Low-temperature soft soldering lead-free solder alloy and preparation method and application thereof

Abstract

The invention discloses a low-temperature soft soldering lead-free solder alloy and a preparation method and application thereof. The invention belongs to the field of brazing materials. The technical problems that in the prior art, low-temperature lead-free brazing filler metal is poor in mechanical property, and welding spot interface IMC overgrows are solved. The prepared low-temperature soft soldering lead-free solder alloy is Sn-Bi-In-Al multi-component lead-free solder, has the advantages of being low in melting point, good in wettability and high in strength and plasticity, does not warp during low-temperature soldering, can be used for electronic packaging and reflow soldering with a Cu substrate, and is thin in IMC of a welding spot interface, uniform in distribution and high in welding spot shearing strength. The low-temperature soft soldering lead-free solder alloy provided by the invention does not contain harmful elements such as Pb, is environment-friendly and eco-friendly, is simple in preparation process, has a good application prospect and is suitable for large-scale industrial production.

Description

Technical Field

[0001] The invention belongs to the field of soldering materials, and in particular relates to a low-temperature soft soldering lead-free solder alloy and a preparation method and application thereof. Background Art

[0002] As the critical dimensions of integrated circuits continue to shrink, the sustainability of Moore's Law has been increasingly challenged. Three-dimensional (3D) packaging technology improves packaging density by stacking multiple independent chips together in the vertical direction, and is considered an important technology to continue Moore's Law. With the promotion and application of 3D packaging technology, the thickness of devices is gradually reduced, which requires materials such as chips and printed circuit boards (PCBs) to be thinner. During the reflow soldering process, the warping phenomenon caused by the mismatch of thermal expansion coefficients will be more significant. The use of low-melting-point lead-free solder to reduce the soldering temperature is particularly important for reducing the warping of materials such as chips and PCB boards. In addition, the reflow soldering of temperature-sensitive components also requires the use of low-temperature lead-free solder to reduce damage to electronic components.

[0003] Currently, the commonly used low-temperature lead-free solders include Sn-Bi and In-based lead-free solders. Sn-Bi lead-free solder alloy has the advantages of low melting point, good wettability, and high tensile strength, and is widely used in the field of low-temperature soft soldering. However, Sn-Bi solder alloy is brittle and has poor ductility. In addition, under isothermal aging conditions, the brittle Bi phase in the Sn-Bi solder is prone to coarsening, resulting in poor plasticity of the solder and peeling. During high-temperature service, the Sn-Bi solder is also prone to Bi phase segregation at the interface and excessive growth of interfacial intermetallic compounds (IMC), resulting in reduced mechanical properties and reliability of the solder joint, and even causing solder joint failure. In-based lead-free solders usually have good ductility and thermal fatigue resistance, but their strength and hardness are relatively low, which restricts their application in the field of low-temperature soft soldering.

[0004] Different from traditional alloys, multi-component alloys exhibit many excellent physical and chemical properties due to lattice distortion effect, hysteresis diffusion effect and "cocktail" effect, such as ultra-high strength, good ductility, high wear resistance, high oxidation resistance and high corrosion resistance. Multi-component alloys have great potential as solders for electronic packaging soft soldering. Their lattice distortion effect is conducive to improving the mechanical properties of solders, and the hysteresis diffusion effect can inhibit the formation of interfacial brittle IMC, thereby improving the mechanical properties and reliability of solder joints. However, the research on multi-component lead-free solders is still in its early stages, and the existing multi-component lead-free solders generally have poor toughness, which is not conducive to the service reliability of solder joints. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problems of poor mechanical properties of low-temperature lead-free solder and excessive growth of IMC at the solder joint interface in the prior art. The present invention provides a low-temperature soft soldering lead-free solder alloy and a preparation method and application thereof.

[0006] The technical solution of the present invention is as follows:

[0007] One of the purposes of the present invention is to provide a low-temperature soft soldering lead-free solder alloy, the alloy chemical composition includes: Sn: 30-60%, Bi: 25-35%, In: 5-35%, Al: 2-5% by atomic percentage, and the sum of the atomic percentages of the constituent elements is 100%.

[0008] Further defined, the alloy chemical composition includes, by atomic percentage, Sn: 57%, Bi: 31%, In: 9% and Al: 3%.

[0009] Further defined, the alloy chemical composition includes, by atomic percentage, Sn: 34%, Bi: 30%, In: 31% and Al: 5%.

[0010] Further defined, the alloy chemical composition includes, by atomic percentage, Sn: 51%, Bi: 34%, In: 12% and Al: 3%.

[0011] Further defined, the alloy chemical composition includes, by atomic percentage, Sn: 58%, Bi: 27%, In: 10% and Al: 5%.

[0012] The second object of the present invention is to provide a method for preparing a lead-free solder alloy for low-temperature soldering, the method comprising:

[0013] S1: Weigh Sn, Bi, In and Al metal particles, mix them and put them into a smelting furnace;

[0014] S2: The mixed metal particles of S1 are covered with molten eutectic salt of LiCl and KCl, smelted, and cast to obtain a low-temperature soft soldering lead-free solder alloy.

[0015] It is further defined that the metal purity of Sn, Bi, In and Al in S1 is ≥99.9%.

[0016] It is further defined that the mass ratio of LiCl to KCl in S2 is 1:1-2.

[0017] It is further defined that the melting temperature in S2 is 600° C., the temperature is kept for 60 to 100 min, and mechanical stirring is performed once every 20 min during the melting period.

[0018] A third object of the present invention is to provide a low-temperature soft soldering lead-free solder alloy for use in low-temperature soft soldering of electronic packaging.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] (1) The low-temperature soft soldering lead-free solder alloy prepared by the present invention is a Sn-Bi-In-Al multi-component lead-free solder alloy. The melting point of the alloy is between 80°C and 150°C. Under the reflow soldering condition of 180°C, it exhibits good wettability and will not warp during low-temperature soldering. Therefore, it can be used in electronic packaging.

[0021] (2) Compared with the traditional low-temperature solder alloy, the low-temperature soft soldering lead-free solder alloy prepared by the present invention has the mechanical properties of high strength and high plasticity, which can improve the service reliability of the solder joint.

[0022] (3) When the low-temperature soft soldering lead-free solder alloy prepared by the present invention is reflow-welded with a Cu substrate, the IMC thickness of the solder joint interface is ≤1.4 μm and is evenly distributed, and the maximum shear strength of the solder joint is ≥80 MPa.

[0023] (4) The low-temperature soft soldering lead-free solder alloy provided by the present invention does not contain harmful elements such as Pb, is environmentally friendly and ecologically friendly, and has a simple preparation process, good application prospects, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a DSC test chart of the Sn-Bi-In-Al lead-free solder prepared in Example 1;

[0025] Figure 2 is the wetting angle of the Sn-Bi-In-Al lead-free solder prepared in Example 1;

[0026] Figure 3 (a) is a SEM image of the Sn-Bi-In-Al lead-free solder prepared in Example 1 at 500 times magnification. Figure 3 (b) is a SEM image of the Sn-Bi-In-Al lead-free solder prepared in Example 1 at 1000 times magnification. Figure 3 (c) is the Sn element distribution diagram of the Sn-Bi-In-Al lead-free solder prepared in Example 1, Figure 3 (d) is the In element distribution diagram of the Sn-Bi-In-Al lead-free solder prepared in Example 1, Figure 3 (e) is the Bi element distribution diagram of the Sn-Bi-In-Al lead-free solder prepared in Example 1, Figure 3 (f) Al element distribution diagram of Sn-Bi-In-Al lead-free solder prepared in Example 1;

[0027] Figure 4 This is a DSC test chart of the Sn-Bi-In-Al lead-free solder prepared in Example 2;

[0028] Figure 5 The tensile stress-strain curve of the Sn-Bi-In-Al lead-free solder prepared in Example 2;

[0029] Figure 6 is the wetting angle of the Sn-Bi-In-Al lead-free solder prepared in Example 3;

[0030] Figure 7 The tensile stress-strain curve of the Sn-Bi-In-Al lead-free solder prepared in Example 3;

[0031] Figure 8 (a) is the solder joint interface microstructure of the Sn-Bi-In-Al lead-free solder prepared in Example 4 after reflow soldering, Figure 8 (b) is the distribution diagram of Sn element at the solder joint interface after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4, Figure 8 (c) is the In element distribution diagram of the solder joint interface after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4, Figure 8 (d) is the Bi element distribution diagram of the solder joint interface after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4, Figure 8 (e) is the Al element distribution diagram of the solder joint interface after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4, Figure 8 (f) is the Cu element distribution diagram of the solder joint interface after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4;

[0032] Fig. 9 This is the shear strength of the solder joints after reflow soldering of the Sn-Bi-In-Al lead-free solder prepared in Example 4. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0034] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained through commercial channels by those skilled in the art.

[0035] Embodiment 1:

[0036] The low temperature soldering lead-free solder alloy prepared in this embodiment has an atomic percentage of 57% Sn, 31% Bi, 9% In and 3% Al. The mass percentages of each element are calculated to be 47.1% Sn, 45.1% Bi, 7.2% In and 0.6% Al. According to the mass percentage, Sn with a purity of 99.9% and Bi, In and Al with a purity of 99.99% are weighed, and the mixed metal particles are poured into a ceramic crucible melting furnace. Then, LiCl and KCl molten eutectic salt (LiCl: KCl = 1:1.3) are covered on the mixed metal to prevent the solder from oxidizing. The melting temperature is 600°C, and the temperature is kept for 80 minutes. During the melting period, mechanical stirring is performed once every 20 minutes. After the temperature is kept, the molten solder is cast in a graphite mold and cooled into an ingot to obtain a Sn-Bi-In-Al multi-component lead-free solder.

[0037] Embodiment 2:

[0038] The low-temperature soft soldering lead-free solder alloy prepared in this embodiment has an atomic percentage of 34% Sn, 30% Bi, 31% In and 5% Al. The mass percentage of each element is calculated to be 28.8% Sn, 44.8% Bi, 25.4% In and 1.0% Al. The solder preparation process is the same as that in Example 1.

[0039] Embodiment 3:

[0040] The low-temperature soft soldering lead-free solder alloy prepared in this embodiment has an atomic percentage of 51% Sn, 34% Bi, 12% In and 3% Al. The mass percentage of each element is calculated to be 41.4% Sn, 48.6% Bi, 9.4% In and 0.6% Al. The solder preparation process is the same as that in Example 1.

[0041] Embodiment 4:

[0042] The low-temperature soft soldering lead-free solder alloy prepared in this embodiment has an atomic percentage of 58% Sn, 27% Bi, 10% In and 5% Al. The mass percentage of each element is calculated to be 49.8% Sn, 40.9% Bi, 8.3% In and 1.0% Al. The solder preparation process is the same as that in Example 1.

[0043] Melting point measurement method: Differential scanning calorimetry (DSC) was used to measure the melting characteristics of the solder alloy in an argon environment, with a temperature range of 25 to 250°C and a heating and cooling rate of 10°C / min.

[0044] Wetting performance test: Take 50g of solder and place it on a Cu substrate coated with flux, then heat it to 110-180℃, keep it warm for 5 minutes and then air cool it to room temperature. Grind and polish the brazed sample, then use a scanning electron microscope (SEM) to take a cross-sectional view of the solder joint and measure the wetting angle between the solder and the Cu substrate.

[0045] Tensile performance test: According to the national standard GB / T228-2002 "Room temperature tensile test method for metallic materials", the brazing alloy is processed into a standard tensile specimen and subjected to a tensile test at room temperature to obtain its tensile stress-strain curve, tensile strength and total elongation after fracture.

[0046] The melting characteristics of the Sn-Bi-In-Al lead-free solder prepared in Example 1 were measured by DSC. The results showed that the melting point of the solder was 127.5°C. Figure 1 As shown; the wetting performance test was carried out, and the wetting angle measured at 160°C for 5 minutes was 38.9°, as shown Figure 2 As shown; SEM was used to observe the solder microstructure and conduct element distribution tests. The results are shown in Figure 3 As shown, from Figure 3 We can see that the Sn-Bi-In-Al lead-free solder prepared in Example 1 contains a dark gray matrix structure Sn-rich phase, a large amount of white dendrite Bi-rich phase and a small amount of light gray blocky InBi phase, and the elements are evenly distributed.

[0047] The melting characteristics of the Sn-Bi-In-Al lead-free solder prepared in Example 2 were measured by DSC. The results showed that the melting point of the solder was 90.5°C. Figure 4 The solder is prepared into a standard tensile specimen and subjected to a tensile performance test. The tensile stress-strain curve of the solder is shown in Figure 5 As shown, from Figure 5 It can be seen that the Sn-Bi-In-Al lead-free solder prepared in Example 2 has a tensile strength of 54.5 MPa and a total elongation after fracture of 49.4%.

[0048] The wetting performance of the Sn-Bi-In-Al lead-free solder prepared in Example 3 was tested. The wetting angle measured at 160°C for 5 minutes was 37.1°. Figure 6 The solder is prepared into a standard tensile specimen for tensile performance testing. The tensile stress-strain curve of the solder is shown in Figure 7 As shown, from Figure 7 It can be seen that the Sn-Bi-In-Al lead-free solder prepared in Example 3 has a tensile strength of 58.1 MPa and a total elongation after fracture of 46.7%.

[0049] The Sn-Bi-In-Al lead-free solder prepared in Example 4 was tested for solder joint interface element distribution and solder joint shear performance: a solder ball with a diameter of 600 μm was prepared by stencil printing, and the Cu pad of the printed circuit board (PCB) had a diameter of 450 μm. The solder ball was placed on the Cu pad coated with flux, and reflow soldered at 180°C for different times to prepare Sn-Bi-In-Al / Cu solder joints of ball grid array (BGA) structure. The solder joints were inlaid, ground and polished, and then the microstructure of the solder joint interface was observed by SEM, and element distribution tests were performed, such as Figure 8 As shown in the figure, after reflow soldering at 180℃ for 5min, a continuous and evenly distributed IMC layer with a thickness of about 1.31μm is formed at the interface between the solder and the Cu pad. The interface IMC is composed of scallop-shaped and short columnar Cu6Sn5. For the Sn-Bi-In-Al / Cu solder joints of the BGA structure after reflow at 180℃ for different times, the Dage shear tester is used to test their shear resistance. The shear height is 30μm and the shear rate is 300μm / s. The shear force of the solder joint is obtained and divided by the pad area to obtain its shear strength. Fig. 9 As shown in the figure, after reflow soldering at 180℃ for different times, the shear strength of the solder joint can reach up to 83.6MPa.

[0050] Through the above tests on Examples 1-4, it can be seen that the constituent elements of the Sn-Bi-In-Al lead-free solder prepared by the present invention are evenly distributed, and after being welded with the Cu pad, the IMC layer formed at the interface between the solder and the Cu pad is evenly distributed and continuous without excessive growth, the solder joint shear strength is very high, will not lead to a decrease in the mechanical properties and reliability of the solder joint, and also avoids problems such as solder joint failure; the melting points of the Sn-Bi-In-Al lead-free solder prepared by the present invention are between 80°C and 150°C, and under the reflow soldering condition of 180°C, they all show good wettability, meet the requirements for low-temperature soft soldering in electronic packaging, and will not warp when used in electronic packaging; the tensile strength and total elongation after fracture of the Sn-Bi-In-Al lead-free solder prepared by the present invention are both excellent, and have the mechanical properties of high strength and high plasticity, which can improve the service reliability of the solder joint.

[0051] The above are only preferred specific embodiments of the present invention, which are all different implementations based on the overall concept of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A low temperature soft soldering lead-free solder alloy, characterized in that: The chemical composition of the alloy includes, by atomic percentage, Sn: 30-60%, Bi: 25-35%, In: 5-35%, Al: 2-5%, and the total atomic percentage of each component element is 100%.

2. The lead-free solder alloy according to claim 1, characterized in that The alloy chemical composition includes, by atomic percentage: Sn: 57%, Bi: 31%, In: 9% and Al: 3%.

3. The lead-free solder alloy according to claim 1, characterized in that: The alloy chemical composition includes, by atomic percentage: Sn: 34%, Bi: 30%, In: 31% and Al: 5%.

4. The lead-free solder alloy according to claim 1, characterized in that: The alloy chemical composition includes, by atomic percentage: Sn: 51%, Bi: 34%, In: 12% and Al: 3%.

5. The lead-free solder alloy according to claim 1, characterized in that: The alloy chemical composition includes, by atomic percentage: Sn: 58%, Bi: 27%, In: 10% and Al: 5%.

6. The method for preparing a low temperature soft soldering lead-free solder alloy according to any one of claims 1 to 5, characterized in that: The method: S1: Weigh Sn, Bi, In and Al metal particles, mix them and put them into a smelting furnace; S2: The mixed metal particles of S1 are covered with molten eutectic salt of LiCl and KCl, smelted, and cast to obtain a low-temperature soft soldering lead-free solder alloy.

7. The preparation method according to claim 6, characterized in that: The metal purity of Sn, Bi, In and Al in S1 is ≥99.9%.

8. The preparation method according to claim 6, characterized in that: The mass ratio of LiCl to KCl in S2 is 1:1~2.

9. The preparation method according to claim 6, characterized in that: The melting temperature in S2 is 600°C, and the temperature is kept for 60 to 100 minutes. Mechanical stirring is performed once every 20 minutes during the melting period.

10. Application of the low temperature soft soldering lead-free solder alloy according to claims 1 to 5 in low temperature soft soldering of electronic packaging.

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