Solder alloy, solder ball, preform solder, solder joint and circuit

Abstract

Provided are a solder alloy, a solder ball, a preformed solder, a solder joint, and a circuit, which are excellent in wettability and shear strength, suitable in failure mode, capable of suppressing the growth of an intermetallic compound at a bonding interface even after multiple reflow soldering, excellent in drop impact resistance, and suitable in the shape of a bump. The solder alloy has an alloy composition comprising, in mass%, 0.10 to 3.00% of Ag, 0.80 to 6.00% of Cu, 0.08 to 0.60% of Ni, 0.0010 to 0.0150% of Ge, and the balance of Sn. It is preferable that the alloy composition further contains 0.1% or less of at least one of Bi, Sb, In, Zn, Ga, Mn, Cr, Co, Si, Ti, and rare earths in total.

Description

Technical Field

[0001] This invention relates to solder alloys, solder balls, preformed solder, brazed joints, and circuits. Background Technology

[0002] In recent years, electronic devices have demanded high integration, thinness, and miniaturization. Electronic components mounted in these devices also require miniaturization and thinning. As a semiconductor package that meets these requirements, the Ball Grid Array (BGA) is the mainstream surface mount package, belonging to the area array type. A BGA has external electrode terminals on its mounting substrate, where solder balls are arranged in a grid pattern at equal intervals. After being placed on the electrodes, the solder balls, along with the mounting substrate, are heated and melted in a reflow oven, thereby forming solder bumps.

[0003] To meet the increasing demands for high integration and miniaturization in electronic devices, electronic components are sometimes connected to both sides of a substrate. To connect these components, a first reflow soldering is performed on one side, followed by a second reflow soldering on the other. Furthermore, in power semiconductor substrates, a third reflow soldering is performed to further connect heat sinks. Thus, with multiple reflow soldering operations, the solder alloy used in the first or second soldering process repeatedly melts and solidifies.

[0004] On the other hand, conventional BGAs have used Sn-Ag-Cu based solder alloys, such as Sn-3.0Ag-0.5Cu. However, if multiple reflow soldering processes are performed as described above, porosity will form at the interface between the solder alloy forming the solder joint and the electrode. Porosity growth leads to a deterioration in shear strength and drop impact resistance. In addition, repeated melting and solidification of the solder alloy can sometimes exacerbate oxidation, thereby deteriorating wettability.

[0005] Therefore, Patent Document 1 discloses a Sn alloy containing 1-9% Cu and various arbitrary elements, designed to suppress reliability degradation of brazed joints caused by repeated reflow soldering. Specifically, Sn-Cu-Ni and Sn-Cu-Ni-Ge solder alloys are disclosed. These solder alloys also contain P, etc. Furthermore, Ag can be included as an arbitrary element. In this document, the chip tilt, porosity growth, and shear strength after repeated reflow soldering are evaluated.

[0006] Patent document 2 discloses a Sn-Ag-Cu-Ni solder alloy used to improve drop impact resistance by reducing the hardness of the solder alloy. The invention described in this document includes P and Ge as elements to suppress discoloration of the solder alloy.

[0007] Patent Document 3 discloses a Sn-Ag-Cu-Ni-Ge solder alloy as a pentagonal solder alloy with good bonding properties and low cost. In the invention described in this document, the porosity and elongation at break were evaluated. Furthermore, this document describes how reducing porosity and thinning the Cu3Sn film thickness improves the elongation at break. Moreover, this document discloses that the reason for adding Ge is to suppress oxidation.

[0008] Patent Document 4 discloses a solder alloy that, in order to reduce the Ag content and improve the bonding strength, contains at least one of La, Ce, Pr, Nd, Sm, and Yb, or Y, Gd, and Dy, in a specified amount in the Sn-Ag-Cu solder alloy. This document also discloses that, in order to refine the crystal structure of the solder alloy and improve its mechanical strength, Ni and Fe may be further included. Furthermore, this document discloses that, in order to suppress the formation of slag caused by the oxidation of La, Ce, Pr, Nd, Sm, and Yb, or Y, Gd, and Dy, as well as Ni and Fe, P, Ga, and Ge, which are elements that are easily oxidized, may also be included.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: International Publication No. 2020 / 135932

[0012] Patent Document 2: International Publication No. 2007 / 102588

[0013] Patent Document 3: Japanese Patent Application Publication No. 2008-93701

[0014] Patent Document 4: Japanese Patent Application Publication No. 2013-049073 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] Patent Document 1 evaluated various alloy compositions containing Sn and Cu. However, it evaluated these compositions using alloy compositions with different constituent elements for each evaluation criterion. For example, in evaluating chip tilt, a solder alloy with constituent elements that did not melt during the second reflow soldering, or even if melted, exhibited a high viscosity at a melting point. Similarly, porosity growth and shear strength after multiple reflow solderings were evaluated using solder alloys with different constituent elements.

[0017] As can be seen, in the invention described in Patent Document 1, although various evaluations were conducted, the constituent elements other than Sn and Cu varied depending on the evaluation criteria. Therefore, an alloy composition that satisfies all the evaluation criteria disclosed in Patent Document 1 was not disclosed. Furthermore, as mentioned above, this document discloses that Ag may be included. However, for alloy compositions containing Ag, not even one component was evaluated. Therefore, in the invention described in Patent Document 1, the effect of adding Ag was not demonstrated.

[0018] In the inventions described in Patent Documents 2 and 3, improvements in drop impact resistance and other properties are achieved by reducing the Ni content and improving the properties of the bonding interface. In the invention described in Patent Document 4, the bonding interface is miniaturized using expensive rare metals.

[0019] As mentioned above, patent documents 1-4 all contain research on the bonding interface of brazed joints. Improving the properties of the bonding interface is, of course, an important objective in brazing filler alloys. However, even with increased load on the brazed joint, the failure mode should not be such that the fracture occurs at the bonding interface. In brazed joints, since physical and electrical loads are primarily applied to the bonding interface, a strong bond at the bonding interface is desirable.

[0020] Thus, the evaluation of the brazed joint under extreme conditions becomes an important indicator in confirming the formation of a good brazed joint. However, the failure modes were not studied in Patent Documents 1-4, so it is unclear whether a brazed joint with a good joint interface was formed.

[0021] Furthermore, the evaluation items studied in Patent Documents 1-4 are all evaluation items conducted after the brazed joint has been formed. As a characteristic of the solder alloy before the brazed joint is formed, the excellent wettability of the molten solder is also important. While wettability is mentioned in Patent Documents 1-4, no specific evaluation of the alloy composition is provided.

[0022] Furthermore, regarding the bonding of BGA isotropic substrates, the shape of the solder bumps needs to be appropriate as a form prior to the formation of the brazing joint. This is because if the solder bumps are irregularly shaped, there is a high possibility of certain problems occurring during soldering. However, the shape of the solder bumps used to form the brazing joint is not studied at all in Patent Documents 1-4.

[0023] Solder bumps are formed by the solidification of solder balls after melting on the electrode, thus bonding with the electrode. Therefore, a generally elliptical surface is preferred. However, sometimes the following situations occur: holes appear on the solder bump causing surface irregularities; needle-like intermetallic compounds protrude from the surface of the solder bump; or it becomes an irregular shape that severely deviates from the generally elliptical shape. In such solder bumps, poor bonding and short circuits between electronic components and the substrate are prone to occur. In recent years, due to the miniaturization and thinning of electronic components, bonding defects caused by irregularly shaped bumps have been increasing year by year, therefore, improvement is desired as soon as possible.

[0024] Therefore, the objective of this invention is to provide a solder alloy, solder ball, preformed solder, brazed joint, and circuit that have excellent wettability and shear strength, appropriate failure mode, inhibit the growth of intermetallic compounds at the joint interface even after multiple reflow solderings, and have excellent resistance to drop impacts, and thus have an appropriate bump shape.

[0025] Solution for solving the problem

[0026] In order to solve the problem of the present invention, the inventors studied various alloy compositions of the solder alloys disclosed in Patent Documents 2 to 4. Specifically, an evaluation was conducted using a Ge-free Sn-1.0Ag-1.0Cu-0.1Ni solder alloy composed of the alloy composition of Example 10 of Patent Document 2. As a result, insights were obtained regarding abnormal solder bump formation.

[0027] Therefore, the inventors evaluated a Sn-1.0Ag-1.0Cu-0.1Ni-0.02Ge solder alloy in which Ge was added to the Sn-1.0Ag-1.0Cu-0.1Ni solder alloy. The results showed observed abnormalities in solder bumps and deterioration in wettability.

[0028] Next, the study focused on the Ni content. In paragraphs 0072 and 0139 of Patent Document 2, it is stated that if the Ni content exceeds 0.07%, hardening the solder alloy before reflow soldering, to a practically harmless extent, will lead to a deterioration in drop impact resistance. On the other hand, paragraph 0063 of the same document describes the strain between the (Cu,Ni)6Sn5 intermetallic compound where Ni moderates the partial substitution of Cu for Ni, and the adjacent Cu3Sn intermetallic compound. Based on these findings, it is speculated that the Sn-Ag-Cu-Ni-Ge solder alloy described in this document only discloses an alloy composition with a Ni content close to 0.07% or 0.05%.

[0029] Furthermore, Figures 3 and 4 of Patent Document 3 disclose that if the Ni content exceeds 0.05%, the formation of pores cannot be suppressed. Moreover, Figure 5 of the same document discloses that the elongation at break of an alloy composition containing 0.03% Ni is the same as that of an alloy composition containing 0.07% Ni, indicating that it is not necessary to contain more than 0.05% Ni.

[0030] Therefore, based on the above description, the inventors evaluated an alloy composition in Sn-Ag-Cu-Ni-Ge solder alloy with the Ni content reduced to 0.05%. The results showed that, similar to the alloy composition with a Ni content of 0.07%, the shear strength deteriorated, and abnormal solder bumping occurred.

[0031] Furthermore, since Patent Document 4 describes a reduction in Ag content, an evaluation was conducted on a Sn-Ag-Cu-Ni-Ge solder alloy with the Ag content reduced to 0.05%. The results showed that while solder bumps were abnormally reduced, wettability and shear strength were poor.

[0032] In view of the above, it can be considered that even with reference to the tendencies of conventional alloy compositions and effects, it is difficult to solve the problems of the present invention for Sn-Ag-Cu-Ni-Ge solder alloys. Therefore, in order to obtain a Sn-Ag-Cu-Ni-Ge solder alloy that simultaneously satisfies all the effects listed in the problems of the present invention, the inventors decided to start from a different direction and study the alloy composition by increasing the Ni content to 0.08% or more. In addition, the contents of Ag, Cu, and Ge were also studied in detail.

[0033] It should be noted that, given that, as described in paragraphs 0072 and 0139 of Patent Document 2, even with slight degradation, the characteristics still exhibit a degree of practical applicability, it can be considered that it is not necessary to achieve all the problems disclosed in the present invention at a level higher than necessary. Therefore, the above-mentioned research was conducted in order to achieve the effect of being practically unproblematic.

[0034] As a result, the following insights were obtained: with the Ni content increased and the constituent elements within the specified range, the wettability and shear strength are excellent, the failure mode is appropriate, the growth of intermetallic compounds at the joint interface can be suppressed even after multiple reflow soldering, and it has excellent resistance to drop impact. Consequently, no abnormalities were observed in the shape of the bumps, thus completing the present invention.

[0035] The present invention, derived from the above insights, is as follows.

[0036] (0) A soft solder alloy, characterized in that, by mass %, it comprises Ag: 0.10-3.00%, Cu: 0.80-6.00%, Ni: 0.08-0.60%, Ge: 0.0010-0.0150%, and the balance consists of Sn and unavoidable impurities.

[0037] (1) A soft solder alloy, characterized in that it has the following alloy composition: Ag: 0.10-3.00%, Cu: 0.80-6.00%, Ni: 0.08-0.60%, Ge: 0.0010-0.0150%, and the balance is composed of Sn.

[0038] (2) The solder alloy according to (0) or (1) above, wherein the alloy composition further contains at least one of Bi, Sb, In, Zn, Ga, Mn, Cr, Co, Si, Ti and rare earth elements in total of less than 0.1%.

[0039] (3) The solder alloy according to any one of (0) to (2) above, wherein the alloy composition satisfies the following formulas (1) and (2).

[0040] 0.00043≤Cu×Ni×Ge≤0.00149 (1)

[0041] 0.09≤Ag×Cu×Ni≤0.11 (2)

[0042] In equations (1) and (2), Ag, Cu, Ni and Ge represent the content (mass %) of the alloy composition.

[0043] (4) A solder ball comprising a solder alloy as described in any one of (0) to (3) above.

[0044] (5) A preformed solder alloy, which is composed of any one of the solder alloys described in (0) to (3) above.

[0045] (6) A brazing joint having a soft solder alloy as described in any one of (0) to (3) above.

[0046] (7) A circuit having the brazing joint described in (6) above. Attached Figure Description

[0047] Figure 1 This is a surface SEM image of the intermetallic compound formed at the joint interface of the brazed joint made using the soft solder alloy of Example 3.

[0048] Figure 2This is a surface SEM image of the intermetallic compound formed at the joint interface of the brazed joint made using the soft solder alloy of Comparative Example 4. Detailed Implementation

[0049] 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.

[0050] 1. Soft solder alloy

[0051] (1) Ag: 0.10~3.00%

[0052] Ag contributes to improved wettability. Furthermore, the precipitation of fine Ag3Sn enhances shear strength. If the Ag content is less than 0.10%, wettability decreases, leading to poor bonding between the solder alloy and the electrode, resulting in reduced shear strength. The lower limit for Ag content is 0.10% or more, preferably 0.50% or more, and more preferably 0.90% or more.

[0053] On the other hand, if the Ag content exceeds 3.00%, the shear strength will decrease due to the precipitation of coarse Ag3Sn, and the drop impact resistance will decrease after multiple reflow solderings. The upper limit of Ag content is 3.00% or less, preferably 2.50% or less, more preferably 2.00% or less, further preferably 1.50% or less, particularly preferably 1.10% or less, and most preferably 1.00% or less.

[0054] (2) Cu: 0.80–6.00%

[0055] Cu contributes to improved shear strength by controlling the growth of intermetallic compounds precipitated at the bonding interface. Furthermore, although a thin and brittle compound forms on the surface of the molten solder along with Ge (described later), this compound is easily disrupted by the convection of the molten solder, resulting in a clean surface. Therefore, the increased surface tension of the molten solder helps suppress solder bump irregularities. Consequently, Cu can achieve optimal failure modes while improving drop impact resistance and wettability.

[0056] If the Cu content is less than 0.80%, it cannot effectively inhibit the growth of intermetallic compounds at the joint interface, resulting in reduced shear strength. Furthermore, a thin compound of Ge and Cu cannot form on the surface of the molten solder, causing the solder bumps to become irregularly shaped. The lower limit for the Cu content is 0.80% or more, preferably 0.95% or more.

[0057] On the other hand, if the Cu content exceeds 6.00%, the liquidus temperature will rise excessively, leading to deterioration of wettability. Furthermore, coarse intermetallic compounds will precipitate at the bonding interface, resulting in decreased resistance to drop impact and shear strength, and an inappropriate failure mode. Consequently, thick compounds of Ge and Cu will precipitate extensively on the surface of the molten solder, reducing the surface tension of the molten solder and causing the solder bumps to become irregularly shaped. The upper limit for the Cu content is 6.00% or less, preferably 5.00% or less, more preferably 4.00% or less, further preferably 3.00% or less, even more preferably 2.00% or less, particularly preferably 1.20% or less, most preferably 1.05% or less, and may also be 1.00% or less.

[0058] (3) N: 0.08~0.60%

[0059] Like Cu, Ni contributes to improved shear strength and suppression of solder bump irregularities. Furthermore, Ni can improve drop impact resistance and wettability while optimizing the failure mode. If the Ni content is less than 0.08%, it cannot effectively suppress the growth of intermetallic compounds at the joint interface, resulting in reduced shear strength. Additionally, a thin layer of Ge and Ni compounds cannot form on the surface of the molten solder, leading to irregular solder bump shapes. The minimum Ni content is 0.08% or more, preferably 0.09% or more.

[0060] On the other hand, if the Ni content exceeds 0.60%, the wettability deteriorates similarly to Cu. Furthermore, coarse intermetallic compounds precipitate at the bonding interface, leading to decreased drop impact resistance and shear strength, and an unsuitable failure mode. Consequently, a large amount of Ge and Ni compounds precipitate on the surface of the molten solder, resulting in irregularly shaped solder bumps. The upper limit for Ni content is 0.60% or less, preferably 0.50% or less, more preferably 0.40% or less, further preferably 0.30% or less, even more preferably 0.20% or less, particularly preferably 0.12% or less, most preferably 0.11% or less, and may also be 0.10% or less.

[0061] (4) Ge: 0.0010~0.0150%

[0062] Ge forms compounds with Cu and Ni, which migrate towards the surface of the molten solder. However, these compounds are disrupted by the convection of the molten solder. Cleaning the surface of the molten solder helps suppress solder bump irregularities. If the Ge content is less than 0.0010%, Cu and Ni compounds cannot be sufficiently precipitated on the surface of the molten solder, resulting in irregularly shaped solder bumps. The lower limit for the Ge content is 0.0010% or more, preferably 0.0020% or more, more preferably 0.0050% or more, and even more preferably 0.0060% or more.

[0063] On the other hand, if the Ge content exceeds 0.0150%, the Cu and Ni compounds on the surface of the molten solder thicken, and these compounds remain on the surface of the solder alloy during solidification, resulting in irregularly shaped solder bumps. Consequently, wettability also decreases. The upper limit for Ge content is 0.0150% or less, preferably 0.0120% or less, more preferably 0.0100% or less, and particularly preferably 0.0080% or less.

[0064] (5) less than 0.1% of at least one of Bi, Sb, In, Zn, Ga, Mn, Cr, Co, Si, Ti and rare earth elements.

[0065] Without affecting the effects of the present invention, the solder alloy of the present invention may contain at least one of Bi, Sb, In, Zn, Ga, Mn, Cr, Co, Si, Ti, and rare earth elements as arbitrary elements in a total amount of less than 0.1%. In the present invention, rare earth elements refer to a total of 17 elements, including Sc and Y belonging to Group 3 of the periodic table and 15 elements of the lanthanum group corresponding to atomic numbers 57-71. The upper limit of the content of any element is not particularly limited, but is preferably less than 0.1%, more preferably less than 0.01%. The lower limit is also not particularly limited, but is preferably more than 0.001%.

[0066] Equations (6)(1) and (2)

[0067] 0.00043≤Cu×Ni×Ge≤0.00149 (1)

[0068] 0.09≤Ag×Cu×Ni≤0.11 (2)

[0069] In equations (1) and (2), Ag, Cu, Ni and Ge represent the content (mass %) of the alloy composition.

[0070] The solder alloy of the present invention preferably satisfies equations (1) and (2). When the above equations are satisfied, the failure mode is further optimized, the shear strength is further improved, the drop impact resistance is further improved even after multiple reflow soldering, the intermetallic compounds precipitated at the joint interface become finer, and the deformation of solder bumps is suppressed. Equation (1) is composed of Cu, Ni, and Ge as elements that suppress the deformation of solder bumps. Although the content of Ge is less than that of Cu and Ni, since Ge forms compounds with Cu and Ni, it has a great influence on the behavior of the alloy structure during solidification. Therefore, a small amount of Ge can greatly contribute to the properties of the solder alloy. Therefore, the content of each constituent element can be directly expressed in the form of an equation without multiplying Ge by a coefficient.

[0071] Equation (2) is a formula composed of Ag, Cu, and Ni that contributes to improved wettability and shear strength. The brazed joint must not fracture at the joint interface, but the strength of the solder alloy itself cannot be too low. In this regard, Ag is a precipitation-strengthening element, and by satisfying equation (2), the strength of the solder alloy itself can be increased to a certain extent without making the failure mode inappropriate. Furthermore, Cu and Ni contribute to the formation of fine intermetallic compounds at the joint interface and the liquidus temperature, thus influencing all properties related to the brazed joint together with Ag.

[0072] These formulas are derived from the interdependence of the constituent elements. This is because an alloy is an integrated substance composed of all its constituent elements, and these elements influence each other. Thus, the solder alloy of the present invention, which satisfies formulas (1) and (2) based on the optimal content of each constituent element, is set to a range that fully considers the interdependence of the constituent elements. Therefore, the alloy composition, in which each constituent element is within the above range and satisfies formulas (1) and (2), exhibits excellent drop impact resistance even after multiple soldering operations, and can suppress the coarsening of intermetallic compounds precipitated at the joint interface. In addition, it also exhibits excellent wettability and shear strength, appropriate failure mode, and can simultaneously play a high-level role in suppressing the irregularization of solder bumps.

[0073] The lower limit of formula (1) is preferably 0.00043 or more, more preferably 0.00060 or more, further preferably 0.00064 or more, particularly preferably 0.00072 or more, and most preferably 0.00076 or more. The upper limit of formula (1) is preferably 0.00149 or less, more preferably 0.00120 or less, further preferably 0.00100 or less, even more preferably 0.00096 or less, particularly preferably 0.00088 or less, and most preferably 0.00086 or less. It can also be 0.00084 or less, or 0.00080 or less.

[0074] The lower limit of formula (2) is preferably 0.09 or more, more preferably 0.095 or more. The upper limit of formula (2) is preferably 0.11 or less, more preferably 0.105 or less, and even more preferably 0.10 or less.

[0075] (7) Balance: Sn

[0076] The balance of the solder alloy of the present invention is Sn. In addition to the aforementioned elements, unavoidable impurities may also be present. The balance of the solder alloy of the present invention may also consist of Sn and unavoidable impurities. Even when unavoidable impurities are present, the aforementioned effects will not be affected. It should be noted that P is preferably not present in the solder alloy of the present invention because it forms compounds with Cu, Ni, and Ge on the surface of the molten solder, leading to deterioration of wettability and bump deformation. Fe forms an oxide film that is difficult to remove with flux, leading to deterioration of wettability and bump deformation; therefore, it is preferably not present in the solder alloy of the present invention. Ce raises the melting point of the solder alloy, forms oxide films and compounds, and forms coarse intermetallic compounds at the bonding interface, preventing the effects of the present invention from being achieved; therefore, it is preferably not present in the solder alloy of the present invention.

[0077] 2. Solder ball

[0078] 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.

[0079] 3. Pre-formed soft solder

[0080] The shape of the preformed solder of the present invention is not particularly limited, and it can be used in the form of plates, rings, cylinders, ribbons, squares, discs, washers, chips, wires, etc. The preformed solder may also contain high-melting-point metal particles (such as Ni particles, Cu particles, and alloy powders with Ni and Cu as the main components) that have a higher melting point than the solder alloy and are easily wetted by the molten solder.

[0081] 4. Brazed joints

[0082] The brazing joint of the present invention is suitable for joining at least two or more components. There are no particular limitations on the components to be joined, as long as they are semiconductors, power modules, inverter products, etc., that use the soft solder alloy of the present invention for electrical connection, such as components, substrates, electronic parts, printed circuit boards, insulating substrates, heat sinks, lead frames, electrode terminals, etc.

[0083] The joining method using the solder alloy of the present invention can be performed, for example, by reflow soldering and following conventional methods. The melting temperature of the solder alloy during flow soldering should be approximately 20°C higher than the liquidus temperature. Furthermore, when joining using the solder alloy of the present invention, the alloy 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 2–3°C / s or higher. Other joining conditions can be appropriately adjusted according to the alloy composition of the solder alloy.

[0084] 5. Circuit

[0085] The circuit of this invention is an electrical circuit. Because it incorporates the solder joints described above, it is preferably used in automotive electronic circuits requiring excellent reliability, and particularly preferably in hybrid semiconductor circuits. Furthermore, this invention, which includes a soft solder alloy, can also be used in power modules.

[0086] 6. Other

[0087] The solder alloy of the present invention can be manufactured as a low-alpha-ray alloy by using low-alpha-ray materials as its raw materials. When such a low-alpha-ray alloy is used for solder bumps around memory, it can suppress soft errors.

[0088] Example

[0089] The present invention will be described through the following embodiments, but the present invention is not limited to the following embodiments.

[0090] To demonstrate the effectiveness of the present invention, the soft solder alloys listed in Table 1 were used to evaluate (1) wettability, (2) shear strength, (3) failure mode, (4) drop impact, (5) IMC grain diameter and (6) irregular bumps.

[0091] (1) Wettability

[0092] Solder balls with a diameter of 0.3 mm were made using the solder alloys described in Tables 1 and 2 below. Flux (manufactured by Senju Metal Industries, Ltd.: WF-6317) was applied to a substrate (S / F: Cu-OSP), and the resulting solder balls were mounted on it. Reflow soldering was then performed using a reflow soldering apparatus (manufactured by Senju Metal Industries, Ltd.: SNR-615) (heating temperature: 220°C or higher, holding time: 40 seconds, peak temperature 245°C). After reflow soldering, the maximum length of the distance between two parallel lines in the projected image of the planar shape containing the wetting spread was measured using a digital microscope (manufactured by Keyence Corporation: VHX-7000), i.e., the wetting spread length. The substrate material used was a 1.2 mm thick glass epoxy board (FR-4). A wetting spread length of 900 μm or more is denoted as "◎", a wetting spread length of 800 μm or more but less than 900 μm is denoted as "〇", and a wetting spread length of less than 800 μm is denoted as "×". In this embodiment, "〇" and "◎" represent evaluation results that are practically sound.

[0093] (2) Shear strength

[0094] Solder balls with a diameter of 0.3 mm were prepared using the solder alloys described in Tables 1 and 2 below. These solder balls were placed on a substrate with a thickness of 1.2 mm and an electrode diameter of 0.5 mm (Cu-OSP) and then soldered. Ten balls were soldered. The soldering conditions were as follows: flux (Senju Metal Industries, Ltd.: WF-6400) was applied to the electrodes; a reflow soldering profile was used with a heating temperature of 220°C or higher, a holding time of 40 seconds, a peak temperature of 245°C, and a cooling rate of 2°C / s; and soldering was performed using a reflow soldering apparatus (Senju Metal Industries, Ltd.: SNR-615). The prepared substrates were then subjected to shear strength testing using a shear strength tester (Nordson Dage: SERIES4000HS) at a shear rate of 1000 mm / s. An average bond strength of 10 bonds of 5.5 N or more is marked with "◎", a bond strength of 4.6 N or more but less than 5.5 N is marked with "〇", and a bond strength less than 4.6 N is marked with "×". In this embodiment, "〇" and "◎" represent evaluation results that are practically sound.

[0095] (3) Destruction Mode

[0096] Using a digital microscope (Keyence Corporation VHX-7000), the failure sites of the specimens after the above shear strength test were observed to be either the body or the joint interface. A “◎” was marked when all 10 failures occurred at the solder alloy (body); a “○” was marked when 9 out of 10 failures occurred at the solder alloy (body); and a “×” was marked when 8 or fewer out of 10 failures occurred at the solder alloy (body). In this embodiment, “○” and “◎” represent evaluation results that are practically acceptable.

[0097] (4) Falling impact

[0098] Solder balls with a diameter of 0.3 mm were fabricated using the solder alloys described in Tables 1 and 2 below. These solder balls were then soldered onto a 12x12 mm CSP module substrate using flux WF-6317 manufactured by Senju Metal Industries, Ltd., to create a CSP using the respective solder alloys as electrodes. Solder paste was printed onto a 30×120 mm, 0.8 mm thick glass epoxy board (FR-4) according to the electrode pattern. The fabricated CSP was mounted on the substrate, and soldering was performed using a reflow soldering apparatus (Senju Metal Industries, Ltd.: SNR-615) with a reflow temperature profile of 220°C or higher, a holding time of 40 seconds, and a peak temperature of 245°C to create an evaluation substrate.

[0099] For the fabricated evaluation substrate, a drop impact test was performed under the following conditions. As a test method, a special fixture was used to fix both ends of the substrate at a position that caused the evaluation substrate to float 10 mm off the base. Following JEDEC standards, impacts with an acceleration of 1500G were repeatedly applied, and the point at which the resistance value increased by 1.5 times was considered the point of breakage. The number of drops was recorded. 210 or more breaks were marked with "◎", 150 or more breaks but less than 209 breaks were marked with "〇", and less than 149 breaks were marked with "×". In this embodiment, "〇" and "◎" represent evaluation results that are practically sound.

[0100] (5) IMC grain diameter

[0101] For the substrate after observing the aforementioned damage mode, a metal stripping agent was used to remove the residual solder body on the surface, exposing the grains composed of intermetallic compounds. Then, a scanning electron microscope (JEM Co., Ltd.: JSM-7000F) was used to photograph the exposed intermetallic compounds at 3000x magnification. The image analysis software accompanying the scanning electron microscope (EMSIS GmbH: Scandium) was then used to calculate the total area of ​​each grain based on the captured image, divided by the number of grains, to calculate the average grain area (S). The square root of (4S / π) was calculated from this average area (S) as the grain diameter. IMC grains with a diameter of 3 μm or less were rated as "◎", those exceeding 3 μm but less than 6 μm were rated as "〇", and those exceeding 6 μm were rated as "×". In this embodiment, "〇" and "◎" represent evaluation results that are practically acceptable.

[0102] (6) Irregular bumps

[0103] Solder balls with a diameter of 0.3 mm were prepared using the solder alloys described in Tables 1 and 2 below. These solder balls were placed on a substrate with a thickness of 1.2 mm and an electrode diameter of 0.5 mm (Cu-OSP) and then soldered. Thirty balls were soldered. The soldering conditions were as follows: flux (Senju Metal Industries, Ltd.: WF-6400) was applied to the electrodes; a reflow soldering profile was used with a heating temperature of 220°C or higher, a holding time of 40 seconds, a peak temperature of 245°C, and a cooling rate of 2°C / s; and soldering was performed using a reflow soldering apparatus (Senju Metal Industries, Ltd.: SNR-615). After soldering, the samples were observed using a scanning electron microscope (JEOL Ltd.: JSM-7000F). Of the 30 irregular bumps, 0 are marked with "◎", 1 to 2 are marked with "〇", and 3 or more are marked with "×". In this embodiment, "〇" and "◎" represent evaluation results that are practically problem-free.

[0104] The evaluation results are shown in Tables 1 and 2.

[0105] [Table 1]

[0106]

[0107] [Table 2]

[0108]

[0109] *The underlined part indicates that it falls outside the scope of this invention.

[0110] As shown in Tables 1 and 2, the contents of Ag, Cu, Ni, and Ge in Examples 1 to 40 are all appropriate, and therefore are marked as "0" or "◎" in all evaluation items. Furthermore, it can be seen that in Examples 2 to 4, 8, 9, 17, 18, 26, 27, and 29 to 40, which satisfy equations (1) and (2), the failure mode, shear strength, drop impact, and irregular bumps, which are evaluation items for brazed joints, are all marked as "◎", demonstrating excellent performance.

[0111] On the other hand, Comparative Example 1 exhibited poor wettability and shear strength due to its low Ag content. Comparative Example 2, with its high Ag content, showed poor shear strength and drop impact resistance. Comparative Example 3, with its low Cu content, experienced poor shear strength and the formation of multiple irregular bumps. Comparative Example 4, with its high Cu content, resulted in a deterioration in all evaluations.

[0112] Comparative Example 5, lacking Ni, exhibited an inappropriate failure mode and deteriorated shear strength. Comparative Examples 6 and 7, with low Ni content, also showed deteriorated shear strength and the formation of multiple irregular bumps. Comparative Example 8, with its high Ni content, resulted in a worse overall evaluation. Comparative Examples 9 and 10, with their low Ge content, also produced multiple irregular bumps. Comparative Example 11, with its high Ge content, suffered from deteriorated wettability and the formation of multiple irregular bumps. Comparative Examples 12 and 13, containing P and Fe respectively, exhibited deteriorated wettability and the formation of multiple irregular bumps. Comparative Examples 14–16, containing Ce, resulted in a worse overall evaluation.

[0113] Figure 1 This is a surface SEM image of the intermetallic compound formed at the joint interface of the brazed joint made using the soft solder alloy of Example 3. Figure 2 This is a surface SEM image of the intermetallic compound at the joint interface of the brazed joint made using the soft solder alloy of Comparative Example 4. Figure 1 and Figure 2 The particles shown are IMC (intermetallic compounds), and the image is a SEM image taken in "(5) IMC grain diameter" above. Figure 1 and Figure 2 It can be seen that the IMC of Example 3 is finer than that of Comparative Example 4. Furthermore, compared to the IMC of Comparative Example 4, the IMC of Example 3 has a more rounded grain shape and less surface roughness. Therefore, Example 3 exhibits a suitable failure mode, high shear strength, and excellent resistance to drop impact. This is also true in other examples, particularly in Examples 2, 4, 8, 9, 17, 18, 26, 27, and 29-40, where excellent results were observed.

Claims

1. A soft solder alloy, characterized in that, It has the following alloy composition by mass%: Ag: 0.10-3.00%, Cu: 0.80-6.00%, Ni: 0.08-0.60%, Ge: 0.0010-0.0150%, and the balance is Sn.

2. The solder alloy according to claim 1, wherein, The alloy composition also contains a total of less than 0.1% of at least one of Bi, Sb, In, Zn, Ga, Mn, Cr, Si, Ti, and rare earth elements.

3. The solder alloy according to claim 1 or 2, wherein, The alloy composition satisfies the following equations (1) and (2). 0.00043≤Cu×Ni×Ge≤0.00149 (1) 0.09≤Ag×Cu×Ni≤0.11 (2) In equations (1) and (2), Ag, Cu, Ni and Ge represent the content (mass %) of the above alloy composition.

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

5. A preformed solder, comprising the solder alloy of claim 1 or 2.

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

7. A circuit having the brazing joint of claim 6.

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