Mid temperature alloys for solder hierarchy

Abstract

A lead-free solder alloy comprising: from 0.5 to 4 wt.% silver, from 0.5 to 12 wt.% bismuth, from 0.1 to 1 wt.% copper, from 0.005 to 0.2 wt.% nickel, from 12 to 22 wt.% indium, optionally one or more of: from greater than 0 to 0.02 wt.% germanium, from greater than 0 to 0.03 wt.% titanium, from greater than 0 to 5.5 wt.% antimony, from greater than 0 to 1 wt.% manganese, from greater than 0 to 1 wt.% cobalt, from greater than 0 to 1 wt.% rare earths, from greater than 0 to 1 wt.% aluminium, from greater than 0 to 1 wt.% silicon, from greater than 0 to 1 wt.% chromium, from greater than 0 to 1 wt.% iron, from greater than 0 to 1 wt.% zinc, from greater than 0 to 1 wt.% gallium, and the balance tin, together with any unavoidable impurities.

Description

[0001] Mid Temperature Alloys For Solder Hierarchy

[0002] The present invention relates generally to the field of metallurgy, and more particularly to a solder alloy. The solder alloy is particularly, though not exclusively, suitable for use in electronic soldering applications such as wave soldering, surface mounting technology, hot air leveling and ball grid arrays, land grid arrays, bottom terminated packages, LEDs and chip scale packages.

[0003] Lead-free solders were initially developed due to environmental and health concerns, and as replacements for conventional soft solder alloys. Many conventional lead-free solder alloys are based around the Sn-0.7 wt.% Cu eutectic composition. The tin-silver-copper system has also been embraced by the electronics industry as a lead-free alternative for soldering materials. For example, the near-eutectic 96.5Sn3.0Ag0.5Cu, exhibits superior fatigue life compared to the eutectic Sn-Pb solder, while having melting point in the range of about 217 to 220 °C.

[0004] As use of lead-free soldering materials becomes widespread, either due to environmental directives or pressure from the end users, so does the range of applications for such materials. In some fields, such as automotive, high-power electronics and energy, including LED lighting for example, it is desirable for solder alloys to operate at higher temperatures, for example at 150 °C or higher, for a relatively longer time. However, the 96.5Sn3.0Ag0.5Cu alloy does not perform well at such temperatures.

[0005] As electronic devices become more complicated, their manufacture may involve more complicated soldering hierarchies, each soldering step of which needs a different peak reflow temperature, and therefore a different solder alloy liquidus temperature. There is therefore a need for a range of lead-free soldering alloys having different liquidus temperatures. In particular, there is a need for lead-free soldering alloys having liquidus temperatures lower than that of 96.5Sn3.0Ag0.5Cu, for example liquidus temperature of around 190 °C or around 165 °C. Furthermore, there is a need for such solder alloys to exhibit a combination of good reliability at high operating temperatures and favourable mechanical properties.

[0006] SnBi and Snln solder alloys are described in W02020047481A1. The example alloys described in this document each have a liquidus temperature below 200 °C. However, such example alloys, particularly ones with liquidus temperatures of around 190 °C, exhibit large solidus-liquidus temperature gaps, typically 30 °C or more. A large solidus-liquidus temperature gap may lead to soldering defects due to the mushy zone formed before the complete solidification of the solder.

[0007] The present invention aims to solve at least some of the problems associated with the prior art or to provide a commercially acceptable alternative.

[0008] Accordingly, in a first aspect, the present invention provides a lead-free solder alloy comprising: from 0.5 to 4 wt.% silver, from 0.5 to 12 wt.% bismuth, from 0.1 to 1 wt.% copper, from 0.005 to 0.2 wt.% nickel, from 12 to 22 wt.% indium, optionally one or more of: from greater than 0 to 0.02 wt.% germanium, from greater than 0 to 0.03 wt.% titanium, from greater than 0 to 5.5 wt.% antimony, from greater than 0 to 1 wt.% manganese, from greater than 0 to 1 wt.% cobalt, from greater than 0 to 1 wt.% rare earths, from greater than 0 to 1 wt.% aluminium, from greater than 0 to 1 wt.% silicon, from greater than 0 to 1 wt.% chromium, from greater than 0 to 1 wt.% iron, from greater than 0 to 1 wt.% zinc, from greater than 0 to 1 wt.% gallium, and the balance tin, together with any unavoidable impurities.

[0009] The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0010] The term “solder alloy” used herein encompasses a fusible metal alloy with a melting point in the range of from 90 to 400 °C. The alloys are lead-free meaning that no lead is added intentionally. Thus, the lead content is zero or at no more than accidental impurity levels.

[0011] The solder alloy may exhibit improved high temperature reliability and may be capable of withstanding maximum operational peak temperatures of typically at least 110 °C or at least 120 °C or even at least 130 °C or even at least 140 °C. The solder alloy may exhibit improved mechanical properties and high temperature creep resistance compared to the conventional 96.5SnAg3.0Cu0.5 alloy.

[0012] The solder alloy may exhibit liquidus and solidus temperatures suitable for forming solder joints in a mid-temperature soldering method that are capable of withstanding high operating temperatures.

[0013] The solder alloy may be suitable for forming a high-quality solder joint after reflow at a peak temperature of 210 °C or less, typically at a peak temperature of from greater than 190 °C to 210 °C, more typically at a peak temperature of from 195 °C to 205 °C. The solder alloy may have a liquidus temperature above 175 °C and below 200 °C. The solder alloy preferably has a liquidus temperature of greater than 180 °C, more preferably greater than 183 °C, even more preferably greater than or equal to 187 °C. Such a liquidus temperature may enable the alloy to be used in a midtemperature soldering process. The solder alloy preferably has a liquidus temperature of less than 200 °C, more preferably less than 197 °C , even more preferably less than 195 °C. Such liquidus temperatures may be advantageous because peak reflow temperatures are typically 25 to 30 °C above the liquidus temperature, and reflow temperatures higher than about 260 °C may lead to various issues during soldering, such as damaging printed circuit boards and components.

[0014] The solder alloy may have a solidus temperature of greater than 160 °C. This may improve the high-temperature reliability of a solder joint formed of the solder alloy. The solder alloy preferably has a solidus temperature of greater than 165 °C, more preferably of 169 °C or greater.

[0015] Advantageously, the solder alloy may exhibit a single melting peak. Multiple peaks during melting may indicate the formation of different phases, which may adversely affect the reliability of a solder joint formed of the solder alloy.

[0016] The solder alloy may exhibit favourable mechanical properties and favourable solderability. The solder alloy may exhibit superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times. The solder alloys may exhibit superior thermal cycling and / or thermal shock performance in harsh environmental conditions such as, for example, from -40 to 125 °C, with 15 minutes dwell time at each temperature.

[0017] The alloying additions are used to modify the alloy microstructure and, consequently, its properties due to physical metallurgy mechanisms such as precipitation strengthening, solid solution strengthening, grain refinement and diffusion control.

[0018] Advantageously, the mechanical properties of the solder alloy, such as hardness, tensile strength and high temperature creep, may be superior to that of 96.5SnAg3.0Cu0.5. Bismuth, antimony and indium, for example, affect the solidus and / or liquidus temperatures of the solder alloy. These elements also have high solid solubility in tin and thus may contribute significantly towards solid solution strengthening of the matrix. The changes in solidus or liquidus temperatures do not appear to affect adversely the mechanical properties of the alloys.

[0019] Diffusion-dependent creep deformation depends on the homologous temperature, i.e. ratio of the test temperature to melting temperature of the material in absolute scale. The homologous temperature of the solder alloy may be less than 0.90. The melting temperature of the solder alloy therefore has no significant effect on the mechanical properties.

[0020] An optimum combination of solid solution and precipitation strengthening may result in a distributed network of precipitate particles in a strong matrix. Precipitate particles may include, for example, AgsSn and (Cu,Ni)eSn5. The precipitate network resists the movement of grain boundaries during creep deformation, thus enhancing the creep strength.

[0021] A significant proportion of the cost of a tin alloy relates to the cost of the alloying elements. For example, incorporating large amounts of alloying elements that are higher cost than tin typically results in a relatively high-cost alloy. Based on the typical prices of alloying elements, the solder alloys of the present invention may exhibit the advantages discussed above without being significantly more expensive than conventional tin alloys, or even being lower cost. The solder alloy comprises from 0.5 to 4 wt.% silver. The solder alloy preferably comprises from 1 to 3.8 wt.% silver, more preferably from 1 .5 to 3.5 wt.% silver, even more preferably from 1.8 to 3.2 wt.% silver, still even more preferably from 1 .9 to 3.1 wt.% silver. The solder alloy preferably comprises from 2.5 to 3.5 wt.% silver, more preferably from 2.8 to 3.2 wt.% silver, even more preferably from 2.9 to 3.1 wt.% silver, still even more preferably about 3 wt.% silver. The solder alloy preferably comprises from 1 .5 to 2.5 wt.% silver, more preferably from 1.8 to 2.2 wt.% silver, even more preferably from 1.9 to 2.1 wt.% silver, still even more preferably about 2 wt.% silver. The presence of silver in the specified amount may serve to improve mechanical properties, for example strength, through the formation of network-like intermetallic compounds such as, for example, AgsSn.

[0022] In addition, the presence of silver may improve wetting and spread. Higher levels of silver, in particular levels higher than 4 wt.% silver, may increase the liquidus temperature and larger precipitates of AgsSn formed in the solder matrix act as sites of crack initiation and subsequent failure. Lower contents of silver may not form enough AgsSn precipitates that may be helpful in improving the strength.

[0023] The solder alloy comprises from 0.5 to 12 wt.% bismuth. The solder alloy preferably comprises from 1 to 4 wt.% bismuth, more preferably from 1 .5 to 3.8 wt.% bismuth, even more preferably from 2 to 3.7 wt.% bismuth, still even more preferably from 2.5 to 3.5 wt.% bismuth, still even more preferably from 2.8 to 3.2 wt.% bismuth, still even more preferably from 2.9 to 3.1 wt.% bismuth, still even more preferably about 3 wt.% bismuth. The presence of bismuth in the specified amount may serve to improve mechanical properties through solid solution strengthening. Bismuth may also act to improve creep resistance. Bismuth may also improve wetting and spread. However, bismuth addition in excess of the specified amount may result in precipitation of bismuth in tin resulting in a more brittle alloy.

[0024] The solder alloy comprises from 0.1 to 1 wt.% copper. The solder alloy preferably from comprises from 0.3 to 0.9 wt.% copper, more preferably from 0.4 to 0.8 wt.% copper, even more preferably from 0.5 to 0.8 wt.% copper. The presence of copper in the specified amount may serve to improve mechanical properties, for example strength, through the formation of Cu-Sn intermetallic compounds such as, for example, CueSns. Copper addition in the specified range results in the optimum quantity of intermetallic compound precipitates required for strengthening the alloy.

[0025] The solder alloy comprises from 0.005 to 0.2 wt.% nickel. The solder alloy preferably comprises from 0.01 to 0.18 wt.% nickel, more preferably from 0.05 to 0.15 wt.% nickel, even more preferably from 0.09 to 0.13 wt.% nickel. The presence of nickel in the specified amount may serve to improve mechanical properties through the formation of intermetallic compounds with tin and copper, which can result in precipitation strengthening. In this regard, the solder alloy preferably comprises both copper and nickel. In addition, the presence of nickel may act to reduce the copper dissolution rate. Nickel may also increase thermal reliability by decreasing IMC growth at the substrate / solder interface.

[0026] The solder alloy comprises from 12 to 22 wt.% indium. The solder alloy preferably comprises from 13 to 22 wt.% indium, more preferably from 15 to 21 .5 wt.% indium, even more preferably from 17 to 21 wt.% indium, still even more preferably from 17.5 to 20.5 wt.% indium. The solder alloy preferably comprises from 17 to 19 wt.% indium, more preferably from 17.5 to 18.5 wt.% indium. The solder alloy preferably comprises from 19 to 21 wt.% indium, more preferably from 19.5 to 20.5 wt.% indium. The presence of indium in the specified amount may act to improve mechanical properties through solid solution and or precipitate strengthening. Addition of indium may also decrease the solidus and liquidus temperatures, with a greater effect at reducing the solidus temperature. Higher levels of indium may result in the formation of low temperature phases that will adversely affect the long-term reliability of the alloys.

[0027] The solder alloy may comprise germanium in an amount of from greater than 0 to 0.02 wt.% germanium. The solder alloy preferably comprises germanium. The solder alloy preferably comprises from 0.001 to 0.015 wt.% germanium, even more preferably from 0.002 to 0.010 wt.% germanium, still even more preferably from 0.003 to 0.009 wt.% germanium. The presence of germanium may act to improve mechanical properties through particle dispersion. Germanium may also help in deoxidation and may also improve wettability and solder joint strength and appearance. In addition, germanium in combination with nickel and / or titanium, may improve the thermo-mechanical fatigue properties.

[0028] The solder alloy may comprise titanium in an amount of from greater than 0 to 0.03 wt.% titanium. The solder alloy preferably comprises titanium. The solder alloy preferably comprises from 0.001 to 0.029 wt.% titanium, more preferably from 0.010 to 0.027 wt.% titanium, even more preferably from 0.015 to 0.025 wt.% titanium, still even more preferably from 0.020 to 0.024 wt.% titanium. Titanium has very limited solubility in Sn. This makes it difficult to incorporate titanium in the alloy. During manufacture, there is always some loss of titanium through the dross. As a result, one is required to add additional titanium in order to achieve the desired levels in the alloy, thereby increasing the cost and wastage of the manufacturing process. Accordingly, manufacturers of tin solder alloys have thus far been reluctant to incorporate titanium into tin solder alloys. However, the inventors have surprisingly found that the presence of titanium in the recited amounts, in combination with the other elements, may result in particularly pronounced improvements in strength and thermo-mechanical reliability. This may outweigh the difficulties in manufacturing.

[0029] The solder alloy preferably comprises one or both of titanium and germanium, more preferably both. Surprisingly, the presence of both titanium and germanium may result in a synergistic effect of improving the thermo-mechanical reliability.

[0030] The solder alloy may comprise antimony in an amount of from greater than 0 to 5.5 wt.% antimony. The solder alloy preferably comprises antimony. The solder alloy preferably comprises from 0.001 to 2 wt.% antimony, more preferably from greater than 0.001 to 2 wt.% antimony. The presence of antimony in the specified amount may serve to improve mechanical properties through solid solution strengthening. Antimony may also act to improve creep resistance and thermal fatigue resistance. Antimony may also increase the liquidus temperature of the alloy. Antimony addition lower than the specified range may not have the required improvement of mechanical strength and thermal fatigue resistance. Antimony additions higher than the specified range may increase the liquidus temperature such that the prescribed reflow temperature also increases. Reflow temperatures higher than 260 °C can lead to various issues during soldering, such as damaging printed circuit boards and components.

[0031] The solder alloy may comprise manganese in an amount of from greater than 0 to 1 wt.% manganese. The solder alloy preferably comprises manganese. The solder alloy preferably comprises from greater than 0.001 to 1 wt.% manganese. The presence of manganese in the recited amount may act to improve strength and interfacial reactions, for example by intermetallic compound formation.

[0032] The solder alloy may comprise cobalt in an amount of from greater than 0 to 1 wt.% cobalt. The solder alloy preferably comprises cobalt. The solder alloy preferably comprises from greater than 0.001 to 1 wt.% cobalt. The presence of cobalt in the specified amounts may improve the strength and / or high temperature properties of the solder.

[0033] The solder alloy may comprise rare earths in an amount of from greater than 0 to 1 wt.% rare earths. The solder alloy preferably comprises rare earths. The solder alloy preferably comprises from 0.001 to 1 wt.% rare earths. The term rare earth element as used herein 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. Preferred rare earths include cerium, neodymium and lanthanum. The presence of rare earths may act to improve mechanical properties by particle dispersion and / or microstructure modification. Rare earths may act to improve spread and wettability. Cerium has been found to be particularly effective in this regard. The solder alloy preferably comprises from 0.001 to 1 wt.% cerium, more preferably from 0.01 to 0.5 wt.% cerium. The solder alloy may comprise aluminium in an amount of from greater than 0 to 1 wt.% aluminium. The solder alloy preferably comprises aluminium. The solder alloy preferably comprises from 0.001 to 1 wt.% aluminium. The presence of aluminum in the recited amounts may improve fatigue life of the solder. Aluminium may act as a deoxidizer and may also improve wettability and solder joint strength.

[0034] The solder alloy may comprise silicon in an amount of from greater than 0 to 1 wt.% silicon. The solder alloy preferably comprises silicon. The solder alloy preferably comprises from 0.001 to 1 wt.% silicon. The presence of silicon in the recited amounts may improve mechanical properties, fatigue life, and thermal and electrical conductivity of the solder.

[0035] The solder alloy may comprise chromium in an amount of from greater than 0 to 1 wt.% chromium. The solder alloy preferably comprises chromium. The solder alloy preferably comprises from 0.001 to 1 wt.% chromium. The presence of chromium in the recited amounts may act to improve strength and interfacial reactions.

[0036] The solder alloy may comprise iron in an amount of from greater than 0 to 1 wt.% iron. The solder alloy preferably comprises iron. The solder alloy preferably comprises from 0.001 to 1 wt.% iron. The presence of iron in the recited amounts may act to improve strength and interfacial reactions.

[0037] The solder alloy may comprise zinc in an amount of from greater than 0 to 1 wt.% zinc. The solder alloy preferably comprises zinc. The solder alloy preferably comprises from 00.01 to 1 wt.% zinc. The presence of zinc in the recited amounts may reduce the melting point, improve the wettability and refine the microstructure. Zinc is also relatively low cost. The solder alloy may comprise gallium in an amount of from greater than 0 to 1 wt.% gallium. The solder alloy preferably comprises gallium. The solder alloy preferably comprises from 0.001 to 1 wt.% gallium. Gallium may act as a deoxidizer and may also improve wettability and solder joint strength.

[0038] In a preferred embodiment, the solder alloy comprises: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, one or both of: from 0.001 to 0.01 wt.% germanium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

[0039] In a particularly preferred embodiment, the solder alloy consists of: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

[0040] In a particularly preferred embodiment, the solder alloy consists of: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, and the balance tin, together with any unavoidable impurities.

[0041] In a particularly preferred embodiment, the solder alloy consists of: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

[0042] The solder alloy preferably has a liquidus temperature of from 180 °C to less than 200 °C, more preferably from 185 °C to 195 °C, even more preferably from 187 to 193 °C.

[0043] The solder alloy preferably has a solidus temperature of greater than 150 °C, preferably greater than 160 °C, even more preferably of 165 °C or greater, even more preferably of 169 °C or greater.

[0044] The solder alloy preferably has a solidus-liquidus temperature gap of less than 25 °C, preferably less than or equal to 20 °C, more preferably less than or equal to 17 °C. A too large solidus-liquidus temperature gap may lead to soldering defects due to the mushy zone formed before the complete solidification of the solder. A small solidus-liquidus gap may result in a more homogeneous microstructure and thereby better reliability of an electronic device containing solder joints formed of the solder alloy.

[0045] In a preferred embodiment, the solder alloy consists of: from 2.5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.05 to 0.15 wt.% nickel, from 17 to 19 wt.% indium, from 0.001 to 0.01 wt.% germanium, the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.08 to 0.12 wt.% nickel, from 17.5 to 18.5 wt.% indium, from 0.003 to 0.009 wt.% germanium, the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of about 3 wt.% silver, from 2.8 to 3.2 wt.% bismuth, about 0.7 wt.% copper, about 0.1 wt.% nickel, about 18 wt.% indium, about 0.006 wt.% germanium, the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0046] In a preferred embodiment, the solder alloy consists of: from 2.5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.01 to 0.1 wt.% nickel, from 19 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 19.5 to 20.5 wt.% indium, from 0.003 to 0.009 wt.% germanium, the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of about 3 wt.% silver, about 3 wt.% bismuth, about 0.7 wt.% copper, about 0.05 wt.% nickel, about 20 wt.% indium, about 0.006 wt.% germanium, the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times. In a preferred embodiment, the solder alloy consists of: from 1 .5 to 2.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.5 to 0.18 wt.% nickel, from 19 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, from 0.014 to 0.028 wt.% titanium, and the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 1 .8 to 2.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.10 to 0.14 wt.% nickel, from 19.5 to 20.5 wt.% indium, from 0.003 to 0.009 wt.% germanium, from 0.019 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of about 2 wt.% silver, about 3 wt.% bismuth, about 0.7 wt.% copper, about 0.12 wt.% nickel, about 20 wt.% indium, about 0.006 wt.% germanium, about 0.022 wt.% titanium, and the balance tin, together with any unavoidable impurities.

[0047] Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermomechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0048] In a preferred embodiment, the solder alloy consists of: from 1 .5 to 2.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.3 to 0.7 wt.% copper, from 0.01 to 0.1 wt.% nickel, from 17 to 19 wt.% indium, from 0.014 to 0.028 wt.% titanium, and the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 1 .8 to 2.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.4 to 0.6 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 17.5 to 18.5 wt.% indium, from 0.019 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of about 2 wt.% silver, about 3 wt.% bismuth, about 0.5 wt.% copper, about 0.05 wt.% nickel, about 18 wt.% indium, about 0.022 wt.% titanium, and the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0049] In a further aspect, the present invention provides a lead-free solder alloy comprising: from 0.5 to 4 wt.% silver, from 5 to 50 wt.% bismuth, from 0.1 to 1 wt.% copper, from 0.005 to 0.2 wt.% nickel, from 1 to 7 wt.% indium, one or more of: from greater than 0 to 0.02 wt.% germanium, from greater than 0 to 5.5 wt.% antimony, from greater than 0 to 0.03 wt.% titanium, from greater than 0 to 1 wt.% manganese, from greater than 0 to 1 wt.% cobalt, from greater than 0 to 1 wt.% rare earths, from greater than 0 to 1 wt.% aluminium, from greater than 0 to 1 wt.% silicon, optionally one or more of: from greater than 0 to 1 wt.% chromium, from greater than 0 to 1 wt.% iron, from greater than 0 to 1 wt.% zinc, from greater than 0 to 1 wt.% gallium, and the balance tin, together with any unavoidable impurities.

[0050] Such a solder alloy may exhibit similar advantages to the alloys of the first aspect. However, in comparison to the alloys of the first aspect, the alloy of the second aspect may have lower liquidus and solidus temperatures. This may make them useful in a solder hierarchy in combination with the solders of the first aspect and / or 96.5Sn3.0Ag0.5Cu. For example, the solder alloys of the second aspect may have a liquidus temperature of from 150 °C to less than 185 °C, typically from 155 °C to 180 °C, more typically from 160 to 175 °C, even more typically from 162 °C to 170 °C, and / or a solidus temperature of greater than 100 °C, typically greater than 110 °C, even more typically of 120 °C or greater, even more typically of 125 °C or greater.

[0051] The solder ally may comprise from 0.5 to 4 wt.% silver. The solder alloy preferably comprises from 1 to 3.5 wt.% silver, more preferably from 1 .2 to 3 wt.% silver, even more preferably from 1 .5 to 2.5 wt.% silver, still even more preferably from 1 .8 to 2.2 wt.% silver. The solder alloy preferably comprises from 1 .5 to 2.5 wt.% silver, more preferably from 1 .8 to 3.2 wt.% silver, even more preferably from 1 .9 to 3.1 wt.% silver. The presence of silver in the specified amount may serve to improve mechanical properties, for example strength, through the formation of network-like intermetallic compounds such as, for example, AgsSn. In addition, the presence of silver may improve wetting and spread. Higher levels of silver, in particular levels higher than 4 wt.% silver, may increase the liquidus temperature and larger precipitates of AgsSn formed in the solder matrix act as sites of crack initiation and subsequent failure. Lower contents of silver may not form enough AgsSn precipitates that may be helpful in improving the strength.

[0052] The solder ally comprises from 5 to 50 wt.% bismuth. The solder alloy preferably comprises from 8 to 48 wt.% bismuth, more preferably from 38 to 47 wt.% bismuth, even more preferably from 39 to 46 wt.% bismuth, still even more preferably from 39.5 to 45.5 wt.% bismuth. The solder alloy preferably comprises from 35 to 45 wt.% bismuth, more preferably from 38 to 42 wt.% bismuth, even more preferably from 39 to 41 wt.% bismuth. The solder alloy preferably comprises from 40 to 50 wt.% bismuth, more preferably from 43 to 47 wt.% bismuth, even more preferably from 44 to 46 wt.% bismuth. The presence of bismuth in the specified amount may serve to improve mechanical properties through solid solution strengthening. Bismuth may also act to improve creep resistance. Bismuth may also improve wetting and spread. However, bismuth addition in excess of the specified amount may result in precipitation of bismuth in tin resulting in a more brittle alloy.

[0053] The solder ally comprises from 0.1 to 1 wt.% copper. The solder alloy preferably comprises from 0.3 to 0.9 wt.% copper, more preferably from 0.4 to 0.8 wt.% copper, even more preferably from 0.5 to 0.8 wt.% copper. The presence of copper in the specified amount may serve to improve mechanical properties, for example strength, through the formation of Cu-Sn intermetallic compounds such as, for example, CueSns. Copper addition in the specified range results in the optimum quantity of intermetallic compound precipitates required for strengthening the alloy.

[0054] The solder alloy comprises from 0.005 to 0.2 wt.% nickel. The solder alloy preferably comprises from 0.01 to 0.1 wt.% nickel, more preferably from 0.02 to 0.08 wt.% nickel, even more preferably from 0.03 to 0.07 wt.% nickel. The presence of nickel in the specified amount may serve to improve mechanical properties through the formation of intermetallic compounds with tin and copper, which can result in precipitation strengthening. In this regard, the solder alloy preferably comprises both copper and nickel. In addition, the presence of nickel may act to reduce the copper dissolution rate. Nickel may also increase thermal reliability by decreasing IMC growth at the substrate / solder interface.

[0055] The solder alloy comprises from 1 to 7 wt.% indium. The solder alloy preferably comprises from 2 to 4 wt.% indium, more preferably from 2.5 to 3.5 wt.% indium, even more preferably from 2.8 to 3.2 wt.% indium. The presence of indium in the specified amount may act to improve mechanical properties through solid solution and or precipitate strengthening. Addition of indium may also decrease the solidus and liquidus temperatures, with a greater effect at reducing the solidus temperature. Higher levels of indium may result in the formation of low temperature phases that will adversely affect the long-term reliability of the alloys.

[0056] The solder alloy may comprise germanium in an amount of from greater than 0 to 0.02 wt.% germanium. The solder alloy preferably comprises germanium. The solder alloy preferably comprises from 0.001 to 0.015 wt.% germanium, more preferably from 0.002 to 0.010 wt.% germanium, even more preferably from 0.003 to 0.009 wt.% germanium. The presence of germanium may act to improve mechanical properties through particle dispersion. Germanium may also help in deoxidation and may also improve wettability and solder joint strength and appearance. In addition, germanium in combination with nickel and / or titanium, may improve the thermo-mechanical fatigue properties.

[0057] The solder alloy may comprise antimony in an amount of from greater than 0 to 5.5 wt.% antimony. The solder alloy preferably comprises antimony. The solder alloy preferably comprises from greater than 0 to 2 wt.% antimony, more preferably from 0.1 to 1 .9 wt.% antimony, even more preferably from 0.5 to 1 .5 wt.% antimony, still even more preferably from 0.8 to 1.2 wt.% antimony, still even more preferably from 0.9 to 1.1 wt.% antimony. The presence of antimony in the specified amount may serve to improve mechanical properties through solid solution strengthening. Antimony may also act to improve creep resistance and thermal fatigue resistance. Antimony may also increase the liquidus temperature of the alloy. Antimony addition lower than the specified range may not have the required improvement of mechanical strength and thermal fatigue resistance. Antimony additions higher than the specified range may increase the liquidus temperature such that the prescribed reflow temperature also increases. Reflow temperatures higher than 260 °C can lead to various issues during soldering, such as damaging printed circuit boards and components.

[0058] The solder alloy may comprise titanium in an amount of from greater than 0 to 0.02 wt.% titanium. The solder alloy preferably comprises titanium. The solder alloy preferably comprises from 0.001 to 0.029 wt.% titanium, preferably from 0.010 to 0.027 wt.% titanium, more preferably from 0.015 to 0.025 wt.% titanium, even more preferably from 0.020 to 0.024 wt.% titanium. Titanium has very limited solubility in Sn. This makes it difficult to incorporate titanium in the alloy. During manufacture, there is always some loss of titanium through the dross. As a result, one is required to add additional titanium in order to achieve the desired levels in the alloy, thereby increasing the cost and wastage of the manufacturing process. Accordingly, manufacturers of tin solder alloys have thus far been reluctant to incorporate titanium into tin solder alloys. However, the inventors have surprisingly found that the presence of titanium in the recited amounts, in combination with the other elements, may result in particularly pronounced improvements in strength and thermo-mechanical reliability. This may outweigh the difficulties in manufacturing.

[0059] The solder alloy may comprise manganese in an amount of from greater than 0 to 0.02 wt.% manganese. The solder alloy preferably comprises manganese. The solder alloy preferably comprises from 0.001 to 1 wt.% manganese. The presence of manganese in the recited amount may act to improve strength and interfacial reactions, for example by intermetallic compound formation.

[0060] The solder alloy may comprise cobalt in an amount of from greater than 0 to 0.02 wt.% cobalt. The solder alloy preferably comprises cobalt. The solder alloy preferably comprises from 0.001 to 1 wt.% cobalt. The presence of cobalt in the specified amounts may improve the strength and / or high temperature properties of the solder.

[0061] The solder alloy may comprise rare earths in an amount of from greater than 0 to 1 wt.% rare earths. The solder alloy preferably comprises rare earths. The solder alloy preferably comprises from 0.001 to 1 wt.% rare earths. The term rare earth element as used herein 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. Preferred rare earths include cerium, neodymium and lanthanum. The presence of rare earths may act to improve mechanical properties by particle dispersion and / or microstructure modification. Rare earths may act to improve spread and wettability. Cerium has been found to be particularly effective in this regard. The solder alloy preferably comprises from 0.001 to 1 wt.% cerium, more preferably from 0.01 to 0.5 wt.% cerium.

[0062] The solder alloy may comprise aluminium in an amount of from greater than 0 to 1 wt.% aluminium. The solder alloy preferably comprises aluminium. The solder alloy preferably comprises from 0.001 to 1 wt.% aluminium. The presence of aluminum in the recited amounts may improve fatigue life of the solder.

[0063] Aluminium may act as a deoxidizer and may also improve wettability and solder joint strength.

[0064] The solder alloy may comprise silicon in an amount of from greater than 0 to 1 wt.% silicon. The solder alloy preferably comprises silicon. The solder alloy preferably comprises from 0.001 to 1 wt.% silicon. The presence of silicon in the recited amounts may improve mechanical properties, fatigue life, and thermal and electrical conductivity of the solder.

[0065] The solder alloy may comprise chromium in an amount of from greater than 0 to 1 wt.% chromium. The solder alloy preferably comprises chromium. The solder alloy preferably comprises from 0.001 to 1 wt.% chromium. The presence of chromium in the recited amounts may act to improve strength and interfacial reactions.

[0066] The solder alloy may comprise iron in an amount of from greater than 0 to 1 wt.% iron. The solder alloy preferably comprises iron. The solder alloy preferably comprises from 0.001 to 1 wt.% iron. The presence of iron in the recited amounts may act to improve strength and interfacial reactions.

[0067] The solder alloy may comprise zinc in an amount of from greater than 0 to 1 wt.% zinc. The solder alloy preferably comprises zinc. The solder alloy preferably comprises from 00.01 to 1 wt.% zinc. The presence of zinc in the recited amounts may reduce the melting point, improve the wettability and refine the microstructure. Zinc is also relatively low cost.

[0068] The solder alloy may comprise gallium in an amount of from greater than 0 to 1 wt.% gallium. The solder alloy preferably comprises gallium. The solder alloy preferably comprises from 0.001 to 1 wt.% gallium. Gallium may act as a deoxidizer and may also improve wettability and solder joint strength.

[0069] In a preferred embodiment, the solder alloy comprises from 1 .5 to 2.5 wt.% silver, from 38 to 47 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.03 to 0.08 wt.% nickel, from 3 to 4 wt.% indium, one or both of: from greater than 0 to 0.02 wt.% germanium, and from greater than 0 to 2 wt.% antimony, and the balance tin, together with any unavoidable impurities.

[0070] The solder alloy preferably comprises one of germanium and antimony.

[0071] The solder alloy preferably has a liquidus temperature of from 150 °C to less than 185 °C, more preferably from 155 °C to 180 °C, even more preferably from 160 to 175 °C, still even more preferably from 162 °C to 170 °C.

[0072] The solder alloy preferably has a solidus temperature of greater than 100 °C, more preferably greater than 110 °C, even more preferably of 120 °C or greater, still even more preferably of 125 °C or greater.

[0073] In a preferred embodiment, the solder alloy consists of: from 1 .5 to 2.5 wt.% silver, from 35 to 45 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.02 to 0.08 wt.% nickel, from 2.5 to 3.5 wt.% indium, from 0.001 to 0.1 wt.% germanium, and the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 1 .8 to 2.2 wt.% silver, from 38 to 42 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 2.8 to 3.2 wt.% indium, from 0.003 to 0.009 wt.% germanium, and the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of: about 2 wt.% silver, about 40 wt.% bismuth, about 0.7 wt.% copper, abut 0.05 wt.% nickel, about 3 wt.% indium, about 0.006 wt.% germanium, and the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermomechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0074] In a preferred embodiment, the solder alloy consists of: from 1 .5 to 2.5 wt.% silver, from 40 to 50 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.02 to 0.08 wt.% nickel, from 2.5 to 3.5 wt.% indium, from 0.5 to 1.5 wt.% antimony, and the balance tin, together with any unavoidable impurities. Preferably such a solder alloy consists of: from 1 .8 to 2.2 wt.% silver, from 43 to 47 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 2.8 to 3.2 wt.% indium, from 0.8 to 1.2 wt.% antimony, and the balance tin, together with any unavoidable impurities. In a particularly preferred embodiment, the solder alloy consists of: about 2 wt.% silver, about 45 wt.% bismuth, about 0.7 wt.% copper, about 0.05 wt.% nickel, about 3 wt.% indium, about 1 wt.% antimony, and the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus-liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times. In a preferred embodiment, the solder alloy consists of: from 1 .5 to 2.5 wt.% silver, from 37 to 47 wt.% bismuth, from 0.3 to 0.7 wt.% copper, from 0.02 to 0.08 wt.% nickel, from 0.5 to 1.5 wt.% indium, from 0.5 to 1.25 wt.% antimony, and the balance tin, together with any unavoidable impurities. Preferably, such a solder alloy consists of: from 1 .8 to 2.2 wt.% silver, from 40 to 44 wt.% bismuth, from 0.4 to 0.6 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 0.8 to 1 .2 wt.% indium, from 0.8 to 1.2 wt.% antimony, and the balance tin, together with any unavoidable impurities. In a preferred embodiment, the solder alloys consists of about 2 wt.% silver, about 42 wt.% bismuth, about 0.5 wt.% copper, about 0.05 wt.% nickel, about 1 wt.% indium, about 1 wt.% antimony, and the balance tin, together with any unavoidable impurities. Such an alloy may exhibit a particularly favourable combination of favourable mechanical properties, favourable solderability, desirable liquidus temperature, desirable solidus temperature, small solidus- liquidus temperature gap, relatively low cost, superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated in thermal cycling or thermal shock tests covering a wide temperature range and long dwell times.

[0075] The solder alloys of the first aspect and the second aspect (the alloys described herein) will typically comprise at least 70 wt.% tin, more typically at least 80 wt.% tin, still more typically at least 84 wt.% tin.

[0076] It will be appreciated that the alloys described herein may contain unavoidable impurities, although, in total, these are unlikely to exceed 1 wt.% of the composition. Preferably, the solder alloys contain unavoidable impurities in an amount of not more than 0.5 wt.% of the composition, more preferably not more than 0.3 wt.% of the composition, still more preferably not more than 0.1 wt.% of the composition, still more preferably not more than 0.05 wt.% of the composition, and most preferably not more than 0.02 wt.% of the composition. It is appreciated that, in reality, the alloys will typically be manufactured using Sn99%. Accordingly, the unavoidable impurities are typically unlikely to exceed 1 wt.% of the composition.

[0077] The solder alloys described herein may consist of the recited elements. Alternatively, the solder alloys described herein may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory (i.e. tin, silver, bismuth, copper, nickel, indium) other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.

[0078] The solder alloys described herein are preferably in the form of a bar, a stick, a solid or flux cored wire, a foil or strip, a film, a preform, a powder or paste (powder plus flux blend), solder spheres for use in ball grid array joints, a preformed solder piece or a reflowed or solidified solder joint, pre-applied on any solderable material such as a copper ribbon for photovoltaic applications or a printed circuit board of any type.

[0079] In a further aspect, the present invention provides a soldered joint comprising the solder alloy described herein.

[0080] The advantages and preferable features of the other aspects of the invention apply equally to this aspect.

[0081] In a further aspect, the present invention provides a solder paste comprising: the solder alloy described herein, and a solder flux.

[0082] The advantages and preferable features of the other aspects of the invention apply equally to this aspect. In a further aspect, the present invention provides a method of forming a solder joint comprising:

[0083] (i) providing two or more work pieces to be joined;

[0084] (ii) providing a solder alloy as described herein or the solder paste as described herein; and

[0085] (iii) heating the solder alloy or solder paste in the vicinity of the work pieces to be joined.

[0086] The advantages and preferable features of the other aspects of the invention apply equally to this aspect.

[0087] The solder alloy or solder paste is typically in contact with both work pieces during heating. “In the vicinity” may be, for example, between the work pieces. The work pieces may be components of a printed circuit board, such as a substrate and a die. Heating the solder alloy or solder paste typically comprises melting the solder alloy or the solder alloy of the solder paste. After melting, the solder alloy is typically solidified. In other words, the solder alloy may be subjected to reflow.

[0088] Heating the solder alloy or solder paste in the vicinity of the work pieces to be joined may comprise disposing the solder alloy or solder paste between the work pieces and in contact with the work pieces.

[0089] In a further aspect, the present invention provides a method of forming a solder joint comprising: providing two or more work pieces to be joined; providing a solder alloy as described herein or the solder paste described herein between and in contact with the two or more work pieces; and heating the solder alloy or solder paste to form a solder joint between the two or more work pieces. The work pieces may be components of a printed circuit board, such as a substrate and a die. Heating the solder alloy or solder paste typically comprises melting the solder alloy or the solder alloy of the solder paste. After melting, the solder alloy is typically solidified. In other words, the solder alloy may be subjected to reflow.

[0090] The advantages and preferable features of the other aspects of the invention apply equally to this aspect.

[0091] A method of joining three or more work pieces to each other comprising: providing a first work piece, a second work piece and a third work piece to be joined; providing a first solder alloy in the vicinity of the first and second work pieces, the first solder alloy being according to any of claims 1 to 26; heating the first solder alloy to form a solder joint between the first and second work pieces; providing a second solder alloy in the vicinity of the second work piece and the third work pieces, the second solder alloy being according to any of claims 27 to 44; and heating the second solder alloy to form a solder joint between the second and third work pieces, wherein heating the second solder alloy is carried out at a lower temperature than heating the first solder alloy.

[0092] The advantages and preferable features of the other aspects of the invention apply equally to this aspect.

[0093] The solder alloy or solder paste is typically in contact with both work pieces during heating. “In the vicinity” may be, for example, between the work pieces. By heating the second solder alloy at a lower temperature than the first solder alloy, the solder joint formed between the first and second work pieces may not be damaged during the second heating step. Preferably, heating the first solder alloy has a peak temperature of from greater than 180 °C to 220 °C, preferably from 185 °C to 210 °C; and / or heating the second solder alloy has a peak temperature of from greater than 165 °C to 190 °C, preferably from 170 °C to 185 °C.

[0094] In a further aspect, the present invention provides use of the solder alloy as described herein or the solder paste as described herein in a soldering method, preferably wherein the soldering method is selected from wave soldering, Surface Mount Technology (SMT) soldering, die attach soldering, thermal interface soldering, hand soldering, laser and RF induction soldering, soldering to a solar module, soldering of level 2 LED package-board, solder dipping, and rework soldering.

[0095] In a further aspect, the present invention provides a method of manufacturing the solder alloy as described herein, the method comprising: providing the recited elements, and melting the recited elements, wherein the recited elements may be provided in the form of individual elements and / or in the form of one or more alloys containing one or more of the recited elements.

[0096] The advantages and preferable features of the other aspects of the invention apply equally to this aspect.

[0097] The present invention will now be described further with reference to the Figure whereby:

[0098] Figure 1 is a drop shock plot for two alloys according to the present invention and a reference example alloy. The present invention will now be described further with reference to the following non-limiting examples.

[0099] Alloys were prepared with compositions as set out in table 1 below:

[0100] Table 1 : Compositions of alloys. ‘Reference examples not within the scope of the invention. The melting points of the alloys were measured, and the results are set out in Table 2 below:

[0101] Table 2: Melting point data. ‘Reference examples not within the scope of the invention. Figure 1 compares the drop shock performance of Alloy D, Alloy G and Sn3AgO.5Cu. Alloy D has equivalent drop shock performance to Sn3AgO.5Cu. Alloy G has 23% better drop shock resistance in comparison to Sn3AgO.5Cu.

[0102] 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 in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

CLAIMS:1 . A lead-free solder alloy comprising: from 0.5 to 4 wt.% silver, from 0.5 to 12 wt.% bismuth, from 0.1 to 1 wt.% copper, from 0.005 to 0.2 wt.% nickel, from 12 to 22 wt.% indium, optionally one or more of: from greater than 0 to 0.02 wt.% germanium, from greater than 0 to 0.03 wt.% titanium, from greater than 0 to 5.5 wt.% antimony, from greater than 0 to 1 wt.% manganese, from greater than 0 to 1 wt.% cobalt, from greater than 0 to 1 wt.% rare earths, from greater than 0 to 1 wt.% aluminium, from greater than 0 to 1 wt.% silicon, from greater than 0 to 1 wt.% chromium, from greater than 0 to 1 wt.% iron, from greater than 0 to 1 wt.% zinc, from greater than 0 to 1 wt.% gallium, and the balance tin, together with any unavoidable impurities.

2. The solder alloy of claim 1 comprising from 1 to 3.8 wt.% silver, preferably from 1 .5 to 3.5 wt.% silver, more preferably from 1 .8 to 3.2 wt.% silver, even more preferably from 1 .9 to 3.1 wt.% silver.

3. The solder alloy of claim 1 or claim 2 comprising from 1 to 4 wt.% bismuth, preferably from 1 .5 to 3.8 wt.% bismuth, more preferably from 2 to 3.7 wt.% bismuth, even more preferably from 2.5 to 3.5 wt.% bismuth, still even morepreferably from 2.8 to 3.2 wt.% bismuth, still even more preferably from 2.9 to 3.1 wt.% bismuth.

4. The solder alloy of any preceding claim comprising from 0.3 to 0.9 wt.% copper, preferably from 0.4 to 0.8 wt.% copper, more preferably from 0.5 to 0.8 wt.% copper.

5. The solder alloy of any preceding claim, comprising from 0.01 to 0.18 wt.% nickel, preferably from 0.05 to 0.15 wt.% nickel, more preferably from 0.09 to 0.13 wt.% nickel.

6. The solder alloy of any preceding claim comprising from 12 to 22 wt.% indium, preferably from 13 to 22 wt.% indium, more preferably from 15 to 21 .5 wt.% indium, even more preferably from 17 to 21 wt.% indium, still even more preferably from 17.5 to 20.5 wt.% indium.

7. The solder alloy of any preceding claim comprising from 0.001 to 0.015 wt.% germanium, preferably from 0.002 to 0.010 wt.% germanium, more preferably from 0.003 to 0.009 wt.% germanium.

8. The solder alloy of any preceding claim comprising from 0.001 to 0.029 wt.% titanium, preferably from 0.010 to 0.027 wt.% titanium, more preferably from 0.015 to 0.025 wt.% titanium, even more preferably from 0.020 to 0.024 wt.% titanium.

9. The solder alloy of any preceding claim comprising one or both of titanium and germanium.

10. The solder alloy of any preceding claim comprising: from greater than 0.001 to 2 wt.% antimony, and / or from greater than 0.001 to 1 wt.% manganese, and / orfrom greater than 0.001 to 1 wt.% cobalt, and / or from 0.001 to 1 wt.% rare earths (like Nd, Ce, La), and / or from 0.001 to 1 wt.% aluminium, and / or from 0.001 to 1 wt.% silicon, and / or from 0.001 to 1 wt.% chromium, and / or from 0.001 to 1 wt.% iron, and / or from 00.01 to 1 wt.% zinc, and / or from 0.001 to 1 wt.% gallium.11 . The lead-free solder alloy of claim 1 comprising: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, one or both of: from 0.001 to 0.01 wt.% germanium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

12. A lead-free solder alloy of claim 11 consisting of: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

13. A lead-free solder alloy of claim 11 consisting of:from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, and the balance tin, together with any unavoidable impurities.

14. A lead-free solder alloy of claim 11 consisting of: from 1 .5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.4 to 0.8 wt.% copper, from 0.02 to 0.14 wt.% nickel, from 17 to 21 wt.% indium, from 0.018 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

15. The solder alloy of any preceding claim having a liquidus temperature of from 180 °C to less than 200 °C, preferably from 185 °C to 195 °C, more preferably from 187 to 193 °C.

16. The solder alloy of any preceding claim having a solidus temperature of greater than 150 °C, preferably greater than 160 °C, even more preferably of 165 °C or greater, even more preferably of 169 °C or greater.

17. The solder alloys of any preceding claim having a solidus-liquidus temperature gap of less than 25 °C, preferably less than or equal to 20 °C, more preferably less than or equal to 17 °C.

18. The solder alloy of claim 1 consisting of: from 2.5 to 3.5 wt.% silver,from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.05 to 0.15 wt.% nickel, from 17 to 19 wt.% indium, from 0.001 to 0.01 wt.% germanium, the balance tin, together with any unavoidable impurities.

19. The solder alloy of claim 18 consisting of: from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.08 to 0.12 wt.% nickel, from 17.5 to 18.5 wt.% indium, from 0.003 to 0.009 wt.% germanium, the balance tin, together with any unavoidable impurities.

20. The solder alloy of claim 1 consisting of: from 2.5 to 3.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.01 to 0.1 wt.% nickel, from 19 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, the balance tin, together with any unavoidable impurities.21 . The solder alloy of claim 20 consisting of: from 2.8 to 3.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 19.5 to 20.5 wt.% indium,from 0.003 to 0.009 wt.% germanium, the balance tin, together with any unavoidable impurities.

22. The solder alloy of claim 1 consisting of: from 1 .5 to 2.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.5 to 0.18 wt.% nickel, from 19 to 21 wt.% indium, from 0.001 to 0.01 wt.% germanium, from 0.014 to 0.028 wt.% titanium, and the balance tin, together with any unavoidable impurities.

23. The solder alloy of claim 22 consisting of: from 1 .8 to 2.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.10 to 0.14 wt.% nickel, from 19.5 to 20.5 wt.% indium, from 0.003 to 0.009 wt.% germanium, from 0.019 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

24. The solder alloy of claim 1 consisting of: from 1 .5 to 2.5 wt.% silver, from 2.5 to 3.5 wt.% bismuth, from 0.3 to 0.7 wt.% copper, from 0.01 to 0.1 wt.% nickel, from 17 to 19 wt.% indium, from 0.014 to 0.028 wt.% titanium, and the balance tin, together with any unavoidable impurities.

25. The solder alloy of claim 24 consisting of: from 1 .8 to 2.2 wt.% silver, from 2.8 to 3.2 wt.% bismuth, from 0.4 to 0.6 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 17.5 to 18.5 wt.% indium, from 0.019 to 0.025 wt.% titanium, and the balance tin, together with any unavoidable impurities.

26. A lead-free solder alloy comprising: from 0.5 to 4 wt.% silver, from 5 to 50 wt.% bismuth, from 0.1 to 1 wt.% copper, from 0.005 to 0.2 wt.% nickel, from 1 to 7 wt.% indium, one or more of: from greater than 0 to 0.02 wt.% germanium, from greater than 0 to 5.5 wt.% antimony, from greater than 0 to 0.03 wt.% titanium, from greater than 0 to 1 wt.% manganese, from greater than 0 to 1 wt.% cobalt, from greater than 0 to 1 wt.% rare earths, from greater than 0 to 1 wt.% aluminium, from greater than 0 to 1 wt.% silicon, optionally one or more of: from greater than 0 to 1 wt.% chromium, from greater than 0 to 1 wt.% iron, from greater than 0 to 1 wt.% zinc, from greater than 0 to 1 wt.% gallium, and the balance tin, together with any unavoidable impurities.

27. The solder alloy of claim 26 comprising from 1 to 3.5 wt.% silver, preferably from 1 .2 to 3 wt.% silver, more preferably from 1 .5 to 2.5 wt.% silver, even more preferably from 1.8 to 2.2 wt.% silver.

28. The solder alloy of claim 26 or claim 27 comprising from 8 to 48 wt.% bismuth, preferably from 38 to 47 wt.% bismuth, more preferably from 39 to 46 wt.% bismuth, even more preferably from 39.5 to 45.5 wt.% bismuth.

29. The solder alloy of any of claims 26 to 28 comprising from 0.3 to 0.9 wt.% copper, preferably from 0.4 to 0.8 wt.% copper, more preferably from 0.5 to 0.8 wt.% copper.

30. The solder alloy of any of claims 26 to 29 comprising from 0.01 to 0.1 wt.% nickel, preferably from 0.02 to 0.08 wt.% nickel, more preferably from 0.03 to 0.07 wt.% nickel.31 . The solder alloy of any of claims 26 to 30 comprising from 2 to 4 wt.% indium, preferably from 2.5 to 3.5 wt.% indium, more preferably from 2.8 to 3.2 wt.% indium.

32. The solder alloy of any of claims 26 to 31 comprising from 0.001 to 0.015 wt.% germanium, preferably from 0.002 to 0.010 wt.% germanium, more preferably from 0.003 to 0.009 wt.% germanium.

33. The solder alloy of any of claims 26 to 32 comprising from greater than 0 to 2 wt.% antimony, preferably from 0.1 to 1.9 wt.% antimony, more preferably from 0.5 to 1 .5 wt.% antimony, even more preferably from 0.8 to 1 .2 wt.% antimony, still even more preferably from 0.9 to 1.1 wt.% antimony.

34. The solder alloy of any of claims 26 to 33 comprising from 0.001 to 0.029 wt.% titanium, preferably from 0.010 to 0.027 wt.% titanium, more preferably from 0.015 to 0.025 wt.% titanium, even more preferably from 0.020 to 0.024 wt.% titanium.

35. The solder alloy of any of claims 26 to 34 comprising: from 0.001 to 1 wt.% manganese from 0.001 to 1 wt.% cobalt from 0.001 to 1 wt.% rare earths, (like Nd, Ce, La) from 0.001 to 1 wt.% aluminium from 0.001 to 1 wt.% silicon from 0.001 to 1 wt.% chromium, from 0.001 to 1 wt.% iron, from 0.001 to 1 wt.% zinc, from 0.001 to 1 wt.% gallium,36. The lead-free solder alloy of claim 26 comprising: from 1 .5 to 2.5 wt.% silver, from 38 to 47 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.03 to 0.08 wt.% nickel, from 3 to 4 wt.% indium, one or both of: from greater than 0 to 0.02 wt.% germanium, and from greater than 0 to 2 wt.% antimony, and the balance tin, together with any unavoidable impurities.

37. The lead-free solder alloy of claim 36 comprising one of germanium and antimony.

38. The solder alloy of any of claims 26 to 37 having a liquidus temperature of from 150 °C to less than 185 °C, preferably from 155 °C to 180 °C, more preferably from 160 to 175 °C, even more preferably from 162 °C to 170 °C.

39. The solder alloy of any of claims 26 to 38 having a solidus temperature of greater than 100 °C, preferably greater than 110 °C, even more preferably of 120 °C or greater, even more preferably of 125 °C or greater.

40. The lead-free solder alloy of claim 26 consisting of: from 1 .5 to 2.5 wt.% silver, from 35 to 45 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.02 to 0.08 wt.% nickel, from 2.5 to 3.5 wt.% indium, from 0.001 to 0.1 wt.% germanium, and the balance tin, together with any unavoidable impurities.41 . The lead-free solder alloy of claim 26 consisting of: from 1 .8 to 2.2 wt.% silver, from 38 to 42 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 2.8 to 3.2 wt.% indium, from 0.003 to 0.009 wt.% germanium, and the balance tin, together with any unavoidable impurities.

42. The lead-free solder alloy of claim 26 consisting of: from 1 .5 to 2.5 wt.% silver, from 40 to 50 wt.% bismuth, from 0.5 to 0.9 wt.% copper, from 0.02 to 0.08 wt.% nickel,40 from 2.5 to 3.5 wt.% indium, from 0.5 to 1 .5 wt.% antimony, and the balance tin, together with any unavoidable impurities.

43. The lead-free solder alloy of claim 42 consisting of: from 1 .8 to 2.2 wt.% silver, from 43 to 47 wt.% bismuth, from 0.6 to 0.8 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 2.8 to 3.2 wt.% indium, from 0.8 to 1 .2 wt.% antimony, and the balance tin, together with any unavoidable impurities.

44. The lead-free solder alloy of claim 26 consisting of: from 1 .8 to 2.2 wt.% silver, from 40 to 44 wt.% bismuth, from 0.4 to 0.6 wt.% copper, from 0.03 to 0.07 wt.% nickel, from 0.8 to 1 .2 wt.% indium, from 0.8 to 1 .2 wt.% antimony, and the balance tin, together with any unavoidable impurities.

45. The solder alloy of any preceding claim in the form of a bar, a stick, a solid or flux cored wire, a foil or strip, a film, a preform, a powder or paste (powder plus flux blend), solder spheres for use in ball grid array joints, a pre-formed solder piece or a reflowed or solidified solder joint, pre-applied on any solderable material such as a copper ribbon for photovoltaic applications or a printed circuit board of any type.

46. A soldered joint comprising the solder alloy of any preceding claim.4147. A solder paste comprising: the solder alloy of any of claims 1 to 44, and a solder flux.

48. A method of forming a solder joint comprising:(i) providing two or more work pieces to be joined;(ii) providing a solder alloy as defined in any of claims 1 to 45 or the solder paste of claim 47; and(iii) heating the solder alloy or solder paste in the vicinity of the work pieces to be joined.

49. A method of joining three or more work pieces to each other comprising: providing a first work piece, a second work piece and a third work piece to be joined; providing a first solder alloy in the vicinity of the first and second work pieces, the first solder alloy being according to any of claims 1 to 25; heating the first solder alloy to form a solder joint between the first and second work pieces; providing a second solder alloy in the vicinity of the second work piece and the third work pieces, the second solder alloy being according to any of claims 27 to 45; and heating the second solder alloy to form a solder joint between the second and third work pieces, wherein heating the first solder alloy is carried out at a higher temperature than heating the second solder alloy.

50. The method of claim 49, wherein: heating the first solder alloy has a peak temperature of from greater than180 °C to 220 °C, preferably from 185 °C to 210 °C; and / or heating the second solder alloy has a peak temperature of from greater than 165 °C to 190 °C, preferably from 170 °C to 185 °C.4251 . Use of a solder alloy of any of claims 1 to 45 or the solder paste of claim 47 in a soldering method, preferably wherein the soldering method is selected from wave soldering, Surface Mount Technology (SMT) soldering, die attach soldering, thermal interface soldering, hand soldering, laser and RF induction soldering, soldering to a solar module, soldering of level 2 LED package-board, solder dipping, and rework soldering.

52. A method of manufacturing the solder alloy of any of claims 1 to 45, the method comprising: providing the recited elements, and melting the recited elements, wherein the recited elements may be provided in the form of individual elements and / or in the form of one or more alloys containing one or more of the recited elements.

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