Lead-free solder alloys and solder joints

Abstract

To provide a lead-free solder alloy which keeps a melting point of an Sn-Bi solder alloy low, which has physical characteristics proper than ever, and which can form a joint improving ductility, distortion characteristics and the like and having reliability higher than ever.SOLUTION: A lead-free solder alloy contains 32-40 mass% of Bi, 0.1-1.0 mass% of Sb, 0.1-1.0 mass% of Cu and 0.001-0.1 pt.mass of Ni, and also contains 0.0005-0.1 mass% of one or more kinds of elements selected from the group consisting of Si, As and Pt. The remnant is composed of Sn and inevitable impurities.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a lead-free solder alloy and a solder joint.

Background Art

[0002] For reducing the environmental load on the earth, lead-free solder has become widespread as a bonding material for electronic components. As typical compositions of lead-free solder, Sn-Ag-Cu-based solder alloys and Sn-Cu-Ni-based solder alloys mainly composed of Sn are known. However, it is known that the melting point of the Sn-Ag-Cu-based solder alloy is 217°C and that of the Sn-Cu-Ni-based solder alloy is 227°C, which is higher than the melting point of 183°C of the conventionally used Sn-Pb eutectic composition. Therefore, for example, on a substrate such as a personal computer that requires bonding of electronic components with low heat resistance, Sn-Bi-based and Sn-In-based lead-free solder alloys with a lowered melting point by containing a predetermined amount of Bi or In are applied.

[0003] By the way, if a large amount of Bi is incorporated into a Sn-Bi-based solder alloy using Bi, the solder alloy becomes brittle and its mechanical strength decreases. In addition, since repeated stress is generated in the solder joint due to repeated thermal expansion and contraction of the electronic components and the substrate by heat, cracks are likely to occur due to thermal fatigue, resulting in low long-term reliability. Since In is expensive, it is disadvantageous in terms of cost.

[0004] Therefore, in order to improve the characteristics of Sn-Bi-based solder alloys, investigations have been carried out (Patent Documents 1 to 4).

[0005] Patent Document 1 discloses a solder alloy represented by Sn-57 or 45 mass%Bi-x mass%M (wherein M is at least one metal selected from the group consisting of copper, silver, nickel, germanium, antimony, and indium, and x is 4.0 mass% or less), and a soldering material characterized by containing an arbitrary amount or 5 to 20 mass% of a thermosetting adhesive in the solder alloy. It is stated that with such a configuration, the reflow temperature can be sufficiently low, and a lead-free solder alloy joint with sufficient properties as a substitute for Sn-Pb solder alloy can be obtained.

[0006] Patent Document 2 discloses a lead-free solder alloy characterized by comprising 20-57% by weight of Bi, 0.2-5% by weight of Sb, 0.01-1% by weight of Ga, and the remainder being Sn. It is stated that this configuration allows soldering at temperatures equivalent to or lower than those of Sn-Pb solder alloys (eutectic compositions), thereby improving workability even when joining heat-sensitive electronic components, and also ensuring good physical properties as a solder alloy.

[0007] Patent Document 3 discloses a lead-free solder alloy having an alloy composition consisting of, by mass%, one or two elements selected from the group consisting of Bi: 31-59%, Sb: 0.15-0.75%, and further Cu: 0.3-1.0% and P: 0.002-0.055%, with the remainder being Sn. It is stated that this configuration has a sufficiently low melting point to suppress thermal distortion of the substrate during soldering, is highly ductile and has high tensile strength, and can improve the shear strength of the solder joint by suppressing the formation of a P-rich layer at the joint interface during soldering to electrodes treated with electroless Ni plating. Furthermore, it is stated that solder joints made with this solder alloy can ensure excellent connection reliability even when using thinner substrates than conventional methods.

[0008] Patent Document 4 discloses a lead-free solder in which 20 to 60 mass% of Bi is added, one or more elements are selected from Cu, Ni, and P, the selected Cu is added in an amount of 0 to 3 mass%, the selected Ni in an amount of 0.005 to 0.5 mass%, the selected P in an amount of 0.005 to 0.05 mass%, and the remainder consists of Sn and unavoidable impurities. It is stated that with such a configuration, a lead-free solder with a lower melting point and superior fatigue resistance than leaded solder can be provided without using Ag, In, and Sb.

[0009] Patent Document 5 describes a technology developed by the present inventor that improves upon the problems of the aforementioned conventional Sn-Bi solder alloy. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2007-90407 [Patent Document 2] Japanese Patent Application Publication No. 7-40079 [Patent Document 3] Patent No. 5679094 [Patent Document 4] Japanese Patent Publication No. 2014-140865 [Patent Document 5] PCT / JP2020 / 16201 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] As described above, the prior art described in Patent Documents 1 to 4 improves the physical properties of Sn-Bi solder alloys to some extent while maintaining their low melting point, but there is still room for improvement. Furthermore, the technology described in Patent Document 5 makes it possible to improve upon the problems of the aforementioned conventional Sn-Bi solder alloys and provides good properties, but depending on the materials to be joined, even better joining properties may be required, and there is a need for a lead-free solder alloy that enables joining with improved ductility and strain properties.Therefore, the object of the present invention is to provide a lead-free solder alloy that maintains the low melting point of Sn-Bi solder alloys, has better physical properties than conventional alloys, and can form a more reliable joint than conventional alloys with improved ductility and strain properties. [Means for solving the problem]

[0012] The inventors of the present invention conducted thorough research to solve the aforementioned problems and found that these problems can be solved by incorporating a specific amount of a specific component into the Sn-Bi solder alloy.

[0013] The first aspect of this invention is a mixture containing 32% to 40% by mass of Bi, 0.1% to 1.0% by mass of Sb, 0.1% to 1.0% by mass of Cu, and 0.001% by mass of Ni. % More than 0.1 mass % This invention relates to a lead-free solder alloy containing the following, and further containing 0.0005 to 0.1 mass% of one or more elements selected from the group Si, As, and Pt, with the remainder consisting of Sn and unavoidable impurities.

[0014] The second aspect of the present invention relates to a solder joint formed using the aforementioned lead-free solder alloy.

[0015] Here, unavoidable impurities refer to substances that are present in the raw materials of solder or that are inevitably introduced during the manufacturing process. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a lead-free solder alloy that maintains the low melting point of the Sn-Bi-based solder alloy, has better physical properties than conventional ones, further improves ductility, strain characteristics, etc., and can form a more reliable joint than conventional ones.

Brief Description of the Drawings

[0017] [Figure 1] It is a figure which showed the shape of the tensile strength test measurement sample used in an Example and a comparative example. [Figure 2] It is a graph which compared the tensile strength measurement values of the Example and the comparative example of this invention. [Figure 3] It is a graph which showed the change rate of the elongation rate of the Example of this invention when the comparative example is made into 100. [Figure 4] It is a figure which showed the change rate of the shear strength and the absorbed energy before and after aging when performing an impact shear test using the solder alloys of Examples 1 to 16 and Comparative Examples 1, 3, and 4. [Figure 5] It is a figure which showed the relative change rate with respect to Comparative Example 1 of the change rate of the shear strength and the absorbed energy before and after aging when performing an impact shear test using the solder alloys of Examples 1 to 16 and Comparative Examples 1, 3, and 4.

Modes for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described.

[0019] The lead-free solder alloy according to the embodiment of the present invention (hereinafter, may be simply referred to as "solder alloy") contains 32% by mass or more and 40% by mass or less of Bi, 0.1% by mass or more and 1.0% by mass or less of Sb, 0.1% by mass or more and 1.0% by mass or less of Cu, and 0.001% by mass % or more and 0.1% by mass % or less, further contains 0.0005 to 0.1% by mass of one or more elements selected from the group of Si, As, and Pt, and the balance consists of Sn and unavoidable impurities.

[0020] Thus, by using Sn as the matrix phase and incorporating Bi, Sb, Cu, and Ni within specific ranges, the melting point reduction effect of Bi can be well maintained, while the deterioration of physical properties due to Bi can be significantly suppressed. In particular, degradation during long-term use can be suppressed, and good thermal fatigue resistance can be achieved. Therefore, it becomes possible to form highly reliable solder joints. Furthermore, since Sb, Cu, and Ni are elements with lower volume resistivity than Bi, their lower content compared to when Bi is included in a composition close to that of a eutectic composition with Sn makes it possible to lower the volume resistivity of the solder alloy compared to conventional methods.

[0021] Furthermore, by using the aforementioned Sn as the matrix phase, incorporating Bi, Sb, Cu, and Ni within specific ranges, and adding one or more elements selected from the group of Si, As, and Pt within specific ranges, it is possible to suppress the Cu6Sn5η-η' transformation that occurs at 186°C, for example, at the interface between the metal wiring on the substrate and the solder alloy, and to further improve ductility and strain characteristics. As a result, it is possible to prevent the occurrence of cracks inside the solder alloy and delamination at the interface between the solder alloy and the metal wiring, and to form a highly reliable joint that is resistant to stresses generated by external stimuli.

[0022] Furthermore, having such a specific component composition can suppress the occurrence of electromigration. Although the mechanism of action is not clear, it is thought that by suppressing the movement of metal atoms in the solder alloy that may occur due to current flow and high-temperature environments, it is possible to suppress IMC coarsening and void formation in the solder joint and form a highly reliable solder joint.

[0023] The Bi content is between 32% by mass and 40% by mass. Below 32% by mass, the effect of lowering the melting point tends to be insufficient. Above 40% by mass, the solder alloy becomes brittle, and the desired physical properties tend not to be obtained. While the Bi content can be within this range, 36% by mass and 38% by mass is preferred.

[0024] The Sb content is between 0.1% by mass and 1.0% by mass. Preferably, it is between 0.1% by mass and 0.6% by mass. It is known that Sb imparts ductility to Sn-Bi solder alloys and suppresses volume changes by inhibiting the phase transformation from β-Sn to α-Sn. However, in relation to other metal components, the various effects described above are only achieved when Sb is included within this range.

[0025] The Cu content is between 0.1% by mass and 1.0% by mass. Preferably, it is between 0.3% by mass and 0.7% by mass. It is known that Cu imparts ductility to Sn-Bi solder alloys. However, it is only by including Cu within this range, in relation to other metal components, that the various effects described above are exhibited.

[0026] The Ni content is 0.001% by mass or more and 0.1% by mass or less. Preferably, it is 0.005% by mass or more and 0.07% by mass or less, and more preferably, 0.01% by mass or more and 0.05% by mass or less. It is known that Ni has the effect of suppressing the formation of intermetallic compounds between Cu and Sn, thereby suppressing the rise in melting point and the decrease in fluidity, and improving the strength and joinability of the alloy. However, in relation to other metal components, the effect of suppressing the Cu6Sn5η-η' transformation that occurs at 186°C is only achieved by including Ni within this range.

[0027] Furthermore, by including one or more elements selected from the group Si, As, and Pt in specific ranges, in addition to Sn, Bi, Sb, Cu, and Ni, ductility and strain properties are improved, leading to alloy strengthening. In addition, improved wettability, suppression of intermetallic compound growth, and improved impact resistance can also be expected.

[0028] Si is expected to improve the ductility and strain characteristics of solder joints. The Si content is 0.0005 to 0.1 mass%, preferably 0.001 to 0.05 mass%, and more preferably 0.003 to 0.02 mass%.

[0029] As improves wetting spread during soldering and enhances the strength of the solder joint. The As content is 0.0005 to 0.1% by mass, preferably 0.001 to 0.05% by mass, and more preferably 0.003 to 0.02% by mass.

[0030] Pt is expected to suppress the growth of intermetallic compounds in solder joints and improve impact resistance. The Pt content is 0.0005 to 0.1 mass%, preferably 0.001 to 0.05 mass%, and more preferably 0.003 to 0.02 mass%.

[0031] The solder alloy according to this embodiment may contain unavoidable impurities. However, even if unavoidable impurities are present, the aforementioned effects can still be achieved.

[0032] There are no particular limitations on the form of the solder alloy, and it can be appropriately selected depending on the application. For example, when soldering by dip soldering, it can be in the shape of a rod. When soldering by reflow soldering, it may be in the form of an amorphous paste, or it may have the shape of a ball or preform. When soldering using a soldering iron, a rosin-cored solder formed into a wire can be used.

[0033] The solder joint according to the embodiment of the present invention is formed using the aforementioned solder alloy. More specifically, the solder joint is formed by joining, for example, the electrode terminal of an electronic component to the electrode terminal of a metal wiring on a substrate using the aforementioned solder alloy. That is, the solder joint is composed of the aforementioned solder alloy and the electronic component and substrate joined via the solder alloy. Because the solder joint is formed using the aforementioned solder alloy, it has the properties of the solder alloy of the present invention, and the deterioration of the solder joint during long-term use is suppressed, it has good thermal fatigue resistance and high reliability. [Examples]

[0034] The present invention will be described in detail below based on examples.

[0035] [Test Example 1] Solder alloys with the compositions shown in Table 1 were prepared and used for tensile strength testing. As part of the evaluation of the solder alloy of the present invention, its tensile strength and elongation were measured, and its superiority was confirmed. Tensile strength represents the strength of a solder joint; a higher value indicates better mechanical properties. Elongation represents the ductility of a solder joint; a higher value indicates better stress and strain relaxation. During actual operation, the heat and cooling of electronic components and changes in the operating environment repeatedly load stress and strain onto the joints of electronic components due to the difference in the coefficient of linear expansion between the component and the substrate. This repeated loading causes damage to the joint, a phenomenon known as thermal fatigue. The solder joining the component and the substrate plays a buffering role against stress and strain due to its ductility. Therefore, improving the ductility (elongation) of the solder means that the stress and strain generated at the joint are alleviated, making it possible to maintain a joint with excellent thermal fatigue resistance. Therefore, it is believed that a solder alloy possessing both high tensile strength and high elongation will form a highly reliable solder joint.

[0036] [Table 1]

[0037] (Test method) 1. Tensile strength test (1) After melting the solder alloy shown in Table 1, a sample for measurement is prepared by casting it into a mold in the shape shown in Figure 1. (2) Using the AG-IS testing machine manufactured by Shimadzu Corporation, each sample was pulled until it broke under the following conditions, and the tensile strength and elongation of the samples were measured. ·Room temperature (20℃~25℃) • Tensile speed: 10 mm / min

[0038] 2. Evaluation of measurement results (1) Tensile strength Regarding the results measured using the method described above, please specify the sample cross-sectional area (mm²) before measurement.2 The following formula was obtained and calculated from the ( ) and the maximum load stress (N) after measurement. Tensile strength (MPa) = Maximum load stress (N) / Sample cross-sectional area (mm²) before measurement 2 ) (2) Growth rate The rate of change in the length of the scored area L (length when the fractured parts are butted together) after measurement, relative to the score length (1-A11) L0 before measurement, was calculated using the following formula. Growth rate (%) = {(L - L0) / L0} × 100 The results showing the growth rate of each sample, with Comparative Example 1 set to 100, are expressed as a percentage as the "rate of change in growth rate".

[0039] 3. Evaluation Criteria (1) Tensile strength: The minimum passing value was set at 80 MPa, which is more than 1.5 times the value of SAC305 (3.0 mass% Ag, 0.5 mass% Cu, the remainder Sn), a lead-free solder with a proven track record. (2) Growth rate: Products with a growth rate change exceeding 100% were deemed acceptable.

[0040] [Table 2]

[0041] Furthermore, a comparison of the tensile strength results is shown in Figure 2, and the rate of change in elongation is shown in Figure 3. Table 2, Figure 2, and Figure 3 show that all of the examples of the solder alloy of the present invention exceeded the acceptance criteria, proving that highly reliable solder joints can be obtained.

[0042] [Test Example 2] Solder alloys were prepared by mixing each metal according to standard methods to obtain the compositions shown in Tables 3 and 4. Impact shear tests were then conducted using the obtained solder alloys according to the method described below. Note that each solder alloy contains unavoidable impurities due to the mixing of the raw materials to achieve the compositions shown in Tables 3 and 4. Furthermore, solder alloys with substantially the same composition as Test Example 1 in Tables 3 and 4 were assigned the same example number and comparative example number.

[0043] [Table 3]

[0044] [Table 4]

[0045] (Test method) <Impact Share Test> 1) Spherical solder balls with a diameter of 0.5 mm were prepared from the solder alloys obtained in Examples 1-16 and Comparative Examples 1, 3, and 4. 2) A copper foil substrate was prepared, and 0.01g of "Flux RM-5" (manufactured by Nippon Superior Co., Ltd.) was applied to the mounting area, after which solder balls were placed. 3) The samples were reflow heated at a temperature increase of 1.5°C / second and a maximum temperature of 200°C or 250°C for 50 seconds to bond the materials. After bonding, they were cooled and washed with IPA to remove the flux, and these samples were used for measurement. 4) A portion of the measurement samples prepared using the above procedure was left in an electric furnace maintained at 150°C for 100 hours to undergo aging treatment. 5) Measurement samples that were not subjected to aging treatment (initial) and measurement samples that were subjected to aging treatment (aged) were placed in an impact shear tester (DAGE 4000HS). 6) Measurements were performed at a shear speed of 100 mm / sec, and shear intensity (N) and absorbed energy (mJ) were measured. The maximum value of the shear intensity was evaluated as the shear intensity. In addition, the rate of change of each measured value of shear intensity and absorbed energy before and after aging (after aging / before aging × 100) and the relative rate of change calculated by setting the value of Comparative Example 1 to 100 ((rate of change for each example and each comparative example) / (rate of change for Comparative Example 1) × 100) were calculated to evaluate the effect of aging. The results are shown in Tables 5 to 7. Furthermore, Figure 4 shows a bar graph summarizing the "rate of change" shown in Table 6. Furthermore, Figure 5 shows a bar graph summarizing the "relative rate of change" shown in Table 7.

[0046] [Table 5]

[0047] [Table 6]

[0048] [Table 7]

[0049] As shown in Tables 6 and 7, and Figures 4 and 5, it can be seen that by using a solder alloy of a predetermined composition, the shear strength after aging treatment is better than before treatment, and good energy absorption characteristics are observed. In particular, as shown in Table 7 and Figure 5, when compared to the solder alloy described in Patent Document 5, which is Comparative Example 1, the solder alloys of each example have good characteristics equivalent to or better than Comparative Example 1, whereas the conventional eutectic Sn-Bi solder alloys of Comparative Examples 3 and 4 do not achieve the effect of containing Si, As, and Pt within a predetermined range. Thus, it can be seen that by using Sn as the matrix phase and including Bi, Sb, Cu, and Ni in a specific range of content, as well as one or more elements selected from the group of Si, As, and Pt in a specific range of content, better physical properties are observed than with conventional solder alloys. Furthermore, it can be seen that the solder alloy exhibits good physical properties equivalent to or better than those described in Patent Document 5.

[0050] As described above, the results of Test Example 1 suggest that a solder alloy of a given composition possesses both good tensile strength and elongation, making it possible to form a highly reliable solder joint. The results of Test Example 2 further support this point. [Industrial applicability]

[0051] The lead-free solder alloy according to the present invention enables soldering at low temperatures, thus eliminating the need to apply thermal load to electronic components. Furthermore, solder joints made with this solder alloy have higher reliability compared to solder joints made with conventional low-temperature solder alloys, making it widely applicable to the joining of electronic components that require low-temperature joining. [Explanation of symbols]

[0052] 1. Sample for tensile strength testing. 1-A Grading Section 1-A11 Rating Section Length 1-A12 Score width 1-B Device fixing part

Claims

1. A lead-free solder alloy containing 32% to 40% by mass of Bi, 0.1% to 1.0% by mass of Sb, 0.1% to 1.0% by mass of Cu, 0.001% to 0.1% by mass of Ni, and further containing 0.0005 to 0.1% by mass of Si, with the remainder consisting of Sn and unavoidable impurities.

2. A solder joint formed using the lead-free solder alloy described in claim 1.

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