Low temperature soldering molten activated coating forming method

Abstract

This invention discloses a low-temperature solder molten activation coating method, comprising: a preliminary cleaning step: immersing the metal casing to be soldered sequentially in solvent-based oil stain cleaner, acetone, and DI water for cleaning, followed by drying with a nitrogen gun; a plasma cleaning step: subjecting the pre-cleaned metal casing to plasma cleaning in a plasma cleaner; a solder paste application step: applying solder paste multiple times to the surface of the metal casing to be soldered, ensuring complete coverage, then placing the solder wire on the solder paste and ensuring close adhesion; and a molten activation step: placing the coated metal casing in a chain sintering furnace for pre-melting of the solder wire to obtain a molten activated metal casing. This invention achieves reduced surface tension and increased surface wetting through a series of cleaning operations, including preliminary cleaning, secondary plasma cleaning for surface activation, decomposition of the oxide layer by the solder paste flux, and hydrogen atmosphere reduction.

Description

Technical Field

[0001] This invention relates to the field of semiconductor optoelectronic device manufacturing technology, and specifically to a low-temperature solder molten state activation coating forming method. Background Technology

[0002] Semiconductor optoelectronic device packaging technology is a key process to ensure the reliability and performance of optoelectronic devices. It involves encapsulating semiconductor chips in a protective casing to provide mechanical support, optical pathways, electrical connections, and environmental protection. Among these processes, the sealing between the semiconductor metal casing and the optical components is a crucial step in optoelectronic device manufacturing, achieving a highly reliable connection between the metal structural components and the optical elements.

[0003] To meet the functional requirements of optoelectronic devices, the metal casing and optical components need to be hermetically sealed and maintain the required vacuum level over a long period. Brazing is one of the commonly used methods for sealing metal casings, and it has the following core characteristics: First, because the brazing temperature is much lower than the melting point of the base material, it can effectively avoid damage to the antireflective coating of the optical components caused by high temperatures, making it particularly suitable for packaging temperature-sensitive optical components. Second, using a vacuum environment or reducing atmosphere for brazing can significantly reduce oxidation and ensure the sealing of the weld joint, which is crucial for optoelectronic devices that require long-term stable operation. Third, the molten brazing filler metal uniformly fills the interface, reducing deformation and ensuring the flatness of the optical window, which is essential for maintaining optical performance. Fourth, the brazed joint has superior mechanical properties, better resists external forces, and minimizes stress concentration. Therefore, compared with low-temperature adhesive bonding, high-frequency sealing, and high-temperature sealing methods, it has advantages such as low-temperature processing, meeting high hermeticity requirements, precise flatness control, and excellent mechanical properties.

[0004] When brazing metal casings, the oxide layer on the surface to be brazed must first be removed to obtain a weldable surface, ensuring sufficient flow and wetting of the solder during the welding process. Common methods include: First, electroplating the metal casing with gold and pre-fabricating a solder ring. This results in low void rates and good quality. However, the gold plating process and solder ring pre-fabrication are costly, and the solder ring specifications are not widely applicable, leading to a long preparation cycle and unsuitability for multi-specification product manufacturing. Second, obtaining a composite film layer on the metal casing surface to be brazed through vacuum deposition. However, the utilization rate of the film material or target material is low and consumption is extremely high, resulting in high costs and making mass production impossible; this method is suitable for research and experimentation. Third, manually applying molten solder, using a flame or soldering iron to apply molten solder to the metal casing surface to be brazed. This method has lower equipment requirements, eliminates the need for metal casing plating, and eliminates the need for custom solder rings, significantly reducing process costs. However, its process efficiency and yield are highly dependent on the operator's skill level.

[0005] Considering the above methods, the first two solutions rely too heavily on gold plating of the metal casing and pre-formed welding rings, resulting in high costs and long cycles. From the perspectives of increasing production capacity and reducing costs and improving efficiency, their optimization potential is limited. The third solution, which replaces manual operation with equipment, improves efficiency while ensuring product process consistency and increasing yield, is a more feasible option. However, relying solely on vacuum brazing furnaces or reducing atmosphere eutectic furnaces for low-temperature molten activation coating of the solder cannot guarantee sufficient flow and wetting of the solder surface during melting. The former only ensures a vacuum environment during pre-melting, preventing further oxidation of the solder surface and solder, but it cannot remove the oxide layer from already oxidized surfaces and solder. While the latter provides some reduction capability, it cannot completely remove the oxide layer, ensuring sufficient solder flow and wetting. Therefore, this situation needs to be considered, and certain measures need to be taken to address the problems analyzed above. Summary of the Invention

[0006] The metal casing, influenced by both machining and storage environment, develops intrinsic surface structures with varying physicochemical properties. From the inside out, these consist of a substrate, a work-hardened layer, an oxide layer, an adsorption layer, and a contamination layer. The latter three surface layers significantly affect the spread of molten solder on the metal casing surface. For high-precision sealing of infrared detector windows, it is essential to thoroughly remove surface organic impurities and oxide films to ensure an extremely clean surface, thus enabling subsequent high-precision processes such as welding and achieving the required ultra-high hermeticity and long-term vacuum life.

[0007] The removal of the surface layer of a metal casing must follow a "from the outside in, layer by layer" sequence, meaning the contaminant layer, adsorption layer, and oxide layer must be removed step by step. Skipping the contaminant and adsorption layers and directly removing the oxide layer will lead to: ① Oil and organic matter obstructing effective contact between the reducing agent and the oxide layer, resulting in incomplete and uneven reduction; ② Some surface contaminants may react with the reducing agent, producing toxic gases, splashes, or heat, posing safety hazards; ③ Even if the oxide layer is barely removed, the exposed fresh metal surface will be secondary contaminated by residual contaminants, forming a weak interface layer. This will result in extremely poor adhesion of subsequent interfaces, significantly impacting the airtight sealing.

[0008] Therefore, this invention first uses solvent-based oil stain cleaners, acetone, and DI water to remove grease, dust, moisture, and loose particles with relatively weak surface adhesion. Then, plasma cleaning is used to remove residual contaminants and organic matter with stronger adhesion, ensuring that the physicochemical barriers created by the contamination and adsorption layers are completely broken down. Based on this cleaning, the reducing agent in the solder paste removes the dense oxide layer and passivation film, exposing the highly reactive base metal. At this point, the metal surface is very active and easily re-oxidizes. Under the hydrogen reducing atmosphere of the chain sintering furnace, oxygen from the surrounding environment is effectively eliminated, preventing direct contact with the base metal. At its melting point temperature, it tightly bonds with the molten solder in the solder paste, providing a certain degree of isolation from base metal oxidation. Simultaneously, the hydrogen reducing atmosphere effectively reduces the oxide layer on the surface of the solder wire, ensuring compensation for insufficient solder in the solder paste and effectively wetting the previously molten solder, achieving the purpose of molten activation and coating of the solder.

[0009] Specifically, the low-temperature solder molten activation coating method includes:

[0010] Preliminary cleaning steps: Immerse the metal casing to be welded in solvent-based oil stain cleaner, acetone and DI water in sequence, and then dry it with a nitrogen gun.

[0011] Plasma cleaning steps: The pre-cleaned metal casing is then subjected to plasma cleaning in a plasma cleaner. The working atmosphere of the plasma cleaner is a mixture of argon and oxygen. Oxygen, as a highly reactive gas, can effectively decompose organic pollutants or organic substrate surfaces. However, its ions are relatively small, and its bond-breaking and bombardment capabilities are limited. After adding a certain proportion of argon, the generated plasma will have a stronger bond-breaking and decomposition capability on organic pollutants or organic substrate surfaces, thus accelerating the cleaning and activation efficiency.

[0012] During plasma cleaning, physical and chemical reactions occur simultaneously, and the principle is as follows:

[0013] Physical cleaning:

[0014] Ar+e - →Ar + +2e - ;

[0015] Ar + +pollutant → volatile pollutant;

[0016] Chemical cleaning:

[0017] O2+ e - →2O * +e - ;

[0018] O* +Organic matter → CO2 + H2O.

[0019] Solder paste application steps: Apply solder paste multiple times to the metal casing surface to be soldered, ensuring the solder paste completely covers the surface. Then place the welding wire on the solder paste and ensure it adheres tightly to the solder paste.

[0020] Melt activation step: The coated metal shell is placed in a chain sintering furnace for pre-melting of the welding wire to obtain a molten activated metal shell.

[0021] The working atmosphere of the chain sintering furnace is a reducing protective atmosphere.

[0022] The plasma cleaning includes:

[0023] One-step cleaning: Ion cleaner parameters are: vacuum degree 1.0 × 10⁻⁶ 2 ~1.5×10 2 Pa, oxygen 100±5 sccm, argon 100±5 sccm, power 100±2W, time 30±1s;

[0024] Secondary cleaning: Ion cleaner parameters are: vacuum degree 0.35×10 2 ~0.5×10 2 Pa, oxygen 250±5 sccm, argon 500±10 sccm, power 240±5 W, time 25±1 s.

[0025] Two plasma cleaning processes achieve a progressive treatment from basic cleaning to deep activation. Its core purpose is to utilize different gas ratios, vacuum levels, and power combinations to target the two key steps of organic contaminant removal and inorganic residue removal / surface activation, respectively, to obtain better surface treatment results.

[0026] Furthermore, in the solder paste application step, the solder paste is a tin-silver-copper solder paste with added hydrogenated rosin flux. Hydrogenated rosin has a carbonization point at least 50°C higher than ordinary rosin. When used in a hydrogen reducing atmosphere, it exhibits strong thermal stability and its residues are less prone to carbonization at high temperatures, thus improving its tolerance in reducing atmospheres. Its liquidus temperature is approximately 217°C.

[0027] The metal oxides on the surface of the metal casing are mainly iron oxides, including FeO, Fe2O3, and Fe3O4. The principle of rosin flux in removing the metal oxide film is as follows:

[0028] 2RCOOH + FeO → Fe(RCOO)2 + H2O (under heating conditions);

[0029] 6RCOOH + Fe₂O₃ → 2Fe(RCOO)₃ + 3H₂O (under heating conditions);

[0030] 16RCOOH + 2Fe3O4 → 2Fe(RCOO)2 + 4Fe(RCOO)3 + 8H2O (under heating conditions);

[0031] The above reaction is carried out at a temperature above 170°C. Fe(RCOO)2 and Fe(RCOO)3 are soluble in the flux solvent, thereby removing metal oxides.

[0032] Furthermore, in the solder paste application step, the metal shell is cylindrical, and the welding wire is a tin-silver-copper alloy welding wire wound into a coil.

[0033] Furthermore, in the solder paste application step, after the solder wire is placed on the solder paste, it is compacted with a pressure block.

[0034] Furthermore, the reducing protective atmosphere in the molten activation step is a mixture of nitrogen and hydrogen. The parameters of the chain sintering furnace are: belt speed 150±5 mm / min, pre-melting temperature 250±5℃, and inlet / outlet process atmosphere flow rate 60±3 L / min.

[0035] At high temperatures, the metal oxides of iron (including FeO, Fe2O3, and Fe3O4) reduced by hydrogen are pure metals. In the following reaction process, hydrogen needs to be continuously introduced until cooling to prevent the metal from being oxidized again.

[0036] H2 + FeO → Fe + H2O (high temperature conditions);

[0037] 3H₂ + Fe₂O₃ → 2Fe + 3H₂O (high temperature conditions);

[0038] 4H₂ + Fe₃O₄ → 3Fe + 4H₂O (high temperature conditions).

[0039] The beneficial effects of this invention are:

[0040] (1) This invention achieves the effect of reducing surface tension and increasing surface wetting by a series of cleaning operations on the surface to be soldered, including preliminary cleaning, secondary plasma cleaning for surface activation, decomposition of oxide layer by solder paste flux, and reduction in hydrogen atmosphere. At the same time, the equipment realizes semi-automated pre-melting production, which improves production efficiency, can replace the existing manual operation method, and the process results are more consistent.

[0041] (2) Traditional welding methods require the simultaneous realization of interfacial metallurgical bonding between the solder and the base materials on both sides. If a welding defect occurs on one side, it may lead to failure of airtightness. However, the present invention can effectively ensure that the solder on one side forms a highly reliable interfacial bond with the substrate, and is applicable to a variety of subsequent welding methods, including but not limited to vacuum reflow soldering, high-frequency vacuum brazing, and high-energy beam brazing.

[0042] (3) The oxide layer on the metal shell to be soldered is removed more thoroughly, resulting in a clean and highly active surface, which ensures that the solder flows and wets the surface when it melts. Attached Figure Description

[0043] Figure 1 This is a schematic diagram illustrating the wetting principle.

[0044] Figure 2 This is a flowchart of the low-temperature solder molten state activation coating forming method in Example 1;

[0045] Figure 3 This is a schematic diagram of the working principle of the plasma cleaner in Example 1;

[0046] Figure 4 This is a schematic diagram of the solder paste application on the metal casing surface to be soldered in Example 1;

[0047] Figure 5 This is a photograph of the solder wetting condition on the metal casing surface to be soldered in Example 1.

[0048] Figure 6 This is a photograph of the solder wetting condition on the metal casing surface to be soldered in Example 2;

[0049] Figure 7 This is a photograph of the solder wetting condition on the metal casing surface to be soldered in Example 3. Detailed Implementation

[0050] The present invention will be further described in detail below through specific embodiments.

[0051] like Figure 1 As shown, wetting refers to the ability of a liquid to spread or adhere to a solid surface, and the degree of wetting is usually measured by the wetting angle. The wetting angle is the angle between the tangent of the liquid surface and the solid surface at the point of contact between the solid, liquid, and gas phases (usually pointing towards the interior of the liquid). The relationship between them can be expressed by Young's equation:

[0052] ;

[0053] Where, γ SL γ is the surface tension between solid and liquid. SA γ represents the surface tension between solid and gas. LA Let θ be the surface tension between the liquid and the gas, and θ be the wetting angle.

[0054] It is generally believed that:

[0055] When θ < 90°, the liquid can wet the solid surface; the smaller the angle, the better the wettability.

[0056] When θ > 90°, the liquid cannot wet the solid surface; the larger the angle, the worse the wettability.

[0057] When θ = 0°, it is called complete wetting or spreading; when θ = 180°, it is completely non-wetting.

[0058] Plasma cleaners use a high-frequency voltage to ionize a working gas (Ar, O2, N2, H2, CF4, etc.) into plasma. This plasma contains numerous active particles (such as ions, free electrons, and activated atoms). These particles collide and react with the dirt on the surface of the object being cleaned. Through physical bombardment and chemical reaction, organic contaminants and oxide layers are removed from the surface. Volatile substances generated during the cleaning process are removed by a vacuum pump, thus achieving the cleaning effect. The principle is as follows: Figure 2 As shown.

[0059] Chain sintering furnace: also known as mesh belt / chain belt continuous sintering furnace, is a high-efficiency sintering equipment. The material is evenly distributed on the conveyor belt of the chain sintering furnace. Through high-temperature heating and sintering process, the material particles are combined with each other to form a dense sintered body.

[0060] Example 1

[0061] like Figure 3 As shown, this embodiment discloses a low-temperature solder molten state activation coating forming method, including:

[0062] Preliminary cleaning steps: Immerse the cylindrical metal casing (4J29 alloy) to be welded in solvent-based degreaser, acetone, and DI water sequentially. Each step takes approximately 3 minutes. After cleaning, dry the casing with a nitrogen gun. The preliminary cleaning mainly removes oil stains, adhesive residue, dirt, and rust from the metal surface, ensuring there are no visible surface defects.

[0063] Plasma cleaning steps: The pre-cleaned metal casing undergoes plasma cleaning in a plasma cleaner. The working atmosphere of the plasma cleaner is a mixture of argon and oxygen. The plasma cleaning process includes two steps: a primary cleaning and a secondary cleaning. The parameters for the primary cleaning are: vacuum 100 Pa, oxygen 100 sccm, argon 100 sccm, power 100 W, and time 30 s. The parameters for the secondary cleaning are: vacuum 35 Pa, oxygen 250 sccm, argon 500 sccm, power 240 W, and time 25 s.

[0064] Solder paste application steps: as follows Figure 4As shown, apply solder paste (a tin-silver-copper solder paste with added hydrogenated rosin flux) multiple times to the surface of the metal casing to be soldered, ensuring complete coverage with approximately 0.1mm of solder paste. Then, cut a coil of tin-silver-copper alloy solder wire (φ1.0mm) and, ensuring the solder coil is flat, place it on top of the solder paste and press it firmly with a pressure block to ensure a tight fit between the solder coil and the solder paste.

[0065] Molten activation step: The coated metal shell is placed in a chain sintering furnace for pre-melting of the welding wire to obtain a molten activated metal shell. The working atmosphere of the chain sintering furnace is a reducing protective atmosphere of a mixture of nitrogen and hydrogen, with a volume percentage of approximately 90% nitrogen and 10% hydrogen. The parameters of the chain sintering furnace are: belt speed 150 mm / min, pre-melting temperature 250℃±5℃, and inlet and outlet process atmosphere flow rate 60 L / min.

[0066] The existing vacuum brazing furnace ( Figure 5 (a) shown) and reducing atmosphere eutectic furnace ( Figure 5 (b) The solder activation coating method shown in this invention is similar to that of the present invention. Figure 5 By comparing (c) and (d), it can be seen that the solder of the present invention flows and wets the surface to be soldered when it melts.

[0067] Example 2

[0068] Based on Example 1, in the plasma cleaning step, the parameters of the ion cleaner for the first cleaning were set as follows: vacuum degree 120 Pa, oxygen 100 sccm, argon 100 sccm, power 100 W, and time 30 s; the parameters for the second cleaning were set as follows: vacuum degree 42.5 Pa, oxygen 250 sccm, argon 500 sccm, power 240 W, and time 25 s. The remaining processes remained unchanged. The final test sample wetting condition was as follows: Figure 6 As shown, both sample #1 and sample #2 were well wetted.

[0069] Example 3

[0070] Based on Example 1, the parameters for the primary cleaning ion cleaner are: vacuum 120 Pa, oxygen 100 sccm, argon 100 sccm, power 100 W, and time 30 s; the parameters for the secondary cleaning ion cleaner are: vacuum 42.5 Pa, oxygen 250 sccm, argon 500 sccm, power 240 W, and time 25 s. Figure 7 As shown, both sample #3 and sample #4 were well wetted.

[0071] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. A method for low-temperature solder molten activation coating and forming, characterized in that, include: Preliminary cleaning steps; Plasma cleaning steps; Solder paste application steps: Apply solder paste to the metal casing surface to be soldered multiple times to completely cover the surface. Then place the welding wire on the solder paste and ensure it adheres tightly to the solder paste. Melt activation step: The coated metal shell is placed in a chain sintering furnace for pre-melting of the welding wire to obtain a molten activated metal shell.

2. The method as described in claim 1, characterized in that, The preliminary cleaning steps include: immersing the metal casing to be welded in solvent-based oil stain cleaner, acetone, and DI water in sequence for cleaning, and then drying it with a nitrogen gun.

3. The method as described in claim 1, characterized in that, The plasma cleaning step includes: performing plasma cleaning on the pre-cleaned metal casing in a plasma cleaner, wherein the working atmosphere of the plasma cleaner is a mixture of argon and oxygen.

4. The method as described in claim 3, characterized in that, The plasma cleaning includes: One-step cleaning: Ion cleaner parameters are: vacuum degree 1.0 × 10⁻⁶ 2 ~1.5×10 2 Pa, oxygen 100±5 sccm, argon 100±5 sccm, power 100±2W, time 30±1s; Secondary cleaning: Ion cleaner parameters are: vacuum degree 0.35×10 2 ~0.5×10 2 Pa, oxygen 250±5 sccm, argon 500±10 sccm, power 240±5 W, time 25±1 s.

5. The method as described in claim 1, characterized in that, In the solder paste application step, the solder paste is a tin-silver-copper solder paste with added hydrogenated rosin flux.

6. The method as described in claim 1, characterized in that, In the solder paste application step, the metal shell is cylindrical, and the welding wire is a tin-silver-copper alloy welding wire wound into a coil.

7. The method as described in claim 1, characterized in that, In the solder paste application step, after the solder wire is placed on the solder paste, it is compacted with a pressure block.

8. The method as described in claim 1, characterized in that, The reducing protective atmosphere in the melt activation step is a mixture of nitrogen and hydrogen.

9. The method as described in claim 1, characterized in that, In the molten activation step, the parameters of the chain sintering furnace are: belt speed 150±5mm / min, pre-melting temperature 250±5℃, and inlet and outlet process atmosphere flow rate 60±3L / min.

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