Lead-free solder compositions

Abstract

A lead-free solder alloy comprising: 1 to 9 wt.% copper; at least one of the following: greater than 0 to 1 wt.% nickel, greater than 0 to 10 wt.% germanium, greater than 0 to 1 wt.% manganese, greater than 0 to 10 wt.% aluminum, greater than 0 to 10 wt.% silicon, greater than 0 to 9 wt.% bismuth, greater than 0 to 5 wt.% indium, greater than 0 to 1 wt.% titanium, greater than 0 to 2 wt.% lanthanum, greater than 0 to 2 wt.% neodymium; optionally one or more of the following: up to 1 wt.% chromium, up to 1 wt.% gallium, up to 1 wt.% cobalt, up to 1 wt.% iron, up to 1 wt.% phosphorus, up to 1 wt.% gold, up to 1 wt.% tellurium, up to 1 wt.% selenium, up to 1% calcium, up to 1% vanadium, up to 1% molybdenum, up to 1% platinum, up to 1% magnesium, up to 5 wt.% silver, up to 1 wt.% zinc, up to 2 wt.% rare earth metals, excluding lanthanum and neodymium, as well as the remaining tin and any unavoidable impurities.

Description

Technical Field

[0001] The present invention relates generally to the field of metallurgy and more particularly to solder alloys particularly, but not exclusively, suitable for use in electronics soldering applications such as wave soldering, surface mount technology, hot air leveling, ball grid arrays, land grid arrays, bottom terminal packages, LEDs, and chip scale packages. Prior Art

[0002] The typical packaging / assembly process for LEDs or any other semiconductor devices involves a series of steps. Various types of electrical / thermal interconnect materials are used in each of these steps. The most common materials used for interconnects are conductive adhesives, sintered materials, solder alloys, epoxies, and polymers. Conductive adhesives, sintered materials, epoxies, and polymers undergo irreversible phase transitions during contact formation. Solder, however, undergoes a quasi-physical, largely reversible phase transition during processing. Solder is the most commonly used material in electronic assembly. Solder's consistent and reproducible melting point is an advantage when it comes to interconnect formation. However, this uniformity becomes a liability if the assembly process involves multiple steps using the same solder. Solder used in the first assembly step will undergo repeated melting and cooling cycles in the second, third, and other process steps. The quality of the interconnect degrades after multiple melting / cooling cycles, resulting in a shortened lifespan or compromised reliability of the final product. Therefore, solder materials with varying melting / cooling temperatures are needed.

[0003] Before the electronics industry transitioned to lead-free solders, SnPb solders of varying compositions were used to establish a soldering temperature hierarchy. For example, 10Sn90Pb or 05Sn95Pb, or some of their Ag-added variants, were used as die attach materials (also known as level I interconnects in LED packaging and assembly terminology) due to their high melting temperatures (10Sn90Pb melts at 299°C, while 05Sn95Pb melts at 310°C). 63Sn37Pb, or some of its variants with melting points around 183°C, was used for circuit board assembly (also known as the II-V stage in the LED industry). Following restrictions on lead content in solders, the most commonly used lead-free solders are SnAg or SnAgCu (SAC) solders, which have melting temperatures ranging from 217 to 228°C. The only viable alternative to high-lead solders as a high-temperature lead-free alternative is 80Au20Sn. Au20Sn, containing 80% gold, is one of the most expensive solder materials. Furthermore, AuSn is a high-modulus and relatively brittle material, resulting in high stress in interconnects. Similar material groups are also used in other semiconductor packaging and electronic assemblies. For packaging and assemblies of high-power electronic components (such as power diodes, MOSFETs, and IGBTs), certain properties (such as high thermal conductivity and high reliability) are even more critical.

[0004] One of the lead-free alloy combinations currently used in the electronics industry for multi-layer interconnects is the eutectic Sn-Cu alloy (Sn-0.7Cu) and Sn-Ag-Cu alloy (SAC305) system. However, this alloy combination inherently has several disadvantages. The typical second reflow temperature is approximately 240 to 250°C, which is higher than the melting peak of the Sn-0.7Cu system, which is 232°C. Since the Sn-0.7Cu solder joints are in the liquid phase during the second reflow, this can lead to reliability issues for the first-level interconnects. Similarly, wafer tilt or movement can occur during the second reflow or solder joint rework process. Therefore, there is a need for an alloy that can potentially overcome all of these disadvantages and partially or completely mitigate these reliability risks. Summary of the Invention

[0005] The present invention is intended to address at least some of the problems associated with the prior art or to provide a commercially acceptable alternative.

[0006] Accordingly, in a first aspect, the present invention provides a lead-free solder alloy comprising: 1 to 9 wt.% copper, At least one of the following: greater than 0 to 1 wt.% nickel, greater than 0 to 10 wt.% germanium, greater than 0 to 1 wt.% manganese, greater than 0 to 10 wt.% aluminum, greater than 0 to 10 wt.% silicon, greater than 0 to 9 wt.% of bismuth, greater than 0 to 5 wt.% indium, greater than 0 to 1 wt.% titanium, greater than 0 to 2 wt.% of lanthanum, Greater than 0 to 2 wt.% of neodymium, Optionally one or more of the following: Up to 1 wt.% chromium, Up to 1 wt.% gallium, Up to 1 wt.% cobalt, Up to 1 wt.% iron, Up to 1 wt.% phosphorus, Up to 1 wt.% gold, Up to 1 wt.% tellurium, Up to 1 wt.% of selenium, Up to 1 wt.% calcium, Up to 1 wt.% vanadium, Up to 1 wt.% of molybdenum, Up to 1 wt.% of platinum, Up to 1 wt.% of magnesium, Up to 5 wt% silver, Up to 1 wt.% zinc, Up to 2 wt.% of rare earth metals, excluding lanthanum and neodymium, and The remainder is tin and any unavoidable impurities.

[0007] The present invention will now be described further. In the following paragraphs, various aspects of the present invention are defined in more detail. Each aspect thus defined may be combined with any other aspect(s) unless expressly indicated to the contrary. Specifically, any feature indicated as preferred or advantageous may be combined with any other feature(s) indicated as preferred or advantageous.

[0008] As used herein, the term "solder alloy" encompasses alloys of fusible metals with a melting point in the range of 90 to 400°C. These alloys are lead-free, meaning that no lead is intentionally added. Therefore, their lead content is zero or at a level no greater than that of an unintentional impurity. The alloys are also generally antimony-free, meaning that no antimony is intentionally added. Therefore, their antimony content is generally zero or at a level no greater than that of an unintentional impurity.

[0009] The solder alloy may have a melting peak at or above 232° C., may form a robust first level interconnect (die attach), and may reduce the risk of die tilting or movement during secondary reflow.

[0010] This solder alloy is suitable for use in Pb-free soldering stages in multi-stage, multi-step electronic assemblies. It is suitable for reflow as a first reflow alloy or a Level I interconnect alloy and is a promising alternative to Sn-Cu, SAC, or SnAg eutectic alloys. During die attach processes, the solder alloy's reduced fluidity at secondary reflow process temperatures can provide stability to semiconductor die attachment layers.

[0011] Without wishing to be bound by theory, it is believed that the presence of optimized volume fractions of different types of intermetallic compounds and other alloying additives increases the viscosity of the solder melt during the Stage II reflow process of a printed circuit board (PCB) without affecting the solderability of the Stage I die attach process. Consequently, die tilt can be reduced.

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

[0013] Compared to conventional lead-free solder alloys such as SnCu, SAC, and SnAg, this solder alloy can exhibit similar or improved thermo-mechanical reliability. Compared to conventional lead-free solder alloys such as SnCu, SAC, and SnAg, this solder alloy can exhibit similar or improved mechanical properties.

[0014] The solder alloy may exhibit favorable wetting characteristics, such as wetting characteristics that are substantially similar to or better than conventional solder alloys, such as SAC solder alloys and Sn0.7Cu solder alloys.

[0015] The solder comprises 1 to 9 wt.% copper. Preferably, the solder alloy comprises 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 comprises approximately 2.5 wt.% copper. Copper has limited solubility in tin. Sn-0.7% wt. Cu forms a eutectic phase. Any further addition of copper beyond 0.7 wt.% will increase the volume fraction of intermetallic compounds (e.g., Cu3Sn and Cu6Sn5). These intermetallic compounds can be used to increase the viscosity of the alloy at a given temperature. These intermetallic compounds can also enhance the mechanical and thermal reliability of the alloy.

[0016] The solder alloy optionally (and preferably) contains greater 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 0.01 to less than 0.02 wt.% nickel. Nickel does not form a solid solution with tin. However, it does form intermetallic compounds that can improve the alloy's mechanical properties and increase its viscosity at a given temperature. Nickel can also be used to reduce copper dissolution.

[0017] The solder alloy optionally (and preferably) contains greater than 0 to 10 wt.% germanium (e.g., 0.0005 to 10 wt.% germanium). Preferably, the alloy contains 0.0005 to 5 wt.% germanium, more preferably 0.001 to 4.5 wt.% germanium, even more preferably 0.001 to 0.005 wt.% germanium, and even more preferably 0.002 to less than 0.005 wt.% germanium. In one preferred embodiment, the alloy contains 0.02 to 0.08 wt.% germanium. In another preferred embodiment, the alloy contains 3.5 to 4.5 wt.% germanium. Germanium can act as a deoxidizer and can also improve wettability. Germanium also has good bonding properties with semiconductor grains. In one preferred embodiment, the alloy includes both nickel and germanium. Such an alloy can provide a favorable combination of high viscosity, low copper solubility, and good wettability.

[0018] The solder alloy optionally (and preferably) contains greater than 0 to 1 wt.% manganese (e.g., 0.001 to 1 wt.% manganese). Preferably, the alloy contains 0.005 to 0.5 wt.% manganese, more preferably 0.01 to 0.3 wt.% manganese, and even more preferably 0.05 to 0.1 wt.% manganese. Manganese has limited solubility in tin and copper. In the presence of tin alone, manganese forms intermetallic compounds such as, for example, Mn3Sn, Mn2Sn, and MnSn2. In the presence of both copper and tin, manganese can also form intermetallic compounds such as, for example, 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 enhance the mechanical and thermal reliability of the alloy.

[0019] The solder alloy optionally (and preferably) contains greater than 0 to 10 wt.% aluminum (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 one 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 act as a deoxidizer and may also improve wettability. Aluminum also has good bonding properties with semiconductor grains. The alloy preferably contains both germanium and aluminum. The solubility of both aluminum and germanium in tin is very limited, but aluminum and germanium have good mutual solubility and form a eutectic mixture in which germanium accounts for 28.4 atomic % (~51.6 wt.%).

[0020] The solder alloy optionally (and preferably) contains greater than 0 to 10 wt.% silicon (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 one 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 act as a deoxidizer and may also improve wettability.

[0021] The solder alloy optionally (and preferably) contains greater 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.%. In one 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 has some solubility in tin and can provide improved mechanical properties through solid solution strengthening. Bismuth can also improve creep resistance. Bismuth can also improve wetting and spreading.

[0022] The solder alloy optionally (and preferably) contains greater than 0 to 5 wt.% indium. Preferably, the solder alloy contains 0.2 to 3 wt.% indium, more preferably 1.5 to 2.5 wt.% indium, and even more preferably 1.8 to 2.2 wt.% indium. Indium has some solubility in tin, which provides an opportunity to improve mechanical properties through solid solution strengthening. Furthermore, indium can be used to lower the liquidus temperature of the solder alloy, allowing the cast alloy to reflow at a lower temperature than the base alloy. However, higher amounts of indium can reduce the solder alloy's beneficial properties, such as its mechanical strength, solderability, and long-term thermomechanical stability. Furthermore, higher amounts of indium can make the solder alloy unfavorably susceptible to oxidation.

[0023] The solder alloy optionally (and preferably) contains greater 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 has limited solubility in tin and can form intermetallic compounds, which can further reduce the fluidity of the solder melt during reflow. Titanium can improve strength and interfacial reactions. It can also improve drop impact performance by controlling copper diffusion at the substrate / solder interface.

[0024] The solder alloy may optionally (and preferably) contain greater 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. Lanthanum may improve spreading and wettability.

[0025] The solder alloy may optionally (and preferably) contain greater 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 may improve spreading and wettability.

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

[0027] The solder alloy may optionally include one or more of: 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., from 0.01 to 1 wt.% tellurium), up to 1 wt.% selenium (e.g., 0.01 to 1 wt.% selenium), up to 1 wt.% calcium (e.g., 0.01 to 1 wt.% calcium), up to 1 wt.% tantalum (e.g., 0.01 to 1 wt.% tantalum), wt.% vanadium (e.g., 0.01 to 1 wt.% vanadium), up to 1 wt.% molybdenum (e.g., 0.01 to 1 wt.% molybdenum), up to 1 wt.% platinum (e.g., 0.01 to 1 wt.% platinum), up to 1 wt.% magnesium (e.g., 0.01 to 1 wt.% magnesium), up to 5 wt.% silver (e.g., 0.01 to 5 wt.% silver), up to 1 wt.% zinc (e.g., 0.01 to 1 wt.% zinc), and up to 2 wt.% rare earth metals, excluding lanthanum and neodymium (e.g., 0.01 to 2 wt.% rare earth metals). Gallium and zinc have some solubility in tin, which may provide opportunities 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 potential deoxidizers and may also be used to improve wettability. Gold, chromium, iron, molybdenum, platinum, selenium, and tellurium can be used to improve strength and interfacial reactivity. Silver and rare earth elements can improve spreading and wettability. As used herein, the term rare earth element 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 can be used to provide more favorable mechanical properties and increase the viscosity of the solder alloy at a given temperature by forming intermetallic compounds.

[0028] The alloy will generally contain at least 80 wt.% tin, more generally at least 85 wt.% tin, still more generally at least 90 wt.% tin, and still more generally at least 95 wt.% tin. The alloy will generally contain up to 98 wt.% tin, more generally up to 97 wt.% tin.

[0029] It will be understood that the alloys described herein may contain unavoidable impurities, although these are unlikely to exceed 1 wt.% of the composition in total. Preferably, the solder alloy contains unavoidable impurities in an amount of no more than 0.5 wt.% of the composition, more preferably no more than 0.3 wt.% of the composition, even more preferably no more than 0.1 wt.% of the composition, even more preferably no more than 0.05 wt.% of the composition, and most preferably no more than 0.02 wt.% of the composition.

[0030] The solder alloys described herein may consist of the elements recited. Alternatively, the solder alloys described herein may consist essentially of the elements recited. It should be understood that, in addition to the mandatory elements (i.e., tin, copper, and at least one of nickel, germanium, manganese, aluminum, silicon, bismuth, indium, titanium, lanthanum, and neodymium), other non-specified elements may be present in the composition, provided that the basic properties of the composition are not significantly affected by their presence.

[0031] In a particularly preferred embodiment, the solder alloy comprises 2 to 3 wt.% copper, 0.005 to 0.02 wt.% nickel, 0.001 to 0.005 wt.% germanium, and the remainder tin plus any unavoidable impurities. Compared to conventional SAC305 and Sn0.7Cu solder alloys, this solder alloy exhibits minimal grain tilt between the first and fourth reflows, reduces the percentage of voids between the first and fourth reflows, increases grain shear strength, and improves thermal cycling performance.

[0032] In a particularly preferred embodiment, the solder alloy consists of 1 to 7 wt.% copper, 0.1 to 1 wt.% nickel, 0.05 to 0.5 wt.% manganese, and 0.05 to 0.1 wt.% phosphorus, with the remainder being tin and unavoidable impurities. This solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, this solder alloy exhibits minimal grain tilt between the first and fourth reflows, reduces the void percentage between the first and fourth reflows, increases grain shear strength, and improves thermal cycling performance.

[0033] In a particularly preferred embodiment, the solder alloy consists of 3 to 7 wt.% copper, 0.1 to 1 wt.% aluminum, 0.1 to 1 wt.% germanium, 0.05 to 0.1 wt.% nickel, 2 to 5 wt.% bismuth, and the remainder tin and unavoidable impurities. This solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, this solder alloy exhibits minimal grain tilt between the first and fourth reflows, reduces the void percentage between the first and fourth reflows, increases grain shear strength, and improves thermal cycling performance.

[0034] In a particularly preferred embodiment, the solder alloy consists of 4 to 7 wt.% copper, 0.1 to 2 wt.% indium, 0.1 to 0.5 wt.% titanium, 0.1 to 0.5 wt.% lanthanum, 0.1 to 0.2 wt.% nickel, and the remainder being tin and unavoidable impurities. This solder alloy advantageously has a melting peak at 232°C. Compared to conventional SAC305 and Sn0.7Cu solder alloys, this solder alloy exhibits minimal grain tilt between the first and fourth reflows, reduces the void percentage between the first and fourth reflows, increases grain shear strength, and improves thermal cycling performance.

[0035] The solder alloy may be in the form of a bar, rod, solid wire with or without flux, foil or ribbon, film, preform, powder, or paste (powder plus flux blend), or solder spheres for ball grid array contacts, or preformed solder sheets, or reflowed or cured solder dots, or pre-applied on any solderable material, such as copper ribbon for photovoltaic applications.

[0036] In a further aspect, the present disclosure provides a solder joint comprising the solder alloy described herein.

[0037] In a further aspect, the present invention provides a solder paste comprising: The solder alloys described herein, and A flux.

[0038] In a further aspect, the present invention provides a method of forming a solder joint, comprising: (i) providing two or more workpieces to be joined; (ii) providing a solder alloy as described herein; and (iii) heating the solder alloy adjacent the workpieces to be joined.

[0039] The workpieces may be components of a printed circuit board, such as substrates and dies.

[0040] In a further aspect, the present invention provides a use of the solder alloy 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 aspect, the present invention provides a solder alloy or solder joint comprising: (a) 1 to 9 wt.% copper (b) one or more of the following 0 to 1 wt.% nickel 0 to 1 wt.% manganese (c) optionally one or more of the following elements Up to 1 wt.% titanium Up to 2 wt.% of one or more rare earths, cerium, lanthanum, neodymium Up to 1 wt.% chromium Up to 10 wt.% germanium Up to 1 wt.% gallium Up to 1 wt.% cobalt Up to 1 wt.% iron Up to 10 wt.% aluminum Up to 1 wt.% phosphorus Up to 1 wt.% gold Up to 1 wt.% tellurium Up to 1 wt.% selenium Up to 1 wt.% calcium Up to 1 wt.% vanadium Up to 1 wt.% molybdenum Up to 1 wt.% platinum Up to 1 wt.% magnesium Up to 10 wt.% silicon Up to 9 wt.% bismuth Up to 5 wt% silver Up to 5 wt.% indium (d) The remainder is tin and unavoidable impurities.

[0042] The advantages and preferred features of the first aspect also apply to this aspect.

[0043] In a further aspect, the present invention provides a solder alloy or solder joint comprising: (a) 1 to 9 wt.% copper (b) one or more of the following 0 to 10 wt.% aluminum 0 to 10 wt.% germanium 0 to 10 wt.% silicon 0 to 9 wt.% bismuth (c) optionally one or more of the following elements Up to 1 wt.% nickel Up to 1 wt.% titanium Up to 2 wt.% of rare earth elements, such as cerium, lanthanum, neodymium Up to 1 wt.% chromium Up to 1 wt.% manganese Up to 1 wt.% gallium Up to 1 wt.% cobalt Up to 1 wt.% iron Up to 1 wt.% phosphorus Up to 1 wt.% gold Up to 1 wt.% tellurium Up to 1 wt.% selenium Up to 1 wt.% calcium Up to 1 wt.% vanadium Up to 1 wt.% molybdenum Up to 1 wt.% platinum Up to 1 wt.% magnesium Up to 5 wt% silver Up to 5 wt.% indium (d) The remainder is tin and unavoidable impurities.

[0044] The advantages and preferred features of the first aspect also apply to this aspect.

[0045] In a further aspect, the present invention provides a solder alloy or solder joint comprising: (a) 1 to 9 wt.% or less copper (b) one or more of the following 0 to 1 wt.% nickel 0 to 5 wt.% indium 0 to 1 wt.% titanium 0 to 2 wt.% lanthanum 0 to 2 wt.% neodymium (c) optionally one or more of the following elements Up to 2 wt.% of rare earth elements, cerium Up to 1 wt.% chromium Up to 1 wt.% manganese Up to 1 wt.% gallium Up to 1 wt.% cobalt Up to 1 wt.% iron Up to 1 wt.% phosphorus Up to 1 wt.% gold Up to 1 wt.% tellurium Up to 1 wt.% selenium Up to 1 wt.% calcium Up to 1 wt.% vanadium Up to 1 wt.% molybdenum Up to 1 wt.% platinum Up to 1 wt.% magnesium Up to 10 wt.% silicon Up to 10 wt.% germanium Up to 9 wt.% bismuth Up to 5 wt% silver Up to 10 wt.% aluminum (d) The remainder is tin and unavoidable impurities.

[0046] The advantages and preferred features of the first aspect also apply to this aspect. Simple diagram description

[0047] The present invention will now be further described by way of a summary of several non-limiting examples of these alloys and their properties with reference to the following figures, in which:

[0048] FIG1 shows the relationship between the grain tilt (µm) and the number of devices / reflow for conventional solder SAC305.

[0049] FIG. 2 shows a graph showing the relationship between grain tilt (µm) and device # / reflow for the SnCuNiGe solder alloy of Example 3.

[0050] FIG. 3 shows the relationship between BLT (µm) and device number / reflow for conventional solder SAC305.

[0051] FIG. 4 shows a graph of BLT (µm) versus device # / reflow for the SnCuNiGe solder alloy of Example 3.

[0052] FIG. 5 shows a graph showing changes in void percentage (%) between the first reflow and the fourth reflow for conventional solder SAC305 (left) and the SnCuNiGe solder alloy of Example 3 (right).

[0053] 6 shows box plots 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 FR4 board.

[0054] 7 shows box plots 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 plate. Implementation Method

[0055] Example 1

[0056] An alloy with the following composition was prepared: Alloy ID – HC1: Cu: 1 to 7 wt.%, Ni: 0.1 to 1 wt.%, Mn: 0.05 to 0.5 wt.%, P: 0.05 to 0.1 wt.%, The rest: Sn and unavoidable impurities.

[0057] This solder alloy advantageously has a melting peak at 232°C. Alloy ID – HC2: Cu: 3 to 7 wt.%, Al: 0.1 to 1 wt.%, Ge:0.1 to 1 wt.%, Ni: 0.05 to 0.1 wt.%, Bi:2 to 5 wt.%, The rest: Sn and unavoidable impurities.

[0058] This solder alloy advantageously has a melting peak at 232°C. Alloy ID – HC3: Cu: 4 to 7 wt.%, In: 0.1 to 2 wt.%, Ti: 0.1 to 0.5 wt.%, La: 0.1 to 0.5 wt.%, Ni: 0.1 to 0.2 wt.%, The rest: Sn and unavoidable impurities.

[0059] This solder alloy advantageously has a melting peak at 232°C. Example 2

[0060] Alloys having the compositions listed in Table 1 below were prepared. [Alloy ID] [Sn] [Cu] [Co] [Ni] [Ti] [Al] [Si] [Ge] [P] [Mn] [In] [Bi] [Ce] HC4 the remaining 1 0.1 0.05 0.005 HC5 the remaining 3 0.1 0.05 0.05 0.005 HC6 the remaining 5 0.1 0.05 0.1 HC7 the remaining 7 0.1 0.05 0.1 HC8 the remaining 5 0.1 0.1 0.05 HC9 the remaining 7 0.1 0.1 0.05 0.01 HC10 the remaining 3 0.1 0.1 0.05 0.005 2 HC11 the remaining 5 0.1 0.1 0.05 0.005 2 HC12 the remaining 7 0.1 0.1 0.05 0.005 2 HC13 the remaining 3 0.1 0.1 0.05 2 HC14 the remaining 5 0.1 0.1 0.05 2 HC15 the remaining 7 0.1 0.1 0.05 2 HC16 the remaining 3 0.1 0.05 0.005 0.05 HC17 the remaining 5 0.1 0.05 0.005 0.05 HC18 the remaining 7 0.1 0.05 0.005 0.05 HC19 the remaining 3 0.1 0.05 0.01 0.005 HC20 the remaining 3 0.05 0.05 0.01 0.005 HC21 the remaining 5 0.05 0.05 0.01 0.005 HC22 the remaining 7 0.05 0.05 0.01 0.005 HC23 the remaining 4 0.05 0.01 0.005 HC24 the remaining 5 0.05 0.01 0.005 HC25 the remaining 7 0.05 0.01 0.005 HC26 the remaining 3 0.05 0.05 0.05 0.1 HC27 the remaining 5 0.05 0.05 0.05 0.1 HC28 the remaining 7 0.05 0.05 0.05 0.1 HC29 the remaining 5 0.1 0.05 0.05 HC30 the remaining 7 0.1 0.05 0.05 HC31 the remaining 6 0.1 0.05 0.005 HC32 the remaining 3 0.1 0.05 0.01 0.1 HC33 the remaining 4 0.1 0.05 0.01 0.1 HC34 the remaining 7 0.1 0.05 0.01 0.1 HC35 the remaining 3 0.1 0.05 0.01 HC36 the remaining 5 0.1 0.05 0.01 HC37 the remaining 7 0.1 0.05 0.01 HC38 the remaining 3 0.1 0.05 0.01 2 HC39 the remaining 5 0.1 0.05 0.01 2 HC40 the remaining 7 0.1 0.05 0.01 2 HC41 the remaining 3 0.1 0.005 2 0.1 HC42 the remaining 5 0.1 0.005 2 0.1 HC43 the remaining 7 0.1 0.005 2 0.1 HC44 the remaining 3 0.05 0.01 0.05 HC45 the remaining 5 0.05 0.01 0.05 HC46 the remaining 7 0.05 0.01 0.05 HC47 the remaining 3 0.05 0.05 1 5 0.1 HC48 the remaining 5 0.05 0.05 1 5 0.1 HC49 the remaining 7 0.05 0.05 1 5 0.1 HC50 the remaining 3 0.05 4 4 HC51 the remaining 5 0.05 0.05 4 4 HC52 the remaining 7 0.05 0.05 4 4 HC53 the remaining 3 0.1 0.005 4 HC54 the remaining 5 0.1 0.005 4 HC55 the remaining 7 0.1 0.005 4 HC56 the remaining 3 0.1 0.05 0.05 0.005 4 HC57 the remaining 5 0.1 0.05 0.05 0.005 4 HC58 the remaining 7 0.1 0.05 0.05 0.005 2 0.1 HC59 the remaining 5 0.1 0.05 0.05 0.05 0.005 2 0.1 HC60 the remaining 7 0.1 0.05 0.05 0.05 0.005 2 0.1 Table 1: Composition of solder alloys.

[0061] Between 225 and 280°C, the liquid fraction of the alloys in Table 1 is equal to or higher than 50%. The liquid fraction is theoretically calculated as liquid fraction (%) = 100 - solid fraction (%). Example 3

[0062] An alloy having the following composition was prepared: Cu:2.5 wt.% Ni:<0.05 wt.% Ge:<0.005 wt.% The rest: Sn and unavoidable impurities. Grain tilt analysis:

[0063] Grain tilt analysis was performed to compare the alloys described above with the conventional SAC305 solder alloy. The results are shown in Figures 1 and 2. SnCuNiGe exhibited minimal change in grain tilt between the first reflow (circles) and the fourth reflow (triangles), while SAC305 showed a significant change. Void Analysis:

[0064] Void analysis was conducted to compare the alloys described above with 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 exhibited the smallest 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 for SnCuNiGe was less pronounced than for SAC305. Shear and thermal testing:

[0065] The following tests and conditions were used: – Mid-power Lumileds 3535L LEDs were selected to directly compare the optical, thermal, and shear performance of the solder pastes – Design identical Al-MCPCB and FR4-PCB boards for the selected LEDs, with contact pads with ENIG coating – Shear test: 1833 thermal cycles

[0066] As can be seen from Figures 6 and 7, among both MCPCB and FR4 assemblies, SnCuNiGe shows the most stable shear strength value as the thermal cycle increases.

[0067] The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments described herein will be apparent to those skilled in the art and still fall within the scope of the appended claims and their equivalents.

[0068] none

Claims

1. A lead-free solder alloy comprising: 2 to 3 wt.% copper, 0.005 to 0.02 wt.% nickel, 0.001 to 0.005 wt.% germanium, and the remainder being tin and any unavoidable impurities.

2. The solder alloy of claim 1, wherein the copper content is 2.5 to 3 wt.%.

3. The solder alloy of claim 1, wherein the nickel content is 0.01 to 0.02 wt.%.

4. The solder alloy of claim 1, wherein the germanium content is 0.002 to 0.005 wt.%.

5. The solder alloy of any of the claims 1 to 4 is in the form of a rod, bar, solid wire, foil or strip with or without flux, film, preform, powder or paste, or solder ball for ball grid array contacts, or preformed solder sheet, or reflow or cured solder dot, or pre-coated on any solderable material.

6. A solder joint comprising a solder alloy as claimed in any one of claims 1 to 5.

7. A method of forming a solder joint, comprising: (i) providing two or more workpieces to be joined; (ii) providing a solder alloy as defined in any one of claims 1 to 5; and (iii) heating the solder alloy in the vicinity of the workpieces to be joined.

8. Use of a solder alloy as claimed in any one of claims 1 to 5 in a soldering method, wherein the soldering method is selected from surface mount technology (SMT) soldering, grain attachment soldering, thermal interface soldering, manual soldering, laser and RF induction soldering, or heavy-duty soldering.

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