Method for surface purification and microstructure stabilization of alloy material for soldering sheet

Abstract

The application discloses a solder sheet alloy material surface purification and structure stabilization treatment method, and relates to the technical field of electronic packaging welding materials; Sn, Ag, Cu and additive materials containing Ce, La and Mg are smelted under a low-oxygen or micro-reduction atmosphere to obtain Sn-Ag-Cu-based molten alloy; the molten alloy is solidified into a cast blank or a cast strip, and is subjected to multi-pass cold rolling, edge shaping and intermediate softening annealing to form a solder sheet plate strip; the plate strip is subjected to low-temperature aging at 95-115 DEG C for 3-6 h in a low-oxygen atmosphere, so that Ce, La and Mg are enriched along the cold rolling defect channels to the surface and the cold rolling structure is stabilized; then, the surface is exposed to a controlled atmosphere with a relative humidity of 35-45% at 22-28 DEG C for 30-90 min, so that a nanoscale self-passivation layer containing rare earth oxides and MgO is formed on the surface; finally, the solder sheet is obtained through punching, detection and protective packaging. The solder sheet alloy material has the advantages of storage and transportation stability, surface purifiability and flux-free welding applicability.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging soldering materials technology, specifically to a method for surface cleaning and microstructure stabilization treatment of alloy materials for solder sheets. Background Technology

[0002] Applications such as power semiconductor modules, SiC power devices, optical communication modules, and high-power laser packaging place high demands on solder joint void ratio, ion residue, thermal resistance, and long-term reliability. In fluxless or low-residue soldering processes, the thickness and composition of the oxide layer on the solder sheet surface directly affect the vacuum formic acid activation load, wetting stability, and solder joint quality.

[0003] Sn-Ag-Cu solders offer good cost and process compatibility, but they are prone to forming a surface layer dominated by Sn oxides during storage, transportation, and exposure before use. This surface layer typically requires strong reduction or activation conditions to remove, which can increase formic acid consumption, equipment corrosion load, and batch-to-batch fluctuations in solder joint void ratio.

[0004] In existing technologies, rare earth elements are mostly used as grain refiners in Sn-based solders or as intermetallic compound regulators, and the controllable enrichment, controlled oxidation, and subsequent cleanability of rare earth elements and Mg on the solder sheet surface have not been fully utilized. Although adding an external organic protective layer can isolate air in the short term, it may introduce organic residues, ionic contamination, or additional cleaning burdens in fluxless or vacuum formic acid soldering scenarios.

[0005] Therefore, there is a need for a comprehensive design approach that integrates alloy composition, smelting process, deformation heat treatment, controlled surface film formation, and protective packaging, so that the alloy material for solder sheets has a stable ultra-thin protective surface layer during storage and can achieve surface purification through existing vacuum formic acid processes during the soldering stage. Summary of the Invention

[0006] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a method for surface purification and microstructure stabilization of alloy materials for solder sheets. Addressing the problems of uncontrollable surface oxidation, high activation load after storage, and insufficient wetting stability in fluxless soldering of existing Sn-Ag-Cu solder sheets, this invention provides a method for surface purification and microstructure stabilization of alloy materials for solder sheets.

[0007] (II) Technical Solution To achieve the above objectives, the present invention is implemented through the following technical solution: a method for surface purification and microstructure stabilization of alloy materials for solder sheets, including low-oxygen smelting, rapid solidification, multi-pass cold rolling, edge shaping, low-temperature aging, controlled atmospheric film formation, stamping inspection, and protective packaging.

[0008] The alloy material used for the solder sheet is preferably a Sn-Ag-Cu based alloy, comprising, by mass, 0.3–4.0 wt% Ag, 0.1–1.0 wt% Cu, Ce greater than 0 and not exceeding 250 ppm, La greater than 0 and not exceeding 250 ppm, a combined Ce and La content of 30–500 ppm, Mg 50–800 ppm, with the balance being Sn and unavoidable impurities. More preferably, Ag is 2.5–3.5 wt%, Cu is 0.5–0.8 wt%, Ce is 70–150 ppm, La is 30–100 ppm, and Mg is 200–400 ppm.

[0009] Low-temperature aging is used to promote the enrichment of Ce, La and Mg along dislocation, grain boundary and free surface directions, while controlling the degree of recovery and recrystallization of β-Sn cold-rolled structure; controlled atmospheric exposure is used to form a composite nanoscale self-passivation layer containing rare earth oxides and MgO on the enriched surface.

[0010] The total thickness of the self-passivation layer is preferably 1.5–5 nm, more preferably 2.0–3.0 nm. The ratio of RE+Mg concentration in the 0–3 nm region of the surface to the RE+Mg concentration in the internal reference region 80–120 nm from the surface is not less than 10:1, preferably 20:1–50:1, where RE is Ce and La.

[0011] When the treated solder sheet is used in a vacuum formic acid atmosphere, the rare earth oxides and MgO can undergo an acid-base reaction with formic acid to generate the corresponding formate salts and remove them from the surface, thereby exposing the wettable Sn-Ag-Cu matrix.

[0012] (III) Beneficial Effects This invention provides a method for surface cleaning and microstructure stabilization of alloy materials for solder sheets, which has the following beneficial effects: By synergistically adding Ce / La and Mg at ppm levels, combined with low-temperature aging and controlled atmospheric exposure, a controlled-thickness rare-earth oxide and MgO composite self-passivation layer is formed on the solder sheet surface, preventing the continuous disorderly thickening of Sn oxides during storage. Combining low-temperature aging with microstructure stabilization helps to build an RE / Mg surface enrichment gradient while maintaining the processability of cold-rolled strip, providing a controllable prerequisite for subsequent surface cleaning.

[0013] By controlling the temperature, humidity, cleanliness, and corrosive gas content of the atmosphere, the traditional approach of avoiding oxidation is transformed into the formation of a controlled protective oxide layer, which helps improve the consistency of the surface condition of different batches of solder sheets.

[0014] It can be used with existing melting, rolling, stamping, aluminum-plastic packaging and vacuum formic acid welding equipment, without the need to introduce rosin, halogen or precious metal solder systems, and is suitable for power semiconductor and optoelectronic device packaging. Attached Figure Description

[0015] Figure 1 This is a schematic flowchart of the method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to the present invention. Figure 2 This is a schematic diagram of the process scenario of the continuous processing production line for alloy materials for solder sheets in Embodiment 1 of the present invention; Figure 3 This is a partial cross-sectional schematic diagram of the self-passivation structure of the solder sheet surface after the present invention and the surface purification result after vacuum formic acid activation. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Please see Figures 1-3 This invention provides a method for surface cleaning and microstructure stabilization treatment of alloy materials for solder sheets. Unless otherwise specified, ppm refers to mass ppm. Surface area refers to the area within 0 to 3 nm from the free surface of the solder sheet; internal reference area refers to the area within 80 to 120 nm from the surface after depth analysis.

[0018] The thickness of the self-passivation layer can be determined by XPS-Ar+ etching depth analysis; the relative contents of RE, Mg, Sn and O can be determined by XPS, GDOES, SIMS or equivalent surface analysis methods.

[0019] The composition of the matrix alloy can be determined by ICP-MS, ICP-OES, OES or equivalent methods; rapid adjustments during the melting process can be made by XRF or direct-reading photoelectric spectroscopy.

[0020] Solder wettability, void ratio, shear strength, high-temperature aging, and thermal cycling reliability can be tested according to common methods in the electronic packaging industry or the applicant's enterprise standards.

[0021] Example 1: Treatment of alloy materials for 50 μm solder sheets This embodiment takes a 1,000 kg heat cycle as an example, and the target finished alloy composition is shown in the table below.

[0022]

[0023] Considering the presence of Sn in the master alloy, the basic feed quality in this embodiment is shown in the table below. If the previous trimmed scrap is used as remelting material, the new material is deducted based on the measured content of Sn, Ag, Cu, Ce, La, and Mg in the remelting material, and the master alloy is added according to the target composition; if the melting process shows that Ce, La, or Mg is below the target range, compensatory feeding is carried out using the corresponding master alloy.

[0024]

[0025] Master alloy preparation Sn-10 wt% Ce master alloy can be prepared in a vacuum induction furnace. First, a vacuum is drawn to a low pressure, then Ar is introduced, and high-purity Sn is melted at approximately 400°C. Next, metallic Ce, whose surface oxide layer has been removed under an inert atmosphere, is added. After homogenization by electromagnetic stirring, it is cast into bars and stored in an air-isolated environment. Sn-10 wt% La master alloy can be prepared using the same method.

[0026] Sn-5 wt% Mg master alloy can be prepared by a combination of mechanical alloying and rapid solidification. High-purity Sn is heated to approximately 300°C, and high-purity Mg chips or granules are added under nitrogen protection and vigorous stirring. After dispersion, the mixture is rapidly solidified to form thin strips or bars. The master alloy is preferably used on the same day it is prepared, or stored for a short period under mineral oil, an inert atmosphere, or vacuum packaging to reduce oxidation loss.

[0027] Smelting and process control High-purity Sn ingots were added to an induction melting furnace. After evacuation, high-purity N2 or a mixture of N2 and 0.5 vol% H2 was introduced to replace the residual oxygen in the furnace until it was less than 1 ppm. The Sn ingots were heated to approximately 315°C and held at that temperature. Ag particles and Cu particles were then added sequentially and stirred until homogenized.

[0028] A master alloy of Sn-10 wt% Ce, Sn-10 wt% La, and Sn-5 wt% Mg was added at approximately 315°C. After addition, the mixture was briefly intensified and then gently homogenized. After removing scum, a sample was taken from the center of the melt and rapidly solidified into a disc sample. XRF or direct-reading photoelectric spectroscopy was used for process control; ICP-MS or ICP-OES was used for offline final analysis. If necessary, the master alloy was replenished based on the test results to bring the Ce, La, and Mg contents to the target range. Compensatory feeding can be determined based on test deviations and scum loss; for example, the initial compensation window for Ce and La can be set at 1.00–1.06 times the target amount, and for Mg at 1.00–1.15 times the target amount, with final correction based on the composition test results.

[0029] Continuous casting The molten alloy is fed into a twin-roll continuous casting apparatus via an insulated flow channel, and the thickness of the cast strip is controlled to be 3–8 mm; in this embodiment, the thickness of the cast strip is approximately 4.8 mm, and the solidification cooling rate is approximately 350°C / min. The resulting cast strip should be free of visible coarse rare earth tin oxide particles.

[0030] Multi-pass cold rolling, edge shaping and recycle material management The cast strip is cold-rolled in multiple passes to produce a strip approximately 50 μm thick. The cold rolling process includes main rolling, finishing rolling, and final rolling stages; the main rolling stage can roll the 4.8 mm cast strip to approximately 0.48 mm, followed by finishing rolling to approximately 0.08 mm, and then final rolling to approximately 0.05 mm. The reduction rate for each pass is controlled according to the equipment capacity.

[0031] When plasticity needs to be restored, intermediate softening annealing can be performed at 60–95°C for 30–120 minutes; in this embodiment, intermediate softening annealing at 80°C for about 50 minutes can be used. In the final rolling stage, the rolling direction of the strip into the next rolling mill can be changed by the guide rollers to achieve continuous bidirectional cross re-rolling, thereby reducing the β-Sn preferred texture caused by unidirectional rolling.

[0032] After the first stage of cold rolling, the edges of the strip are shaped, and approximately 5 mm can be removed from one side using a disc shearing machine; after finishing rolling, approximately 2 mm can be removed from one side again. The removed edge scrap is sealed and recycled as subsequent smelting remelting material, and new Sn, Ag, Cu, and RE / Mg master alloy are added based on the measured composition of the remelting material.

[0033] Low temperature aging The cold-rolled strip is placed in a low-oxygen atmosphere for low-temperature aging. Preferred conditions are: N2 + 0.5 vol% H2 atmosphere, residual oxygen less than 1 ppm, dew point below -60°C; heating to 105°C at approximately 2.5°C / min, holding at that temperature for 4 h, and then slowly cooling to room temperature at a rate not exceeding 2°C / min. This step promotes the enrichment of Ce, La, and Mg along the defect channels formed during cold rolling towards the surface and stabilizes the cold-rolled microstructure.

[0034] In a continuous conveyor belt cryogenic aging furnace, a combination structure of a preheating section, a core isothermal section, and a slow cooling section can be adopted. The conveyor belt speed in the core isothermal section should be matched with the effective length of the furnace cavity so that the conveyor belt stays in the core isothermal section for about 4 hours; for example, when the effective length of the core isothermal section is 12 m, the conveyor belt speed can be controlled to be about 0.050 m / min.

[0035] Controlled atmospheric exposure After low-temperature aging, the strips are transferred to a clean, controlled exposure environment using air that has undergone HEPA filtration and corrosive gas adsorption or catalytic purification. The exposure conditions are: temperature 25°C, relative humidity 40%, air cleanliness level Class 1000 or higher, preferably Class 100, laminar flow velocity approximately 0.4 m / s, and exposure time 60 min.

[0036] After controlled exposure, a self-passivation layer comprising rare earth oxides and MgO is formed on the surface of the strip. Preferably, the thickness of the self-passivation layer is controlled to be about 2.5 nm; the enrichment ratio of the total RE+Mg concentration in the 0-3 nm region of the surface to the enrichment ratio of the internal reference region 80-120 nm from the surface is controlled to be 20:1-50:1; and the ratio of the total molar amount of rare earth oxides and MgO to the molar amount of SnO2 is in the range of 2-4.

[0037] Stamping, testing and packaging The strip with the self-passivation layer is punched or cut to obtain solder sheets of the target size. For example, it can be punched to a size of 10 mm × 10 mm × 0.05 mm. The punching process is preferably carried out in a nitrogen-purged environment.

[0038] Finished products can be batch-wise analyzed by ICP-MS, XPS depth profiling, and GDOES scanning. The tests include the content of Ce, La, and Mg in the matrix, the thickness of the self-passivation layer, the surface RE+Mg enrichment ratio, and the composition of surface oxides.

[0039] After passing the inspection, the finished product is placed in an aluminum-plastic composite barrier bag, vacuumed, filled with high-purity nitrogen, and sealed with a desiccant and deoxidizer. Preferably, the water vapor transmission rate of the inner packaging is less than 0.05 g / m2·day, and the oxygen transmission rate is less than 0.5 cc / m2·day·atm; the vacuum degree before sealing is less than 500 Pa, and the purity of the nitrogen gas is not less than 99.999%.

[0040] Example 2: Treatment scheme with lower rare earth / Mg content In another embodiment, the base alloy comprises 2.5 wt% Ag, 0.6 wt% Cu, 70 ppm Ce, 30 ppm La, and 200 ppm Mg, with the balance being Sn and unavoidable impurities. Unless otherwise specified, the melting, casting, cold rolling, edge shaping, controlled atmosphere exposure, stamping inspection, and packaging steps are the same as in Example 1; the cold-rolled strip is held at 95°C for 6 h, followed by exposure at 25°C and 35% relative humidity for 90 min. This embodiment is suitable for products requiring lower Mg content and extended aging time to achieve surface enrichment.

[0041] Example 3: Treatment scheme for higher rare earth / Mg content In another embodiment, the matrix alloy comprises 3.5 wt% Ag, 0.8 wt% Cu, 150 ppm Ce, 100 ppm La, and 400 ppm Mg, with the balance being Sn and unavoidable impurities. Unless otherwise specified, the melting, casting, cold rolling, edge shaping, controlled atmosphere exposure, stamping inspection, and packaging steps are the same as in Example 1; the cold-rolled strip is held at 115°C for 3 h, followed by exposure at 22–28°C and 40% relative humidity for 30–60 min. This embodiment is suitable for products requiring a high surface enrichment rate.

[0042] Example 4: Matching vacuum formic acid activation and welding When using the solder sheet treated as described above for soldering, it can be placed between a chip, substrate, housing, or other parts to be soldered, and surface activation can be performed in a vacuum formic acid atmosphere. Preferred activation conditions include: preheating to 130–145°C and holding for about 60 s; introducing formic acid vapor at 190–210°C, with a formic acid vapor partial pressure of 800–2500 Pa, a chamber back pressure of 1500–3000 Pa, a nitrogen carrier gas flow rate of 3–8 slm, and activation holding for 60–180 s.

[0043] In this process, rare earth oxides and MgO undergo an acid-base reaction with formic acid to generate corresponding formates, which are then removed from the solder surface. For example, La₂O₃ can react with formic acid to form La(HCOO)₃, and MgO can react with formic acid to form Mg(HCOO)₂. Due to the thin thickness and controlled composition of the self-passivation layer, the Sn-Ag-Cu substrate can be exposed after activation, which is beneficial for subsequent wetting and solder joint formation.

[0044] Scale settings To illustrate the function of each technical feature, a comparative example can be set up according to the table below. Except for the differences in the table, the comparative example preferably uses the same total deformation, cold rolling process, testing method, and terminal vacuum formic acid activation conditions as Example 1.

[0045]

[0046] The comparative examples described above were used to investigate the effects of rare earth elements, magnesium, controlled atmospheric film formation steps, intrinsically derived passivation layer, and low-temperature aging window on the surface layer structure of the finished product. The results of the comparative examples can be evaluated using indicators such as passivation layer thickness, oxide composition, RE+Mg depth gradient, oxide layer growth after storage, residual oxide layer after formic acid activation, wettability, void ratio, and shear strength.

[0047] Testing items

[0048] Industrial applicability The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets of the present invention can be used for the mass production of electronic packaging soldering materials, and is particularly suitable for power semiconductor chip mounting, SiC power module packaging, hermetically sealed packaging of optical communication modules, and high-power laser packaging. This invention can be integrated with existing melting, rolling, stamping, aluminum-plastic packaging, and vacuum formic acid welding equipment, and has industrial application value.

[0049] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for surface cleaning and microstructure stabilization treatment of alloy materials for solder sheets, characterized in that: The process includes the following steps: S1, melting Sn, Ag, Cu, and additives containing Ce, La, and Mg in a low-oxygen or slightly reducing atmosphere to obtain a Sn-Ag-Cu based molten alloy, wherein the Sn-Ag-Cu based molten alloy comprises, by mass, 0.3–4.0 wt% Ag, 0.1–1.0 wt% Cu, Ce greater than 0 and not exceeding 250 ppm, La greater than 0 and not exceeding 250 ppm, a total content of Ce and La of 30–500 ppm, Mg of 50–800 ppm, with the balance being Sn and unavoidable impurities; S2, solidifying the molten alloy to form a billet or strip; S3, subjecting the billet or strip to multiple cold rolling passes and edge shaping to obtain a strip for solder sheet; S4, subjecting the strip to a low-oxygen atmosphere at 95–115°C for 3–6 passes. h. Low-temperature aging treatment to enrich Ce, La, and Mg on the surface of the strip and stabilize the cold-rolled structure; S5. Expose the low-temperature aged strip to a controlled atmosphere at a temperature of 22-28°C and a relative humidity of 35-45% for 30-90 min to form a nanoscale self-passivation layer containing rare earth oxides and MgO on its surface; S6. Punch or cut the strip with the self-passivation layer and perform protective packaging.

2. The alloy material surface purification and microstructure stabilization treatment method for a solder sheet according to claim 1, characterized by: The Sn-Ag-Cu based molten alloy comprises, by mass: 2.5–3.5 wt% Ag, 0.5–0.8 wt% Cu, 70–150 ppm Ce, 30–100 ppm La, and 200–400 ppm Mg, with the balance being Sn and unavoidable impurities; the Ce / La mass ratio is 1.5–3.0, and the RE / Mg molar ratio is 0.05–2.0, wherein RE is Ce and La.

3. The alloy material surface purification and microstructure stabilization treatment method for a solder sheet according to claim 2, characterized by: In step S1, Ce, La and Mg are added as Sn-Ce master alloy, Sn-La master alloy and Sn-Mg master alloy respectively; the master alloys are prepared, stored and fed under an inert atmosphere or a low oxygen atmosphere, and the feeding is compensated according to the oxidation loss of rare earth elements and the volatilization loss of Mg during the smelting process.

4. The alloy material surface purification and microstructure stabilization treatment method for a solder sheet according to claim 3, characterized by: In step S1, the melting temperature is 305–325°C; the melting atmosphere is high-purity nitrogen with an oxygen content of less than 5 ppm, or nitrogen with an oxygen content of less than 1 ppm and containing 0.1–0.5 vol% hydrogen; XRF or photoelectric direct-reading spectroscopy is used for rapid process control during the melting process, and ICP-MS or ICP-OES is used for offline final inspection.

5. The alloy material surface purification and microstructure stabilization treatment method for a solder sheet according to claim 4, characterized by: In step S2, the thickness of the billet or strip is 3-8 mm, and the solidification cooling rate is 300-400°C / min. In step S3, the total reduction in the thickness direction of the multi-pass cold rolling is 70-95%, the single-pass reduction is 8-25%, the roll temperature is room temperature to 80°C, and the bidirectional cross-rolling is achieved by changing the rolling direction through guide rolls in the final rolling stage.

6. The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to claim 5, characterized in that: In step S3, the strip is edge-cut after roughing or finishing rolling, and the edge waste is sealed and recycled as subsequent smelting remelting material; during cold rolling, intermediate softening annealing is set, the intermediate softening annealing temperature is 60-95°C, and the time is 30-120 min.

7. The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to claim 6, characterized in that: In step S4, the heating rate of low-temperature aging is 2-4°C / min; in the continuous conveyor belt aging furnace, the conveyor belt speed of the core isothermal section allows the strip to remain in the core isothermal section for 3-6 hours. When the effective length of the core isothermal section is 12 m and the target residence time is 4 hours, the conveyor belt speed is controlled at 0.050 m / min.

8. The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to claim 7, characterized in that: In step S5, the controlled atmosphere is filtered by HEPA and treated with adsorption or catalytic purification of corrosive gases, with an air cleanliness level of Class 1000 or higher and an air velocity of 0.05 to 2 m / s.

9. The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to claim 8, characterized in that: The surface area of ​​the alloy material used for the treated solder sheet is within a range of 0 to 3 nm from the surface; the thickness of the self-passivation layer is 1.5 to 5 nm; the ratio of RE+Mg concentration in the surface area to the RE+Mg concentration in the internal reference area 80 to 120 nm from the surface is not less than 10:1; the ratio of the total molar amount of rare earth oxides and MgO in the self-passivation layer to the molar amount of SnO2 is not less than 1.

5.

10. The method for surface cleaning and microstructure stabilization of alloy materials for solder sheets according to claim 1, characterized in that: The protective packaging includes aluminum-plastic composite barrier packaging and nitrogen-filled encapsulation. Before encapsulation, the vacuum degree is less than 10 kPa, and the purity of the nitrogen gas is not less than 99.5%. Before welding, the solder sheet is surface activated with alloy material in a vacuum formic acid atmosphere. The activation temperature is 170-230°C, the formic acid vapor partial pressure is 100-8000 Pa, and the activation holding time is 30-600 s.

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