollution problems caused by traditional chromium-containing treatment; the parameters of the entire production process are clear and highly controllable, which facilitates automated and continuous production and is suitable for large-scale industrial promotion and application, with high economic and social benefits. Attached Figure Description

[0019] Figure 1 These are microscope photographs of Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0020] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. The following embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0021] This invention provides a method for producing tin-plated copper-clad steel wire with anti-tin whisker properties, comprising the following steps: S1: Steel wire substrate pretreatment: The steel wire is degreased, pickled, rinsed and dried to remove surface oil and oxides; Specifically, through pretreatment steps such as degreasing, pickling, rinsing and drying, oil and oxide scale on the surface of the steel wire can be effectively removed, significantly improving the adhesion of subsequent copper or tin layers, reducing interference from interface impurities, preventing subsequent interlayer peeling and stress concentration, and contributing to the overall structural stability and electrical reliability of the material.

[0022] S2: Copper coating: A copper layer with a thickness of 5-30µm is formed on the surface of the steel wire by electroplating to obtain copper-clad steel wire; Specifically, the copper layer is electroplated using an acidic copper sulfate electroplating process. The electroplated copper layer can improve conductivity and provide good tin affinity, providing a surface with high adhesion for subsequent hot-dip tin plating.

[0023] S3: Intermediate Annealing: Place the copper-clad steel wire in a protective atmosphere and anneal it at a temperature range of 450℃-600℃ for 10-30 minutes. Specifically, annealing can eliminate residual stress introduced during the cold working of copper-clad steel wire, reduce the hardness of the wire, improve the plasticity of the wire, and provide a uniform substrate without internal stress concentration for the subsequent hot-dip tin plating process, which helps to form a dense and continuous tin layer structure.

[0024] S4: Hot-dip tin plating: The annealed copper-clad steel wire is hot-dip tin-plated through molten tin. The temperature of the tin liquid is controlled at 250℃-290℃, and 0.1%-0.5% of bismuth or zinc is added to the tin liquid. Specifically, hot-dip tin plating can form a dense metallic tin coating with good solderability and oxidation resistance. Adding bismuth or zinc can effectively inhibit the growth of tin whiskers. These microalloying elements can form fine precipitates in the tin layer, interfering with the growth path of tin whiskers.

[0025] The growth of tin whiskers is related to local crystal slip and dislocation migration in a pure tin layer under long-term stress. Trace amounts of Bi or Zn can form solid solutions or micro / nano alloy structures, hindering dislocation movement and thus suppressing tin whisker formation.

[0026] S5: Tin layer stress relief treatment: The tin-plated wire is subjected to low-temperature heat treatment at 90℃-130℃ for 2-6 hours in an inert atmosphere.

[0027] Specifically, low-temperature heat treatment of tin-plated wire can further release the deposition stress and solidification stress of the tin layer, especially in an inert atmosphere to prevent oxidation, while promoting the stability of the microcrystalline structure and significantly reducing the probability of tin whisker formation in later use.

[0028] S6: Surface passivation treatment: The heat-treated wire is passivated in a passivation solution; Specifically, passivation can form a stable anti-oxidation protective film on the surface of the tin layer, effectively delaying the oxidation process of the metal surface, improving the environmental adaptability and storage life of the wire, reducing corrosion-induced microcracks, thereby reducing stress concentration-induced tin whiskers. Using chromium-free passivation solutions such as organophosphate solutions is not only environmentally friendly and harmless, but also forms a chemically stable complex protective layer with the tin layer, playing a dual role of corrosion inhibition and rust prevention.

[0029] S7: Film coating: A 1-5 μm thick polymer protective film is coated on the passivated wire surface.

[0030] Specifically, the polymer coating provides the wire with an additional physical barrier and flexible covering, effectively blocking external moisture, oxygen and mechanical friction, delaying the oxidation and corrosion process of the tin layer, and at the same time playing a physical sealing role against tin whiskers induced by thermal stress or micromechanical stress.

[0031] Example 1 This embodiment is performed in the following order: S1: Pretreatment of steel wire substrate: High-strength carbon steel wire with a diameter of 0.8mm was selected as the base material, and the following processing steps were performed in sequence: Degreasing: Soak in 5% NaOH solution at 60℃ for 10 minutes to remove oil stains; Pickling: Immerse in 15% HCl solution for 2 minutes to remove oxide scale; Rinse: Rinse 3 times with running deionized water to prevent acid residue; Drying: The steel wire substrate is dried for 10 minutes at 100°C using a hot air drying device to obtain a clean and dry substrate.

[0032] S2: Copper layer coating: The pretreated steel wire is placed in an acidic copper sulfate electroplating bath. The electroplating solution consists of: The copper-clad steel wire is obtained by electroplating CuSO4·5H2O 220 g / L, H2SO4 50 g / L, chloride ion (Cl⁻) content controlled at 80 ppm, current density controlled at 3A / dm², and electroplating time of 12 minutes to form a uniform copper layer with a thickness of 10μm.

[0033] S3, Intermediate Annealing: The copper-clad steel wire is placed in a protective atmosphere containing 90% nitrogen and 10% hydrogen, with the temperature controlled at 550℃ and the annealing time at 20 minutes to relieve the internal stress of the copper layer and improve the bonding strength.

[0034] S4, Hot-dip tin plating: The annealed copper-clad steel wire is continuously fed into a tin bath at a temperature of 270°C. 0.3 wt% of bismuth is added to the tin bath. The wire speed is controlled at 40 m / min and the tin plating time is about 4 seconds to form a tin layer with a thickness of 6 μm.

[0035] S5. Stress relief treatment for tin layer: After tin plating, the wire is placed in an inert atmosphere (nitrogen) environment at a temperature of 110°C for 4 hours, which effectively relieves the internal stress of the tin layer and inhibits the growth of tin whiskers.

[0036] Bismuth can alter the crystal structure of tin, inhibit grain boundary migration, effectively reduce internal stress and atomic diffusion rate, and reduce the tendency of tin whisker formation from the source.

[0037] S6. Surface passivation treatment: Use a chromium-free organophosphate passivation solution (composed of 0.3% polyphosphate + 0.2% sodium tripolyphosphate + a small amount of nonionic surfactant) for 3 minutes.

[0038] S7, Film Coating: A 1.5 μm thick polyimide-imide copolymer protective film was uniformly sprayed onto the passivated surface and dried at 80°C for 10 minutes.

[0039] The final product was observed by scanning electron microscopy (SEM). After being stored in an environment with a room temperature of 25°C and a relative humidity of 60% for 180 days, there was no tin beard growth, and the surface showed no cracks or peeling.

[0040] Example 2 The difference between this embodiment and Embodiment 1 is that 0.5% zinc is added to the tin bath instead of bismuth in the hot-dip tin plating step. The tin plating temperature is 260℃, and the tin layer thickness is 8μm. The remaining process steps are the same. A 1.2μm thick modified polyimide is used as the surface protective film.

[0041] The test results showed that after the product was stored for 120 days under high humidity (85% RH) and high temperature (60℃) aging test, there were no obvious cracks in the surface tin layer and no tin whiskers were observed, indicating that zinc also has a good tin whisker suppression effect.

[0042] Example 3 The difference between this embodiment and Embodiment 1 is that the following automatic cleaning and surface activation process is added between steps S3 and S4: Automatic cleaning section: Passing through in sequence Ultrasonic cleaning tank (40 kHz, 60℃, 2 minutes); Alkali spray tank (2% Na2CO3, 1 minute); Weak acid activation bath (0.5% H2SO4, 30 seconds); Deionized water cleaning tank (two sections); Hot air drying section (120℃, 3 minutes).

[0043] Surface activation step: Immerse in 2% citric acid solution for 15 seconds to remove the surface oxide layer and activate the copper layer.

[0044] Other process parameters are the same as in Example 1.

[0045] After alternating damp heat test (40℃±2℃, RH 95%±5%, cycle 168 hours), no tin whiskers or cracks were observed on the surface of the wire; compared with the control sample without this treatment process, the uniformity of the tin layer on the surface was significantly improved and the adhesion was enhanced.

[0046] Example 4 Based on Example 1, the polymer protective film material was replaced with a modified polyimide (MPI) film with a thickness of 2 μm. This polymer has excellent thermal stability and flexibility, further enhancing the mechanical stress relief capability and corrosion resistance.

[0047] The final product was observed by scanning electron microscopy (SEM). After being stored in an environment with a room temperature of 25°C and a relative humidity of 60% for 180 days, there was no tin beard growth, and the surface showed no cracks or peeling.

[0048] Comparative Example 1 The entire process of Example 1 is performed, but bismuth or zinc is not added to the molten tin.

[0049] The final product was observed by scanning electron microscopy (SEM). After being stored in an environment with a room temperature of 25°C and a relative humidity of 60% for 180 days, the tin layer grains grew and the structure became uneven, forming a residual stress concentration area.

[0050] Comparative Example 2 The entire process of Example 1 is followed, but step S5 is omitted, i.e., no stress relief treatment is performed on the tin layer after tin plating.

[0051] After being stored at room temperature and 65% humidity for 90 days, electron microscopy revealed needle-like tin whiskers on the surface of the tin layer, with a length of up to 10 μm, indicating that the stress was not effectively released, leading to the formation of tin whiskers.

[0052] Comparative Example 3 Refer to Example 2, but omit S6 and S7, that is, do not perform passivation and protective film treatment.

[0053] After the product underwent a temperature and humidity aging test (60℃, 90% RH, for 96 consecutive hours), it was found that the tin layer surface underwent slight oxidation and blackening, and rust spots appeared in some areas, indicating that the tin layer surface is prone to oxidation and has poor long-term stability.

[0054] The performance of the tin-plated copper-clad steel wires produced in Examples 1-4 and Comparative Examples 1-3 was tested, and the results are detailed in Table 1.

[0055] Table 1 As shown in Table 1, all embodiments of the present invention exhibit excellent performance, especially Embodiment 1, which has the most balanced performance in all aspects. Comparative Examples 1-4 generally suffer from problems such as tin whisker growth, poor adhesion, and weak corrosion resistance. Trace metal elements (bismuth or zinc), stress relief treatment, and film coating steps are the core key factors for effectively suppressing tin whiskers. At the same time, surface passivation and heat treatment steps can further improve the stability and oxidation resistance of the tin plating layer.

[0056] The foregoing description illustrates and describes preferred embodiments of the present invention. As previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A method for producing tin-plated copper-clad steel wire with anti-tin whisker properties, characterized in that, Includes the following steps: S1: Steel wire substrate pretreatment: The steel wire is degreased, pickled, rinsed and dried to remove surface oil and oxides; S2: Copper coating: A copper layer with a thickness of 5-30µm is formed on the surface of the steel wire by electroplating to obtain copper-clad steel wire; S3: Intermediate Annealing: Place the copper-clad steel wire in a protective atmosphere and anneal it at a temperature range of 450℃-600℃ for 10-30 minutes. S4: Hot-dip tin plating: The annealed copper-clad steel wire is hot-dip tin-plated through molten tin. The temperature of the tin liquid is controlled at 250℃-290℃, and 0.1%-0.5% of bismuth or zinc is added to the tin liquid. S5: Tin layer stress relief treatment: The tin-plated wire is subjected to low-temperature heat treatment at 90℃-130℃ for 2-6 hours in an inert atmosphere. S6: Surface passivation treatment: The heat-treated wire is passivated in a passivation solution; S7: Film coating: A 1-5 μm thick polymer protective film is coated on the passivated wire surface.

2. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 1, characterized in that, The copper layer coating is achieved using an acidic copper sulfate electroplating process, with the current density controlled at 1-5 A / dm².

3. The method for producing tin-plated copper-clad steel wire with anti-tin whisker as described in claim 1, characterized in that, The molten tin contains 0.3% bismuth.

4. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 1, characterized in that, The passivation solution is a chromium-free organic phosphate solution, and the passivation time is 1-5 minutes.

5. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 1, characterized in that, The polymer is selected from polyethylene terephthalate, modified polyimide, or polyamide-imide copolymer.

6. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 1, characterized in that, The tin layer thickness of the wire after tin plating is 2-10μm.

7. The method for producing tin-plated copper-clad steel wire with anti-tin whisker according to claim 6, characterized in that, The linear speed during hot-dip galvanizing is 20-60 m / min.

8. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 1, characterized in that, An automatic cleaning and surface activation step is provided between step S3 and step S4.

9. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 8, characterized in that, The automatic cleaning process includes sequentially passing through an ultrasonic cleaning tank, an alkaline spray tank, a weak acid activation tank, a deionized water cleaning tank, and a hot air drying section.

10. The method for producing tin-plated copper-clad steel wire with anti-tin whiskers according to claim 8, characterized in that, The surface activation step includes treatment with 1-3% citric acid for 10-30 seconds.

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