Method for improving cracking of thin inductor by regulating and controlling hardness of copper wire through annealing

Abstract

The invention discloses a method for improving cracking of a thin inductor by regulating and controlling hardness of a copper wire through annealing, and belongs to the technical field of power inductors, in the method, lattice distortion and dislocation accumulation of the copper wire are thoroughly eliminated through high-temperature annealing treatment, and material plasticity is remarkably recovered. The annealing temperature is far higher than the recrystallization temperature of pure copper, grain coarsening is accelerated, and the hardness of the copper wire is greatly reduced. And the softened copper wire generates extremely small internal stress in the subsequent high-pressure forming process, so that the problem of cracking of a printed surface caused by stress concentration is directly avoided, and the structural integrity and yield of a product are improved. A traditional open type cavity is replaced with the closed type cavity, and it is ensured that each corner of the cavity is evenly filled with the soft magnetic powder in the pressing process. By means of the design, the defect of structure looseness caused by insufficient density of powder in a local area is completely eradicated, the overall density of the formed inductor is consistent, no weak area exists, the cracking risk caused by uneven density is fundamentally eliminated, and the mechanical stability of a product is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of power inductors, and specifically provides a method for improving cracking of thin inductors by regulating the hardness of copper wires through annealing. Background Art

[0002] Power inductors generally refer to inductors used in power circuits such as DC-DC converters. They primarily perform voltage stabilization, filtering, and signal processing functions within these circuits. With the advancement of electronic technology, the frequencies used by CPUs and GPUs are increasing, placing greater demands on stable power supply, filtering, and responsiveness in circuits. This is driving the development of inductors towards miniaturization, thinness, and lightweighting. At the same time, as server power usage increases, cooling methods are gradually shifting from air cooling to immersion cooling, placing higher demands on inductor reliability. The structure of existing integrated copper-magnetic co-fired inductors includes a conductor, a magnet, and an insulating coating. 1. Magnetic powder is introduced into a molding die for pre-pressing, followed by the addition of a conductor and secondary powder filling. High-pressure molding is used. The resulting molded product has significant internal stress, so it undergoes high-temperature annealing to remove this stress. This preparation method is effective for producing products thicker than 2mm. Products thinner than 2mm are prone to cracking due to excessive internal stress, resulting in reduced reliability.

[0003] However, the prior art lacks a method for preparing an integrated co-fired inductor with high reliability, low loss, high heat dissipation, high magnetic permeability, and high conversion efficiency for a thin inductor. Summary of the Invention

[0004] The purpose of the present invention is to solve the above-mentioned problem and provide a method for improving cracking of thin inductors by regulating the hardness of copper wires through annealing.

[0005] The technical solution adopted by the present invention is as follows: a method for improving cracking of thin inductors by regulating the hardness of copper wires through annealing, the method comprising the following steps: S1: Using pure copper as raw material, unclad copper terminals are produced through a stamping process. This step provides the base material for subsequent high-temperature annealing, ensuring that the copper wire structure meets design requirements.

[0006] S2: Perform dimensional inspection on the formed copper terminals to select those that meet the predetermined specifications, ensuring the dimensional consistency and assembly accuracy of the copper terminals in subsequent processes.

[0007] S3: Thoroughly clean the copper terminal surface with acetone or alcohol to remove any oil or lubricant residue. This step provides a clean copper surface for high-temperature annealing, preventing impurities from affecting the annealing effect.

[0008] S4: Place the cleaned copper terminals in an annealing furnace with a nitrogen atmosphere. First, heat the temperature from room temperature to 600-700°C at a rate of 5-10°C / min. Keep the temperature for 0.5-2 hours. After turning off the heating, cool the terminals to below 200°C in the furnace, remove them from the furnace, and allow them to stand in air until they reach room temperature.

[0009] This step eliminates lattice distortion through recrystallization, significantly reduces the hardness of the copper wire, and reduces the internal stress of subsequent forming, thereby suppressing the risk of cracking.

[0010] S5: Mix ferrosilicon powder (60% to 70% of the mixed powder mass), sendust powder, an insulating agent, a coating agent, and a lubricant in appropriate proportions, controlling the ferrosilicon powder particle size (D50) to 10–20 μm. This ratio ensures powder fluidity and compacting density, providing a uniform material for compaction.

[0011] S6: Fill the closed-cavity mold with soft magnetic powder; first place the annealed copper terminals; then fill with soft magnetic powder a second time; and then form the mold under high pressure at 2000MPa to ensure the copper wire is completely embedded in the powder. The closed-cavity design avoids localized density deficiencies, and the softened copper wire further reduces internal stress, minimizing cracking on the printed surface.

[0012] S7: Under 99.99% pure nitrogen: Raise the temperature at 5-10°C / min to 300°C and hold for 30 minutes. Then, cool the temperature at 3-6°C / min to 40°C. Then, raise the temperature at 5-10°C / min to 600-700°C and hold for 1-3 hours. Cool the furnace to room temperature. This step completely eliminates internal molding stress and improves the structural stability of the product.

[0013] S8: Use an automatic grinder (purple sand wheel, 300–500 grit, 600–1000 rpm) to grind the product surface, controlling the feed depth to 0.05 mm, exposing the copper wire pads. This step provides a conductive interface for subsequent processes, while also improving heat dissipation efficiency by designing the gap between the pads.

[0014] S9: The polished products are inspected for electrical properties and appearance, and qualified products are transferred to subsequent assembly or packaging processes. This step ensures that the final product meets reliability requirements and achieves high-yield production of thin inductors.

[0015] In a preferred embodiment, in step S1, T2 grade pure copper strip (thickness 0.30 mm ± 0.02 mm) is used and stamped using a precision progressive die. The die gap is controlled to 8% of the material thickness, and the stamping speed is set to 120 times / minute. After stamping, the copper terminal edge burr height is ≤ 0.01 mm, the surface roughness Ra is ≤ 1.6 μm, and the resistivity is ≤ 0.0171 Ω·mm. 2 / m. The stamping oil is low-viscosity synthetic ester oil (viscosity 12 cSt at 40°C), and the residual oil film thickness after stamping is ≤0.5μm.

[0016] In a preferred embodiment, in step S2, a three-dimensional coordinate measuring machine (accuracy ±0.003mm) is used to measure the terminal bend angle (90° ± 0.3°), pin pitch (tolerance ±0.03mm), and flatness (≤0.05mm / 100mm). The sorting process is carried out in a Class 1000 clean environment, with unqualified products isolated and then melted and recycled. Qualified products are transferred to a nitrogen-protected cabinet (oxygen content ≤10ppm) for a temporary storage time of ≤4 hours.

[0017] In a preferred embodiment, in step S3, a two-stage cleaning process is adopted: first, ultrasonic cleaning is performed in an alkaline degreasing agent (pH 10.5±0.5) at 60°C for 8 minutes (frequency 40kHz, power density 0.5W / cm 2 ), followed by rinsing in ultrapure water (resistivity ≥ 18MΩ·cm); the second stage is ultrasonic cleaning in anhydrous ethanol (water ≤ 0.1%) for 5 minutes. After cleaning, the residual hydrocarbon on the surface is ≤ 20μg / cm 2 , chloride ion content <0.5ppm (verified by ion chromatography).

[0018] In a preferred embodiment, in step S4, the annealing furnace temperature uniformity is ±3°C, and the oxygen content within the furnace is ≤3 ppm. The copper wires are stacked in a molybdenum boat, separated by zirconia spacers (1 mm thick). During the heating phase, a hydrogen and nitrogen mixture (5% H2, 95% N2) is introduced. When held at 650°C, the grain size increases to 40-55 μm, and the Vickers hardness decreases from 125 HV to 65-70 HV. Upon cooling to 150°C, pure nitrogen protection is switched, with a cooling rate of ≤15°C / min.

[0019] In a preferred embodiment, in step S5, the iron silicon powder (FeSi6.5%) is air-classified to a D90 value of ≤22 μm and a coercivity of ≤7 A / m (as measured by a vibrating sample magnetometer). The lubricant, zinc stearate, is pre-dried in a vacuum at 110°C for 4 hours (vacuum ≤10 Pa). The mixing equipment is equipped with a temperature-controlled jacket (25 ± 1°C). After mixing, the powder has a Hall effect flow rate of 23 ± 2 s / 50 g and a bulk density of 2.8 ± 0.1 g / cm. 3 .

[0020] In a preferred embodiment, in step S6, the mold is made of tungsten steel (hardness 93 HRA), and the cavity surface is coated with a diamond-like carbon coating (thickness 2 μm). During pressing, the mold temperature is maintained at 85 ± 2°C, the upper punch speed is 15 mm / s, and the pressure fluctuation during the holding phase is ≤ 1%. The green density after molding is 7.25 ± 0.05 g / cm 3, radial bending strength ≥18MPa (three-point bending test).

[0021] In a preferred embodiment, in step S7, the cooling zone of the sintering furnace is equipped with a high-pressure nitrogen injection device (cooling rate ≥ 80°C / min). During the 300°C holding stage, a nitrogen mixture containing 8% hydrogen is introduced, with a dew point ≤ -40°C. During the 650°C holding stage, the furnace pressure is maintained at a slightly positive pressure of +15 Pa. The post-sintering product has a density deviation of ≤ 0.3%, and a grain size distribution coefficient of ≤ 8% (electron backscatter diffraction analysis).

[0022] In a preferred embodiment, in step S8, a diamond dressing roller (grit size D107) is used to dress the grinding wheel, with a dressing feed rate of 0.0015 mm / stroke. The grinding fluid is a water-based emulsion (pH 8.8±0.2, conductivity ≤50 μS / cm), with a flow rate controlled at 10 L / min. The height tolerance of the copper pad after grinding is ±0.01 mm, and the edge chipping area is ≤0.01 mm. 2 (Detected by laser confocal microscopy).

[0023] In a preferred embodiment, the electrical performance test in step S9 is conducted in a constant temperature oven at 23±0.5°C, with a test frequency of 1MHz±1kHz and a current of 0.5A RMS. The X-ray detection resolution is 3μm (pixel size 5μm), and the internal crack detection threshold is 10μm. Qualified products are stored in a drying oven (nitrogen-filled) with a dew point of ≤-30°C and transported in anti-static aluminum foil bags (surface resistance ≤10 6 Ω).

[0024] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. In this invention, high-temperature annealing completely eliminates lattice distortion and dislocation accumulation in the copper wire, significantly restoring the material's plasticity. The annealing temperature, far exceeding the recrystallization temperature of pure copper, accelerates grain coarsening and significantly reduces the copper wire's hardness. The softened copper wire generates minimal internal stress during the subsequent high-pressure forming process, directly preventing cracking on the printed surface due to stress concentration, thereby improving product structural integrity and yield.

[0025] 2. This invention uses a closed cavity design instead of the traditional open cavity, ensuring that the soft magnetic powder evenly fills every corner of the cavity during the pressing process. This design completely eliminates structural defects caused by insufficient powder density in local areas. After molding, the inductor has a consistent overall density and no weak areas. This fundamentally eliminates the risk of cracking caused by uneven density and enhances the mechanical stability of the product.

[0026] 3. In this invention, the optimized pad structure features a raised design with reserved heat dissipation gaps, effectively resolving the potential risk of copper wire being buried by magnetic powder. The raised pads precisely expose the copper conductor surface during the grinding process, ensuring the reliability of subsequent welding or conductive connections. Furthermore, the gaps between the pads create natural heat dissipation channels, accelerating heat dissipation during device operation and preventing localized overheating that can cause material expansion and cracking, significantly improving the long-term reliability of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart for preparing the thin inductor of the present invention; Figure 2 Schematic diagram of the inductor structure of the present invention, wherein 1 is soft magnetic metal powder and 2 is copper conductor. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0029] Example: Reference Figure 1-2 A method for improving cracking of thin inductors by regulating the hardness of copper wires through annealing comprises the following steps: This thin co-fired inductor consists of metal soft magnetic powder and processed copper conductors. The copper wire is first prepared and high-temperature annealed. The copper buckle and powder are then formed at 1900-2000MPa to obtain the product, which is then sintered. After sintering, the copper wire is exposed by grinding to facilitate the flow of subsequent processes.

[0030] Pure copper is used as the copper wire raw material, and the copper wire raw material is stamped and formed to obtain a non-clad copper terminal; Detect and sort the copper terminals to obtain copper terminals of predetermined size; Use acetone or alcohol to remove oil and lubricant residue on the surface of the copper wire.

[0031] The cleaned copper wire is placed in an annealing furnace for high-temperature annealing. Under nitrogen atmosphere, the wire is heated from room temperature at a rate of 5-10°C to 600-700°C. After 0.5-2 hours of heating, the wire is turned off and cooled naturally to below 200°C in a nitrogen atmosphere. The wire is then placed in air and allowed to cool to room temperature before forming.

[0032] Powder processing: Soft magnetic metal materials including iron silicon, sendust, iron nickel powder, insulating agent, coating agent, and lubricant are mixed and proportioned. The mass of the mixed powder of iron silicon powder and sendust is used as the calculation basis. The iron silicon powder accounts for 60% to 70% of the mass of the mixed powder, and the particle size D50 is 10-20μm.

[0033] Compression Molding: The mold cavity is closed, which facilitates the molding of the powder within the cavity. Combined with the annealed copper terminals, this reduces cracking on the printed surface of the product after molding. The mold cavity is first filled with soft magnetic metal powder, and the copper buckle is placed in the mold. The soft magnetic metal powder is then refilled and formed using a pressure of 2000MPa to achieve a product without exposed copper wires.

[0034] High-temperature sintering: After forming, the product undergoes high-temperature annealing in a protective atmosphere of nitrogen with a purity of 99.99%. The annealing method is to increase the temperature from room temperature to 300°C at a rate of 5°C-10°C and hold for 30 minutes. After holding, the temperature is lowered to 40°C at a rate of 3°C-6°C. The temperature is then increased to 600°C-700°C at a rate of 5°C-10°C and held for 1-3 hours. After holding, the heating is turned off under the protection of a nitrogen atmosphere. The product is then removed from the furnace after cooling to room temperature.

[0035] Grinding treatment: At this time, the copper wire of the product's solder pad is covered by magnetic powder, and it needs to be ground to expose the copper wire before proceeding to the next process. The grinding method is to use an automatic grinder, the grinding wheel is a purple sand grinding wheel with a mesh size of 300-500, a rotation speed of 600-1000r / min, and the feed amount of the grinder is 0.05mm.

[0036] The above technical solution has the following advantages or beneficial effects: 1. During cold working, copper wire experiences lattice distortion and dislocation accumulation, increasing its hardness. Annealing eliminates these defects through recrystallization and grain growth at high temperatures, restoring the material's plasticity and thus reducing its hardness. The recrystallization temperature of pure copper is typically between 200°C and 400°C. The 600°C to 700°C temperature used in this design is significantly higher than this, accelerating recrystallization and significantly coarsening the grains, further softening the material. This softening significantly reduces the internal stress caused by the copper wire during the forming process, thereby improving the problem of cracking on the printed surface after forming.

[0037] 2. The molding cavity is a sealed cavity rather than an open cavity. This closed cavity is different from the cavity with the copper wire partially open, and will not cause the problem of insufficient molding density in some areas.

[0038] 3. After forming, the copper wire part of the product may be buried by powder, so the pad is designed to be raised. Figure 2As shown, it is convenient to expose the copper wire after grinding, and there is a certain gap between the two pads, which helps to dissipate heat of the device.

[0039] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further limitations, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device that includes the element.

[0040] The above description is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for improving cracking of thin inductors by regulating the hardness of copper wires through annealing, characterized by: The method comprises the following steps: S1: Using pure copper as raw material, a copper terminal without coating is prepared by stamping process; S2: Dimensional inspection of formed copper terminals is performed to select those that meet predetermined specifications, ensuring dimensional consistency and assembly accuracy in subsequent processes. S3: Use acetone or alcohol to thoroughly clean the surface of the copper terminal to remove oil and lubricant residue; S4: Place the cleaned copper terminal in an annealing furnace with a nitrogen atmosphere, first heating from room temperature to 600-700°C at a rate of 5-10°C / min; hold the temperature for 0.5-2 hours; turn off the heating, cool the terminal to below 200°C, remove the terminal from the furnace, and allow it to stand in air until it reaches room temperature; S5: Mix the iron silicon powder, sendust powder, insulating agent, coating agent and lubricant in proportion, and control the iron silicon powder particle size D50 to 10-20μm; this ratio ensures the fluidity and molding density of the magnetic powder, and provides a uniform material for pressing and molding; S6: Fill the closed cavity mold with soft magnetic powder; first place the annealed copper terminal; then fill it with soft magnetic powder for the second time; and then press it into shape under high pressure of 2000MPa to ensure that the copper wire is completely buried in the powder. S7: Under 99.99% pure nitrogen: heat to 300°C at 5°C-10°C / min and hold for 30 minutes; then cool to 40°C at 3°C-6°C / min; then heat to 600°C-700°C at 5°C-10°C / min and hold for 1-3 hours; then cool to room temperature. S8: Use an automatic grinder to grind the product surface, control the feed amount to 0.05mm, and expose the copper wire pad; S9: Conduct electrical performance and appearance inspection on the polished products, and qualified products will be transferred to subsequent assembly or packaging processes.

2. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S1, a T2-grade pure copper strip is used and stamped using a precision progressive die; the die gap is controlled to 8% of the material thickness, and the stamping speed is set to 120 times / minute; after stamping, the burr height of the copper terminal edge is ≤0.01mm, the surface roughness Ra is ≤1.6μm, and the resistivity is ≤0.0171Ω·mm 2 / m; low-viscosity synthetic ester oil is used as the stamping oil, and the residual oil film thickness after stamping is ≤0.5μm.

3. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S2, a three-dimensional coordinate measuring machine is used to detect the bending angle, pin spacing and flatness of the terminals. The sorting process is carried out in a Class 1000 clean environment, and unqualified products are isolated and then melted and recycled. Qualified products are transferred to a nitrogen protection cabinet and temporarily stored for ≤4 hours.

4. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S3, a two-stage cleaning process is used: first, ultrasonic cleaning in an alkaline degreasing agent at 60°C for 8 minutes, followed by rinsing in ultrapure water; second, ultrasonic cleaning in anhydrous ethanol for 5 minutes; after cleaning, the residual hydrocarbons on the surface are ≤20μg / cm 2 , chloride ion content <0.5ppm.

5. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S4, the temperature uniformity of the annealing furnace is ±3°C, and the oxygen content in the furnace is ≤3ppm; the copper wires are placed in a molybdenum boat in a stacked manner, and the layers are separated by zirconia spacers; a hydrogen and nitrogen mixture is introduced during the heating stage, and when the temperature is maintained at 650°C, the grain size increases to 40-55μm, and the Vickers hardness decreases from 125HV to 65-70HV; when cooling to 150°C, pure nitrogen protection is switched, and the cooling rate is ≤15°C / min.

6. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S5, the iron silicon powder is subjected to airflow classification to control D90≤22μm and coercivity≤7A / m; the lubricant zinc stearate is pre-dried in vacuum at 110°C for 4 hours; the mixing equipment is equipped with a temperature control jacket, and the powder Hall flow rate after mixing is 23±2s / 50g and the bulk density is 2.8±0.1g / cm 3 .

7. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S6, the mold is made of tungsten steel, and the cavity surface is coated with diamond-like coating; the mold temperature is maintained at 85±2°C during pressing, the upper punch speed is 15 mm / s, and the pressure fluctuation during the holding stage is ≤1%; the green density after molding is 7.25±0.05 g / cm 3 , radial bending strength ≥18MPa.

8. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S7, the cooling zone of the sintering furnace is equipped with a high-pressure nitrogen injection device; a nitrogen mixture containing 8% hydrogen is introduced during the 300°C holding stage, with a dew point of ≤-40°C; the furnace pressure is maintained at a slightly positive pressure of +15Pa during the 650°C holding stage; the density deviation of the sintered product is ≤0.3%, and the grain size distribution coefficient is ≤8%.

9. The method of improving cracking of thin inductors by regulating copper wire hardness through annealing as claimed in claim 1, characterized in that: In step S8, a diamond dressing roller is used to dress the grinding wheel, with a dressing feed of 0.0015 mm / stroke; the grinding fluid is a water-based emulsion, with a flow rate of 10 L / min; the height tolerance of the copper pad after grinding is ±0.01 mm, and the edge chipping area is ≤0.01 mm 2 .

10. The method of improving cracking of thin inductors by regulating the hardness of copper wires through annealing as claimed in claim 1, characterized in that: In step S9, the electrical performance test is performed in a constant temperature box at 23±0.5°C, with a test frequency of 1MHz±1kHz and a current of 0.5A RMS; The X-ray detection resolution is 3μm, and the internal crack detection threshold is 10μm; qualified products are stored in a drying oven with a dew point ≤-30℃, and anti-static aluminum foil bags are used during transportation.

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