Lead-free solder composition

Abstract

To provide a lead-free solder alloy for multilevel interconnection.SOLUTION: A solder alloy comprises: 1-9 wt.% Cu; at least one of >0 to 1 wt.% Ni, >0 to 10 wt.% Ge, >0 to 1 wt.% Mn, >0 to 10 wt.% Al, >0 to 10 wt.% Si, >0 to 9 wt.% Bi, >0 to 5 wt.% In, >0 to 1 wt.% Ti, >0 to 2 wt.% La, >0 to 2 wt.% Nd; optionally one or more of maximum 1 wt.% Cr, maximum 1 wt.% Ga, maximum 1 wt.% Co, maximum 1 wt.% Fe, maximum 1 wt.% P, maximum 1 wt.% Au, maximum 1 wt.% Te, maximum 1 wt.% Se, maximum 1 wt.% Ca, maximum 1 wt.% V, maximum 1 wt.% Mo, maximum 1 wt.% Pt, maximum 1 wt.% Mg, maximum 5 wt.% Ag, maximum 1 wt.% Zn, maximum 2 wt.% rare-earth metals excluding La and Nd; and the balance Sn with any unavoidable impurities.SELECTED DRAWING: Figure 2

Description

Technical Field

[0002] , , ,

[0003]

[0001] The present invention generally relates to the field of metallurgy, and more specifically to solder alloys. The solder alloys are not particularly limited, but are suitable for use in electronic soldering applications such as wave soldering, surface mount technology, hot air leveling, and ball grid array, land grid array, bottom termination package, LED, and chip scale package.

Background Art

[0002] A typical packaging / assembly process for an LED or any other semiconductor device involves a series of several steps. In each of these steps, several types of electrical / thermal interconnect materials are used. The most common materials used for interconnects are conductive adhesives, sintered materials, soldering alloys, epoxies, and polymers. Among these, conductive adhesives, sintered materials, epoxies, and polymers undergo irreversible process phase transitions during the contact formation process. However, solder undergoes a quasi-physical and mainly reversible phase transition during processing. Solder is the most common material used in electronics assembly. The fixed and reproducible melting point of solder is an advantage as far as the formation of interconnects is concerned. However, when the assembly process involves multiple steps using the same solder, the same properties of the solder become disadvantageous. The solder used in the first assembly process undergoes repeated melting and solidification cycles during secondary, ternary, etc. process steps. The quality of the interconnects that undergo multiple melting / solidification cycles deteriorates, resulting in a reduced lifespan of the final product or a product with impaired reliability. Therefore, there is a need for soldering materials having different melting / solidification temperatures.

[0003] Before the electronics industry transitioned to lead-free solder, different compositions of SnPb solder were used to create a soldering temperature hierarchy. For example, 10Sn90Pb or 05Sn95Pb, or some of their variations with some Ag addition, were used as die attach materials (also referred to as Level I interconnects in LED packaging and assembly terminology) due to their high melting temperatures (299°C for 10Sn90Pb and 310°C for 05Sn95Pb). 63Sn37Pb, or some of its variations with a melting point of approximately 183°C, was used in circuit board assemblies (also known as Level II-V in the LED industry). Following the limitations on lead in solder, the most commonly used lead-free solders were SnAg or SnAgCu(SAC) solders with melting points in the range of 217-228°C. The only viable option for a high-temperature lead-free solder to replace high-lead solder was 80Au20Sn. Containing 80% gold, Au20Sn is one of the most expensive solders. In addition, AuSn is a highly elastic, relatively brittle material that results in interconnects with high stress. Furthermore, a similar set of materials is also used in other semiconductor packaging and electronic assemblies. Some of its properties, such as high thermal conductivity and high reliability, are even more important for the packaging and assembly of high-power electronic components such as power diodes, MOSFETs, and IGBTs.

[0004] One of the lead-free alloy combinations currently used in the electronics industry for multilevel interconnects is the eutectic Sn-Cu alloy (Sn-0.7Cu) and Sn-Ag-Cu alloy (SAC305) system. However, several drawbacks are inherently related to the above alloy combination. A typical second reflow temperature is around 240-250°C, which is higher than the melting peak of the Sn-0.7Cu system, i.e., 232°C. Since the Sn-0.7Cu solder joint is in a liquidus state during the second reflow, it can lead to reliability problems associated with the reliability of the first level interconnect. In this case, die tilting or movement may also occur during the secondary reflow or during rework of the solder joint. Therefore, there is a need for an alloy that can potentially overcome all of the above drawbacks and partially or completely mitigate the reliability risks. [Overview of the Initiative]

[0005] The present invention aims to solve at least some of the problems related to the prior art, or to provide a commercially acceptable alternative.

[0006] Therefore, in the first aspect, the present invention is a lead-free solder alloy, 1-9% by weight of copper, At least one of the following: Nickel of more than 0 to 1% by weight, Germanium in a quantity of 0 to 10% by weight, Manganese in amounts greater than 0 to 1% by weight, Aluminum with a weight of 0 to 10% Silicon in amounts greater than 0 to 10% by weight, Bismuth of 0 to 9% by weight, Indium exceeding 0-5% by weight, Titanium with a weight of 0 to 1 percent, Lanterns weighing between 0% and 2% Neodymium with a weight of 0 to 2% Choose one or more of the following: Up to 1% by weight of chromium, Up to 1% by weight of gallium, Up to 1% by weight of cobalt, Up to 1% by weight of iron, Up to 1% by weight of phosphorus, Up to 1% by weight of gold, Up to 1% by weight of tellurium, Up to 1% by weight of selenium, Up to 1% by weight of calcium, Up to 1% by weight of vanadium, Up to 1% by weight of molybdenum, Up to 1% by weight of platinum, Up to 1% by weight of magnesium, Up to 5% by weight of silver, Up to 1% by weight of zinc, A maximum of 2% by weight of rare earth metals, excluding lanthanum and neodymium, and The present invention provides a solder alloy containing tin as the remainder, along with any unavoidable impurities.

[0007] The present invention will now be described further. Different aspects of the present invention will be defined in more detail in the following sections. Each of the aspects defined in this way may be combined with any other aspects or more of the invention unless otherwise explicitly indicated. In particular, any feature shown to be preferred or advantageous may be combined with any other features or more of the features shown to be preferred or advantageous.

[0008] As used herein, the term "solder alloy" encompasses fusible metal alloys having a melting point in the range of 90 to 400°C. Alloys are lead-free, meaning that lead is not intentionally added. Therefore, the lead content is either zero or below the level of accidental impurities. Alloys are also typically antimony-free, meaning that antimony is not intentionally added. Therefore, the antimony content is typically either zero or below the level of accidental impurities.

[0009] The solder alloy may have a melting peak of 232°C or higher, potentially forming a robust first-level interconnect (die attach) and reducing the risk of die tilting or migration during secondary reflow.

[0010] Solder alloys can be suitably used in lead-free soldering layers of multi-level, multi-process electronic assemblies. The alloys may be suitable for reflow as a first reflow alloy or I-level interconnect alloy and may be a better alternative to Sn-Cu, SAC, or SnAg eutectic alloys. In die-attach processes, due to the reduced fluidity at secondary reflow temperatures, the solder alloys can provide stability to the semiconductor die-attach layer.

[0011] While not bound by theory, the presence of optimal volume fractions of different types of intermetallic compounds and other alloy additives is thought to improve the viscosity of the solder molten during Level II reflow of printed circuit boards (PCBs) without impairing solderability during Level I die attach processes. As a result, die tilt may be reduced.

[0012] Advantageously, the solder alloy can melt within a temperature range of 225°C to 260°C, making it suitable for reflow soldering in typical electronic assembly processes.

[0013] Solder alloys may exhibit similar or improved thermomechanical reliability compared to conventional lead-free solder alloys such as SnCu, SAC, and SnAg. Solder alloys may exhibit similar or improved mechanical properties compared to conventional lead-free solder alloys such as SnCu, SAC, and SnAg.

[0014] Solder alloys can exhibit advantageous wetting properties, such as those of conventional solder alloys, such as SAC solder alloys and Sn0.7Cu solder alloys, which are substantially similar to or better than those of conventional solder alloys.

[0015] The solder contains 1 to 9 wt% copper. Preferably, the solder alloy contains 1.2 to 7 wt% copper, more preferably 1.5 to 3.5 wt% copper, and even more preferably 2 to 3 wt% copper. In a particularly preferred embodiment, the solder alloy contains about 2.5 wt% copper. The solubility of copper in tin is limited. Sn-0.7 wt% Cu forms a eutectic phase. Any further addition of copper exceeding 0.7 wt% increases the volume fraction of intermetallic compounds such as Cu3Sn and Cu6Sn5. These intermetallic compounds may help increase the viscosity of the alloy at a given temperature. These intermetallic compounds may also improve the mechanical and thermal reliability of the alloy.

[0016] The solder alloy optionally (and preferably) contains more than 0 to 1 wt% nickel (e.g., 0.001 to 1 wt% nickel). Preferably, the solder alloy contains 0.001 to 0.2 wt% nickel, more preferably 0.005 to 0.02 wt% nickel, and even more preferably less than 0.01 to 0.02 wt% nickel. Nickel does not form solid solutions with tin. However, it can form intermetallic compounds that can result in improved mechanical properties of the alloy and increase the viscosity of the alloy at a given temperature. Nickel can help reduce copper dissolution.

[0017] The solder alloy optionally (and preferably) contains from greater than 0 to 10 wt% germanium (e.g., from 0.0005 to 10 wt% germanium). Preferably, the alloy contains from 0.0005 to 5 wt% germanium, more preferably from 0.001 to 4.5 wt% germanium, even more preferably from 0.001 to 0.005 wt% germanium, and still even more preferably from 0.002 to less than 0.00 and preferably) contains from greater than 0 to 1 wt% manganese (e.g., from 0.001 to 1 wt% manganese). Preferably, the alloy contains from 0.005 to 0.5 wt% manganese, more preferably from 0.01 to 0.3 wt% manganese, and even more preferably from 0.05 to 0.1 wt% manganese. Manganese has a limited solubility in tin and copper. Manganese forms intermetallic compounds such as Mn3Sn, Mn2Sn, and MnSn2 in the presence of only tin. In the presence of both copper and tin, manganese can also form intermetallic compounds such as MnCu5Sn2, MnCu2Sn, and MnCu4Sn. These intermetallic compounds can increase the viscosity of the Sn-Cu alloy at a given temperature. These intermetallic compounds can also improve the mechanical and thermal reliability of the alloy.

[0018] The solder alloy optionally (and preferably) contains from greater than 0 to 1 wt% manganese (e.g., from 0.001 to 1 wt% manganese). Preferably, the alloy contains from 0.005 to 0.5 wt% manganese, more preferably from 0.01 to 0.3 wt% manganese, and even more preferably from 0.05 to 0.1 wt% manganese. Manganese has a limited solubility in tin and copper. Manganese forms intermetallic compounds such as Mn3Sn, Mn2Sn, and MnSn2 in the presence of only tin. In the presence of both copper and tin, manganese can also form intermetallic compounds such as MnCu5Sn2, MnCu2Sn, and MnCu4Sn. These intermetallic compounds can increase the viscosity of the Sn-Cu alloy at a given temperature. These intermetallic compounds can also improve the mechanical and thermal reliability of the alloy.

[0019] The solder alloy optionally (and preferably) contains aluminum in an amount greater than 0 to 10 wt% (e.g., 0.005 to 10 wt% aluminum). Preferably, the solder alloy contains 0.01 to 5 wt% aluminum, more preferably 0.05 to 2 wt% aluminum. In a preferred embodiment, the alloy contains 0.005 to 0.015 wt% aluminum. In another preferred embodiment, the alloy contains 0.02 to 0.08 wt% aluminum. In another preferred embodiment, the alloy contains 0.8 to 1.2 wt% aluminum. In another preferred embodiment, the alloy contains 3.5 to 4.5 wt% aluminum. Aluminum can function as a deoxidizer and can also improve wettability. Aluminum also has good adhesion properties with semiconductor dies. The alloy preferably contains both germanium and aluminum. Both aluminum and germanium have very limited solubility in tin, but aluminum and germanium have good mutual solubility and form a eutectic mixture with 28.4 atomic% (about 51.6 wt%) germanium.

[0020] The solder alloy optionally (and preferably) contains silicon in an amount greater than 0 to 10 wt% (e.g., 0.005 to 10 wt% silicon). Preferably, the solder alloy contains 0.01 to 8 wt% silicon, more preferably 0.02 to 6 wt% silicon. In a preferred embodiment, the alloy contains 0.02 to 0.08 wt% silicon. In another preferred embodiment, the alloy contains 3.5 to 4.5 wt% silicon. Silicon can function as a deoxidizer and can also improve wettability.

[0021] The solder alloy optionally (and preferably) contains more than 0 to 9 wt% bismuth (e.g., 0.005 to 9 wt% bismuth). Preferably, the solder alloy contains 0.01 to 7 wt% bismuth, more preferably 0.05 to 6 wt% bismuth. In a preferred embodiment, the solder alloy contains 0.07 to 0.13 wt% bismuth. In another preferred embodiment, the solder alloy contains 2 to 5 wt% bismuth, preferably 3.5 to 4.5 wt% bismuth. Bismuth has some solubility in tin, which can provide improved mechanical properties through solid solution strengthening. Bismuth can also act to improve creep resistance. Bismuth can also improve wetting and diffusion.

[0022] The solder alloy optionally (and preferably) contains more than 0 to 5% by weight of indium. Preferably, the solder alloy contains 0.2 to 3% by weight of indium, more preferably 1.5 to 2.5% by weight of indium, and even more preferably 1.8 to 2.2% by weight of indium. Indium has some solubility in tin, which may offer the opportunity for improved mechanical properties through solid solution strengthening. In addition, indium may help lower the liquidus temperature of the solder alloy, thereby allowing the alloy to be reflowed at a lower temperature than the base alloy. However, higher amounts of indium may reduce the advantageous properties of the solder alloy, such as its mechanical strength, solderability, and long-term thermomechanical stability. In addition, higher amounts of indium may make the solder alloy more susceptible to unfavorable oxidation.

[0023] The solder alloy optionally (and preferably) contains more than 0 to 1 wt% titanium (e.g., 0.01 to 1 wt% titanium). Preferably, the solder alloy contains 0.015 to 0.5 wt% titanium, more preferably 0.02 to 0.08 wt% titanium. Titanium has limited solubility in tin and can form intermetallic compounds, which can further reduce the fluidity of the solder molten material during reflow. Titanium can improve strength and interfacial reaction. Titanium can also improve drop impact performance by controlling copper diffusion at the substrate / solder interface.

[0024] The solder alloy optionally (and preferably) contains more than 0 to 2 wt% lanthanum (e.g., 0.01 to 2 wt% lanthanum). Preferably, the solder alloy contains 0.05 to 1 wt% lanthanum, more preferably 0.1 to 0.5 wt% lanthanum. The lanthanum may act to improve diffusion and wetting properties.

[0025] The solder alloy optionally (and preferably) contains more than 0 to 2 wt% neodymium (e.g., 0.01 to 2 wt% neodymium). Preferably, the solder alloy contains 0.05 to 1 wt% neodymium, more preferably 0.1 to 0.5 wt% neodymium. Neodymium may act to improve diffusion and wetting properties.

[0026] The solder alloy contains at least one of nickel, germanium, manganese, aluminum, silicon, bismuth, indium, titanium, lanthanum, and neodymium. Preferably, the solder alloy contains at least two of these elements. In a preferred embodiment, the solder alloy contains two of these elements. In another preferred embodiment, the solder alloy contains three of these elements. In another preferred embodiment, the solder alloy contains four of these elements. In another preferred embodiment, the solder alloy contains nickel and germanium. In another preferred embodiment, the solder alloy contains nickel, manganese, and phosphorus. In another preferred embodiment, the solder alloy contains aluminum, germanium, nickel, and bismuth. In another preferred embodiment, the solder alloy contains indium, titanium, lanthanum, and nickel.

[0027] The solder alloy may optionally contain one or more of the following: up to 1 wt% chromium (e.g., 0.01 to 1 wt% chromium), up to 1 wt% gallium (e.g., 0.01 to 1 wt% gallium), up to 1 wt% cobalt (e.g., 0.01 to 1 wt% cobalt), up to 1 wt% iron (e.g., 0.01 to 1 wt% iron), up to 1 wt% phosphorus (e.g., 0.01 to 1 wt% phosphorus), up to 1 wt% gold (e.g., 0.01 to 1 wt% gold), up to 1 wt% tellurium (e.g., 0.01 to 1 wt% tellurium), up to 1 wt% selenium (e.g., 0.01 to 1 wt% selenium), up to 1 wt% It contains calcium (e.g., 0.01–1 wt% calcium), up to 1 wt% vanadium (e.g., 0.01–1 wt% vanadium), up to 1 wt% molybdenum (e.g., 0.01–1 wt% molybdenum), up to 1 wt% platinum (e.g., 0.01–1 wt% platinum), up to 1 wt% magnesium (e.g., 0.01–1 wt% magnesium), up to 5 wt% silver (e.g., 0.01–5 wt% silver), up to 1 wt% zinc (e.g., 0.01–1 wt% zinc), and up to 2 wt% rare earth metals (e.g., 0.01–2 wt% rare earth metals), excluding lanthanum and neodymium. Gallium and zinc have some solubility in tin, which may provide an opportunity for solid solution strengthening. However, higher levels of zinc may reduce the solderability of the solder alloy. Cobalt may be used to reduce copper dissolution. Calcium, magnesium, phosphorus, and vanadium are deoxidizing agents that may also be used to improve wetting properties. Gold, chromium, iron, molybdenum, platinum, selenium, and tellurium may be used for improved strength and interfacial reactions. Silver and rare earth elements may improve diffusion and wetting properties. As used herein, the term rare earth elements refers to one or more elements selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Silver may help provide more favorable mechanical properties and may increase the viscosity of the solder alloy at a given temperature through the formation of intermetallic compounds.

[0028] The alloy typically contains at least 80% by weight of tin, more typically at least 85% by weight of tin, more typically at least 90% by weight of tin, and more typically at least 95% by weight of tin. The alloy typically contains up to 98% by weight of tin, and more typically up to 97% by weight of tin.

[0029] The alloys described herein may contain unavoidable impurities, but it will be understood that in total, these are unlikely to exceed 1% by weight of the composition. Preferably, the solder alloy contains unavoidable impurities in an amount of 0.5% by weight or less of the composition, more preferably 0.3% by weight or less of the composition, even more preferably 0.1% by weight or less of the composition, even more preferably 0.05% by weight or less of the composition, and most preferably 0.02% by weight or less of the composition.

[0030] The solder alloys described herein may consist of the enumerated elements. Alternatively, the solder alloys described herein may essentially consist of the enumerated elements. Therefore, it will be understood that, in addition to these essential elements (i.e., tin, copper, and at least one of nickel, germanium, manganese, aluminum, silicon, bismuth, indium, titanium, lanthanum, and neodymium), other unspecified elements may be present in the composition, provided that the essential properties of the composition are not substantially affected by the presence of the other unspecified elements.

[0031] In a particularly preferred embodiment, the solder alloy consists of 2-3 wt% copper, 0.005-0.02 wt% nickel, 0.001-0.005 wt% germanium, and the remainder tin with any unavoidable impurities. Compared to conventional SAC305 and Sn0.7Cu solder alloys, such solder alloys exhibit minimal die tilt between the first and fourth reflows, reduced void percentage between the first and fourth reflows, increased die shear strength, and improved thermal cycling behavior.

[0032] In a particularly preferred embodiment, the solder alloy consists of 1-7 wt% copper, 0.1-1 wt% nickel, 0.05-0.5 wt% manganese, and 0.05-0.1 wt% phosphorus, with the remainder being tin along with unavoidable impurities. Such a solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, such a solder alloy exhibits minimal die gradient between the first and fourth reflows, reduced void percentage between the first and fourth reflows, increased die shear strength, and improved thermo-cycle behavior.

[0033] In a particularly preferred embodiment, the solder alloy consists of 3-7 wt% copper, 0.1-1 wt% aluminum, 0.1-1 wt% germanium, 0.05-0.1 wt% nickel, 2-5 wt% bismuth, and the remainder tin with unavoidable impurities. Such a solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, such a solder alloy exhibits minimal die gradient between the first and fourth reflows, reduced void percentage between the first and fourth reflows, increased die shear strength, and improved thermo-cycle behavior.

[0034] In a particularly preferred embodiment, the solder alloy consists of 4-7 wt% copper, 0.1-2 wt% indium, 0.1-0.5 wt% titanium, 0.1-0.5 wt% lanthanum, 0.1-0.2 wt% nickel, and the remainder tin with unavoidable impurities. Such a solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, such a solder alloy exhibits minimal die gradient between the first and fourth reflows, reduced void percentage between the first and fourth reflows, increased die shear strength, and improved thermo-cycle behavior.

[0035] The solder alloy may be in the form of a pre-applied coating on any solderable material, such as bars, rods, solid wires with or without flux, foils or strips, films, preforms, or powders or pastes (blends of powder and flux), or solder spheres for use in ball grid array joints, or pre-formed solder pieces or reflowed or solidified solder joints, or copper ribbons for photovoltaic applications.

[0036] In a further embodiment, the present invention provides a solder joint comprising a solder alloy as described herein.

[0037] In a further embodiment, the present invention is Solder alloys described herein, and We provide solder paste, including solder flux.

[0038] In a further embodiment, the present invention provides a method for forming a solder joint, the method being: (i) to provide two or more workpieces to be joined, (ii) To provide solder alloys as described herein, (iii) including heating the solder alloy in the vicinity of the workpiece to be joined.

[0039] The workpiece may be a component of a printed circuit board, such as a substrate and a die.

[0040] In a further embodiment, the present invention provides the use of solder alloys as described herein in a soldering method. Preferably, the soldering method is selected from surface mount technology (SMT) soldering, die attach soldering, thermal interface soldering, manual soldering, laser and RF induction soldering, and rework soldering.

[0041] In a further embodiment, the present invention provides a solder alloy or solder joint, including the following: (a) 1-9% by weight of copper (b) One or more of the following 0-1% by weight of nickel 0-1% by weight of manganese (c) Selectively one or more of the following elements Up to 1% by weight of titanium Up to 2% by weight of one or more rare earth elements, cerium, lanthanum, neodymium Up to 1% by weight of chromium Up to 10% by weight of germanium Up to 1% by weight of gallium Up to 1% by weight of cobalt Up to 1% by weight of iron Up to 10% by weight of aluminum Up to 1% by weight of phosphorus Up to 1% by weight of gold Up to 1% by weight of tellurium Up to 1% by weight of selenium Up to 1% by weight of calcium Up to 1% by weight of vanadium Up to 1% by weight of molybdenum Up to 1% by weight of platinum Up to 1% by weight of magnesium up to 10% by weight of silicon Up to 9% by weight of bismuth Up to 5% by weight of silver Up to 5% by weight of indium (d) The remainder is tin, along with unavoidable impurities.

[0042] The advantages and preferred features of the first embodiment apply equally to this embodiment.

[0043] In a further embodiment, the present invention provides a solder alloy or solder joint, including the following: (a) 1-9% by weight of copper (b) One or more of the following 0-10% by weight of aluminum 0-10% by weight germanium 0-10% by weight of silicon 0-9% by weight of bismuth (c) Selectively one or more of the following elements Up to 1% by weight of nickel Up to 1% by weight of titanium Rare earth elements, excluding cerium, lanthanum, and neodymium, up to 2% by weight. Up to 1% by weight of chromium Up to 1% by weight of manganese Up to 1% by weight of gallium Up to 1% by weight of cobalt Up to 1% by weight of iron Up to 1% by weight of phosphorus Up to 1% by weight of gold Up to 1% by weight of tellurium Up to 1% by weight of selenium Up to 1% by weight of calcium Up to 1% by weight of vanadium Up to 1% by weight of molybdenum Up to 1% by weight of platinum Up to 1% by weight of magnesium Up to 5% by weight of silver Up to 5% by weight of indium (d) The remainder is tin, along with unavoidable impurities.

[0044] The advantages and preferred features of the first embodiment apply equally to this embodiment.

[0045] In a further embodiment, the present invention provides a solder alloy or solder joint, including the following: (a) Copper 1 to 9% by weight or less (b) One or more of the following 0-1% by weight of nickel 0-5% by weight of indium 0-1% by weight of titanium Lanterns weighing 0-2% 0-2% by weight neodymium (c) Selectively one or more of the following elements Up to 2% by weight of rare earth elements, cerium Up to 1% by weight of chromium Up to 1% by weight of manganese Up to 1% by weight of gallium Up to 1% by weight of cobalt Up to 1% by weight of iron Up to 1% by weight of phosphorus Up to 1% by weight of gold Up to 1% by weight of tellurium Up to 1% by weight of selenium Up to 1% by weight of calcium Up to 1% by weight of vanadium Up to 1% by weight of molybdenum Up to 1% by weight of platinum Up to 1% by weight of magnesium up to 10% by weight of silicon Up to 10% by weight of germanium Up to 9% by weight of bismuth Up to 5% by weight of silver Up to 10% by weight of aluminum (d) The remainder is tin, along with unavoidable impurities.

[0046] The advantages and preferred features of the first embodiment apply equally to this embodiment. [Brief explanation of the drawing]

[0047] The present invention will be further described here with reference to the following drawings, by some non-limiting examples of these alloys and an overview of their properties. [Figure 1] This is a plot of die tilt (μm) versus device # in reflow soldering with the conventional solder SAC305. [Figure 2] This is a plot of die tilt (μm) versus device # in the reflow of the SnCuNiGe solder alloy of Example 3. [Figure 3] This is a plot of BLT (μm) versus device # in reflow soldering with conventional solder, SAC305. [Figure 4] This is a plot of BLT (μm) versus device # in the reflow of the SnCuNiGe solder alloy of Example 3. [Figure 5]This plot shows the change in void (%) between the first and fourth reflows for conventional solder SAC305 (left) and the SnCuNiGe solder alloy of Example 3 (right). [Figure 6] This is a box plot of shear force (kgf) versus cycles on an FR4 substrate for conventional solder alloy SAC305 (left), conventional solder alloy SnCu0.7 (center), and the SnCuNiGe solder alloy of Example 3 (right). [Figure 7] This is a box plot of shear force (kgf) versus cycles for conventional solder alloy SAC305 (left), conventional solder alloy SnCu0.7 (center), and the SnCuNiGe solder alloy of Example 3 (right) on a metal substrate. [Modes for carrying out the invention]

[0048] Example 1 An alloy having the following composition was prepared: Alloy ID-HC1: Cu: 1~7% by weight, Ni: 0.1~1% by weight, Mn: 0.05~0.5% by weight, P:0.05~0.1% by weight, Along with unavoidable impurities, the remainder is Sn.

[0049] Such solder alloys advantageously have a melting peak at 232°C.

[0050] Alloy ID-HC2: Cu: 3~7% by weight, Al: 0.1~1% by weight, Ge: 0.1~1% by weight, Ni: 0.05~0.1% by weight, Bi: 2~5% by weight, Along with unavoidable impurities, the remainder is Sn.

[0051] Such solder alloys advantageously have a melting peak at 232°C.

[0052] Alloy ID-HC3: Cu: 4~7% by weight, In: 0.1~2% by weight, Ti: 0.1~0.5% by weight, La: 0.1~0.5% by weight, Ni: 0.1~0.2% by weight, Along with unavoidable impurities, the remainder is Sn.

[0053] Such solder alloys advantageously have a melting peak at 232°C.

[0054] Example 2 Alloys having the compositions listed in Table 1 below were prepared.

[0055] [Table 1]

[0056] Between 225 and 280°C, the alloys in Table 1 have a liquid fraction of 50% or more. Theoretically, the liquid fraction is calculated as: liquid fraction (%) = 100 - solid fraction (%).

[0057] Example 3 An alloy having the following composition was prepared. Cu:2.5% by weight Ni:<0.05wt% Ge: <0.005% by weight Along with unavoidable impurities, the remainder is Sn.

[0058] Dye gradient analysis: Die tilt analysis was performed on the alloys compared to the conventional SAC305 solder alloy. The results are shown in Figures 1 and 2. SnCuNiGe demonstrated minimal change in die tilt between the first reflow (circular) and the fourth reflow (triangular), while SAC305 showed considerable variation.

[0059] Void analysis: The alloy was subjected to void analysis compared to the conventional SAC305 solder alloy. The results are shown in Figures 3, 4, and 5. As shown in Figures 3 and 4, in contrast to SAC305, SnCuNiGe demonstrates the minimum change in void percentage between the first and fourth reflows. Figure 5 shows that the change in void percentage between the first and fourth reflows in SnCuNiGe is less significant than in SAC305.

[0060] Shear and thermal tests: The following tests and conditions will be used. • To directly compare the optical, thermal, and shear performance of solder paste, a selection of medium-power Lumileds 3535L LEDs were used. Identical Al-MCPCB and FR4-PCB substrates designed for selected LEDs, featuring contact pads with an ENIG finish. • Shear test: 1833 thermal cycle

[0061] As shown in Figures 6 and 7, SnCuNiGe exhibited the most stable shear strength values ​​in both MCPCB and FR4 assemblies as the thermal cycle increased.

[0062] The details provided herein are provided by description and illustration and are not intended to limit the scope of the appended claims. Many modifications of the currently preferred embodiments shown herein will be obvious to those skilled in the art and remain within the scope of the appended claims and their equivalents.

Claims

1. 2 to 3% by mass of copper, 0.005 to 0.02 mass% nickel, 0.001 to 0.005 mass% germanium, and A lead-free solder alloy consisting of tin as the remainder, along with any unavoidable impurities.

2. The solder alloy according to claim 1, comprising less than 0.01 to 0.02 mass% nickel.

3. The solder alloy according to claim 1, comprising less than 0.002 to 0.005 mass% germanium.

4. The solder alloy according to any one of claims 1 to 3, in the form of a powder or paste (a blend of powder and flux).

5. A solder joint comprising the solder alloy described in any one of claims 1 to 4.

6. A method for forming a solder joint, (i) To provide two or more workpieces to be joined, (ii) To provide a solder alloy as defined in any one of claims 1 to 4, (iii) A method comprising heating the solder alloy in the vicinity of the workpiece to be joined.

7. Use of a solder alloy according to any one of claims 1 to 4 in a soldering method, wherein the soldering method is selected from surface mount technology (SMT) soldering, die attach soldering, thermal interface soldering, manual soldering, laser and RF induction soldering, and rework soldering.

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