Solder alloys, solder balls, solder pastes and soldered joints

Abstract

The present application provides a soft solder alloy, solder balls, solder paste and brazed joints having excellent thermal conductivity, drop impact resistance and thermal cycle resistance. The soft solder alloy has the following alloy composition: Ag: 1.0-4.0%, Cu: 0.10-1.00%, Sb: 1.0-7.0%, Co: 0.001-0.030%, Fe: 0.005-0.050%, and the balance consisting of Sn, in mass %.

Description

Technical Field

[0001] This invention relates to soft solder alloys, solder balls, solder paste, and brazed joints used in various electronic devices. Background Technology

[0002] In recent years, with the advancement of high functionality in electronic devices such as mobile terminals, the performance of electronic components mounted on substrates has improved dramatically. As electronic components become more high-performance, they carry large currents, thus exposing the solder joints used in the substrates of electronic devices to high temperatures due to Joule heating. On the other hand, when electronic devices are used in cold regions, the solder joints are exposed to low-temperature environments.

[0003] Sn-3Ag-0.5Cu solder alloy is widely used as a soldering alloy for brazing joints. In the past, its use did not pose significant problems in environments with low power consumption and small temperature variations. However, in recent years, as the application range of solder alloys has expanded and the operating environments have become more demanding, reliability issues have begun to emerge. As mentioned earlier, the temperature rise during power-on due to the increasing performance of electronic components, coupled with the widening temperature difference when exposed to cold air in cold regions, has led to a rapid temperature rise in the solder joint when electronic components are exposed to cold air and then used indoors.

[0004] When electronic devices are exposed to such temperature differences, the difference in thermal expansion coefficients between the electronic components and the substrate causes stress to concentrate on the small cross-sectional area of ​​the brazed joints. Furthermore, when the electronic device is subjected to external vibrations in such an environment, the brazed joints are further subjected to loads. Moreover, electronic devices such as mobile terminals are intended to be used for at least several years. Therefore, the solder alloy constituting the brazed joints must possess high reliability, ensuring that the brazed joints will not experience problems even after long-term use in harsh environments.

[0005] For example, Patent Document 1 discloses a solder alloy designed to suppress cracking in brazed joints even under harsh environments such as temperature differences and vibration loads, and also to suppress cracking near the interface with electrodes that have not been properly plated. Patent Document 2 discloses a solder alloy designed to suppress cracking under harsh environments such as temperature differences and vibration loads, and to suppress peeling at through-holes. To achieve these objectives, Patent Documents 1 and 2 disclose Sn-Ag-Cu-Sb-Ni solder alloys in which Sb and Ni are added to a Sn-Ag-Cu solder alloy.

[0006] Patent Document 3 discloses a solder alloy, similar to Patent Document 1, whose purpose is to suppress the cracking progress of brazed joints even under harsh environments such as temperature differences and vibration loads, and also to suppress the cracking progress near the interface with electronic components that have not been plated as specified. Patent Document 4 discloses a solder alloy, similar to Patent Document 2, whose purpose is to suppress cracking progress under harsh environments such as temperature differences and vibration loads, and to suppress peeling at through-holes. To achieve this purpose, Patent Documents 3 and 4 disclose a Sn-Ag-Cu-Sb-Bi-Ni solder alloy in which Sb, Bi, and Ni are added to a Sn-Ag-Cu solder alloy.

[0007] Patent Document 5 discloses a solder alloy designed to provide excellent reliability and prevent adverse effects on audio quality. To achieve this objective, Patent Document 5 discloses a Sn-Ag-Cu-Sb-Bi-Co-Fe solder alloy containing Sb, Bi, Co, and Fe in a Sn-Ag-Cu solder alloy.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2017-170465

[0011] Patent Document 2: Japanese Patent Application Publication No. 2019-058950

[0012] Patent Document 3: Japanese Patent No. 6719443

[0013] Patent Document 4: Japanese Patent No. 6795630

[0014] Patent Document 5: Japanese Patent No. 6889387 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] Paragraph 0008 of Patent Document 1 describes that solder alloys with increased strength due to the addition of Bi have the disadvantages of deteriorated ductility and increased brittleness. Furthermore, paragraph 0010 of Patent Document 2 discloses that during the cooling of the solder alloy, the concentration of Bi increases near the interface with the pad, where solidification is difficult within the solder joint. Moreover, the same paragraph describes that near the interface with high Bi concentration, the solder joint becomes even more difficult to solidify, and when a contraction force perpendicular to the substrate is generated, the solder joint is more likely to peel off from the surface of the pad. From this perspective, the solder alloys described in Patent Documents 1 and 2 are preferably Bi-free.

[0017] Furthermore, in the solder alloy described in the aforementioned literature, by including a specified amount of Ni as an essential element, (Cu,Ni)6Sn5 is formed in the solder alloy and near the bonding interface, thereby inhibiting the progression of cracking.

[0018] However, when the brazed joint is exposed to the same environment as the aging treatment, a phenomenon called "splling" occurs, where the alloy layer separates from the interface. This "splling" is the phenomenon where the solid phase of the intermetallic compound constituting the IMC layer formed at the joint interface diffuses into the solder mass when the corresponding heat treatment (temperature / time) is applied to the brazed joint. This "splling" becomes more likely to occur when the solder alloy, as described in Patent Documents 1-4, contains Ni, which promotes the miniaturization of the IMC layer.

[0019] Generally, thermal conduction in metals is primarily based on the interlocking of atomic vibrations and the energy transfer of conduction electrons, with conduction electrons contributing significantly. Therefore, metals are excellent conductors of heat. However, when the amount of "free" intermetallic compounds increases, the relatively low thermal conductivity of these compounds locally hinders the movement of conduction electrons and the interlocking of atomic vibrations. As a result, the thermal conduction of the brazed joint deteriorates. It is speculated that the finer particle size of intermetallic compounds in Ni-containing solder alloys facilitates "free" conduction, thus reducing the thermal conductivity of the brazed joint.

[0020] Furthermore, it is known that in Bi-containing solder alloys, the heat generated during the operation of electronic devices causes Bi segregation, resulting in the partial precipitation of the Bi phase in the eutectic. This phenomenon occurs even when the Bi content is below 58% by mass, which is the eutectic composition. Therefore, in the Bi-containing solder alloys described in Patent Documents 3-5, the solder alloy becomes brittle, and the brazed joints are prone to breakage due to drop impacts.

[0021] All the solder alloys disclosed in Patent Documents 1-5 exhibit excellent performance even after thermal cycling tests. According to these documents, the solder alloys described in Patent Documents 1-5 can withstand external stresses caused by thermal expansion and contraction. However, since the stress application rate in thermal cycling tests is determined by the expansion and contraction rates of the solder alloy and the substrate, it is slower than the stress application rate of instantaneous stresses such as drop impacts. For existing electronic devices such as mobile terminals, since drops are unavoidable, solder joints that can also withstand rapidly applied external stresses are required.

[0022] The subject of this invention is to provide soft solder alloys, solder balls, solder pastes and brazed joints with excellent thermal conductivity, drop impact resistance and thermal cycling resistance.

[0023] Solution for solving the problem

[0024] The inventors have investigated in detail the problems of the Sn-Ag-Cu-Sb-Ni solder alloys described in Patent Documents 1 and 2. In Patent Documents 1 and 2, the reason for adding Ni is stated as being to form (Cu,Ni)6Sn5 in the molten solder alloy and disperse it in the base material, thereby suppressing the progression of cracks in the solder mass. Furthermore, the same documents describe that during solder bonding, Ni migrates to the vicinity of the bonding interface to form fine (Cu,Ni)6Sn5, thus suppressing the growth of the Cu3Sn layer at the bonding interface.

[0025] Thus, it can be seen that Ni in the Sn-Ag-Cu-Sb-Ni solder alloy contributes to the refinement of the solder alloy's microstructure. However, since (Cu,Ni)6Sn5 is dispersed in the base material, it is believed that the thermal conductivity of the brazed joint is actually reduced.

[0026] In Patent Documents 3 and 4, the reason for adding Ni, in addition to being the same as in Patent Documents 1 and 2, is to form (Cu,Ni)6Sn5 in the molten solder alloy and disperse it in the base material, and also describes a synergistic effect with Co. Furthermore, since it is also stated that the dispersion of (Cu,Co)6Sn5 in the base material inhibits the progression of cracking, it is believed that the thermal conductivity will be further reduced in alloy compositions containing both Ni and Co. Because the Sn-Ag-Cu-Sb-Bi-Ni disclosed in Patent Documents 3 and 4 contains Bi, there are concerns about a reduction in drop impact resistance due to Bi.

[0027] In view of the above, the inventors investigated the removal of Ni and Bi from Sn-Ag-Cu-Sb-Bi-Ni solder alloys. However, while the thermal conductivity of Sn-Ag-Cu-Sb solder alloys improved due to the absence of Ni, the microstructure of the solder alloy did not become finer, and its drop impact resistance and thermal cycling resistance deteriorated. On the other hand, when the microstructure of the solder alloy was finer than desired, although the drop impact resistance and thermal cycling resistance improved, the thermal conductivity decreased due to the release of intermetallic compounds. Therefore, the elements that could achieve a moderately fine alloy microstructure were investigated in detail.

[0028] It was previously thought that adding Co to the Sn-Ag-Cu-Sb solder alloy would decrease thermal conductivity. However, it was found that while the thermal conductivity did not decrease significantly without Ni, insufficient micronization led to deterioration in drop impact resistance. Furthermore, it was found that adding Fe to the Sn-Ag-Cu-Sb solder alloy suppressed free radicals, resulting in finer microstructure compared to Co, although less effective than Ni. Therefore, it did not significantly reduce drop impact resistance, but thermal cycling performance was insufficient. Here, the Sn-Ag-Cu-Sb-Bi-Co-Fe alloy disclosed in Patent Document 5 contains Bi. The aforementioned solder alloy is a useful invention for excellent sound quality. However, since audio products are not intended for handling, drop impact is not a consideration.

[0029] The inventors investigated the elements and their contents in Sn-Ag-Cu-Sb solder alloys that, while resulting in a moderately finer alloy microstructure, did not achieve the desired fineness as when Ni was added, and where the release of intermetallic compounds was suppressed. The results revealed that solder alloys containing Fe in addition to Co exhibit superior thermal conductivity, thermal cycling resistance, and drop impact resistance. However, it was also found that adding only Co and Fe resulted in alloy compositions where these effects could not be simultaneously achieved to a practically acceptable degree. Generally, all constituent elements of an alloy influence each other, so it is rare for a single element to simultaneously exhibit all the superior effects. Therefore, in addition to the Co and Fe contents, the contents of Ag, Cu, and Sb were further investigated in detail.

[0030] Previously, as mentioned earlier, when considering the microstructure of solder alloys, there was an inverse relationship between microstructure refinement and the suppression of free radicals. Therefore, it was considered difficult to provide solder alloys that simultaneously satisfy the three effects of thermal conductivity, drop impact resistance, and thermal cycling resistance. Especially for solder alloys with a large number of elements added to Sn, the thermal conductivity decreased due to the formation of various compounds, so there was almost no research on thermal conductivity in the past. However, the inventors conducted detailed research and obtained the following insight: thermal conductivity, drop impact resistance, and thermal cycling resistance can be simultaneously improved, provided that the contents of Ag, Cu, Sb, Co, and Fe are within specified ranges, thus completing the present invention.

[0031] Based on the above insights, the present invention is as follows.

[0032] (0) A soft solder alloy, characterized in that, by mass %, it comprises Ag: 1.0-4.0%, Cu: 0.10-1.00%, Sb: 1.0-7.0%, Co: 0.001-0.030%, Fe: 0.005-0.050%, and the balance is composed of Sn.

[0033] (1) A soft solder alloy, characterized in that it has the following alloy composition: Ag: 1.0-4.0%, Cu: 0.10-1.00%, Sb: 1.0-7.0%, Co: 0.001-0.030%, Fe: 0.005-0.050%, and the balance being Sn.

[0034] (2) The solder alloy according to (1) above further contains at least one of P, Ge, Ga and Mn in a total of less than 0.1% by mass.

[0035] (3) The solder alloy according to (1) or (2) above also contains less than 6.0% In by mass%.

[0036] (4) The solder alloy according to (1) or (2) above, wherein the alloy composition contains In: 0 to 4.0%, and satisfies the following formulas (1) and (2):

[0037] 0.040≤Ag×Cu×Sb×Co≤0.075 (1)

[0038] 1.6≤Ag×Cu×Sb×Co / Fe≤5.7 (2)

[0039] In equations (1) and (2), Ag, Cu, Sb, Co and Fe represent the content (mass%) of the alloy composition.

[0040] (5) A solder ball made of the solder alloy described in (1) or (2) above.

[0041] (6) A solder ball made of the solder alloy described in (3) above.

[0042] (7) A solder paste having a solder powder composed of a solder alloy as described in (1) or (2) above.

[0043] (8) A solder paste having a solder powder composed of the solder alloy described in (3) above.

[0044] (9) A brazing joint having the soft solder alloy described in (1) or (2) above.

[0045] (10) A brazing joint having the soft solder alloy described in (3) above. Attached Figure Description

[0046] Figure 1 These are SEM images of the cross-section of the brazed joint. Figure 1(a) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Comparative Example 18; Figure 1 (b) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Example 16.

[0047] Figure 2 These are SEM images of the cross-section of the brazed joint. Figure 2 (a) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Comparative Example 18. Figure 2 (b) is a photograph showing the cross-sectional mapping analysis results of the brazed joint obtained using the soft solder alloy of Comparative Example 18. Figure 2 (c) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Example 16. Detailed Implementation

[0048] The present invention will now be described in more detail. In this specification, the "%" in relation to the composition of the solder alloy is "mass %" unless otherwise specified.

[0049] 1. Soft solder alloy

[0050] (1) Ag: 1.0–4.0%

[0051] Ag forms a network of Ag3Sn, which helps improve thermal cycling resistance and drop impact resistance. Furthermore, Ag helps improve the wettability of the molten solder. When the Ag content is less than 1.0%, the thermal cycling resistance decreases due to the low amount of Ag3Sn precipitation. The lower limit of the Ag content is 1.0% or more, preferably 2.0% or more, more preferably 3.0% or more, and even more preferably 3.2% or more. On the other hand, when the Ag content exceeds 4.0%, coarse primary crystals of Ag3Sn precipitate, deteriorating drop impact resistance. Sometimes, thermal conductivity also deteriorates. The upper limit of the Ag content is 4.0% or less, preferably 3.8% or less, more preferably 3.5% or less.

[0052] (2) Cu: 0.10~1.00%

[0053] Cu forms compounds with Sn in the solder mass and near the bonding interface. Therefore, adding Cu improves thermal conductivity, thermal cycling resistance, and drop impact resistance. It also improves the wettability of the molten solder. When the Cu content is less than 0.10%, drop impact resistance decreases due to the low precipitation of Cu6Sn5 and the like. Additionally, thermal cycling resistance may sometimes also decrease. The lower limit for Cu content is 0.10% or more, preferably 0.30% or more, and more preferably 0.50% or more. On the other hand, when the Cu content exceeds 1.00%, the intermetallic compounds formed at the bonding interface become free, leading to a deterioration in thermal conductivity. Drop impact resistance may also sometimes deteriorate. Furthermore, the liquidus temperature increases. The upper limit for Cu content is 1.00% or less, preferably 0.80% or less, and more preferably 0.70% or less.

[0054] (3) Sb: 1.0~7.0%

[0055] Since Sb dissolves in Sn and precipitates SnSb compounds, it contributes to improved thermal cycling resistance. When the Sb content is less than 1.0%, the amount of SnSb compound precipitated is small, and thermal cycling resistance is not improved. The lower limit of the Sb content is 1.0% or more, preferably 2.0% or more, and more preferably 3.0% or more. On the other hand, when the Sb content exceeds 7.0%, the thermal conductivity decreases due to excessive SnSb precipitation. The upper limit of the Sb content is 7.0% or less, preferably 6.0% or less, more preferably 5.0% or less, and even more preferably 4.0% or less.

[0056] (4) Co: 0.001~0.030%

[0057] Co refines Sn grains, contributing to improved drop impact resistance and thermal cycling resistance. When the Co content is less than 0.001%, Sn grains do not refine, and thermal cycling resistance is not improved. Furthermore, the liquidus temperature increases. The lower limit for the Co content is 0.001% or more, more preferably 0.003% or more, and even more preferably 0.006% or more. On the other hand, when the Co content exceeds 0.030%, the amount of compound becomes excessive, further forming coarse compounds, leading to microstructural deterioration and decreased drop impact resistance. The upper limit for the Co content is 0.030% or less, preferably 0.010% or less, and even more preferably 0.008% or less.

[0058] (5) Fe: 0.005~0.050%

[0059] Fe refines the intermetallic compounds precipitated at the bonding interface to a certain extent and suppresses the release of intermetallic compounds into the solder mass. Therefore, the addition of Fe improves thermal conductivity and drop impact resistance. When the Fe content is less than 0.005%, the intermetallic compounds precipitated at the bonding interface cannot be refined, and drop impact resistance deteriorates. Furthermore, since the release of intermetallic compounds is not suppressed, thermal conductivity is not improved. The lower limit of the Fe content is 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Fe content exceeds 0.050%, the intermetallic compounds are not refined, and drop impact resistance decreases. In addition, the liquidus temperature increases. The upper limit of the Fe content is 0.050% or less, preferably 0.040% or less, and more preferably 0.025% or less.

[0060] (6) In: below 6.0%

[0061] In is an optional element that may be included within a range that does not impair the aforementioned effects of the solder alloy of the present invention. The upper limit of the In content is preferably 6.0% or less, more preferably 5.0% or less. There is no particular limitation on the lower limit, but it is preferably more than 0%, more preferably more than 0.1%, and even more preferably more than 0.5%.

[0062] (7) At least one of P, Ge, Ga and Mn in a total of less than 0.1%.

[0063] P, Ge, Ga, and Mn are optional elements that can be included to inhibit oxidation in solder alloys. The total content of these elements is preferably 0.1% or less. There is no particular limitation on the lower limit; a total content of 0.001% or more is acceptable. Furthermore, the content of these elements is more preferably 0.001% to 0.1%, and even more preferably 0.001% to 0.050%.

[0064] (8) Balance: Sn

[0065] The balance of the solder alloy of the present invention is Sn. In addition to the aforementioned elements, it may contain unavoidable impurities. Even in the presence of unavoidable impurities, the aforementioned effects will not be affected.

[0066] It should be noted that the solder alloy of the present invention preferably does not contain Ni and Bi. Ni causes the alloy microstructure to become finer than desired, and intermetallic compounds can leach from the bonding interface into the solder mass, resulting in reduced thermal conductivity. Bi forms an enriched layer in the solder alloy, thus reducing drop impact resistance and contributing to peeling.

[0067] Equations (9)(1) and (2)

[0068] 0.040≤Ag×Cu×Sb×Co≤0.075 (1)

[0069] 1.6≤Ag×Cu×Sb×Co / Fe≤5.7 (2)

[0070] In equations (1) and (2) above, Ag, Cu, Sb, Co and Fe represent the content (mass %) of the alloy composition.

[0071] Equation (1) is a formula related to elements that improve thermal cycling resistance. In Equation (1), although the contents of Ag, Cu, and Sb are higher than those of Co, even a low content of Co significantly contributes to improving thermal cycling resistance. Therefore, although the proportion of Co is about 1 / 10000 to 1 / 10 of that of Ag, Cu, and Sb, it is considered to contribute to the properties of the solder alloy to the same degree. Therefore, in this invention, in order to further improve thermal cycling resistance, it is not meaningful to calculate the total amount of the constituent elements in Equation (1), and it is preferable to set a balanced content.

[0072] When calculating equations (1) and (2), the numerical values ​​indicated in the measured values ​​of the alloy composition shown in Tables 1 and 2 below are used. That is, in the calculation of equations (1) and (2), all digits in the measured values ​​shown in Tables 1 and 2 below that are smaller than the number of significant figures are considered as 0. For example, when the measured value of the Co content is “0.008” mass%, the Co content used in the calculation of equations (1) and (2) is not in the range of 0.0075 to 0.0084%, but is considered as “0.008000…”. Equation (1) is calculated up to the third decimal place, and equation (2) is calculated up to the first decimal place.

[0073] It should be noted that when calculating equations (1) and (2) based on the alloy composition specifically disclosed in the patent documents and other documents recorded in this specification, the same treatment is applied.

[0074] Equation (2) is a formula relating to the ratio of elements that contribute to improving drop impact resistance and elements that contribute to improving thermal conductivity. Similar to equation (1), equation (2) also includes a higher content of Ag, Cu, and Sb, and a lower content of Co and Fe. However, although the content of Co is lower, it still contributes to the refinement of the alloy structure, thus greatly contributing to drop impact resistance. Although the content range of Fe is similar to that of Co, it contributes more to the refinement of the alloy structure and also helps to suppress free radicals. Furthermore, among the elements constituting the solder alloy of the present invention, Fe is an element that can actively improve thermal conductivity. In order to achieve a higher level of full performance in terms of thermal conductivity, thermal cycling resistance, and drop impact resistance, it is desirable to satisfy equation (2), which takes into account thermal conductivity and thermal cycling resistance, in addition to equation (1).

[0075] Furthermore, in the case of In, the In content is expected to be 4.0% or less, based on satisfying equations (1) and (2). In the alloy composition, even if the In content is 0%, the above-mentioned effect will be achieved as long as equations (1) and (2) are satisfied. Since In forms compounds with Sn, as long as the In content is 4.0% or less, the decrease in thermal conductivity caused by the formed compounds will be suppressed, and the effect of satisfying equations (1) and (2) will be more easily achieved.

[0076] As mentioned earlier, the constituent elements in an alloy do not function individually, but rather all the constituent elements together constitute a substance. Therefore, it is rare for a single element to simultaneously exhibit all the superior effects. Thus, as stated above, in order to exhibit superior properties within the optimal content range of each constituent element, it is necessary to study the constituent elements holistically. In the solder alloy of the present invention, in order to simultaneously satisfy all the properties of heat cycling resistance, thermal conductivity, and drop impact resistance at a higher level, the In content is preferably 4.0% or less, and satisfies equations (1) and (2).

[0077] The lower limit of formula (1) is preferably 0.040 or higher, more preferably 0.042 or higher, even more preferably 0.043 or higher, even more preferably 0.046 or higher, particularly preferably 0.047 or higher, most preferably 0.050 or higher, and may be 0.055 or higher. The upper limit of formula (1) is preferably 0.075 or lower, more preferably 0.070 or lower, even more preferably 0.069 or lower, even more preferably 0.058 or lower, and particularly preferably 0.056 or lower.

[0078] (1) The preferred value is 0.042 to 0.070, more preferably 0.043 to 0.069, even more preferably 0.046 to 0.058, and particularly preferably 0.047 to 0.056.

[0079] The lower limit of formula (2) is preferably 1.6 or more, more preferably 1.7 or more, even more preferably 1.8 or more, even more preferably 1.9 or more, particularly preferably 2.0 or more, most preferably 2.2 or more, and can be 2.3 or more. The upper limit of formula (2) is preferably 5.7 or less, more preferably 5.6 or less, even more preferably 5.2 or less, even more preferably 3.7 or less, particularly preferably 3.5 or less, and most preferably 2.7 or less.

[0080] (2) The preferred value is 1.7 to 5.6, more preferably 1.8 to 5.2, even more preferably 1.9 to 3.7, and particularly preferably 2.0 to 3.5. The most preferred value is 2.2 to 2.7, but 2.3 to 2.7 is also acceptable.

[0081] The upper limit of In is preferably 4.0% or less, more preferably 3.0% or less, further preferably 2.0% or less, and most preferably 1.0% or less. The lower limit of In is preferably 0% or more, more preferably 0.3% or more, further preferably 0.5% or more, and most preferably 0.7% or more.

[0082] 2. Solder ball

[0083] The solder alloy of the present invention can be used as solder balls. The solder balls of the present invention are used to form electrodes and substrate bumps in semiconductor packages such as BGAs (Ball Grid Arrays). The diameter of the solder balls of the present invention is preferably in the range of 1 to 1000 μm. The solder balls can be manufactured using conventional solder ball manufacturing methods.

[0084] 3. Solder paste

[0085] The solder alloy of the present invention can be used as solder paste. Solder paste is a paste-like substance made by mixing solder alloy powder with a small amount of flux. The solder alloy of the present invention can be used as solder paste when mounting electronic components onto a printed circuit board by reflow soldering. The flux used in the solder paste can be either water-soluble or non-water-soluble. Typically, a rosin-based flux, which is a non-water-soluble flux with a rosin matrix, is used.

[0086] 4. Brazed joints

[0087] The brazing joint of the present invention connects electronic components to their substrate (intermediate layer) or bonds electronic components to a printed circuit board. That is, the brazing joint of the present invention refers to the connection portion of electrodes, which can be formed using general soldering conditions.

[0088] Regarding the bonding method of the solder alloy of the present invention, reflow soldering can be used, for example, and performed according to conventional methods. The heating temperature can be appropriately adjusted according to the heat resistance of the chip and the liquidus temperature of the solder alloy. Furthermore, when bonding using the solder alloy of the present invention, the microstructure can be further refined by considering the cooling rate during solidification. For example, the brazed joint can be cooled at a cooling rate of 2 to 3°C / second or higher. Other bonding conditions can be appropriately adjusted according to the alloy composition of the solder alloy.

[0089] The solder alloy of the present invention can be manufactured as a low-low alpha alloy by using low-alpha materials as its raw materials. When such a low-low alpha alloy is used to form solder bumps around memory, it can suppress soft errors.

[0090] Example

[0091] A soft solder alloy with the alloy composition shown in Tables 1 and 2 was prepared. The thermal conductivity was measured as evaluation 1, the resistance value after thermal cycling test was measured as evaluation 2, and the resistance value after drop impact test was measured as evaluation 3.

[0092] • Evaluation 1: Measurement of thermal conductivity

[0093] After casting the solder alloys listed in Tables 1 and 2, the samples were machined into test pieces approximately φ10mm × 3mm in size, and their thermal conductivity was determined using the laser flash method based on JIS R 1611. The measuring apparatus used was an LF / TCM-8510B (trade name) manufactured by Rigaku Co., Ltd. The measuring temperature was 23 ± 1℃. Thermal conductivity of 52 W / m / K or higher was rated as "◎", 50 W / m / K or higher but less than 52 W / m / K was rated as "○", and less than 50 W / m / K was rated as "×".

[0094] Evaluation 2: Measurement of resistance value after thermal cycling test (TCT)

[0095] The solder alloys listed in Tables 1 and 2 were atomized to prepare solder powder. This powder was then mixed with a soldering flux made from rosin, solvent, activator, thixotropic agent, and organic acid to prepare solder paste for each solder alloy. The solder paste was printed onto a 0.8mm thick printed circuit board (material: FR-4) using a 100μm thick metal mask. Fifteen BGA components were then mounted using a pick-and-place machine, and reflow soldering was performed at a maximum temperature of 240°C for 60 seconds to create a test substrate.

[0096] Test substrates soldered with various soft solder alloys were placed in a thermal cycling test apparatus with conditions set at a low temperature of -40°C and a high temperature of +125°C for 10 minutes. The number of cycles was determined from an initial resistance value of 3 to 5 Ω to a resistance value exceeding 15 Ω for at least one BGA component. 750 cycles or more were rated as "◎", 700 to 749 cycles as "○", and less than 700 cycles as "×".

[0097] Evaluation 3: Resistance value determination after drop impact test

[0098] The solder alloys listed in Tables 1 and 2 were pulverized to prepare solder powder. This powder was then mixed with a soldering flux made from rosin, solvent, activator, thixotropic agent, and organic acid to prepare solder paste for each solder alloy. The solder paste was printed onto a 0.8 mm thick printed circuit board (material: FR-4) using a 100 μm thick metal mask. Five LGA components were then mounted using a pick-and-place machine and reflow soldered at a maximum temperature of 240°C for 60 seconds to create a test substrate. Afterward, the LGA components were cut into individual units of five.

[0099] Next, with each monolithic LGA component facing the base, the two ends of each substrate are fixed to the base with bolts. In this state, drop impact resistance is evaluated by applying an impact with an acceleration of 1500G according to JEDEC standards. In this drop test, the progress of cracking in the brazed joint is confirmed by whether the resistance value increases by 50% from the initial value. Regarding the judgment of cracking progress, a case where the resistance value does not increase by 50% from the initial value after more than 85 drops is rated as "◎", a case where the number of drops exceeding 50% of the initial value is 80 to 84 drops is rated as "○", and a case where the number of drops exceeding 50% of the initial value is less than 79 drops is rated as "×".

[0100] The evaluation results are shown in Tables 1 and 2.

[0101] [Table 1]

[0102]

[0103] [Table 2]

[0104]

[0105] *The underlined part indicates that it is outside the scope of this invention.

[0106] As can be clearly seen from Tables 1 and 2, the high thermal conductivity and numerous TCT and drop impact tests are observed in Examples 1-41 due to the appropriate content of the constituent elements. In particular, Examples 3-8, 11, 12, 16, 22, 23, 26, 27, 30, and 32-40, which satisfy equations (1) and (2), showed exceptionally excellent results in all evaluations. It should be noted that Example 31 has a thermal conductivity of "○" because the In content is 5%.

[0107] On the other hand, Comparative Example 1, lacking both Co and Fe, exhibited poor TCT and drop impact resistance. Comparative Example 4, with its lower Ag content, showed poor thermal cycling resistance. Comparative Example 5, with its higher Ag content, exhibited poor drop impact resistance. Comparative Example 6, with its lower Cu content, also showed poor drop impact resistance. Comparative Example 7, with its higher Cu content, resulted in the freeing of intermetallic compounds from the bonding interface into the solder mass, leading to a decrease in thermal conductivity.

[0108] Comparative Example 8 has poor thermal cycling resistance due to its low Sb content. Comparative Example 9 has poor thermal conductivity due to its high Sb content. Comparative Example 10 has poor thermal cycling resistance due to its low Co content. Comparative Example 11 has poor drop impact resistance due to its high Co content.

[0109] Comparative Examples 12 and 13 exhibited poor drop impact resistance due to unsuitable Fe content. Comparative Example 14 also exhibited poor drop impact resistance due to the presence of Bi. Comparative Examples 15-17 exhibited poor thermal conductivity due to the presence of Ni. Furthermore, Comparative Example 18 also exhibited poor drop impact resistance due to the presence of Bi.

[0110] The results confirming the presence or absence of free radicals are shown below. Figure 1 . Figure 1 These are SEM images of the cross-section of the brazed joint. Figure 1 (a) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Comparative Example 18; Figure 1 (b) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Example 16. Five samples were prepared for both the Example and Comparative Examples, covering a region 0–20 μm from the interface between the alloy layer and the surface treatment. Figure 1 The region (represented as the "object region") is observed.

[0111] Powders of the solder alloys shown in Example 16 and Comparative Example 18 were prepared by atomization. The alloy powder was mixed with a flux (GLV manufactured by Senju Metal Industries, Ltd.) containing rosin, solvent, thixotropic agent, organic acid, etc., to prepare solder paste. The solder paste, containing 88% by mass of alloy powder and 12% by mass of flux, was printed onto the Cu pads of a 6-layer printed circuit board (FR-4, Cu-OSP) using a 150 μm metal mask, and then 3216 chip resistors were mounted using a pick-and-place machine. Afterwards, the substrate was melted and reflow soldered at a maximum temperature of 245°C for 40 seconds to create a test substrate.

[0112] Next, the test substrate was cut out and ground, and the area near the joint interface of the cross-section was magnified 1000 times for observation. The overall area and the area of ​​the CuSn-based compound were calculated using image analysis software. Scandium was used as the image analysis software. The area ratio of the CuSn-based compound was calculated using the calculated area results by (Area Ratio of CuSn-based Compound) (%) = (Area of ​​CuSn-based Compound) × 100 / (Area of ​​Object Region).

[0113] from Figure 1 As can be clearly seen in (b), the percentage of CuSn compounds is 0% in the corresponding region of Example 16. Figure 1 Image (a) is from Comparative Example 18, where the percentage of CuSnNi compounds in the corresponding region is 15-20%. This clearly demonstrates that no compound release occurred in the examples. Other examples yielded the same results as Example 16.

[0114] Figure 2 These are SEM images of the cross-section of the brazed joint. Figure 2 (a) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Comparative Example 18. Figure 2 (b) is a photograph showing the cross-sectional mapping analysis results of the brazed joint obtained using the soft solder alloy of Comparative Example 18. Figure 2 (c) is a cross-sectional SEM image of the brazed joint obtained using the soft solder alloy of Example 16.

[0115] and Figure 1 Similarly, a test substrate was prepared, and the area near the interface of the cross section was photographed using FE-SEM at 350x magnification. Then, the cross section was qualitatively analyzed using EDS accompanying the SEM to identify Bi, and the surface analysis of Bi was determined using image analysis software (Scandium, manufactured by EMSIS GmbH). Figure 2 (a) and Figure 2 (c) is a cross-sectional SEM image, in which the presence of a Bi-enriched layer is confirmed. On the other hand, in Figure 2 As can be seen from (b), a Bi-enriched layer is formed near the interface, which is the reason for the poor impact resistance due to embrittlement.

[0116] Industrial availability

[0117] The solder of the present invention can be used in consumer electronic devices such as personal computers, but it can also be used, for example, in vehicle electronic circuits such as ECUs (Engine Control Units) that control the electronic circuits of automobiles by computers to improve fuel efficiency, and it can achieve excellent results.

Claims

1. A solder alloy characterized by comprising, in mass %, The alloy has the following composition by mass: Ag: 1.0~4.0%, Cu: 0.10~1.00%, Sb: 1.0~7.0%, Co: 0.001~0.030%, Fe: 0.005~0.050%, In: 0~4.0%, with the balance being Sn, and the alloy composition satisfies the following equations (1) and (2): 0.040≤Ag×Cu×Sb×Co≤0.075 (1) 1.6≤Ag×Cu×Sb×Co / Fe≤5.7 (2) In equations (1) and (2), Ag, Cu, Sb, Co and Fe represent the mass percentage content of the alloy composition.

2. The solder alloy according to claim 1, further comprising, by mass % less than 0.1% of at least one of P, Ge, Ga and Mn.

3. A solder ball comprising the solder alloy of claim 1 or 2.

4. A solder paste having a solder powder composed of a solder alloy as described in claim 1 or 2.

5. A brazing joint having the soft solder alloy as described in claim 1 or 2.

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