A high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging

Abstract

The present invention discloses a high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging. The solder alloy comprises the following components in mass percentage: 3.4% Ag, 0.4% Cu, 1.4-2.8% In, 1.3-2.6% Bi, 0.8-1.5% Sb, and the balance is Sn; the atomic number ratio of the sum of Bi atoms and Sb atoms to In atoms is 1:1; and the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula is satisfied: Sn 260‑4x Ag 10 Cu2In 2x Bi x Sb x The lead-free solder alloy of the present invention has process performance compatible with the commonly used Sn-Ag-Cu alloy, good wettability, and excellent mechanical properties, and is particularly suitable for SoC chip packaging.

Description

Technical Field

[0001] The invention belongs to the technical field of solder alloys, and in particular relates to a Sn-based lead-free solder alloy with high strength and high plasticity. Background Art

[0002] As chips continue to evolve toward larger sizes, higher computing power, higher-density integration, higher throughput, and higher reliability, SoC (System on a Chip) chips, with their high integration and low power consumption, meet the performance and battery life requirements of mobile devices such as smartphones and tablets. At the same time, SoC chips also face significant reliability challenges: on the one hand, the risks of warping and other issues facing electronic devices during the packaging and manufacturing process are becoming more severe; on the other hand, the reliability requirements for packaging during service are becoming more stringent. The typical Sn-3.0Ag-0.5Cu solder currently used for package interconnects suffers from poor performance in temperature cycling and thermal shock, and no longer meets the requirements for high-reliability packaging. There is an urgent need to optimize the composition of Sn-Ag-Cu lead-free solder alloys to improve the strength and plasticity of the solder joints, thereby enhancing their thermal fatigue reliability, such as those associated with temperature cycling and thermal shock.

[0003] Currently, several representative commercially available doped solder alloys (often containing elements such as Ni, Bi, Co, Ge, Ga, Sb, and In), such as Ecolloy's SAC-R (Sn-0.92Cu-2.46Bi), Cyclomax's SAC-Q (Sn-3.4Ag-0.5Cu-3.3Bi), Alpha's Innolot (Sn-3.8Ag-0.7Cu-3Bi-1.45Sb-0.15Ni), and Senju's M794 (Sn-3.4Ag-0.7Cu-3.2Bi-3Sb-0.06Ni-0.01Co), all offer improved strength over Sn-Ag-Cu alloys. However, these alloys still suffer from shortcomings such as insufficient temperature cycle life and poor ductility, failing to meet the current demand for high service reliability in solder alloys. Therefore, there is an urgent need to design and develop a Sn-based solder alloy with both high strength and high ductility to meet current industrial application requirements. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a high-strength, high-plasticity Sn-based lead-free solder alloy that is suitable for SoC chip packaging, adapts to the current mainstream Sn-3.0Ag-0.5Cu solder alloy reflow process, has the advantages of high welding strength, high plasticity, low cost, lead-free, and environmental protection.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging. The solder alloy comprises the following components in percentage by mass: 3.4% Ag, 0.4% Cu, 1.4-2.8% In, 1.3-2.6% Bi, 0.8-1.5% Sb, and the balance is Sn; the atomic number ratio of the sum of Bi atoms and Sb atoms to In atoms is 1:1; and the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula is satisfied: Sn 260-4x Ag 10 Cu2In 2x Bi x Sb x .

[0007] Preferably, the solder alloy has an In content of 1.4%, a Bi content of 1.3%, and a Sb content of 0.8% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 252 Ag 10 Cu2In4Bi2Sb2.

[0008] Preferably, the solder alloy has an In content of 2.1%, a Bi content of 1.9%, and a Sb content of 1.1% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 248 Ag 10 Cu2In6Bi3Sb3.

[0009] Preferably, the solder alloy has an In content of 2.8%, a Bi content of 2.6%, and a Sb content of 1.5% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 244 Ag 10 Cu2In8Bi4Sb4.

[0010] In view of the characteristics of Sn-Ag-Cu solder alloy, the present invention is based on the theoretical model of "(cluster) connected atoms" and combines the phase diagram with the lever theorem to obtain the Sn-Ag-Cu ternary cluster formula [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn 10]Sn5. According to the principle of strong interaction: compared with Ag and Cu atoms, the mixing enthalpy of Bi, Sb, and In atoms with Sn atoms is the lowest, and Sn atoms serve as the matrix. Therefore, in the fourth, fifth, and sixth component replacement processes, Sn atoms are all replaced. In addition, since the number of outermost electrons of Sn atoms is 4, the number of outermost electrons of Bi and Sb atoms is 5, and the number of outermost electrons of In atoms is 3, in order to ensure that the number of atoms before and after replacement remains unchanged, and the number of outermost electrons of the cluster remains consistent, the atomic ratio of the sum of Bi and Sb atoms to In atoms is determined to be 1:1. Finally, combined with the saturated solid solubility limit of In, Sb, and Bi in β-Sn, a Sn-Ag-Cu-In-Bi-Sb hexa-element alloy with a determined composition is obtained.

[0011] The present invention has the following beneficial effects:

[0012] 1) By regulating the elemental ratio of In, Bi, and Sb, a micron-sized Bi-rich phase or InSb phase is precipitated between the β-Sn crystals. This refines the β-Sn matrix and improves the plasticity of the Sn-Ag-Cu solder alloy, significantly enhancing the overall performance, especially the mechanical properties. With a melting point between 199 and 221°C, the alloy has the advantages of good wettability, high soldering strength, high plasticity, low cost, lead-freeness, and environmental friendliness, making it particularly suitable for SoC chip packaging.

[0013] 2) In the Sn-based solder alloy of the present invention, In can reduce the liquidus temperature of the solder, balance the effect of Sb on the melting point, and adapt to the existing process; generate the Cu6(Sn,In)5 intermetallic compound phase at the interface during brazing, improve the alloy's wettability and interface bonding strength; at the same time, inhibit the generation of Cu3Sn intermetallic compound phase during aging, avoid the formation of Kirkendall voids; improve the joint strength, and can improve the joint ductility, thereby improving the mechanical properties of the solder joint before and after aging. Bi at room temperature (25 o C) has a solid solubility of 3% in β-Sn and at high temperature (125 oC) The solid solubility can be increased to 17%. Therefore, Bi added at room temperature can improve the strength and hardness of the solder through solid solution strengthening. Furthermore, the precipitated Bi can refine the bulk solder microstructure. At high temperatures, especially during aging and temperature cycling, Bi continuously dissolves in the β-Sn matrix, refining the matrix while suppressing the coarsening of the Ag3Sn phase, thereby improving the mechanical properties of the solder alloy at high temperatures. Sb increases the solder hardness through solid solution strengthening and significantly refines the bulk solder microstructure. Furthermore, Sb continues to dissolve in the β-Sn matrix during aging, improving the mechanical properties after aging. Furthermore, In increases the amount of Bi dissolved in the matrix, resulting in micron-sized precipitated Bi-rich phases. The smaller Bi particles exert both grain refinement and dispersion strengthening effects on the solder matrix, enhancing both the strength and ductility of the alloy. In can form an InSb intermetallic compound phase with Sb at the β-Sn grain boundary. The size of the InSb phase is micron-level. The smaller InSb phase can produce fine grain strengthening and dispersion strengthening effects on the solder matrix, thereby improving the strength and plasticity of the alloy at the same time. At the same time, the simultaneous presence of In and Sb can avoid the continuous bidirectional phase transformation between β-Sn and γ (InSn4), thereby improving the alloy's thermal fatigue resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The microstructure diagram of the solder of the comparative example and the embodiment of the present invention;

[0015] Figure 2 The Vickers hardness indentation diagram of the solder of the comparative example and the embodiment of the present invention;

[0016] Figure 3 This is a diagram of the shear strength of BGA solder joints in comparative examples and embodiments of the present invention;

[0017] Figure 4 Graphs showing shear displacement of BGA solder joints in comparative examples and embodiments of the present invention. DETAILED DESCRIPTION

[0018] The present invention will be further described below with reference to specific examples, but the scope of protection of the present invention is not limited in any way. Example 1

[0019] The high-strength, high-plasticity Sn-based solder alloy of this embodiment includes the following components in percentage by mass: Ag 3.4%, Cu 0.4%, In 1.4%, Bi 1.3%, Sb 0.8%, and the balance being Sn.

[0020] The Sn-Ag-Cu ternary cluster model design process of this embodiment is as follows:

[0021] Based on [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn10 ]Sn5=Sn 260 Ag 10 Cu2 model, 4 In atoms are used to replace 4 Sn atoms, and 2 Bi atoms and 2 Sb atoms are added to replace 2 Sn atoms respectively. Finally, Sn 252 Ag 10 Cu2In4Bi2Sb2, that is, Sn-3.4Ag-0.4Cu-1.4In-1.3Bi-0.8Sb (mass percentage).

[0022] The method for preparing the high-strength, high-plasticity Sn-based solder alloy of this embodiment comprises the following steps:

[0023] 1) Weigh Sn, Ag, Cu, In, Bi, and Sb with a purity of 99.99% according to mass ratio and place them in a quartz tube;

[0024] 2) Use hydrogen flame to melt and seal one end of the quartz tube, melt the other end into a narrow opening, and use a vacuum pump to evacuate the tube to a vacuum degree of 1×10 -3 ~ 1×10 -4 Pa, after exhausting the air in the tube, melt and seal the narrow opening;

[0025] 3) Place the quartz tube from step 2) in a resistance furnace and heat it to 800-1000°C for melting. After all components are melted, keep the temperature at 800-1000°C for 8-10 hours to homogenize the alloy. Rotate the quartz tube every hour to ensure a more uniform liquid alloy in the tube.

[0026] 4) After the smelting is completed, the quartz tube is taken out and cooled to room temperature to obtain a high-strength and high-plasticity Sn-based solder alloy. Example 2

[0027] The high-strength, high-plasticity Sn-based solder alloy of this embodiment includes the following components in percentage by mass: Ag 3.4%, Cu 0.4%, In 2.1%, Bi 1.9%, Sb 1.1%, and the balance is Sn.

[0028] The Sn-Ag-Cu ternary cluster model design process of this embodiment is as follows:

[0029] Based on [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn 10 ]Sn5=Sn 260 Ag 10 Cu2 model, 6 In atoms are used to replace 6 Sn atoms, and 3 Bi atoms and 3 Sb atoms are added to replace 3 Sn atoms respectively. Finally, Sn248 Ag 10 Cu2In6Bi3Sb3, that is, Sn-3.4Ag-0.4Cu-2.1In-1.9Bi-1.1Sb (mass percentage).

[0030] The preparation method of the solder alloy is the same as that of Example 1. Example 3

[0031] The high-strength, high-plasticity Sn-based solder alloy of this embodiment includes the following components in percentage by mass: Ag 3.4%, Cu 0.4%, In 2.8%, Bi 2.6%, Sb 1.5%, and the balance is Sn.

[0032] The Sn-Ag-Cu ternary cluster model design process of this embodiment is as follows:

[0033] Based on [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn 10 ]Sn5=Sn 260 Ag 10 Cu2 model, 8 In atoms are used to replace 8 Sn atoms, and 4 Bi atoms and 4 Sb atoms are added to replace 4 Sn atoms respectively. Finally, Sn 244 Ag 10 Cu2In8Bi4Sb4, that is, Sn-3.4Ag-0.4Cu-2.8In-2.6Bi-1.5Sb (mass percentage).

[0034] The preparation method of the solder alloy is the same as that of Example 1.

[0035] Comparative Example 1

[0036] The solder alloy of this comparative example includes the following components in mass percentage: Ag 3.4%, Cu 0.4%, In 3.6%, Bi 3.2%, Sb 1.9%, and the balance is Sn.

[0037] The Sn-Ag-Cu ternary cluster model design process of this comparative example is as follows:

[0038] Based on [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn 10 ]Sn5=Sn 260 Ag 10 Cu2 model, 10 In atoms are used to replace 10 Sn atoms, and 5 Bi atoms and 5 Sb atoms are added to replace 5 Sn atoms respectively. Finally, Sn 240 Ag 10 Cu2In 10Bi5Sb5, namely Sn-3.4Ag-0.4Cu-3.6In-3.2Bi-1.9Sb (mass percentage). Although this comparative example satisfies the cluster model, the elemental composition ranges of In, Bi, and Sb exceed the composition ranges specified in the claims.

[0039] The preparation method of the solder alloy is the same as that of Example 1.

[0040] Comparative Example 2

[0041] The solder alloy of this comparative example includes the following components in mass percentage: Ag 3.4 %, Cu 0.4 %, In 4.2 %, Bi 4.5 %, and Sb 1.9 %, with the balance being Sn.

[0042] The Sn-Ag-Cu ternary cluster model design process of this comparative example is as follows

[0043] Based on [Sn-Ag 10 Cu2]Sn3+16{[Sn-Sn 10 ]Sn5=Sn 260 Ag 10 Cu2 model, using 12 In atoms to replace 12 Sn atoms, adding 7 Bi atoms to replace 7 Sn atoms, and 5 Sb atoms to replace 5 Sn atoms. Finally, Sn 236 Ag 10 Cu2In 12 Bi7Sb5, namely Sn-3.4Ag-0.4Cu-4.2In-4.5Bi-1.9Sb (mass percentage). Although this comparative example satisfies the cluster model, the elemental composition ranges of In, Bi, and Sb exceed the composition ranges specified in the claims.

[0044] The preparation method of the solder alloy is the same as that of Example 1.

[0045] Comparative Example 3

[0046] The solder alloy of this comparative example includes the following components in percentage by mass: 3.0% Ag and 0.5% Cu, with the balance being Sn. This comparative example is a commercial composition that does not satisfy the cluster formula.

[0047] The preparation method of the solder alloy is the same as that of Example 1.

[0048] Comparative Example 4

[0049] The solder alloy of this comparative example includes the following components in percentage by mass: 3.4% Ag, 0.5% Cu, and 3.3% Bi, with the balance being Sn. This comparative example is a commercial composition that does not satisfy the cluster formula.

[0050] The preparation method of the solder alloy is the same as that of Example 1.

[0051] Comparative Example 5

[0052] The solder alloy of this comparative example includes the following components in percentage by mass: 0.92% Cu and 2.46% Bi, with the balance being Sn. This comparative example is a commercial composition that does not satisfy the cluster formula.

[0053] The preparation method of this comparative example is the same as that of Example 1.

[0054] Comparative Example 6

[0055] The solder alloy of this comparative example includes the following components in percentage by mass: 3.8% Ag, 0.7% Cu, 3.0% Bi, 1.45% Sb, and 0.15% Ni, with the balance being Sn. This comparative example is a commercial composition that does not satisfy the cluster formula.

[0056] The preparation method of the solder alloy is the same as that of Example 1.

[0057] Test Method

[0058] The solder alloys of the examples and comparative examples were subjected to DSC testing, hardness testing, and shear performance testing of BGA solder joints. The experimental conditions are as follows:

[0059] 1) DSC testing was performed at a heating rate of 10°C / min. The sample mass was 30 mg using a differential scanning calorimeter. Numerical processing was performed automatically using software. The solidus temperature was defined as the intersection of the tangent line of the endothermic peak with the extended line of the left baseline, while the liquidus temperature was defined as the intersection of the tangent line of the endothermic peak with the extended line of the right baseline. The test results are shown in Table 1.

[0060] 2) Take a solder block of appropriate size and polish it until the surface is shiny and free of visible scratches. Place the solder block on the stage of the Vickers hardness tester, select a uniform location, and apply a load of 50 gf for 10 seconds. The test results are shown in Table 1.

[0061] 3) 600 μm diameter solder balls of the comparative and example solder alloys were prepared. A small amount of flux was applied to the 600 μm solder balls and accurately placed on 500 μm pads on a Cu substrate on a printed circuit board (PCB). After assembly, the balls were placed in a reflow oven using a commercial SAC305 solder alloy reflow method, with a maximum temperature of 240–260°C and a time above liquidus temperature of 60–120 s. The shear strength of the solder joints was tested using a Dage 4000 bondtester, with a blade height of 45 μm and a shear speed of 500 μm / s. The test results are shown in Table 1.

[0062] Table 1 Test results of solder alloy properties of examples and comparative examples

[0063]

[0064] As can be seen from Table 1, the solder alloys of Examples 1, 2, and 3 have better shear strength, shear displacement, and more suitable melting temperature than the solder alloys of Comparative Examples 1-6.

[0065] Figure 1 The microstructure diagrams of Examples 1, 2, 3 and Comparative Examples 1-6. The microstructure of the embodiment alloy consists of β-Sn phase, Ag3Sn phase, Cu6Sn5 phase, and micron-sized Bi phase and InSb phase. The added In element is first dissolved in Ag3Sn; Bi and Sb elements are first dissolved in the β-Sn matrix; In can increase the solid solubility of Bi in the β-Sn matrix, and Bi exceeding the solid solubility will precipitate as a micron-sized Bi-rich phase. It can be seen from the microstructure in the figure that the microstructures of Examples 1, 2, and 3 are more uniform and finer than those of Comparative Examples 1-6.

[0066] Figure 2 The Vickers hardness indentation diagrams of Examples 1, 2, 3 and Comparative Examples 1-6 are shown. Comparative Examples 1-6 all showed obvious deformation and cracks, while the alloy body solders of Examples 1, 2, and 3 showed no obvious cracks. This is because, by proportioning, compound addition of appropriate amounts of In elements and Sb elements, In elements and Bi elements, In elements and Bi, and Sb elements can generate micron-scale InSb phases and Bi phases at the grain boundaries, strengthen the grain boundaries, reduce brittleness while increasing hardness, and effectively hinder the expansion of cracks under stress. In addition, according to the results in Table 1, Examples 1, 2, and 3 have lower Vickers hardness than Comparative Examples 1-6, and are not prone to stress transfer during thermal fatigue service, which has a greater advantage in the practical application of solder alloys.

[0067] Figure 3 is the shear strength diagram of Examples 1, 2, 3 and Comparative Examples 1-6, Figure 4 The shear displacement diagrams of Examples 1, 2, 3 and Comparative Examples 1-6 are shown. As can be seen from the figure, in the solder joints of Examples 1, 2 and 3, in which In, Sb and Bi are added in a specific range, In, Bi and Sb elements can all make the solder joints have higher shear strength, while greatly improving the shear displacement of the solder joints and having good comprehensive mechanical properties. This is due to the synergistic effect of In and Sb elements, which generates micron-sized InSb phases at the grain boundaries, reduces grain boundary energy, reduces the risk of brittle fracture, and improves solder joint ductility. In addition, the In element can increase the solid solubility of the Bi element in the β-Sn matrix, reduce the precipitation of large Bi phases in the matrix, and reduce the brittleness of the solder joints while improving the strength of the solder joints.

[0068] Through the performance comparison of the examples and comparative examples, it can be seen that the Sn-based solder alloy of the present invention has higher welding strength, better plasticity, is lead-free and environmentally friendly, is more suitable for the current mainstream Sn-3.0Ag-0.5Cu solder alloy reflow process, and is particularly suitable for SoC chip packaging.

[0069] The above-described embodiments merely represent several implementation methods of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging, characterized in that: The solder alloy comprises the following components in mass percentage: 3.4% Ag, 0.4% Cu, 1.4-2.8% In, 1.3-2.6% Bi, 0.8-1.5% Sb, and the balance is Sn; the atomic number ratio of the sum of Bi atoms and Sb atoms to In atoms is 1:1; and the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula is satisfied: Sn 260-4x Ag 10 Cu2In 2x Bi x Sb x .

2. A high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging according to claim 1, characterized in that: The solder alloy has an In content of 1.4%, a Bi content of 1.3%, and a Sb content of 0.8% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 252 Ag 10 Cu2In4Bi2Sb2.

3. A high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging according to claim 1, characterized in that: The solder alloy has an In content of 2.1%, a Bi content of 1.9%, and a Sb content of 1.1% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 248 Ag 10 Cu2In6Bi3Sb3.

4. A high-strength and high-plasticity Sn-based lead-free solder alloy suitable for SoC chip packaging according to claim 1, characterized in that: The solder alloy has an In content of 2.8%, a Bi content of 2.6%, and a Sb content of 1.5% by mass; and satisfies the Sn-Ag-Cu-In-Bi-Sb hexagram cluster formula. 244 Ag 10 Cu2In8Bi4Sb4.

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