Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging and preparation method of Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder

Abstract

The invention relates to Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging and a preparation method of the Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder, and belongs to the technical field of electronic packaging materials. The solder comprises, by atomic percent, 25%-35% of tin, 25%-35% of bismuth, 25%-35% of indium, 5% of silver and 5%-10% of zinc, and preparation of the solder is achieved in an induction melting mode. Main phases of the Sn-In-Bi-Ag-Zn high-reliability high-entropy alloy low-temperature solder for electronic packaging are the tin-based solid solution and the indium bismuth phase, bismuth and indium atoms are subjected to high solid solution in the tin-based solid solution, and solid solution strengthening of the solder is promoted. The melting point is only 80-90 DEG C, and the warping problem caused by CET mismatching can be effectively reduced due to the extremely low melting point. The low-temperature wettability is good, and the wetting angle is smaller than or equal to 33 degrees when the copper substrate is wetted at 120 DEG C. Meanwhile, inhibition of interface IMC rapid growth can be achieved without a diffusion barrier layer, a thin IMC layer can still be kept under the conditions of long-time and multiple times of backflow, the volume ratio of the IMC in a micro welding spot can be remarkably reduced, the influence of the IMC on the mechanical property and reliability of the welding spot can be reduced, good mechanical property can still be kept after long-time and multiple times of backflow, and the service life of the welding spot can be prolonged. The method is suitable for advanced packaging such as flexible packaging and 3D IC packaging which require low packaging temperature or high thermal reliability.

Description

Technical Field

[0001] The invention relates to a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging and a preparation method thereof, belonging to the technical field of electronic packaging materials. Technical Background

[0002] As chip manufacturing processes are approaching physical limits, it is becoming increasingly difficult to improve chip performance by shrinking the process, and the cost is increasing dramatically. Advanced packaging has become the main way to continue Moore's Law. The application of technologies such as Filp, Chiplet, 2.5D and 3D packaging has greatly improved the integration of packaging. The increase in the degree of integration has led to the gradual miniaturization of the solder joints in the package. For micro-bumps with a solder height of 5-15μm, their volume is more than 1,000 times different from that of flip-chip solder joints. The hard and brittle IMC occupies a large proportion of the solder joints, thus affecting the reliability of the solder joints. In addition, for 3D packaging, its vertical stacking structure requires multiple reflows during welding to achieve welding. In order to avoid repeated melting of solder joints during multiple reflows, high, medium and low melting point solders are required to be used in combination. At the same time, low-temperature solders can effectively reduce the warpage problem caused by CET mismatch, but compared with mature medium and high temperature solders, low-temperature solders still have a large gap to fill. At present, high and medium temperature solders are relatively mature, but there are certain problems with low-temperature solders. The relatively mature Sn-58Bi also affects the reliability of its solder joints due to the inherent brittleness of the Bi element. The crack propagation caused by the difference in Bi solubility between the SnBi solder and the Cu substrate solder joint interface IMC (Cu6Sn5, Cu3Sn) will seriously threaten the solder joint life.

[0003] High entropy alloy (HEA) refers to a multi-component alloy with a mixing entropy higher than 1.62R or more than 5 main elements, and the atomic ratio of each component is between 5% and 35%. High entropy alloy (HEA) has unique properties, among which its high entropy effect and slow diffusion effect affect the growth of IMC from the thermodynamic and kinetic perspectives respectively. According to the Gibbs free energy formula △G=△H-T△S, the high entropy effect of high entropy alloy makes the system have lower free energy than IMC in the solid solution state, so the system tends to form a solid solution rather than an intermetallic compound. Phase change in high entropy alloy requires the coordinated diffusion of many different types of atoms, but the vacancies that replace the diffusion are of limited concentration, and the vacancies will be surrounded and competed by different atoms during diffusion. They need higher activation energy to migrate through the fluctuating diffusion path, making the diffusive phase in high entropy alloy very slow. Generally, the growth of IMC at the welding interface depends on the diffusion of elements between interfaces, and the slow diffusion effect of high entropy alloys requires a larger diffusion activation energy for the diffusion between elements, which can make the growth of IMC more difficult. In summary, high entropy alloy solder can significantly inhibit the growth of interface IMC, hinder the diffusion of elements between interfaces, and hinder the high consumption rate of copper without the need for a diffusion barrier layer. High entropy alloys have great use value in improving the reliability of packaging solder joints. Summary of the invention

[0004] Aiming at the increase in the volume proportion of brittle IMC of micro solder joints in advanced packaging, which seriously threatens the reliability of advanced packaging such as 3DIC, the present invention provides a high-reliability high-entropy alloy low-temperature solder for Sn-Bi-In-Ag-Zn electronic packaging. The unique high-entropy effect and slow diffusion effect of the high-entropy alloy can inhibit the rapid growth of interface IMC. After long-term reflow, the thickness of the interface IMC can still be maintained at a relatively thin level, thereby ensuring the reliability of the solder joint.

[0005] Aiming at the demand for low-temperature solders in advanced packaging such as 3DIC packaging, the present invention provides a Sn-Bi-In-Ag-Zn electronic packaging high-reliability high-entropy alloy low-temperature solder, which has a low melting point, good wetting properties on a Cu substrate and high interface shear strength, and can achieve reliable copper-copper interconnection at low temperatures.

[0006] In order to achieve the above objectives, the present invention adopts the following technical solutions:

[0007] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging of the present invention comprises, by atomic percentage, 25%-35% of tin, 21%-35% of bismuth, 25%-35% of indium, 5% of silver and 5%-10% of zinc.

[0008] The preparation method of the Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging of the present invention is as follows: (1) Weigh high-purity Sn, In, Bi, and Ag particles according to the designed proportion, and cover them with 1.3:1 KCl and LiCl molten eutectic molten salt for protection, and smelt the Sn-In-Bi-Ag quaternary master alloy. Add Sn, In, Bi, and Ag in the order of low-density metal at the bottom and high-density metal at the top to reduce the specific gravity segregation during the smelting process, keep the temperature at 800°C-900°C for 1h-2h and stir thoroughly, and directly cast it in water for cooling. (2) Weigh high-purity Zn particles and Sn-In-Bi-Ag quaternary master alloy according to the designed proportion, and place the Sn-In-Bi-Ag quaternary master alloy on the Zn particles for protection. Pour molten KCl and LiCl eutectic salt onto the surface of the metal raw material, keep it at 500°C for 1h-2h and stir it thoroughly, then directly cast it into water and cool it to obtain Sn-Bi-In-Ag-Zn high entropy alloy solder.

[0009] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging described in the present invention has main phases of tin-based solid solution and indium-bismuth phase. Bismuth and indium atoms are highly dissolved in the tin-based solid solution, which promotes the solid solution strengthening of the solder. The tensile strength of the cast alloy is ≥37MPa and the elongation is ≥33%.

[0010] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging of the present invention has a melting point between 80°C and 90°C, which significantly reduces the packaging temperature and can effectively reduce the warping problem caused by CET mismatch during the reflow process. At the same time, at a reflow temperature of 120°C to 160°C, the wetting angle on the Cu substrate after wetting is ≤33°, which has good low-temperature wetting performance.

[0011] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging disclosed by the present invention has a unique high-entropy effect and a slow diffusion effect, which significantly inhibits the rapid growth of the interface IMC. After reflow at 120°C-160°C for 5min-20min, the IMC thickness is still ≤1.5μm. The thin IMC layer helps to reduce the influence of the IMC on the mechanical properties and reliability of the solder joint, thereby improving the service life of the solder joint.

[0012] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging of the present invention can realize reliable copper-copper interconnection at low temperature, and after reflow at 120°C-160°C for 5 minutes, the shear strength of the solder joint is ≥30MPa.

[0013] The Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder for advanced packaging of the present invention has good thermal reliability, and the solder joints can still maintain good mechanical properties after long-term reflow. After reflow at 120°C-160°C for 10 minutes, the shear strength decay is ≤5MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the DSC curve of Sn-Bi-In-Ag-Zn low temperature high entropy solder in Example 1 of the present invention;

[0015] Figure 2 This is the engineering stress-strain curve of Sn-Bi-In-Ag-Zn low-temperature high-entropy solder in Example 1 of the present invention;

[0016] Figure 3 This is the fracture morphology of the Sn-Bi-In-Ag-Zn low-temperature high-entropy solder tensile specimen in Example 1 of the present invention;

[0017] Figure 4 This is the X-ray phase analysis of Sn-Bi-In-Ag-Zn low temperature high entropy solder in Example 1 of the present invention;

[0018] Figure 5 This is a SEM image of a Sn-Bi-In-Ag-Zn / Cu solder joint prepared by reflowing Sn-Bi-In-Ag-Zn low-temperature high-entropy solder at 120° C. for 20 minutes in Example 1 of the present invention; Specific implementation plan

[0019] Implementation Case 1:

[0020] The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder of this example is composed of 30% tin, 30% bismuth, 30% indium, 5% silver and 5% zinc in atomic percentage.

[0021] Weigh high-purity Sn, In, Bi, and Ag particles according to the designed ratio, and cover them with 1.3:1 KCl and LiCl molten eutectic molten salt for protection, and melt Sn-In-Bi-Ag quaternary master alloy. Add Sn, In, Bi, and Ag in the order of low-density metal at the bottom and high-density metal at the top to reduce specific gravity segregation during the smelting process, keep the temperature at 850℃ for 2h and stir thoroughly, and directly cast it in water to cool.

[0022] High-purity Zn particles and Sn-In-Bi-Ag quaternary master alloy were weighed according to the designed proportion, and the Sn-In-Bi-Ag quaternary master alloy was placed on the Zn particles for protection. The molten KCl and LiCl eutectic salts were poured on the surface of the metal raw material, kept at 500°C for 1h and fully stirred, and directly cast in water for cooling to obtain the Sn-Bi-In-Ag-Zn high entropy alloy solder.

[0023] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder, the main phases of which are tin-based solid solution and indium-bismuth phase. The alloy has a tensile strength of 38.5 MPa and an elongation of 33.83%.

[0024] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder, which has a melting point of 87.97° C. and a wetting angle of 31.7° when wetting on a Cu substrate at 120° C.

[0025] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder. The average shear strength is 35.7 MPa after reflow at 120°C for 5 minutes. The shear strengths are 35.1 MPa, 34.0 MPa and 30.6 MPa respectively when reflowing at 120°C-160°C for 10 minutes. After reflowing at 160°C for 20 minutes, the interface IMC thickness is still <0.5 μm.

[0026] Implementation Case 2:

[0027] The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder of this example is composed of 30% tin, 25% bismuth, 30% indium, 5% silver and 10% zinc in atomic percentage.

[0028] Weigh high-purity Sn, In, Bi, and Ag particles according to the designed ratio, and cover them with 1.3:1 KCl and LiCl molten eutectic molten salt for protection, and melt Sn-In-Bi-Ag quaternary master alloy. Add Sn, In, Bi, and Ag in the order of low-density metal at the bottom and high-density metal at the top to reduce specific gravity segregation during the smelting process, keep the temperature at 850℃ for 2h and stir thoroughly, and directly cast it in water to cool.

[0029] High-purity Zn particles and Sn-In-Bi-Ag quaternary master alloy were weighed according to the designed proportion, and the Sn-In-Bi-Ag quaternary master alloy was placed on the Zn particles for protection. The molten KCl and LiCl eutectic salts were poured on the surface of the metal raw material, kept at 500°C for 1h and fully stirred, and directly cast in water for cooling to obtain the Sn-Bi-In-Ag-Zn high entropy alloy solder.

[0030] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder, the main phases of which are tin-based solid solution and indium-bismuth phase. The alloy has a tensile strength of 37.5 MPa and an elongation of 35.33%.

[0031] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder, which has a melting point of 87.97° C. and a wetting angle of 32.9° when wetting on a Cu substrate at 120° C.

[0032] This example provides a Sn-Bi-In-Ag-Zn high-reliability high-entropy alloy low-temperature solder. The average shear strength after reflow at 120°C for 5 minutes is 36.4 MPa. The shear strengths after reflow at 120°C, 140°C and 160°C for 10 minutes are 35.0 MPa, 31.9 MPa and 30.7 MPa, respectively. After reflow at 160°C for 20 minutes, the interface IMC thickness is still <0.9 μm.

[0033] The above embodiments are only preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be regarded as equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging, characterized in that: The main elements are tin, bismuth, indium, silver and zinc, and the composition by atomic percentage is 25%-35% tin, 21%-35% bismuth, 25%-35% indium, 5% silver and 5%-10% zinc.

2. The Sn-Bi-In-Ag-Zn full IMC phase high entropy low temperature solder for packaging as claimed in claim 1, characterized in that: The solder contains 25% tin, 5% silver, 5%-10% zinc, ≥27% bismuth and the balance indium.

3. The Sn-Bi-In-Ag-Zn full IMC phase high entropy low temperature solder for packaging as claimed in claim 1, characterized in that: The solder contains 30% tin, 5% silver, 5%-10% zinc, ≥24% bismuth and the balance indium.

4. The Sn-Bi-In-Ag-Zn full IMC phase high entropy low temperature solder for packaging as claimed in claim 1, characterized in that: The solder contains 35% tin, 5% silver, 5%-10% zinc, ≥21% bismuth and the balance indium.

5. The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claim 1, characterized in that: The main phases are tin-based solid solution and indium-bismuth phase. Bismuth and indium atoms are highly dissolved in the tin-based solid solution, with bismuth and indium solubilities of 8%-13% and 10%-13% respectively, which promotes the solid solution strengthening of the solder, with a tensile strength of ≥37MPa and an elongation of ≥3%.

6. The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claims 1-2, characterized in that: It has an extremely low melting point and good low-temperature wetting performance, which significantly reduces the packaging temperature and can effectively reduce the warping problem caused by CET mismatch during the reflow process. Its melting point is only 80℃-90℃, and the wetting angle is ≤33° when wetting on the copper substrate at 120℃-160℃.

7. The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claims 1-3, characterized in that: During packaging, the rapid growth of the interface IMC can be suppressed without a diffusion barrier layer, and a thin IMC layer can be maintained after multiple and long-term reflows. When the peak temperature is 120℃-160℃, the thickness of the interface IMC is ≤1.5μm after reflowing on a Cu substrate for 5-20min.

8. The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claims 1-4, characterized in that: After soldering, the solder can achieve reliable copper-copper interconnection at low temperature. After 5 minutes at 120℃-160℃, the shear strength of the solder joint is ≥30MPa.

9. The Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claims 1-5, characterized in that: The solder joints have good thermal reliability and can maintain good mechanical properties after long-term reflow. After reflow at 120℃-160℃ for 10 minutes, the shear strength decay is ≤5MPa.

10. The method for preparing Sn-Bi-In-Ag-Zn high reliability high entropy alloy low temperature solder for advanced packaging as claimed in claims 1-6, characterized in that: (1) Weigh high-purity Sn, In, Bi, and Ag particles according to the designed ratio, and cover them with 1.3:1 KCl and LiCl molten eutectic molten salt for protection, and melt the Sn-In-Bi-Ag quaternary master alloy. Add Sn, In, Bi, and Ag in the order of low-density metal at the bottom and high-density metal at the top to reduce the specific gravity segregation during the smelting process, keep it at 800℃-900℃ for 1h-2h and stir it thoroughly, and directly cast it into water to cool. (2) Weigh high-purity Zn particles and Sn-In-Bi-Ag quaternary master alloy according to the designed ratio, and place the Sn-In-Bi-Ag quaternary master alloy on the Zn particles for protection. Pour the molten KCl and LiCl eutectic molten salt on the surface of the metal raw material, keep it at 500℃ for 1h-2h and stir it thoroughly, and directly cast it into water to cool to obtain Sn-Bi-In-Ag-Zn high entropy alloy solder.

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