Enhanced sn-ag-cu low silver solder and preparation method and application thereof

Abstract

The application discloses a kind of enhanced Sn-Ag-Cu low silver lead-free solder and its preparation method and application, and the specific components of enhanced technical scheme are as follows: Cu: 0.5%~1.0%, Ag: 0.1%~1.0%, La: 0.002%~1.2%, Ge: 0.002%~1.3%, Co: 0.001%~1.2%, and the rest is Sn, calculated by mass percentage. Under the joint action of the added elements, the microstructure of the solder can be improved, thereby effectively enhancing the creep resistance of the solder, greatly improving the fatigue life of the solder at high temperature, and significantly improving the reliability of the solder.

Description

Technical Field

[0001] This invention relates to microelectronic packaging solders, and more particularly to an enhanced Sn-Ag-Cu low-silver solder, its preparation method, and its application. Background Technology

[0002] In microelectronic products, solder joints play a crucial role in electrical connection, mechanical support, and heat transfer between printed circuit boards and electronic components. Therefore, solder joints are essential in electronic packaging, and their service life determines the lifespan of the microelectronic product. As microelectronic devices continue to miniaturize, device leads become smaller and the spacing narrower. With the reduction in component size, the solder joint size inevitably shrinks as well. This dimensional change causes significant alterations to the chemical reactions and mechanical properties at the metal interface, thus significantly impacting the reliability of microelectronic packaging. Many research reports indicate that one of the main causes of problems or malfunctions in electronic products is solder joint failure after a period of service or after exposure to impacts, drops, or other accidents. These solder joint failures have a substantial impact on the reliability of microelectronic products. Therefore, effectively improving solder joint reliability has become a critical issue in the field of microelectronic packaging.

[0003] Currently, ternary eutectic Sn-Ag-Cu solder is the most competitive in the microelectronics packaging field. Compared with traditional solders, Sn-Ag-Cu solder has better wetting and mechanical properties, and materials such as Sn, Ag, and Cu are relatively easy to obtain, thus it has been widely used in the microelectronics packaging industry. Early Sn-Ag-Cu series lead-free solders typically had a high Ag content, resulting in some significant drawbacks. For example, the reaction between Ag and Sn would generate a large amount of brittle intermetallic compounds in the solder joint. These brittle intermetallic compounds are considered the weakest part of the solder joint, easily leading to voids or cracks during the service life of electronic products, thus threatening the reliability and lifespan of the products.

[0004] Furthermore, due to the continuous rise in the price of Ag, the cost of silver-containing lead-free solders has also increased significantly. To address the issue of brittle intermetallic compounds and reduce costs, the electronic packaging industry has adjusted the composition of Sn-Ag-Cu lead-free solders and specifically reduced the Ag content. While the reduced silver content in Sn-Ag-Cu lead-free solders has indeed alleviated the cost increase to some extent, it has also caused the composition of the Sn-Ag-Cu lead-free solders to deviate from the eutectic point, introducing new problems such as increased melting point, decreased wettability, faster intermetallic compound growth, and a significant decline in creep and fatigue resistance. These performance degradations severely impact the long-term reliability and lifespan of microelectronic products, thus failing to meet the rapidly evolving packaging demands of the microelectronics industry. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing material properties and provide a Sn-Ag-Cu low-silver lead-free solder with good performance, excellent solder joint mechanical properties, and superior creep and fatigue resistance.

[0006] To address the aforementioned problems, the present invention is implemented through the following technical solution.

[0007] An enhanced Sn-Ag-Cu low-silver lead-free solder is provided, in which La, Ge, and Co elements are added. It comprises the following components by mass percentage: Cu 0.5–1.0 wt.%, Ag 0.1–1.0 wt.%, La 0.002–1.2 wt.%, Ge 0.002–1.3 wt.%, Co 0.001–1.2 wt.%, and the remainder being Sn.

[0008] Preferably, the content of Cu is 0.5-1.0 wt.%, the content of Ag is 0.1-1.0 wt.%, the content of La is 0.01-1.0 wt.%, the content of Ge is 0.03-1.0 wt.%, the content of Co is 0.03-1.0 wt.%, and the remaining component is Sn.

[0009] Preferably, the content of Cu is 0.5-1.0 wt.%, the content of Ag is 0.3-1.0 wt.%, the content of La is 0.002-1.2 wt.%, the content of Ge is 0.002-1.3 wt.%, the content of Co is 0.001-1.2 wt.%, and the remaining component is Sn.

[0010] Preferably, the content of La is 0.01–1.0 wt.%, the content of Ge is 0.03–1.0 wt.%, the content of Co is 0.03–1.0 wt.%, the content of Cu is 0.6–1.0 wt.%, the content of Ag is 0.3–1.0 wt.%, and the remainder is Sn.

[0011] Preferably, the mass ratio of Co:La is (2-3):1.

[0012] Preferably, the mass ratio of Cu to Ge is (2-5):1.

[0013] Preferably, the mass ratio of Cu to Ag is (2-7):1.

[0014] The preparation method of the enhanced Sn-Ag-Cu low-silver lead-free solder described in any of the above items is as follows:

[0015] (1) The raw material composition is calculated by mass percentage as follows: Cu: 0.5%~1.0%, Ag: 0.1%~1.0%, La: 0.002%~1.2%, Ge: 0.002%~1.3wt.%, Co: 0.001%~1.2wt.%, and the remaining component is Sn. The raw materials are mixed in proportion and smelted under the protection of inert gas.

[0016] (2) Remelt 3 to 5 times and then anneal;

[0017] (3) An enhanced Sn-Ag-Cu low-silver lead-free solder was obtained by mechanical processing.

[0018] Preferably, the inert gas in step (1) is nitrogen or argon.

[0019] Preferably, the maximum melting temperature in step (1) is such that all materials are completely melted.

[0020] Preferably, the annealing temperature in step (2) is 135℃~155℃.

[0021] More preferably, the annealing temperature in step (2) is 150°C.

[0022] Preferably, the machining process in step (3) is cold rolling or extrusion.

[0023] The present invention also provides the application of an enhanced Sn-Ag-Cu low-silver lead-free solder as described in any of the above claims in electronic packaging.

[0024] The enhanced solder of the present invention has good wettability, excellent fatigue resistance and creep resistance, and can meet the requirements of low silver lead-free solder for micro-miniature precision electronic packaging.

[0025] Compared with the prior art, the present invention has at least the following advantages.

[0026] 1. Because low-silver Sn-Ag-Cu solder deviates from the ternary eutectic point, its melting point is increased, resulting in a thicker intermetallic compound layer, especially the Cu3Sn layer, formed between the solder and the substrate. This Cu3Sn layer is prone to developing micropores during service, significantly increasing the likelihood of brittle fracture at the solder joint, thus severely impacting its mechanical properties and long-term reliability. This invention, by adding Co to the Sn-Ag-Cu low-silver lead-free solder, effectively inhibits the growth of the intermetallic compound layer, especially the Cu3Sn layer, between the solder and the substrate. It also inhibits the growth of intermetallic compound grains within the solder, making the precipitation strengthening effect of the intermetallic compound grains more pronounced, thereby improving the solder's mechanical properties and long-term reliability.

[0027] 2. This invention adds La to the low-silver Sn-Ag-Cu-Co solder. The addition of La improves the wettability of the solder and refines the intermetallic compound grains, resulting in a more uniform distribution of these grains and a more stable microstructure. This further enhances the mechanical properties and reliability of the low-silver Sn-Ag-Cu-Co solder. Experiments show that the addition of La significantly improves the creep resistance of the low-silver Sn-Ag-Cu-Co solder.

[0028] 3. This invention adds Ge element to the low-silver Sn-Ag-Cu-Co-La solder. The addition of Ge element can effectively prevent the oxidation of Sn in the solder and the formation of slag during wave soldering, which greatly helps to improve the reliability of low-silver Sn-Ag-Cu solder. After adding Co, La, and Ge elements, through the synergistic effect of Co, La, Ge, and other elements, the growth of intermetallic compound grains in the solder can be more effectively suppressed, the microstructure of low-silver Sn-Ag-Cu solder can be significantly improved, and the creep resistance and mechanical properties at high temperatures of low-silver lead-free solder can be effectively improved. Attached Figure Description

[0029] Figure 1 This is a micrograph of the intermetallic compound grains in Comparative Example 2.

[0030] Figure 2 This is a micrograph of the intermetallic compound grains from Example 2.

[0031] Figure 3 This is a schematic diagram of the weld joint lap joint sample structure.

[0032] Figure 4 These are creep lifetime diagrams for Examples 2, 4, 7, Comparative Examples 1, 2, and 4.

[0033] Figure 5 Examples 2, 3, 6, Comparative Examples 1, 2, and 3 were tested at 125°C with a total strain of 5% and a frequency of 10. -1 Fatigue life diagram under Hz conditions. Detailed Implementation

[0034] To enable those skilled in the art to better understand the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with embodiments and comparative examples.

[0035] Example 1

[0036] The low-silver lead-free solder of this embodiment contains the following components by mass percentage: Ag 0.4wt.%, Cu 0.5wt.%, Co 0.001wt.%, La 0.002wt.%, Ge 0.002wt.%, with the balance being Sn.

[0037] The preparation method is as follows: Raw materials are mixed in different weight percentages and melted under a nitrogen atmosphere. The melting temperature is such that all raw materials are completely melted. After cooling, the mixture is remelted 2-3 times. Then, after cooling to room temperature, it is annealed at 150℃. Finally, it is processed into strip-shaped solder using machining methods such as cold rolling and extrusion for later use.

[0038] Example 2

[0039] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 1.0 wt.%, Cu 0.5 wt.%, Co 0.2 wt.%, La 0.1 wt.%, Ge 0.2 wt.%, with the balance being Sn.

[0040] Example 3

[0041] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 0.4 wt.%, Cu 0.5 wt.%, Co 0.2 wt.%, La 0.1 wt.%, Ge 0.002 wt.%, with the balance being Sn.

[0042] Example 4

[0043] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 0.4 wt.%, Cu 0.5 wt.%, Co 0.2 wt.%, La 0.002 wt.%, Ge 0.2 wt.%, with the balance being Sn.

[0044] Example 5

[0045] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 0.3 wt.%, Cu 0.7 wt.%, Co 0.1 wt.%, La 0.15 wt.%, Ge 0.2 wt.%, with the balance being Sn.

[0046] Example 6

[0047] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 0.1 wt.%, Cu 0.5 wt.%, Co 1.2 wt.%, La 0.1 wt.%, Ge 0.2 wt.%, with the balance being Sn.

[0048] Example 7

[0049] The low-silver lead-free solder of this embodiment differs from that of Embodiment 1 only in that it contains the following components: Ag 0.3 wt.%, Cu 1 wt.%, Co 0.2 wt.%, La 1.2 wt.%, Ge 1.3 wt.%, with the balance being Sn.

[0050] Comparative Example 1

[0051] The only difference between this comparative example of a low-silver lead-free solder and Example 1 is that it contains the following components: Ag 0.4 wt.%, Cu 0.5 wt.%, and the balance Sn.

[0052] Comparative Example 2

[0053] The only difference between this comparative example of a low-silver lead-free solder and Example 1 is that it contains the following components: Ag 1.0 wt.%, Cu 0.5 wt.%, and the balance Sn.

[0054] Comparative Example 3

[0055] The only difference between this comparative example of a low-silver lead-free solder and Example 1 is that it contains the following components: 0.1 wt.% Ag, 0.5 wt.% Cu, and the balance Sn.

[0056] Comparative Example 4

[0057] The only difference between this comparative example of a low-silver lead-free solder and Example 1 is that it contains the following components: Ag 0.3 wt.%, Cu 1.0 wt.%, and the balance Sn.

[0058] Test 1: Microscopic testing of solder

[0059] The growth thickness and grain size of the intermetallic compound in the cross-section of the solder joints were tested in Examples 1, 2, Comparative Example 1, and Comparative Example 2. The test method was as follows: A suitable amount of solder was placed on a copper substrate, and after reflow soldering for 120 seconds (the reflow time was set to 120 seconds for clearer observation of the intermetallic compound growth), the thickness of the intermetallic compound in the cross-section and the grain size of the intermetallic compound on the substrate were observed using a scanning electron microscope. For observation of the thickness of the intermetallic compound in the cross-section, the solder joint was first cut and made into a metallographic sample before observation and measurement using a scanning electron microscope. For observation of the grain size of the intermetallic compound, the tin substrate on the surface of the solder joint was first etched away with nitric acid before observation using a scanning electron microscope.

[0060] Figure 1 These are micrographs of the intermetallic compound grains in Comparative Example 2. Figure 2 These are micrographs of the intermetallic compound grains from Example 2. From... Figure 1 , Figure 2 It can be observed that after adding appropriate amounts of Co, La, and Ge elements, the grain size of the intermetallic compound is significantly smaller than that without the addition, indicating that the added Co, La, and Ge elements have a good inhibitory effect on the growth of the intermetallic compound.

[0061] The specific measurement results of the growth thickness and grain size of the intermetallic compound at the weld joint cross-section of Examples 1, 2, and 5 and Comparative Examples 1 and 2 are shown in Table 1.

[0062] Table 1

[0063]

[0064] As shown in Table 1, the intermetallic compound growth and maturation of the comparative solder joints without the addition of Co, La, and Ge elements were slower, and the thickness and grain size of the intermetallic compound in the cross-section were higher than those in Examples 1, 2, and 5. Clearly, the addition of Co, La, and Ge elements effectively inhibited the growth of the intermetallic compound. This is because the addition of Co, La, and Ge elements generated a large number of heterogeneous nucleation sites within the solder joint. These heterogeneous nucleation sites accelerated the nucleation process of the intermetallic compound grains, shortened the maturation time, and thus reduced the grain size of the intermetallic compound. Therefore, the addition of Co, La, and Ge elements can reduce the grain size of the intermetallic compound, significantly improving the microstructure of the solder joint, thereby further enhancing the mechanical properties and reliability of the solder joints in microelectronic devices.

[0065] Test 2: Mechanical property testing of solder

[0066] Creep and fatigue tests were performed on the examples and comparative examples.

[0067] To better reflect the actual usage of the solder joints, the inventors designed, as follows: Figure 3 The sample structure shown is formed by overlapping tiny solder joints. (Example) Figure 3 As shown, the solder joint size is 1.0×1.0×0.1mm, and the solder joint is located between two high-purity copper substrates to form an overlapping structure. The preparation method involves placing the copper sheet and solder in a template, and then using a universal testing machine in a reflow oven to conduct creep and low-cycle fatigue tests on the samples prepared with the solders of the examples and comparative examples.

[0068] The creep test conditions were: a constant stress of 10 MPa was applied at room temperature.

[0069] The fatigue test conditions were: temperature 125℃, total strain set at 5%, and frequency 10 Hz. -1 Hz is defined as the number of cycles required for the maximum stress to decrease to 50%.

[0070] Figure 4 The creep lifetimes of Examples 2, 4, 7, Comparative Example 1, Comparative Example 2, and Comparative Example 4 are shown. Figure 4 As can be seen, the creep rupture times of Examples 2, 4, and 7, which added Co, La, and Ge elements, were all longer than those of Comparative Examples 1, 2, and 4, which did not add Co, La, and Ge elements. This indicates that the creep resistance of the solder alloy was significantly improved after adding Co, La, and Ge elements. This is because the addition of Co, La, and Ge elements generates more micro-intermetallic compound particles within the tin matrix of the solder joint. These micro-intermetallic compound particles can interact with dislocations, and the dislocations are pinned by these micro-intermetallic compound particles during movement. The pinned dislocations require greater external stress to move. It can be said that the mechanical properties of the solder joint depend to a certain extent on the distribution, number, and movement of dislocations. For the internal structure of the solder joint to change, there must be an external force that enables these dislocations to overcome obstacles and move. The pinning effect of the micro-intermetallic compound particles makes the obstacles encountered by dislocations during movement stronger, making it more difficult for dislocations to move. Therefore, the internal structure of the solder joint is more stable, thus effectively improving the creep resistance of the solder alloy. In Example 4, the lower La content resulted in a decrease in creep rupture time, indicating that La has a certain impact on creep resistance. Furthermore, the creep rupture time in Comparative Example 1 was lower than that in Comparative Example 2, suggesting that reducing the silver content had a certain impact on the creep resistance of the solder.

[0071] Figure 5 Examples 2, 3, 6, Comparative Examples 1, 2, and 3 are shown at a temperature of 125°C, a total strain of 5%, and a frequency of 10. -1 Fatigue life under Hz conditions. Figure 5As can be seen, after adding Co, La, and Ge, the fatigue lives of Examples 2, 3, and 6 are all higher than those of Comparative Examples 1, 2, and 3 without Co, La, and Ge, indicating that adding Co, La, and Ge has a certain effect on improving the fatigue life of lead-free solder. This is because during low-cycle fatigue, solder joints gradually generate voids and gaps, which accumulate over time to form larger voids and gaps, eventually leading to solder joint fracture and failure. After adding Co, La, and Ge, the numerous tiny intermetallic compound particles generated within the tin matrix of the solder joint, in addition to effectively pinning dislocations, can also effectively prevent grain boundary slip. Grain boundary slip is also one of the main causes of cracks in solder joints. Therefore, by effectively increasing the difficulty of grain boundary slip, the generation and accumulation of voids and gaps are effectively controlled and delayed, thus significantly improving the fatigue life of lead-free solder. Examples 2 and 6 have higher Ge content, and the experimental results show that the fatigue life of Examples 2 and 6 is also higher than that of Example 3 with lower Ge content. This indicates that Ge has a good strengthening effect on improving the fatigue life of lead-free solder at high temperature, and can better improve the mechanical properties and reliability of lead-free solder at high temperature.

[0072] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An enhanced Sn-Ag-Cu low-silver lead-free solder, characterized by, The components include the following by mass percentage: Cu: 0.6%-1.0%, Ag: 0.3%-1.0%, La: 0.01%-1.0%, Ge: 0.03%-1.0%, Co: 0.03%-1.0%, and the rest is Sn; wherein the mass ratio of Co to La is (2-3):1, and the mass ratio of Cu to Ge is (2-5):

1.

2. The method of claim 1, wherein the enhanced Sn-Ag-Cu low silver lead-free solder is prepared by, The method comprises the following steps: (1) each raw material is weighed according to the mass percentage of claim 1, and then mixed and smelted under the protection of nitrogen or argon; (2) the smelted alloy is subjected to back melting for 3-5 times, and then annealing treatment is performed; (3) the alloy after the annealing treatment is subjected to mechanical processing to obtain the enhanced Sn-Ag-Cu low-silver lead-free solder.

3. The production method according to claim 2, characterized by, The temperature of the annealing treatment in step (2) is 135-155°C.

4. The production method according to claim 2, characterized by, The mechanical processing in step (3) is cold rolling or extrusion.

5. Application of the enhanced Sn-Ag-Cu low-silver lead-free solder of claim 1 in electronic packaging.

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