Solder alloys, solder pastes, solder balls, and solder fittings

Abstract

The present invention provides solder alloys, solder pastes, solder balls, and solder joints that have a low melting point, excellent ductility, shear strength, and heat cycle resistance, further suppress the occurrence of open faults, and reduce residual voids even when joining large areas. [Solution] The solder alloy has an alloy composition consisting of Bi: 35.0-68.0%, Sb: 0.1-2.0%, Ni: 0.010-0.050%, Ge: 0.007-0.090% by mass, with the remainder being Sn. Preferably, the alloy composition further contains at least one of Co, Ti, Al, Mn, As, Fe, Pd, Zn, Zr, Pb, In, Ce, P, and Ga in a total amount of 0.1% or less by mass. This solder alloy can be suitably used in solder paste, solder balls, and solder joints.

Description

Technical Field

[0001] The present invention relates to a low-melting-point solder alloy, a solder paste, solder balls, and solder joints.

Background Art

[0002] In recent years, electronic devices such as CPUs (Central Processing Units) have been required to be miniaturized. As the miniaturization of electronic devices progresses, the thermal load during soldering increases, and defects such as warping of the substrate occur. Therefore, soldering at a low temperature is desired. If the soldering temperature is low, it becomes possible to manufacture a highly reliable circuit board. In order to perform soldering at a low temperature, it is necessary to use a low-melting-point solder alloy.

[0003] Examples of low-melting-point solder alloys include Sn-58Bi and Sn-52In as disclosed in JIS Z 3282 (2017). The melting temperatures of these alloys are 139°C and 119°C respectively, and both are alloy compositions representative of low-melting-point solders. In particular, Sn-58Bi is widely used as a solder alloy with low cost and excellent wettability.

[0004] However, since the Bi phase is hard and brittle, it deteriorates the mechanical properties of the solder alloy. When distortion and stress are generated in the substrate due to thermal cycling or drop impact, there is a risk of fracture in the solder alloy part. Therefore, various solder alloys have been studied in order to suppress the rise in melting point and improve the reliability of solder joints.

[0005] For example, in Patent Document 1, a solder alloy that is not only low-melting-point but also improves ductility and shear strength and has excellent heat cycle resistance has been studied. The solder alloy described in the same document is a Sn-Bi-Sb-Ni solder alloy in which Sb and Ni are added to a Sn-Bi solder alloy.

[0006] Furthermore, Patent Document 1 describes that the synergistic effect of Sb and Ni results in a finer alloy structure, simultaneously exhibiting ductility, shear strength, and heat cycle resistance. Patent Document 1 also discloses that Ge may be added as an element to suppress Sn oxidation and improve wettability. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6477965 [Overview of the project] [Problems that the invention aims to solve]

[0008] As mentioned above, the solder alloy described in Patent Document 1 is an excellent solder alloy that can simultaneously exhibit ductility, shear strength, and heat cycle resistance with a single alloy composition, and can also suppress oxidation and improve wettability. However, in recent years, as substrates have become thinner due to multilayering and packages have become larger, defects caused by substrate warping due to heating during soldering have recurred.

[0009] Therefore, when the heating temperature rises from room temperature to the peak mounting temperature, or when it cools down from the peak temperature, the BGA (Ball Grid Array) or LGA (Land Grid Array) packages mounted on the substrate bend due to differences in the thermal expansion coefficients and rigidity of the materials that make up the internal structure, causing them to shift from their original mounting position. Consequently, the electrodes on the substrate and the electrodes on the package solidify at a distance greater than the original distance, and a tear phenomenon occurs, where tiny voids are created as the molten solder alloy constituting the solder joint is torn apart. Even if a tear phenomenon occurs, the solder joint remains electrically connected, and therefore functions as a solder joint.

[0010] However, as the tearing phenomenon progresses, open-circuit defects occur where the solder joint solidifies while separated during melting. These open-circuit defects lead to the loss of electrical connection in the solder joint, resulting in a critical defect.

[0011] Furthermore, as mentioned above, Patent Document 1 contains Ge to improve wettability. Here, the wettability between the substrate and the molten solder alloy is locally improved, which suppresses the generation of voids. However, when large components such as heat sinks are joined to the substrate, local void suppression based on improved wettability alone is not sufficient to suppress the generation of voids.

[0012] In addition, Patent Document 1 states that the fluidity of the molten solder alloy will not be inhibited unless any arbitrary element such as Ge is contained in an amount exceeding 0.1%. When the molten solder alloy has good fluidity, it is expected that the generated voids will be released to the outside. Voids can be generated due to deterioration of wettability, and may be trapped when mounting components on a substrate, but when solder paste is used, voids may be generated due to the components of the paste. In this case, even if the molten solder alloy has fluidity, it may not be able to completely release the voids.

[0013] Even if a solder alloy is excellent, such as the solder alloy described in Patent Document 1, which can exhibit multiple effects with a single alloy composition, it needs to be improved as appropriate in response to changes in technological trends. Therefore, there is a need for an excellent solder alloy that can solve new problems while maintaining the various effects that can be exhibited by conventional solder alloys.

[0014] In particular, Ge has been studied extensively as an element that exhibits oxidation-inhibiting effects, including in Patent Document 1. The liquid properties of molten Sn-Bi solder alloys are affected by the solidification shape, making stable control even more difficult in atmospheric environment packaging. To improve the packaging quality of joints made with Sn-Bi low-temperature solder alloys, it is desirable that the solid properties have an effect of increasing reliability, as described in Patent Document 1, while at the same time being suitable for the packaging process as a liquid and improving joint quality. However, sufficient studies have not been conducted on technologies and guidelines that can achieve both of these.

[0015] The object of the present invention is to provide a solder alloy, solder paste, solder ball, and solder joint that have a low melting point, excellent ductility, shear strength, and heat cycle resistance, further suppress the occurrence of open defects, and reduce residual voids even when joining large areas. [Means for solving the problem]

[0016] The inventors focused on the Sn-Bi-Sb-Ni-Ge solder alloy disclosed in Patent Document 1, noting that it is suitable for the mounting process even as a liquid and exhibits the effect of improving bonding quality. They then investigated the cause of alloy compositions that cause open-circuit defects when warping occurs in thin substrates. As a result, they found that when the oxidation state of the molten solder alloy surface becomes unstable, open-circuit defects occur due to the progression of the tear phenomenon from the liquid phase, whether the alloy is entirely in the liquid phase or a mixture of solid and liquid phases.

[0017] Considering these factors, it might seem that increasing the Ge content would be beneficial in order to increase the stable oxide film on the surface of the molten solder alloy. However, while a high Ge content suppresses open-circuit defects, it has been found that there are alloy compositions in which a large number of voids are generated when a solder joint is formed using paste printed over a large area. This is presumed to be because when a thick Ge oxide film is formed, the active components in the flux constituting the paste are preferentially consumed in reducing the oxide film, and the reduction of the electrodes occurs later, resulting in the generation of a large number of voids at the joint interface.

[0018] Therefore, the inventors focused on the Ge content and found that it is necessary to precisely control the Ge content within a predetermined range. However, they found that in Sn-Bi-Sb-Ni-Ge solder alloys, controlling only the Ge content does not yield sufficient results.

[0019] To prevent open-circuit defects from occurring even when the Ge content is reduced, we also found that by precisely controlling the content of Bi and Ni in addition to Ge, it is possible to suppress the occurrence of open-circuit defects caused by reducing the Ge content.

[0020] In obtaining this knowledge, the inventors focused on the fact that if the surface state of the molten solder alloy is stable, the occurrence of open faults due to the progression of the tear phenomenon can be suppressed. Then, assuming that if the surface state is stable, the droplets of the molten solder alloy will also be stably held, they evaluated the droplet retention properties of the molten solder alloy. As a result, they obtained the finding that solder alloys with droplet retention properties, which are an indicator of how stably droplets are held, tend to suppress the occurrence of open faults.

[0021] Furthermore, in order to sufficiently suppress voids that may occur in a solder joint having a large area by containing Ge to a certain extent, in addition to the contents of Bi, Ni, and Ge, when the content of Sb is within a predetermined range, the finding that the generation of voids can be sufficiently suppressed was also obtained. In addition, it was also found that the Sn-Bi-Sb-Ni-Ge solder alloy that exhibits these effects has a low liquidus temperature, excellent ductility, shear strength, and heat cycle resistance, similar to conventional solder alloys. The present invention obtained based on these findings is as follows.

[0022] (0) A solder alloy characterized in that, by mass%, Bi: 35.0 to 68.0%, Sb: 0.1 to 2.0%, Ni: 0.010 to 0.050%, Ge: 0.007 to 0.090%, and the balance is Sn. (1) A solder alloy characterized by having an alloy composition in which, by mass%, Bi: 35.0 to 68.0%, Sb: 0.1 to 2.0%, Ni: 0.010 to 0.050%, Ge: 0.007 to 0.090%, and the balance is Sn.

[0023] (2) The solder alloy according to (0) or (1) above, wherein the alloy composition (solder alloy) further contains, by mass%, at least one of Co, As, Fe, Pd, Zr, Pb, Ce, and P in a total amount of 0.1% or less.

[0024] (3) The solder alloy according to (0) or (1) above, wherein the alloy composition (solder alloy) further contains, by mass%, at least one of Co, Ti, Al, Mn, As, Fe, Pd, Zn, Zr, Pb, In, Ce, P, and Ga in a total amount of 0.1% or less.

[0025] (4, 8) The solder alloy according to any one of (0) to (3) above, wherein the alloy composition (solder alloy) satisfies the following (1) and (2) formulas. 0.0008 ≦ Bi × Sb × Ni × Ge ≦ 0.0347 (1) 34.7 ≦ (Bi × Ni) / Ge ≦ 154.6 (2) In formulas (1) and (2), Bi, Sb, Ni, and Ge each represent the content as a mass percentage of the solder alloy.

[0026] (5) A solder paste having the solder alloy according to any one of (0) to (3) above.

[0027] (6) A solder ball having the solder alloy according to any one of (0) to (3) above.

[0028] (7) A solder joint having the solder alloy according to any one of (0) to (3) above.

Brief Description of the Drawings

[0029] [Figure 1] FIG. 1 is a schematic diagram showing a method for evaluating droplet retention. FIG. 1(a) is a schematic diagram showing a discharge state where the evaluation is "◎". FIG. 1(b) is a schematic diagram showing a discharge state where the evaluation is "〇". FIG. 1(c) is a schematic diagram showing a discharge state where the evaluation is "×". FIG. 1(d) is a diagram showing the discharge amount and time. [Figure 2] FIG. 2 is a schematic diagram showing a method for evaluating avoidance of open defects in an LGA joint. FIG. 2(a) is a schematic diagram showing a state where a PKG is installed on a substrate. FIG. 2(b) is a schematic diagram showing a state where the PKG is brought into contact with the solder paste printed on the substrate and the temperature is rising. FIG. 2(c) is a schematic diagram showing a state where the PKG is raised upward by 40 μm from the state of FIG. 2(b) in a 190°C environment. FIG. 2(d) is a partial cross-sectional schematic diagram showing the vicinity of the joint of the solder joint before raising upward by 40 μm in FIG. 2(c). FIG. 2(e) is a partial cross-sectional schematic diagram showing the vicinity of the joint of the solder joint in FIG. 2(c). FIG. 2(f) is a partial cross-sectional schematic diagram showing the state from the middle of solidification to the completion of solidification of a solder joint without defects. FIG. 2(g) is a partial cross-sectional schematic diagram showing the state from the middle of solidification to the completion of solidification of a solder joint where a hot tear phenomenon has occurred. FIG. 2(h) is a partial cross-sectional schematic diagram showing the state from the middle of solidification to the completion of solidification of a solder joint where an open defect has occurred due to the progress of the tear phenomenon. [Figure 3] Figure 3 shows X-ray transmission plan images of solder joints after reflow soldering at a peak temperature of 190°C. Figure 3(a) is an X-ray transmission plan image of Example 3, Figure 3(b) is an X-ray transmission plan image of Example 15, and Figure 3(c) is an X-ray transmission plan image of Comparative Example 11. [Modes for carrying out the invention]

[0030] The present invention will be described in more detail below. In this specification, "%" in relation to solder alloy composition refers to "mass%" unless otherwise specified.

[0031] 1. Alloy composition of solder alloys (1) Bi: 35.0~68.0% Bi is an essential element for lowering the melting point of solder alloys. Since Sn-Bi eutectic alloys have a low melting point of 139°C, Bi can lower the liquidus temperature of the solder alloy. Furthermore, solder alloys containing a specific amount of Bi are known to exhibit superplasticity and excellent ductility. Therefore, solder alloys containing a specific amount of Bi have superior ductility and shear strength.

[0032] If the Bi content is less than 35.0%, the liquidus temperature rises. The lower limit of the Bi content is 35% or more, preferably 41.0% or more, more preferably 48.0% or more, and even more preferably 56.0% or more.

[0033] On the other hand, if the Bi content exceeds 68.0%, a large amount of hard, brittle, and coarse Bi phase precipitates, making the solder alloy itself harder and degrading its ductility and shear strength. Furthermore, if the Bi content is significantly high, the melting point rises. In addition, if a large amount of Bi phase precipitates, the surface state of the molten solder alloy becomes unstable, leading to droplet retention problems and open faults due to the progression of the tear phenomenon. The upper limit of the Bi content is 68.0% or less, preferably 65.0% or less, more preferably 63.0% or less, even more preferably 60.0% or less, and particularly preferably 58.0% or less.

[0034] In this invention, the Bi content can be set to a range by appropriately combining the aforementioned lower and upper limits. The preferred range for Bi is 48.0 to 60.0%.

[0035] (2) Sb: 0.1~2.0% Sb is an essential element that contributes to improved ductility and heat cycle resistance, as well as suppressing void formation even when printed as a paste over a large area. At around 200°C, Sb dissolves in β-Sn at a solid level of about 10%, but as the temperature decreases, the solid solubility limit of Sb decreases, and at room temperature, it hardly dissolves at all, and β-SnSb precipitates. During solidification, β-SnSb precipitates around the Sn and Bi phases, exhibiting a pinning effect that can suppress the coarsening of each phase.

[0036] If the Sb content is less than 0.1%, ductility and heat cycle resistance cannot be improved. The lower limit of the Sb content is 0.1% or more, preferably 0.2% or more, more preferably 0.3% or more, even more preferably 0.4% or more, and particularly preferably 0.5% or more.

[0037] On the other hand, if the Sb content exceeds 2.0%, β-SnSb precipitates excessively, raising the melting point. Furthermore, the formation of coarse β-SnSb reduces ductility. In addition, a large amount of voids are generated when printed as a paste over a large area. The upper limit of the Sb content is 2.0% or less, preferably 1.5% or less, more preferably 1.3% or less, even more preferably 1.2% or less, even more preferably 1.0% or less, particularly preferably 0.8% or less, and most preferably 0.7% or less.

[0038] In this invention, the aforementioned lower and upper limits can be appropriately combined to achieve a range of Sb content. The preferred range for Sb is 0.4 to 1.2%.

[0039] (3) Ni: 0.010~0.050% Ni is an essential element for improving the ductility and heat cycle resistance of solder alloys. Furthermore, when Ni is added simultaneously with an appropriate amount of Ge, a thin, uniform Ge oxide layer forms, homogenizing the surface structure of the molten solder alloy. This results in excellent droplet retention, suppresses open defects caused by the progression of tearing, and can even suppress void formation when printed as a paste over a large area.

[0040] If the Ni content is less than 0.010%, the ductility and heat cycle resistance are poor. The lower limit of the Ni content is 0.010% or more, more preferably 0.012% or more, even more preferably 0.013% or more, particularly preferably 0.014% or more, and most preferably 0.015% or more.

[0041] On the other hand, if the Ni content exceeds 0.050%, open defects will occur due to the progression of the tear phenomenon, provided the Ge content is within the range described later. If the content is even higher, a large amount of voids will be generated when printed as a paste over a large area. The upper limit of the Ni content is 0.050% or less, preferably 0.039% or less, more preferably 0.029% or less, even more preferably 0.019% or less, particularly preferably 0.018% or less, and most preferably 0.017% or less.

[0042] In this invention, the Ni content can be set to a range by appropriately combining the aforementioned lower and upper limits. The preferred range for Ni is 0.010 to 0.039%.

[0043] (4) Ge: 0.007~0.090% Ge is an essential element in solder alloys, as it is adapted to the mounting process as a liquid and exhibits the effect of improving bonding quality. When the Ge content is appropriate, only a small amount of the active components in the flux are consumed in reducing the oxides of the solder powder, thus ensuring sufficient reduction to the electrode surface. Therefore, even when printed as a paste over a large area, the generation of voids can be suppressed. In other words, the solder alloy according to the present invention does not contain Ge primarily for its oxidation-inhibiting effect, but rather needs to contain an appropriate amount to suppress the unnecessary consumption of the active components in the flux.

[0044] Furthermore, in order to minimize the occurrence of voids, which are unavoidable when printed as a paste over a large area, and to stabilize the surface state of the molten solder alloy, it is necessary to precisely adjust the Ge content. When the Ge content is at an appropriate level and the surface state of the molten solder alloy is stable, open defects due to the progression of the tear phenomenon can be suppressed. On the other hand, if too much Ge is included to stabilize the surface state, the occurrence of voids will be promoted as described above. Therefore, in the Sn-Bi-Sb-Ni-Ge solder alloy according to the present invention, the Ge content, along with Bi, Sb, and Ni, must be within the aforementioned range, as described above.

[0045] If the Ge content is less than 0.007%, the surface state of the molten solder alloy becomes unstable, droplet retention deteriorates, and as a result, open defects occur due to the progression of the tear phenomenon. The lower limit of the Ge content is 0.007% or more, preferably 0.009% or more, more preferably 0.010% or more, even more preferably 0.012% or more, even more preferably 0.013% or more, particularly preferably 0.014% or more, and most preferably 0.015% or more.

[0046] On the other hand, if the Ge content exceeds 0.090%, a thick oxide film is formed, resulting in a large number of voids when printed as a paste over a large area. The upper limit of the Ge content is 0.090% or less, preferably 0.075% or less, more preferably 0.050% or less, even more preferably 0.030% or less, even more preferably 0.026% or less, particularly preferably 0.025% or less, and most preferably 0.024% or less, 0.022% or less, 0.020% or less, 0.018% or less, or 0.016% or less.

[0047] In this invention, the Ge content range can be determined by appropriately combining the aforementioned lower and upper limits. The preferred range for Ge is 0.010 to 0.050%.

[0048] (5) The alloy composition further contains, by mass%, at least one of Co, Ti, Al, Mn, As, Fe, Pd, Zn, Zr, Pb, In, Ce, P, and Ga in total amount of 0.1% or less.

[0049] The solder alloy according to the present invention may contain arbitrary elements as long as the effects of the present invention are not impaired. The effects of the present invention are maintained if the total amount of these arbitrary elements is 0.1% or less. Among these, the effects of the present invention are not particularly impaired even if at least one of Co, Ti, Al, Mn, As, Fe, Pd, Zn, Zr, Pb, In, Ce, P, and Ga is contained in a total amount of 0.1% or less. Among these, the effects of the present invention are particularly maintained if at least one of Co, As, Fe, Pd, Zr, Pb, Ce, and P is contained in a total amount of 0.1% or less. The lower limit is not particularly limited, but it should be 0.001% or more.

[0050] (6) Equations (1) and (2) 0.0008 ≤ Bi × Sb × Ni × Ge ≤ 0.0347 (1) 34.7 ≤ (Bi × Ni) / Ge ≤ 154.6 (2) In equations (1) and (2), Bi, Sb, Ni, and Ge each represent the mass percentage content of the solder alloy. The solder alloy according to the present invention has even better liquid properties while maintaining the solid properties of a Sn-Bi-Sb-Ni-Ge solder alloy, and therefore it is preferable that it satisfies equations (1) and (2). The technical significance of each equation is as follows.

[0051] Equation (1) represents the relationship for satisfying the solid and liquid properties of a solder alloy to an even higher level, and takes into account the content of all essential elements. Bi, Sb, and Ni are elements that improve the solid properties of a solder alloy, but in order to further improve the liquid properties, it is better to also consider the content of Ge. Because each constituent element of a solder alloy is directly and indirectly related, the whole thing acts as a unified entity and exerts its respective effects. In the case of a Sn-Bi-Sb-Ni-Ge solder alloy, in order to maintain solid properties while exhibiting even better liquid properties, it is preferable to satisfy equation (1), which takes into account the content of each constituent element.

[0052] In detail, Bi forms a eutectic structure with Sn, lowering the melting point, but if the Sb content is too high, the liquidus temperature rises. Furthermore, Bi, Sb, and Ni contribute to ductility, Bi and Sb contribute to shear strength, and Sb and Ni contribute to heat cycle resistance. In addition, Bi, Ni, and Ge contribute to droplet retention and suppression of open defects caused by the progression of tearing. Sb, Ni, and Ge contribute to the generation of voids when printed as a paste over a large area. Thus, since the constituent elements of the solder alloy according to the present invention contribute to each other's effects, it is preferable that equation (1) is satisfied.

[0053] The lower limit of equation (1) is preferably 0.0008 or higher, more preferably 0.0017 or higher, even more preferably 0.0028 or higher, even more preferably 0.0030 or higher, particularly preferably 0.0035 or higher, most preferably 0.0039 or higher, or 0.0044 or higher.

[0054] The upper limits of equation (1), in order of preference, are 0.0326 or less, 0.0317 or less, 0.0290 or less, 0.0261 or less, 0.0236 or less, 0.0218 or less, 0.0209 or less, 0.0174 or less, 0.0168 or less, 0.0157 or less, 0.0153 or less, 0.0122 or less, 0.0117 or less, 0.0109 or less, 0.0104 or less, 0.0096 or less, 0.0088 or less, 0.0087 or less, 0.0084 or less, 0.0070 or less, 0.0067 or less, 0.0065 or less, 0.0062 or less, 0.0061 or less, 0.0059 or less, 0.0057 or less, 0.0053 or less, 0.0052 or less, and 0.0046 or less.

[0055] In this invention, the aforementioned lower and upper limits can be appropriately combined to reach the range of equation (1).

[0056] Equation (2) shows the relationship between the Bi and Ni content, which deteriorates droplet retention and causes open faults when it exceeds the upper limit, and the Ge content, which deteriorates droplet retention and causes open faults when it falls below the lower limit. When the amount of Bi is high, the surface tension decreases, the surface oxidation rate increases, and the surface state of the molten solder alloy becomes unstable. With Ni, the surface state of the molten solder alloy becomes unstable due to the deposition of a large amount of Ni3Sn4. On the other hand, Ge can compensate for these unstable elements by forming a stable film on the surface of the molten solder alloy. For this reason, when the Ge content is low, the surface state of the molten solder alloy becomes unstable. Furthermore, even within the above-mentioned content ranges for Bi, Ni, and Ge, it is preferable that the Bi and Ni content and the Ge content exhibit the appropriate relationship shown in equation (2) in order to further suppress open faults.

[0057] The lower limits of equation (2) are, in order of preference, 34.7 or higher, 34.8 or higher, 36.3 or higher, 39.5 or higher, 42.0 or higher, 43.5 or higher, 48.3 or higher, 49.3 or higher, 54.4 or higher, 55.1 or higher, 58.0 or higher, 62.1 or higher, 66.9 or higher, 72.5 or higher, 84.1 or higher, and 87.0 or higher.

[0058] The upper limit of equation (2) is preferably 154.6 or less, more preferably 145.0 or less, even more preferably 124.3 or less, particularly preferably 113.1 or less, and most preferably 96.7 or less.

[0059] In this invention, the aforementioned lower and upper limits can be appropriately combined to reach the range of equation (2).

[0060] The calculations in equations (1) and (2) used the measured values ​​of the alloy composition shown in Tables 1 and 2. For the values ​​calculated from equations (1) and (2), equation (1) is calculated to four decimal places, and equation (2) is calculated to one decimal place. This calculation rule is used in this application and is intended to be used similarly in calculations concerning further solder alloys described in other literature, as all solder alloys must be treated in the same manner.

[0061] (7) Remainder: Sn The remainder of the solder alloy according to the present invention is Sn. In addition to the aforementioned elements, it may also contain unavoidable impurities. The remainder of the solder alloy according to the present invention may consist of Sn and unavoidable impurities. Even if unavoidable impurities are present, the aforementioned effects will not be affected.

[0062] 2. Solder paste The solder paste according to the present invention is a mixture of solder powder having the alloy composition described above and flux. The flux used in the present invention is not particularly limited as long as it can be soldered by conventional methods. Therefore, a mixture of commonly used rosin, organic acid, activator, thixotropic agent, and solvent may be used as appropriate. In the present invention, the mixing ratio of the metal powder component and the flux component is not particularly limited, but preferably, the metal powder component is 70-90% by mass and the flux component is 10-30% by mass.

[0063] 3. Solder ball The solder alloy according to the present invention can be used as solder balls. When used as solder balls, the solder alloy according to the present invention can be manufactured using the dropping method, which is a common method in the industry. Alternatively, solder joints can be manufactured by processing the solder balls using a method common in the industry, such as by mounting one solder ball on one electrode coated with flux and joining them. The particle size of the solder balls is preferably 1 μm or more, more preferably 10 μm or more, even more preferably 20 μm or more, and particularly preferably 30 μm or more. The upper limit of the particle size of the solder balls is preferably 3000 μm or less, more preferably 1000 μm or less, even more preferably 800 μm or less, and particularly preferably 600 μm or less.

[0064] 4. Solder joints The solder joint according to the present invention is suitably used for joining at least two or more members to be joined. The members to be joined are not particularly limited as long as they are electrically connected using the solder alloy according to the present invention, such as semiconductors using elements, substrates, electronic components, printed circuit boards, insulating substrates, heat sinks, lead frames, electrode terminals, etc., and power modules, inverter products, etc.

[0065] The joining method using the solder alloy according to the present invention can be carried out according to a conventional method, for example, using the reflow method. When performing flow soldering, the melting temperature of the solder alloy may be approximately 20°C higher than the liquidus temperature. Furthermore, when joining using the solder alloy according to the present invention, considering the cooling rate during solidification can further refine the alloy structure. For example, the solder joint should be cooled at a cooling rate of 2-3°C / s or higher. Other joining conditions can be appropriately adjusted according to the alloy composition of the solder alloy.

[0066] 5. Others The solder alloy according to the present invention can be used as a preform. Examples of preform shapes include washers, rings, pellets, discs, ribbons, and wires.

[0067] Furthermore, the solder alloy according to the present invention can be manufactured as a low-alpha-dose alloy by using a low-alpha-dose material as its raw material. When such a low-alpha-dose alloy is used to form solder bumps around memory, it is possible to suppress soft errors. [Examples]

[0068] The present invention will be described with reference to the following embodiments, but the present invention is not limited to these embodiments. To demonstrate the effectiveness of the present invention, the following were evaluated using the solder alloys listed in Tables 1 and 2: (Evaluation 1) liquidus temperature, (Evaluation 2) ductility, (Evaluation 3) shear strength, (Evaluation 4) TCT (heat cycle resistance), (Evaluation 5) droplet retention, (Evaluation 6) avoidance of open defects in LGA joints, and (Evaluation 7) voids in large-area printed areas.

[0069] (Evaluation 1) Liquidus temperature For solder alloys with the alloy compositions listed in Tables 1 and 2, the temperature was determined from the DSC curve. The DSC curves were obtained by heating at 5°C / min in air using a TA Instruments DSC (model: Q2000). The liquidus temperature was determined from the obtained DSC curves and used as the melting temperature. If the liquidus temperature was 185°C or lower, it was evaluated as "○". If the liquidus temperature exceeded 185°C, it was evaluated as "×".

[0070] (Evaluation 2) Ductility Ductility was measured in accordance with JIS Z 3198-2. For each solder alloy having the alloy composition listed in Tables 1 and 2, test specimens with a gauge length of 30 mm and a diameter of 8 mm were prepared by casting into a mold. The prepared test specimens were pulled at room temperature with a stroke of 0.6 mm / min using an Instron Type 5966, and the elongation (ductility) at which the test specimen fractured was measured. In this example, a ductility of 80% or more was judged to be at a level that can accommodate the miniaturization of future electronic devices and was evaluated as "○", while a ductility of less than 80% was evaluated as "×".

[0071] (Rating 3) Market share strength Solder alloys having the alloy compositions listed in Tables 1 and 2 were atomized to produce solder powder (particle size: 20–32 μm). Solder pastes for each solder alloy were prepared by mixing them with soldering flux (manufactured by Senju Metal Industry Co., Ltd., product name: 155HF) consisting of rosin, solvent, activator, thixotropic agent, organic acid, etc. In these solder pastes, the solder powder accounted for 90% of the total mass of the solder paste. Test boards were prepared by printing the solder paste onto Cu electrodes on a 0.8 mm thick printed circuit board (material: FR-4) using a 120 μm thick metal mask, mounting BGA components using a mounter, and then reflow soldering under conditions of a maximum temperature of 190°C and a holding time of 60 seconds.

[0072] The shear strength (N) of this test substrate was measured using a shear strength measuring device (STR-1000, manufactured by RHESCA) under a condition of 6 mm / min. If the shear strength was 60.00 N or higher, it was judged to be at a level where it could be used without practical problems and was evaluated as "○", and if it was less than 60.00 N, it was evaluated as "×".

[0073] (Rating 4) TCT (Heat Cycle Resistance) The solder paste prepared in Evaluation 3 was printed onto a 0.8 mm thick printed circuit board (material: FR-4) using a 100 μm thick metal mask on an OSP-treated Cu electrode. Then, 15 BGA components were mounted using a mounter, and reflow soldering was performed under conditions of a maximum temperature of 190°C and a holding time of 60 seconds to create a test board.

[0074] Test boards soldered with each solder alloy were placed in a heat cycle test apparatus set to low temperature -40°C, high temperature +125°C, and holding time 10 minutes. The number of cycles was determined when the resistance of at least one BGA component exceeded 15Ω, starting from the initial resistance of 3-5Ω. Cycles of 1700 or more were marked "○", and cycles less than 1700 were marked "×". The evaluation results are shown in Table 1.

[0075] (Rating 5) Droplet retention • Evaluation of the separation properties of molten alloy droplets Solder alloys having the alloy compositions listed in Tables 1 and 2 were cast, and alloy pieces measuring φ3 mm and 4 cm in length were prepared by cutting and polishing. The prepared alloy pieces were melted inside the syringe of a surface tensile meter (Kyowa Interface Science Co., Ltd.: Dmo-501), and the separation of the molten solder alloy in the molten state was evaluated using the suspension droplet method, in which the molten solder alloy was discharged from the tip of the syringe in an atmospheric environment at 190°C.

[0076] As shown in Figure 1(d), molten solder alloy was dispensed at a rate of 1.0 μl / s from a syringe tip that was completely free of molten solder alloy droplets. Since the molten solder alloy droplets, whose surface tension and gravity balance was lost, would fall from the syringe tip, the time from when a total volume of 5 μl or more was supplied until the droplets separated from the syringe tip was evaluated.

[0077] As shown in Figures 1(a) and 1(d), a "◎" rating was given if the molten solder alloy droplet 10 did not separate from the syringe tip 11 for 5 seconds or more. As shown in Figures 1(b) and 1(d), a "〇" rating was given if the molten solder alloy droplet 20 did not separate from the syringe tip 21 for 1 second or more but less than 5 seconds. As shown in Figures 1(c) and 1(d), a "×" rating was given if the molten solder alloy droplet 30 separated from the syringe tip 31 in less than 1 second.

[0078] (Rating 6) Avoidance of open faults at LGA connectors Using a metal mask with an aperture diameter of 0.24 mm, a paste-printed area was formed on the test substrate with solder paste prepared in the same manner as in Evaluation 3. Then, as shown in Figure 2(a), the substrate 41 was placed on a solder wettability tester (RHESCA, product name: 5200TN). The solder wettability tester is equipped with a heating stage 40 that moves up and down and a PKG holding arm 42. The tip of the holding arm 42 was used to hold a PKG 45 having an LGA electrode 44 with an opening pattern the same as the paste-printed area 43. Then, as shown in Figure 2(b), the holding arm 42 was lowered to a position where the distance between the electrodes of the PKG 45 and the substrate 41 was 55 μm, and moved to a position where the paste-printed area 43 was in contact with the LGA electrode 44 of the PKG 45. After that, the heating stage was raised to 190°C and held for 30 seconds.

[0079] After confirming that the paste printing area 43 had melted and the electrodes of the substrate and the package were connected by molten solder, the holding arm 42 that holds the package 45 was moved 40 μm upward, as shown in Figure 2(c), to increase the distance between the substrate 41 and the package 45 (distance: 95 μm), and then cooled to room temperature. The mounted substrate 41 was cured with epoxy resin and the vertical cross-section was polished, and the formation state of the solder joints was observed with an electron microscope. Eighteen LGA joints were observed from one polished surface.

[0080] As shown in Figure 2(f), the solder alloy 47 was evaluated as "◎" if it had no defects. As shown in Figure 2(g), even if the solder alloy 48 had a hot tear 48a, which is a shrinkage cavity from the side, it was evaluated as "〇" because the electrical connection was not lost. As shown in Figure 2(h), the solder alloy 49 had an open defect 49a, and the electrical connection was lost, so it was evaluated as "×".

[0081] (Evaluation 7) Voids in large-area printing areas A solder paste was prepared in the same manner as in evaluation 6 above. A metal mask with a 5mm square opening was used, and the thickness was 0.12mm. t After forming the paste-printed area on the test substrate, a QFP component with a 5mm square die-bond area was mounted using a mounter, and a reflow soldering process was performed, holding the peak temperature at 190°C for 90 seconds to form a solder joint.

[0082] For the implemented samples, 25x magnification X-ray transmission plane images were taken using Uniheight Systems Co., Ltd.'s microfocus X-ray system (model number: XVR-160) to observe the residual void state. For the X-ray transmission plane images of 6 samples, the ratio of the void area to the solder joint area (((void area) / (solder joint area)) × 100 (%)) was calculated, and the average of the 6 samples was taken as the average void area ratio. If the average void area ratio was 10% or less, it was evaluated as "◎", if it was between 10% and 20%, it was evaluated as "〇", and if it was 20% or more, it was evaluated as "×". The results are shown in Tables 1 and 2.

[0083] [Table 1]

[0084] [Table 2]

[0085] As shown in Tables 1 and 2, in Examples 1 to 56, the content of the essential elements Bi, Sb, Ni, and Ge was within the range of the present invention, and therefore all evaluations were "○" or "◎". In particular, Examples 1 to 14, 22, 26 to 33, and 35 to 41, which satisfy formulas (1) and (2), as well as Examples 42, 46 to 48, 50, 51, 53, and 54, which satisfy formulas (1) and (2) and also contain Co, As, Fe, Pd, Zr, Pb, Ce, and P respectively, and Example 56, which contains all the optional elements contained in the solder alloys of Examples 42 to 55, all received an evaluation of "◎", showing superior results among the examples.

[0086] In Examples 42-52, the Ti in Example 43, the Al in Example 44, the Mn in Example 45, the Zn in Example 49, the In in Example 52, and the Ga in Example 55 formed relatively thick oxide films. As a result, the reduction of the solder powder and electrode surface was slightly insufficient compared to the "◎" examples, and it is presumed that this led to a slight increase in the area ratio of voids. For this reason, the evaluation 6 in these examples was "〇". However, these examples showed results far superior to the comparative examples.

[0087] On the other hand, Comparative Example 1, due to its low Bi content, exhibited an increased liquidus temperature. Comparative Example 2, due to its high Bi content, showed inferior ductility, shear strength, droplet retention, and open defect avoidance.

[0088] Comparative Example 3 had poor ductility and TCT due to its low Sb content. Comparative Example 4 had a high Sb content, which resulted in an increased liquidus temperature, poor ductility, and poor void formation in large-area printed areas.

[0089] Comparative Example 5 had poor ductility and TCT due to its low Ni content.

[0090] Comparative Examples 6 and 7 exhibited poor droplet retention and open defect avoidance due to their high Ni content. In particular, Comparative Example 7, having a higher Ni content than Comparative Example 6, further exhibited inferior ductility and void formation in large-area printed areas.

[0091] Comparative Examples 8-10, due to the absence of Ge or low Ge content, exhibited poor droplet retention and prevention of open defects. Comparative Examples 11 and 12, due to their high Ge content, showed poor void formation in large-area printed areas.

[0092] Tables 1 and 2 show the results of observing X-ray transmission plane images of Examples 3, 15, and Comparative Example 11. Figure 3 is an X-ray transmission plane image of the solder joint after reflow with a peak temperature of 190°C. Figure 3(a) is the X-ray transmission plane image of Example 3, Figure 3(b) is the X-ray transmission plane image of Example 15, and Figure 3(c) is the X-ray transmission plane image of Comparative Example 11. As shown in Figures 3(a) and 3(b), in Examples 2 and 15, the average void area ratio was less than 20%, indicating a reduction in the occurrence of voids 50 and 51. In particular, in Example 3, the average void area ratio was 10% or less, and the occurrence of void 50 was reduced to a high level. [Explanation of symbols]

[0093] 10, 20, 30 Droplets of molten solder alloy 11, 21, 31 Syringe tip 40 Heating Stages 41 circuit boards 42 Holding arm 43 (Solder) Paste 44 LGA electrode 45 PKG 46-49 Solder alloy 48a (Hot) Tear Phenomenon 49a Open fault 50-52 Void

Claims

1. A solder alloy characterized by having an alloy composition consisting of, by mass%, Bi: 35.0-68.0%, Sb: 0.1-2.0%, Ni: 0.010-0.050%, Ge: 0.007-0.090%, with the remainder being Sn.

2. The solder alloy according to claim 1, wherein the alloy composition further contains, by mass%, at least one of Co, As, Fe, Pd, Zr, Pb, Ce, and P in total amount of 0.1% or less.

3. The solder alloy according to claim 1, wherein the alloy composition further contains, by mass%, at least one of Co, Ti, Al, Mn, As, Fe, Pd, Zn, Zr, Pb, In, Ce, P, and Ga in total amount of 0.1% or less.

4. The solder alloy according to claim 1 or 2, wherein the alloy composition satisfies the following formulas (1) and (2). 0.0008 ≤ Bi × Sb × Ni × Ge ≤ 0.0347 (1) 34.7 ≤ (Bi × Ni) / Ge ≤ 154.6 (2) In equations (1) and (2) above, Bi, Sb, Ni, and Ge each represent the mass percentage content of the solder alloy.

5. A solder paste having the solder alloy according to claim 1 or 3.

6. A solder ball having the solder alloy according to claim 1 or 3.

7. A solder joint having the solder alloy according to claim 1 or 3.

8. The solder alloy according to claim 3, wherein the alloy composition satisfies the following formulas (1) and (2). 0.0008 ≤ Bi × Sb × Ni × Ge ≤ 0.0347 (1) 34.7 ≤ (Bi × Ni) / Ge ≤ 154.6 (2) In equations (1) and (2) above, Bi, Sb, Ni, and Ge each represent the mass percentage content of the solder alloy.

Images