Low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder and preparation method thereof

Abstract

The invention provides a low-melting-point Sn-In-Zn-Ga-Ag multi-principal element alloy lead-free solder and a preparation method of the low-melting-point Sn-In-Zn-Ga-Ag multi-principal element alloy lead-free solder. The lead-free solder comprises the following components in percentage by atom: 34.0% of Sn, 30.0% of In, 20.0% of Zn, 10.0% of Ga and 6.0% of Ag. The preparation process is simple, pollution-free and easy to control; and meanwhile, the conductive brazing filler metal is good in conductivity and brazing performance, and meets the requirements of a brazing welding process in electronic packaging.

Description

Technical Field

[0001] The invention belongs to the field of lead-free solder preparation, and in particular relates to a low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder and a preparation method thereof. Background Art

[0002] With the development of 5G technology, people have higher expectations for electronic products. Current products are no longer able to meet these needs. The design and manufacturing of electronic devices are evolving towards multifunctionality and miniaturization, placing higher demands on electronic packaging technology. However, Moore's Law for silicon chip development and manufacturing is approaching its physical limits, making electronic packaging technology a crucial means of increasing the computing power of future electronic products. To address this challenge, large-scale, high-density, and miniaturized integrated circuit technology is shifting electronic packaging from two-dimensional (2D) to three-dimensional (3D) packaging. Vertical stacking of different chip types is a hallmark of 3D packaging technology. This involves multiple levels of packaging, enabling vertical stacking of different chip types. This process involves different process temperatures for high- and low-melting-point solders. For example, the high-melting-point lead-free solder SAC305 (217°C) is reflowed first, followed by the low-melting-point solder Sn-58Bi (138°C). The process temperature of the latter stage must ensure that the solder joints formed in the previous stage remain stable and that the solder joints formed at the current stage remain stable and reliable.

[0003] Due to limitations in chip size and silicon wafer thickness, there's an increased risk of thermal warpage due to a mismatch in the thermal expansion coefficients of the chip and printed circuit board, potentially leading to serious malfunctions in electronic devices. Therefore, using a low-melting-point solder can lower the soldering process temperature, minimizing thermal stress and warpage during the soldering process. Furthermore, low soldering temperatures can also reduce damage to certain temperature-sensitive electronic components.

[0004] The development of a new lead-free solder with a low melting point, excellent mechanical properties, superior wettability, and slow growth of intermetallic compounds (IMCs) is a major challenge facing the current development of electronic packaging. Despite decades of research, binary eutectic solders with a melting point below 180°C are still unavailable for industrial production. The currently used low-temperature eutectic solder, Sn-58Bi (138°C), exhibits Bi segregation during soldering. In recent years, an increasing number of elements have been introduced into electronic packaging, leading to the development of low-melting-point, multi-principal-component alloy lead-free solders suitable for three-dimensional packaging. Multi-principal-component alloys exhibit four key characteristics: 1. high entropy effect; 2. retarded diffusion effect; 3. lattice distortion effect; and 4. "cocktail" effect. The high entropy effect refers to the high mixing entropy within the system caused by the high number of components in a multi-principal-component alloy, which favors the formation of stable solid solutions rather than ordered intermetallic compounds. The delayed diffusion effect occurs when a multi-principal alloy contains a large variety of constituent atoms, and these atoms differ significantly in size and chemical properties. This increases the activation energy for atomic diffusion, hinders cooperative diffusion between the components, and causes a certain degree of retardation in the diffusion of atoms within the alloy system. The lattice distortion effect occurs when differences in atomic radius, lattice potential energy, and bond types between the components of a multi-principal alloy cause the alloy's lattice to distort, increasing the effects of solid solution strengthening and contributing to improved alloy strength and hardness. The "cocktail" effect is due to the fact that the properties of a multi-principal alloy can be altered by varying the composition or element ratios. Summary of the Invention

[0005] In response to the high melting points of conventional solder alloys, the present invention aims to provide a low-melting-point, lead-free Sn-In-Zn-Ga-Ag multi-element alloy solder and its preparation method. This patented solution boasts a simple, pollution-free, and easily controllable preparation process, as well as excellent conductivity and solderability, making it suitable for 3D IC soldering applications.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A low-melting-point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder has the following composition, calculated in atomic percentage: 34.0% Sn, 30.0% In, 20.0% Zn, 10.0% Ga, and 6.0% Ag. The alloy is prepared by smelting and then directly casting to obtain an alloy ingot.

[0007] The method for preparing the low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder comprises the following steps: preparing Sn, In, Zn, Ga, and Ag in proportion; pouring the mixture into a crucible, and then vacuum melting the mixture to obtain a melt; and molding the melt to obtain the Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder.

[0008] Specifically, the preparation method of the low-melting-point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder comprises the following steps: placing half of the Sn particles at the bottom of the crucible, placing In, Ga, and Zn particles in the middle, and then placing the remaining Sn and Ag particles; vacuuming with a mechanical pump, closing the vacuum pump, and filling with high-purity argon gas to normal atmospheric pressure to complete a purge process, and repeating the purge process three times; after the last purge, using a vacuum pump to vacuum to 3.0×10 -3 Pa, turn on the induction heating power supply, the initial heating power is 5kw, the heating time is 1min, and then the heating power is increased by 2kw every 2 minutes until it reaches 20kw, and wait for the particles to be completely liquefied; after the particles are completely liquefied, keep warm for 5 minutes, cast, and cool to obtain the lead-free solder.

[0009] Furthermore, tin particles with a purity of 99.99% and a particle size of 2-4 mm are selected as raw materials.

[0010] Furthermore, indium particles with a purity of 99.95% and a particle size of 1-3 mm are selected as raw materials.

[0011] Furthermore, zinc particles with a purity of 99.99% and a particle size of 2-6 mm are selected as raw materials.

[0012] Furthermore, bulk silver with a purity of 99.99% is selected as the raw material.

[0013] Furthermore, bulk gallium with a purity of 99.99% is selected as the raw material.

[0014] Furthermore, due to the volatilization of Zn, 20% more Zn is added to the raw material Zn particles.

[0015] Furthermore, the melting point of the low-melting-point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder is ≤102°C.

[0016] The advantages of the present invention are: (1) This patent aims at the requirements of soldering of three-dimensional packaging (3D IC) and proposes a low-melting-point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder and its preparation method. According to the cocktail effect, In and Ga elements are selected, among which In can lower the melting point temperature and Ga element can lower the melting point of the alloy, and a Sn-In-Zn-Ga-Ag solder with a melting point of about 100°C is prepared.

[0017] (2) This patent uses the In element to meet the requirements of solder wettability, where In can improve the wettability of the alloy, and prepares a Sn-In-Zn-Ga-Ag solder with good wettability.

[0018] (3) This patent selects Zn and Ag elements in response to the requirements of the mechanical properties of the solder. The content of Ag affects the mechanical properties of the alloy and the high-temperature oxidation resistance of the alloy, and an appropriate amount of Zn can improve the mechanical properties of the alloy. At the same time, the differences in the lattice potential energy and bond types of Sn, In, Zn, Ga, and Ag elements can cause the lattice of the alloy to be distorted, which is beneficial to improving the strength of the alloy and preparing a Sn-In-Zn-Ga-Ag solder with good mechanical properties.

[0019] (4) This patent aims to meet the requirement of slow growth of intermetallic compounds. Based on the delayed diffusion effect, the synergistic effect between Sn, In, Zn, Ga, and Ag elements inhibits the diffusion of the interface layer and the atomic radius between each element, and prepares a continuous, uniform, and thinner Sn-In-Zn-Ga-Ag solder of intermetallic compounds.

[0020] (5) This patent solution takes into account the volatilization of Zn, so 20% more Zn is added. At the same time, considering the volatilization of In, Zn, and Ga, a portion of Sn particles are placed at the bottom of the crucible, In, Ga, and Zn particles are placed in the middle, and the remaining Sn and Ag particles are added.

[0021] (6) Sn is a commonly used element in electronic packaging; In can lower the melting point and improve the wettability of the alloy; Ga can lower the melting point of the alloy and inhibit the growth rate of the solder interface compound during the aging process; the Ag content affects the mechanical properties of the alloy and the high-temperature oxidation resistance of the alloy, and the addition of Ag can improve the electrical conductivity and thermal conductivity of the solder; an appropriate amount of Zn can improve the mechanical properties of the alloy; adding In and Zn to the alloy at the same time can refine the microstructure of the solder; adding In and Ga at the same time can lower the melting point of the alloy. However, the output of In is small and the price of In is expensive, so adding too much will increase the cost; if too much Ag is added, the liquidus temperature of the solder will rise and increase the cost; if too much Ga is added, the melting point of the solder will be too low, so it will not meet the packaging requirements. Zn is easily oxidized and volatilized during the smelting process, and adding too much Zn will reduce the wettability of the solder. The present invention optimizes the dosage ratio of each element and scientifically compounds each element. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The invention relates to the preparation process of low melting point Sn-In-Zn-Ga-Ag multi-element alloy lead-free solder; Figure 2 DSC curve of the Sn-In-Zn-Ga-Ag alloy prepared in Example 1; Figure 3 This is a schematic diagram of solder paste coating; Figure 4 Schematic diagram of solder paste-substrate wetting. DETAILED DESCRIPTION

[0023] To make the above features and advantages of the present invention more clearly understood, the following embodiments are given for detailed description. Unless otherwise specified, the methods of the present invention are all conventional methods in the art.

[0024] Example 1 Low-temperature eutectic solder Sn-58Bi (138°C) can cause Bi segregation and brittle joints during soldering. Therefore, the problem addressed by this patent solution is how to design a reliable, low-melting-point, multi-principal-element alloy solder for the third-level packaging of three-dimensional packaging.

[0025] A low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder comprises, by atomic percentage, 34.0% Sn, 30.0% In, 20.0% Zn, 10.0% Ga and 6.0% Ag.

[0026] The smelting process is divided into two parts: material preparation and smelting. The material preparation uses an electronic balance. The smelting uses a 3kg suspension smelting furnace ZG2-XF and a water-cooled copper crucible with a crucible size of Φ100*150mm.

[0027] Materials: 99.99% pure tin particles with a particle size of 2-4mm, 99.95% pure indium particles with a particle size of 1-3mm, 99.99% pure zinc particles with a particle size of 2-6mm (20% more is added to account for volatilization); 99.99% pure bulk silver and 99.99% pure bulk gallium.

[0028] like Figure 1 As shown in the figure, this patent solution proposes a process for preparing a low-melting-point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder. Preparation process: Place half of the Sn particles at the bottom of the crucible, add In, Ga, and Zn particles in the middle, and then add the remaining Sn and Ag particles. After vacuuming with a mechanical pump, turn off the vacuum pump and fill with high-purity argon to normal atmospheric pressure to complete a purge process. Repeat the purge process three times. After the last purge, use a vacuum pump to vacuum to 3.0×10 -3 Pa, turn on the induction heating power, the initial heating power is 5kw, the heating time is 1min, then increase the heating power by 2kw every 2 minutes until it reaches 20kw, and wait for the particles to be completely liquefied. After the particles are completely liquefied, keep warm for 5 minutes, press the casting button to start casting. After the cast sample is cooled, perform DSC test in an argon atmosphere with a temperature range of room temperature to 200℃ and a heating rate of 10℃ / min. Figure 2Figure 2 shows the DSC curve of a Sn-In-Zn-Ga-Ag alloy. This sample exhibits two endothermic peaks during heating, corresponding to temperatures of 62.33°C and 101.67°C, respectively. The first endothermic peak is smaller, with a nearby inflection point corresponding to a temperature of 30.33°C. A second endothermic peak appears at 101.67°C during the heating process, with two inflection points near this endothermic peak corresponding to temperatures of 74.33°C and 111.67°C, respectively. Therefore, it can be inferred that the melting point of this alloy is 101.67°C.

[0029] The wettability of the solder is one of the factors that determine soldering quality. The better the wettability, the higher the bond quality between the solder and the base material. The spreading area method has been widely used in industrial production due to its simplicity and practicality. This method evaluates the wettability of the solder by measuring the area of ​​spread of the solder on the substrate surface, reflecting the ability of the liquid alloy to spread on the substrate surface.

[0030] Experimental methods: Experimental equipment: workbench, scraper, heating table, steel mesh Matrix treatment First, a 5 mm × 5 mm × 1 mm Cu sheet was polished with 180#, 40000#, 1200#, 2500#, and 4000# sandpaper in sequence, pickled with 5% dilute hydrochloric acid to remove the surface oxide layer, and then ultrasonically cleaned with anhydrous ethanol and dried for later use.

[0031] Solder paste coating First, take out the solder paste prepared by using the solder mixed flux of the present invention, put the treated copper sheet into a 5 mm × 5 mm × 1 mm hole on the workbench, and then put a steel mesh with a hole diameter of 2 mm and a thickness of 0.2 mm on the workbench, as shown in the following diagram ( Figure 3 ). Use a scraper to take some solder paste and apply it to the copper sheet through the steel mesh.

[0032] Solder paste wetting First, gradually increase the temperature of the heating stage to the set temperature for 5 minutes, and then place the substrate on the heating stage after the temperature of the heating stage stabilizes. After it is completely melted, remove the substrate and cool it to room temperature. Figure 4 The wetted spreading area was photographed and recorded and imported into the software to measure the spreading area.

[0033] Experimental results The solder paste starts to spread at 135°C, and as the temperature increases, the spreading area also increases. The original area is about 3.148mm 2 At 135°C, the spreading area is 3.714 mm 2At 145°C, the spreading area is 5.164 mm 2 At 165°C, the spreading area is 6.412 mm 2 At 200°C, the spreading area is 8.628 mm 2 .

[0034] Shear failure is a key form of solder joint fracture, making shear strength an important indicator for evaluating the mechanical properties of micro-solder joints, reflecting their ability to resist shear deformation. Given the mechanical loads solder joints endure during use, shear testing has become an important method for evaluating solder joint reliability.

[0035] Experimental method: paste brushing machine, heating table, shear force testing instrument Matrix treatment First, 5 mm × 5 mm × 1 mm and 10 mm × 10 mm × 1 mm Cu sheets were polished with 180#, 4000#, 1200#, 2500#, and 4000# sandpapers, pickled with 5% dilute hydrochloric acid to remove the surface oxide layer, and then ultrasonically cleaned with anhydrous ethanol and dried for later use.

[0036] Solder paste coating The solder paste was applied on the steel mesh of the paste brushing machine, and the processed 10 mm × 10 mm × 1 mm Cu sheet was placed on the paste brushing machine. The solder paste was applied by the paste brushing machine. Then the 10 mm × 10 mm × 1 mm Cu sheet coated with the solder paste was removed, and the processed 5 mm × 5 mm × 1 mm Cu sheet was placed on the 10 mm × 10 mm × 1 mm Cu sheet coated with the solder paste to prepare the sample.

[0037] Solder joint preparation First, gradually increase the temperature of the heating table to the set temperature. After the temperature stabilizes, place the sample on the heating table and heat it. After it is completely melted, remove the sample and cool it to room temperature.

[0038] Shear test Place the sample on the shear test bench and measure its shear force using a shear force tester.

[0039] Experimental results After heating at 200°C and cooling to room temperature, the shear force of the sample can reach 50N.

[0040] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A low melting point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder, characterized in that: The composition of the lead-free solder, calculated by atomic percentage, includes: 34.0% Sn, 30.0% In, 20.0% Zn, 10.0% Ga, and 6.0% Ag.

2. The low melting point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder according to claim 1, characterized in that: The melting point of the low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder is ≤102°C.

3. The low melting point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder according to claim 1, characterized in that: The purity of the raw materials of Sn, Zn, Ga and Ag used is ≥99.99wt%; the purity of the raw material of In used is ≥99.95wt%.

4. The low melting point Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder according to claim 1, characterized in that: Due to the volatilization of Zn, 20% more Zn is added to the raw material Zn.

5. The method for preparing the low-melting-point Sn-In-Zn-Ga-Ag multi-principal-element alloy lead-free solder according to any one of claims 1 to 4, characterized in that: The following steps are involved: Sn, In, Zn, Ga and Ag are mixed in proportion; poured into a crucible, and then vacuum smelted to obtain a melt, which is then formed into a Sn-In-Zn-Ga-Ag multi-principal alloy lead-free solder.

6. The preparation method according to claim 5, characterized in that Place half of the Sn particles at the bottom of the crucible, add In, Ga, and Zn particles in the middle, and then add the remaining Sn and Ag particles.

7. The preparation method according to claim 5, characterized in that The specific operation of the vacuum melting includes: turning on the induction heating power supply in a vacuum argon environment, with an initial heating power of 5 kW and a heating time of 1 minute, then increasing the heating power by 2 kW every 2 minutes until it reaches 20 kW, and waiting for the particles to be completely liquefied; after the particles are completely liquefied, keeping the heat for 5 minutes, casting, and cooling to obtain the lead-free solder.

8. The preparation method according to claim 7, characterized in that The vacuum argon environment was evacuated as follows: after evacuating with a mechanical pump, the vacuum pump was turned off and filled with high-purity argon gas to normal atmospheric pressure to complete a purge process, and the purge process was repeated three times; after the last purge, the vacuum pump was used to evacuate to 3.0×10 -3 Pa.

Images