A Cu-Sn based solder for high-reliability welding joints and its preparation method

Abstract

This invention belongs to the field of new materials technology and discloses a Cu-Sn based solder for high-reliability welding joints and its preparation method. The preparation method includes: polishing, cleaning, and glow discharge sputtering the substrate to obtain a clean substrate; depositing a pure copper layer on the clean substrate surface using magnetron sputtering technology under argon atmosphere; simultaneously sputtering a copper target and a tin target to deposit a copper-tin co-sputtered layer on the pure copper layer surface; and sputtering a tin target to deposit a pure tin layer on the copper-tin co-sputtered layer surface, resulting in a Cu-Sn based solder with a multi-layer structure; in the copper-tin co-sputtered layer, the atomic ratio of Cu to Sn is 5-9:1. The preparation method of this invention is simple, pollution-free throughout, cost-effective, and improves efficiency; furthermore, the prepared Cu-Sn based solder can effectively reduce the Cu-Sn interdiffusion distance during transient liquid phase welding, reduce void formation, make welding more complete, and improve the reliability of the welding joint.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, and in particular to a Cu-Sn based solder for high-reliability welding joints and its preparation method. Background Technology

[0002] Compared with first- and second-generation semiconductor materials, third-generation semiconductor materials have a wider bandgap, higher breakdown electric field, higher thermal conductivity, higher electron saturation velocity, and higher radiation resistance. They are more suitable for manufacturing high-temperature, high-frequency, high-power, and radiation-resistant devices and can be widely used in high-voltage, high-frequency, high-temperature, and high-reliability fields, including radio frequency communication, radar, satellite, power management, automotive electronics, and industrial power electronics.

[0003] With the widespread application of third-generation semiconductor materials, there is a growing demand for smaller packaged solder joints, as well as better heat dissipation and high-temperature reliability. Transient liquid phase bonding technology can effectively reduce bonding temperature and bonding time, and the resulting intermetallic compound solder joints have high service temperatures and excellent thermodynamic properties. Therefore, transient liquid phase bonding technology is one of the most promising interconnect technologies currently available.

[0004] Cu-Sn solders possess excellent electrical and thermal conductivity, low reactivity, and are inexpensive and readily available, making them the most cost-effective advanced transient liquid phase soldering encapsulation solder. The intermetallic compounds formed after transient liquid phase welding of Cu-Sn solders mainly consist of two phases: Cu6Sn5 and Cu3Sn. Excessive Cu6Sn5 phase in the weld joint reduces thermal fatigue life, tensile strength, fracture toughness, and isothermal shear fatigue life. In contrast, the Cu3Sn phase exhibits better Young's modulus, fracture toughness, and melting point, as well as higher electrical conductivity and shear strength. Therefore, a single Cu3Sn phase is crucial for achieving high-strength and highly reliable welded joints.

[0005] Currently, traditional Cu-Sn solders are mostly prepared by electroplating. However, electroplating makes it difficult to precisely control the composition and thickness ratio of Cu-Sn solders, and it is impossible to obtain a single Cu3Sn phase intermetallic compound. The subsequent bonding time of the prepared Cu-Sn solder is relatively long, and the weld has voids, which can easily lead to the initiation of cracks in the weld zone and reduce its strength. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a Cu-Sn based solder for high-reliability welding joints and its preparation method. This invention proposes to control the types of Cu-Sn intermetallic compounds by utilizing magnetron sputtering technology to prepare Cu-Sn based solder. This allows for easy control of the elemental ratio of the solder layer through co-sputtering deposition, and the generation of a Cu3Sn phase after bonding. This enables precise control of the solder composition and thickness ratio of the Cu-Sn based solder, effectively reducing porosity in the weld zone, improving the reliability of the welding joint, and the method is simple and easily commercialized.

[0007] The present invention provides a Cu-Sn based solder for high-reliability welding joints and its preparation method, which is achieved through the following technical solution:

[0008] The first objective of this invention is to provide a method for preparing a Cu-Sn based solder for high-reliability welding joints, comprising the following steps:

[0009] Step 1, Pretreatment of the substrate: Polish and clean the substrate to obtain the pretreated substrate.

[0010] It should be noted that this invention does not limit the type of substrate, as long as it meets the requirements of electronic packaging. For example, 6061 aluminum alloy, a commonly used packaging metal, can be used.

[0011] This invention takes into account the possibility of impurities and surface defects such as oil stains on the substrate surface. Therefore, it first polishes and then cleans the substrate surface to remove impurities and improve its surface roughness, so as to improve the formation quality of subsequent Cu-Sn solder.

[0012] It should also be noted that since the subsequent glow discharge sputtering cleaning process and the magnetron sputtering process of each step are all carried out in the magnetron sputtering equipment, the present invention installs the pretreated substrate on the workpiece gantry in the vacuum chamber of the magnetron sputtering equipment, and places the pure copper target and the pure tin target on the target position in the magnetron sputtering vacuum chamber, respectively, so as to facilitate the subsequent glow discharge sputtering cleaning process and the deposition of the pure copper layer, the copper-tin co-sputtering layer and the pure tin layer.

[0013] Step 2, Glow Sputter Cleaning: The pretreated substrate is subjected to glow sputter cleaning to obtain a clean substrate.

[0014] It should be noted that, considering that the surface of the pretreated substrate may still contain adsorbates and oxide layers, which may affect the adhesion between the Cu-Sn solder and the substrate film, the pretreated substrate was further subjected to glow discharge sputtering cleaning.

[0015] Step 3, Deposit a pure copper layer: Under an argon atmosphere, a pure copper layer is deposited on the clean substrate surface using magnetron sputtering technology with a copper target as the sputtering target.

[0016] It should be noted that, considering the high purity, good density, and good uniformity of the thin film prepared by magnetron sputtering technology, this invention uses magnetron sputtering technology to deposit a pure copper layer on the substrate surface, so as to achieve better bonding between Cu-Sn solder and the substrate through the pure copper layer.

[0017] Step 4, depositing a copper-tin co-sputtering layer: using a copper target and a tin target as co-sputtering targets, a copper-tin co-sputtering layer is deposited on the surface of the pure copper layer.

[0018] It should be noted that this invention employs magnetron sputtering technology, using copper and tin targets as co-sputtering targets, enabling the copper sputtered by the copper target and the tin sputtered by the tin target to form a copper-tin co-sputtered layer during the sputtering process. This invention can control the copper-tin composition ratio in the formed copper-tin co-sputtered layer by adjusting the magnetron sputtering process parameters, thereby obtaining a single Cu3Sn phase. This results in the Cu-Sn solder obtained by this invention exhibiting high strength and high reliability. Furthermore, the use of magnetron sputtering technology reduces the diffusion distance of copper and tin during subsequent heat treatment, solving the porosity problem caused by long-distance interdiffusion of copper and tin, further improving the high strength and high reliability of the Cu-Sn solder obtained by this invention.

[0019] It should also be noted that the present invention can adjust the atomic ratio of Cu to Sn in the copper-tin co-sputtered layer by controlling the sputtering power of the copper target and the tin target during the magnetron sputtering process. By controlling the atomic ratio of Cu to Sn in the copper-tin co-sputtered layer to be 5 to 9:1, the composition ratio of copper and tin in the formed copper-tin co-sputtered layer can be controlled, avoiding the problem of incomplete formation of Cu3Sn phase caused by the copper-tin composition ratio being too high or too low, and ensuring that the Cu-Sn solder finally obtained by the present invention can obtain a single Cu3Sn phase.

[0020] In some preferred embodiments of the present invention, when the atomic ratio of Cu to Sn is 6.5:1, the overall atomic ratio of Cu to Sn in the Cu-Sn solder is 3:1, which is easy to form Cu3Sn phase by subsequent heat treatment bonding.

[0021] Step 5, Deposit a pure tin layer: Using a tin target as the sputtering target, deposit a pure tin layer on the surface of the copper-tin co-sputtering layer to form a Cu-Sn solder with a multi-layer structure composed of a pure copper layer, a copper-tin co-sputtering layer and a pure tin layer on the substrate surface.

[0022] It should be noted that, taking into account the need for a low-melting-point tin layer to melt and diffuse with the solid copper-tin co-sputtered layer during the subsequent bonding process, the present invention further deposits a pure tin layer on the surface of the copper-tin co-sputtered layer, so that the pure tin layer melts at low temperature and diffuses with the solid copper-tin co-sputtered layer at low temperature, and the connection is achieved by cooling and solidification.

[0023] It should also be noted that this invention takes into account that traditional Cu-Sn solder consists of two layers: a pure copper layer and a pure tin layer. Existing technologies, when combining pure copper and pure tin layers, require melting the pure tin layer at a low temperature, followed by solid-liquid interdiffusion with the high-melting-point solid copper to form an intermetallic compound, which then solidifies to achieve bonding. However, the Cu-Sn solder prepared by the above method has a long subsequent bonding time, and the weld seam contains voids, easily leading to the initiation of cracks in the weld zone and a decrease in strength. In contrast, this invention, by adding a copper-tin co-sputtering layer between the pure copper and pure tin layers—that is, by sequentially depositing a pure copper layer, a copper-tin co-sputtering layer, and a pure tin layer on the substrate surface—forms a multi-layered Cu-Sn solder composed of pure copper, copper-tin co-sputtering, and pure tin layers. This reduces the diffusion distance between the liquid Sn and solid Cu during subsequent heat treatment, solving the void problem caused by long-distance interdiffusion of copper and tin.

[0024] In some preferred embodiments of the present invention, the atomic ratio of Cu to Sn in the Cu-Sn solder is 3.36:1 to 2.36, in order to adjust the copper-tin ratio and control the formation of a single Cu3Sn phase after subsequent heat treatment.

[0025] In some preferred embodiments of the present invention, during the deposition of the copper-tin co-sputtered layer, argon gas is introduced and the argon gas flow rate is adjusted to 15 sccm to 25 sccm, and the working gas pressure is controlled to be 0.1 Pa to 1 Pa; and during sputtering deposition, the sputtering power of the copper target is controlled to be 100 W to 150 W, the sputtering power of the tin target is controlled to be 65 W to 100 W, and the deposition time is 110 min to 147 min.

[0026] In some preferred embodiments of the present invention, argon gas is introduced during the deposition of the pure copper layer, and the working gas pressure is controlled to be 0.1 Pa to 1 Pa; and during sputtering deposition, the sputtering power of the copper target is controlled to be 140 W to 160 W, and the deposition time is 40 min to 60 min.

[0027] In some preferred embodiments of the present invention, argon gas is introduced during the deposition of the pure tin layer, and the working gas pressure is controlled to be 0.1 Pa to 1 Pa; and during sputter deposition, the sputtering power of the tin target is controlled to be 70 W to 90 W, and the deposition time is 40 min to 60 min.

[0028] In some preferred embodiments of the present invention, the glow discharge sputtering cleaning process is carried out by the following steps: evacuating to a background vacuum of ≤0.005Pa, introducing argon gas, adjusting the argon gas flow rate to 15sccm~25sccm, and controlling the working gas pressure to 0.4Pa~1.6Pa; applying a bias voltage of -500V~-200V to the substrate, and performing glow discharge sputtering cleaning for 10min~20min.

[0029] In some preferred embodiments of the present invention, the polishing is performed by the following steps: polishing with SiC wet sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively, and finally polishing to a mirror finish with diamond spray polishing agents with a particle size of 1.5μm and 0.5μm respectively.

[0030] The second objective of this invention is to provide a Cu-Sn solder prepared by the above-described method, wherein the Cu-Sn solder has a composite structure consisting of a pure copper layer, a copper-tin co-sputtered layer, and a pure tin layer arranged sequentially from the inside out on the surface of the substrate.

[0031] In some preferred embodiments of the present invention, the thickness of the pure copper layer is 0.8 μm to 1.2 μm.

[0032] In some preferred embodiments of the present invention, the thickness of the pure tin layer is 0.6 μm to 1 μm.

[0033] In some preferred embodiments of the present invention, the thickness of the copper-tin co-sputtered layer is 2μm to 4μm.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention addresses the issue of controlling the types of Cu-Sn intermetallic compounds. Utilizing magnetron sputtering technology, a pure copper layer, a copper-tin co-sputtered layer, and a pure tin layer are sequentially deposited on the substrate surface, forming a multilayered Cu-Sn solder composed of these layers. By adding a copper-tin co-sputtered layer between the pure copper and pure tin layers, the diffusion distance between the liquid Sn and solid Cu phases during subsequent heat treatment is reduced, solving the porosity problem caused by long-distance interdiffusion of copper and tin. Furthermore, this invention can control the elemental ratio of the solder layer through co-sputtering deposition and generate a Cu3Sn phase after bonding. This allows for precise control of the solder composition and thickness ratio of the Cu-Sn solder, effectively reducing porosity in the weld zone, improving the reliability of the weld joint, and the method is simple and easily commercialized.

[0036] The preparation method of this invention is simple, pollution-free throughout the process, saves costs, and improves efficiency. Moreover, the copper-tin composition ratio of the prepared Cu-Sn solder can be controlled, which can effectively reduce the interdiffusion distance of Cu-Sn during transient liquid phase welding, reduce the generation of voids, make the welding more complete, and improve the reliability of the welded joint.

[0037] The Cu-Sn solder prepared according to this invention exhibits good crystallinity, high film density and uniformity. Furthermore, test results show that after welding, the Cu-Sn solder of this invention exhibits no pores at the bonding interface, high bonding strength, and excellent welding effect. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of the Cu-Sn solder prepared according to the present invention; in the figure, 1 is the substrate, 2 is the pure copper layer, 3 is the copper-tin co-sputtering layer, and 4 is the pure tin layer.

[0039] Figure 2 The image shows a cross-sectional scanning electron microscope image of the Cu-Sn solder prepared in Example 1. Detailed Implementation

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below.

[0041] Example 1

[0042] This embodiment provides a Cu-Sn based solder for high-reliability welding joints, and it is prepared through the following steps:

[0043] Step 1, Pretreatment of the substrate:

[0044] 1.1) In this embodiment, 6061 aluminum alloy is used as the substrate. It is polished with SiC water-based sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively. Finally, it is polished to a mirror finish with diamond spray polishing agent with a particle size of 1.5μm and 0.5μm respectively to obtain polished 6061 aluminum alloy.

[0045] 1.2) The polished 6061 aluminum alloy was ultrasonically cleaned in acetone, alcohol and deionized water for 20 minutes in sequence, and then dried to obtain the pretreated 6061 aluminum alloy.

[0046] Step 2, Glow sputtering cleaning treatment:

[0047] 2.1) After fixing the pretreated 6061 aluminum alloy onto the sample tray, install it on the workpiece gantry in the vacuum chamber of the magnetron sputtering equipment, and place the pure copper target and the pure tin target on the target positions in the magnetron sputtering vacuum chamber, respectively.

[0048] 2.2) After the vacuum degree of the sputtering chamber of the magnetron sputtering equipment is evacuated to 0.005Pa, argon gas is introduced and its flow rate is adjusted to 20sccm. The gas pressure in the sputtering chamber is controlled to 0.5Pa. The bias power supply is turned on and the power is adjusted to -200W. Glow photolysis is performed for 20 minutes to obtain clean 6061 aluminum alloy.

[0049] Step 3, Deposit a pure copper layer:

[0050] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working pressure was controlled at 0.5 Pa. The power was turned on to sputter the copper target. The sputtering power was adjusted to 150 W and the sputtering time was 50 min. A 1 μm thick pure copper layer was formed on the clean 6061 aluminum alloy surface.

[0051] Step 4, deposit copper-tin co-sponged layer:

[0052] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled at 0.5 Pa. At the same time, the power supply for controlling the copper target and the tin target was turned on. The sputtering power of the copper target was adjusted to 150 W and the sputtering power of the tin target was adjusted to 100 W so that the copper target and the tin target could be deposited on the surface of the pure copper layer simultaneously. The sputtering time was 110 min to deposit a copper-tin co-sputtered layer with a thickness of 3 μm on the surface of the pure copper layer. In this embodiment, the atomic ratio of Cu to Sn in the copper-tin co-sputtered layer is 5:1.

[0053] Step 5, Deposit a pure tin layer:

[0054] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled to 0.5 Pa. The power was turned on to sputter the tin target. The sputtering power was adjusted to 80 W and the sputtering time was 50 min to deposit a pure tin layer with a thickness of 800 nm on the surface of the copper-tin co-sputtered layer.

[0055] After the coating is completed, the gas supply is stopped, and a vacuum state is maintained. After the furnace cools to room temperature, the sample is removed, resulting in a Cu-Sn solder with a multilayer structure formed on the surface of substrate 1, consisting of a pure copper layer 2, a copper-tin co-sputtered layer 3, and a pure tin layer 4. The structural diagram is shown below. Figure 1 As shown.

[0056] Example 2

[0057] This embodiment provides a Cu-Sn based solder for high-reliability welding joints, and it is prepared through the following steps:

[0058] Step 1, Pretreatment of the substrate:

[0059] 1.1) In this embodiment, 6061 aluminum alloy is used as the substrate. It is polished with SiC water-based sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively. Finally, it is polished to a mirror finish with diamond spray polishing agent with a particle size of 1.5μm and 0.5μm respectively to obtain polished 6061 aluminum alloy.

[0060] 1.2) The polished 6061 aluminum alloy was ultrasonically cleaned in acetone, alcohol and deionized water for 20 minutes in sequence, and then dried to obtain the pretreated 6061 aluminum alloy.

[0061] Step 2, Glow sputtering cleaning treatment:

[0062] 2.1) After fixing the pretreated 6061 aluminum alloy onto the sample tray, install it on the workpiece gantry in the vacuum chamber of the magnetron sputtering equipment, and place the pure copper target and the pure tin target on the target positions in the magnetron sputtering vacuum chamber, respectively.

[0063] 2.2) After the vacuum degree of the sputtering chamber of the magnetron sputtering equipment is evacuated to 0.005Pa, argon gas is introduced and its flow rate is adjusted to 20sccm. The gas pressure in the sputtering chamber is controlled to 0.5Pa. The bias power supply is turned on and the power is adjusted to -200W. Glow photolysis is performed for 20 minutes to obtain clean 6061 aluminum alloy.

[0064] Step 3, Deposit a pure copper layer:

[0065] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working pressure was controlled at 0.5 Pa. The power was turned on to sputter the copper target. The sputtering power was adjusted to 150 W and the sputtering time was 50 min. A 1 μm thick pure copper layer was formed on the clean 6061 aluminum alloy surface.

[0066] Step 4, deposit copper-tin co-sponged layer:

[0067] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled at 0.5 Pa. At the same time, the power supply for controlling the copper target and the tin target was turned on. The sputtering power of the copper target was adjusted to 150 W and the sputtering power of the tin target was adjusted to 80 W so that the copper target and the tin target could be deposited on the surface of the pure copper layer simultaneously. The sputtering time was 125 min to deposit a copper-tin co-sputtered layer with a thickness of 3 μm on the surface of the pure copper layer. In this embodiment, the atomic ratio of Cu to Sn in the copper-tin co-sputtered layer is 6.5:1.

[0068] Step 5, Deposit a pure tin layer:

[0069] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled to 0.5 Pa. The power was turned on to sputter the tin target. The sputtering power was adjusted to 80 W and the sputtering time was 50 min to deposit a pure tin layer with a thickness of 800 nm on the surface of the copper-tin co-sputtered layer.

[0070] After the coating is completed, the gas supply is stopped and a vacuum state is maintained. After the furnace cools to room temperature, the sample is taken out, and a Cu-Sn solder with a multi-layer structure composed of a pure copper layer, a copper-tin co-sputtered layer and a pure tin layer is obtained on the surface of the substrate.

[0071] Example 3

[0072] This embodiment provides a Cu-Sn based solder for high-reliability welding joints, and it is prepared through the following steps:

[0073] Step 1, Pretreatment of the substrate:

[0074] 1.1) In this embodiment, 6061 aluminum alloy is used as the substrate. It is polished with SiC wet sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively. Finally, it is polished to a mirror finish with diamond spray polishing agent with a particle size of 1.5μm and 0.5μm respectively to obtain polished 6061 aluminum alloy.

[0075] 1.2) The polished 6061 aluminum alloy was ultrasonically cleaned in acetone, alcohol and deionized water for 20 minutes in sequence, and then dried to obtain the pretreated 6061 aluminum alloy.

[0076] Step 2, Glow sputtering cleaning treatment:

[0077] 2.1) After fixing the pretreated 6061 aluminum alloy onto the sample tray, install it on the workpiece gantry in the vacuum chamber of the magnetron sputtering equipment, and place the pure copper target and the pure tin target on the target positions in the magnetron sputtering vacuum chamber, respectively.

[0078] 2.2) After the vacuum degree of the sputtering chamber of the magnetron sputtering equipment is evacuated to 0.005Pa, argon gas is introduced and its flow rate is adjusted to 20sccm. The gas pressure in the sputtering chamber is controlled to 0.5Pa. The bias power supply is turned on and the power is adjusted to -200W. Glow photolysis is performed for 20 minutes to obtain clean 6061 aluminum alloy.

[0079] Step 3, Deposit a pure copper layer:

[0080] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working pressure was controlled at 0.5 Pa. The power was turned on to sputter the copper target. The sputtering power was adjusted to 150 W and the sputtering time was 50 min. A 1 μm thick pure copper layer was formed on the clean 6061 aluminum alloy surface.

[0081] Step 4, deposit copper-tin co-sponged layer:

[0082] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled at 0.5 Pa. At the same time, the power supply for controlling the copper target and the tin target was turned on. The sputtering power of the copper target was adjusted to 150 W and the sputtering power of the tin target was adjusted to 65 W so that the copper target and the tin target could be deposited on the surface of the pure copper layer simultaneously. The sputtering time was 147 min to deposit a copper-tin co-sputtered layer with a thickness of 3 μm on the surface of the pure copper layer. In this embodiment, the atomic ratio of Cu to Sn in the copper-tin co-sputtered layer is 9:1.

[0083] Step 5, Deposit a pure tin layer:

[0084] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working gas pressure was controlled to 0.5 Pa. The power was turned on to sputter the tin target. The sputtering power was adjusted to 80 W and the sputtering time was 50 min to deposit a pure tin layer with a thickness of 800 nm on the surface of the copper-tin co-sputtered layer.

[0085] After the coating is completed, the gas supply is stopped and a vacuum state is maintained. After the furnace cools to room temperature, the sample is taken out, and a Cu-Sn solder with a multi-layer structure composed of a pure copper layer, a copper-tin co-sputtered layer and a pure tin layer is obtained on the surface of the substrate.

[0086] Comparative Example 1

[0087] This comparative example provides a Cu-Sn based solder, which is prepared through the following steps:

[0088] Step 1, Pretreatment of the substrate:

[0089] 1.1) In this comparative example, 6061 aluminum alloy was used as the substrate. It was polished with SiC wet sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively. Finally, it was polished to a mirror finish with diamond spray polishing agent with a particle size of 1.5μm and 0.5μm respectively to obtain polished 6061 aluminum alloy.

[0090] 1.2) The polished 6061 aluminum alloy was ultrasonically cleaned in acetone, alcohol and deionized water for 20 minutes in sequence, and then dried to obtain the pretreated 6061 aluminum alloy.

[0091] Step 2, Glow sputtering cleaning treatment:

[0092] 2.1) After fixing the pretreated 6061 aluminum alloy onto the sample tray, install it on the workpiece gantry in the vacuum chamber of the magnetron sputtering equipment, and place the pure copper target and the pure tin target on the target positions in the magnetron sputtering vacuum chamber, respectively.

[0093] 2.2) After the vacuum degree of the sputtering chamber of the magnetron sputtering equipment is evacuated to 0.005Pa, argon gas is introduced and its flow rate is adjusted to 20sccm. The gas pressure in the sputtering chamber is controlled to 0.5Pa. The bias power supply is turned on and the power is adjusted to -200W. Glow photolysis is performed for 20 minutes to obtain clean 6061 aluminum alloy.

[0094] Step 3, Deposit a pure copper layer:

[0095] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working pressure was controlled at 0.5 Pa. The power was turned on to sputter the copper target. The sputtering power was adjusted to 150 W and the sputtering time was 150 min. A 3 μm thick pure copper layer was formed on the clean 6061 aluminum alloy surface.

[0096] Step 4, Deposit a pure tin layer:

[0097] Argon gas was introduced into the vacuum chamber and its flow rate was adjusted to 20 sccm. The working pressure was controlled at 0.5 Pa. The power was turned on to sputter the tin target. The sputtering power was adjusted to 80 W and the sputtering time was 63 min to deposit a pure tin layer with a thickness of 1 μm on the surface of the pure copper layer.

[0098] After the coating is completed, the gas supply is stopped and a vacuum state is maintained. After the sample is cooled to room temperature in the furnace, it is taken out, thus obtaining Cu-Sn solder composed of a pure copper layer and a pure tin layer on the surface of the substrate.

[0099] The only difference between this comparative example and Example 1 is that:

[0100] This comparative example does not form a copper-tin co-sputtering layer.

[0101] Experimental Section

[0102] (I) Morphological Test

[0103] This invention takes the Cu-Sn solder prepared in Example 1 as an example and performs cross-sectional scanning electron microscopy (SEM) testing on it. The test results are as follows: Figure 2 As shown.

[0104] Figure 2 The cross-sectional scanning electron microscope image of the Cu-Sn solder prepared in Example 1 shows that there are relatively obvious boundaries between the Cu layer, the Cu / Sn co-sputtered layer and the Sn layer, and the crystallinity is good, with high film density and uniformity.

[0105] (II) Welding effect test

[0106] In this invention, Cu-Sn solder prepared in Examples 1-3 and Comparative Example 1 were used to clamp two 6061 aluminum alloy substrates coated with Cu-Sn solder in a staggered alignment manner using a specific clamp. The bonding area was controlled at 15mm×10mm and the bonding pressure was about 1MPa. Transient liquid phase welding was performed to obtain welded samples.

[0107] The transient liquid phase welding conditions are: holding at 300℃ for 180 minutes.

[0108] The Cu-Sn solders in Examples 1 to 3 all showed good welding results. The Cu-Sn solder in Comparative Example 1 showed poor welding results.

[0109] The present invention also tested the shear strength of the welded samples corresponding to Examples 1 to 3 and Comparative Example 1, respectively, and the test method was to use a universal testing machine of model UTM6409 to test the shear strength.

[0110] Test results show that the strength of Example 1 is 35.4 MPa, the strength of Example 2 is 46.8 MPa, the strength of Example 3 is 17.9 MPa, and the strength of Comparative Example 1 is 10.6 MPa. In Example 2, the copper-tin ratio is just right to form a complete Cu3Sn phase, and the bonding is good and without pores. Therefore, the Cu-Sn solder in Example 2 has the highest bonding strength, at 46.8 MPa. In Example 1, the final intermetallic compound formed is the Cu3Sn phase, but due to a slight excess of Sn, pores were found at the bonding interface after scanning electron microscopy. Therefore, the strength is lower than that of Example 2, at 35.4 MPa. In Example 3, the final intermetallic compound formed is the Cu6Sn5 phase, resulting in a low bonding strength of 17.9 MPa. In Comparative Example 1, the long-distance diffusion of copper and tin leads to the formation of a large number of pores, resulting in the lowest bonding strength, at only 10.6 MPa.

[0111] Obviously, the above embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a Cu-Sn based solder for high-reliability welding joints, characterized in that, Includes the following steps: The substrate is polished and then cleaned to obtain a pretreated substrate. The pretreated substrate is then subjected to glow discharge sputtering cleaning to obtain a clean substrate. Under an argon atmosphere, a pure copper layer is deposited on the clean substrate surface using magnetron sputtering technology with a copper target as the sputtering target. Using copper and tin targets as co-sputtering targets, a copper-tin co-sputtering layer is deposited on the surface of the pure copper layer. Using a tin target as the sputtering target, a pure tin layer is deposited on the surface of the copper-tin co-sputtering layer to form a Cu-Sn solder with a multilayer structure composed of a pure copper layer, a copper-tin co-sputtering layer and a pure tin layer on the substrate surface. In the copper-tin co-sputtered layer, the atomic ratio of Cu to Sn is 5~9:1; In the Cu-Sn solder, the atomic ratio of Cu to Sn is 3.36:1~2.36; During the deposition of the copper-tin co-sputtered layer, argon gas is introduced and the working gas pressure is controlled at 0.1 Pa to 1 Pa; and during sputtering deposition, the sputtering power of the copper target is controlled at 100 W to 150 W, the sputtering power of the tin target is controlled at 5 W to 100 W, and the deposition time is 110 min to 147 min. During the deposition of the pure copper layer, argon gas is introduced and the working gas pressure is controlled at 0.1 Pa to 1 Pa; and during sputter deposition, the sputtering power of the copper target is controlled at 140 W to 160 W and the deposition time is 40 min to 60 min. During the deposition of the pure tin layer, argon gas is introduced and the working gas pressure is controlled at 0.1 Pa to 1 Pa; and during sputter deposition, the sputtering power of the tin target is controlled at 70 W to 90 W and the deposition time is 40 min to 60 min. The glow discharge cleaning process is carried out through the following steps: Evacuate to a background vacuum level ≤0.005Pa, introduce argon gas, adjust the argon gas flow rate to 15sccm~25sccm, and control the working gas pressure to 0.4Pa~1.6Pa; apply a bias voltage of -500V~-200V to the substrate, and perform glow discharge cleaning for 10min~20min.

2. The method for preparing Cu-Sn solder as described in claim 1, characterized in that, The polishing is performed through the following steps: grinding with SiC wet sandpaper of 400#, 600#, 1000#, 1200#, 1500#, 2000#, 3000# and 4000# respectively, and finally polishing to a mirror finish with diamond spray polishing agent.

3. A Cu-Sn based solder for high-reliability welded joints prepared by the method described in claim 1 or 2, characterized in that, The Cu-Sn solder has a multi-layer structure on the substrate surface consisting of a pure copper layer, a copper-tin co-sputtering layer, and a pure tin layer, arranged sequentially from the inside out.

4. The Cu-Sn based solder for high-reliability welding joints as described in claim 3, characterized in that, The thickness of the pure copper layer is 0.8μm~1.2μm.

5. The Cu-Sn based solder for high-reliability welding joints as described in claim 3, characterized in that, The thickness of the pure tin layer is 0.6 μm to 1 μm.

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