Soldering method, solder paste, solder flux and solder joint

Abstract

A soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%.

Description

[0001] SOLDERING METHOD, SOLDER PASTE, SOLDER FLUX AND SOLDER JOINT

[0002] The present invention relates to a soldering method, a solder paste and a paste flux.

[0003] In the manufacture of electronic devices, components and substrates are typically joined using a soldering method to provide both a mechanical and electrical connection. Conventional soldering methods involve disposing solder between two or more workpieces to be joined and reflowing the solder. Different components and substrates may require different types of solder applications. In order to more easily position the solder at a desired location on the component and / or substrate, the solder is typically employed in the form of a solder paste so that it can be applied by, for example, printing. However, the use of solder pastes may result in the presence of a large volume of voids in the final solder joint, which may decrease the electrical and / or mechanical reliability of the solder joint.

[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. Solder alloys capable of operating at such temperatures are described in WO2021 / 043437. However, such solder alloys tend to be incompatible with conventional soldering fluxes used to form solder pastes, and solder pastes comprising such solder alloys and conventional soldering fluxes tend to phase separate over time making their use problematic. Furthermore, solder joints formed using such solder pastes tend to exhibit electrochemical migration. Electrochemical migration (ECM) is the dissolution and movement of metal ions in presence of electric potential, which results in the growth of dendritic structures. The process is commonly observed in printed circuit boards where it may significantly decrease the insulation between conductors.

[0005] The present invention aims to solve 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 soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%.

[0007] 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.

[0008] Advantageously, the method may result in a solder joint exhibiting high strength and high reliability. In particular, the solder joint may exhibit a low level of voids, typically the solder joint may be substantially free of voids.

[0009] The inventors have investigated the problem of voids in solder joints and have discovered that such voids primarily occur due to “out-gassing” of components in the solder flux during the temperature profile, i.e. evaporation and / or decomposition of components such as, for example, a solvent or an activator. In particular, the inventors have discovered that at lower temperatures gases are able to escape from the solder paste. However, once the solder particles begin to melt, gas bubbles become trapped within the solder paste and result in voids in the final solder joint.

[0010] By ensuring that weight loss of the paste flux (resulting from “out-gassing”) is restricted to 25% or less during the range of from the solidus temperature of the solder to the highest temperature in the temperature profile, the method may result in the formation of a solder joint exhibiting a low level of voids, typically the solder joint may be substantially free of voids, and the volume % of void in the solder joint may be low. In addition, any voids that may be present in the solder joint may be small in size.

[0011] The method comprises providing a solder paste between two or more work pieces to be joined. Typically, the solder paste will be in contact with the two or more work pieces to be joined. The wording “between two or more work pieces to be joined” may encompass a situation in which the solder paste is sandwiched between the two or more work pieces. Alternatively, the wording may encompass the situation in which the solder paste is provided above or below adjacent work pieces.

[0012] When the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%. The weight loss percentage is based on the total weight of the paste flux, i.e. the total weight loss of the soldering flux before being subjected to the temperature profile, rather than the total weight of the paste flux at the solidus temperature of the alloy. As will be appreciated, the temperature profile will comprise heating up to the solidus temperature of the alloy. During this time, if the soldering paste comprises solvent, then at least some of the solvent may evaporate. The percentage weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile does not include the weight loss experienced during heating up to the solidus temperature, e.g. as a result of evaporation of solvent.

[0013] When the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature (melting point) of the solder to the highest temperature in the temperature profile is preferably no more than 20%, more preferably no more than 15%, even more preferably no more than 10%, still even more preferably no more than 5%, still even more preferably no more than 2%.

[0014] The temperature profile comprises heating the solder paste to an elevated temperature for a period of time. The temperature may vary over the temperature profile, and the rate of increase or decrease of temperature may vary. The solder paste may be held at the elevated temperature for a period of time. The temperature profile typically ends with cooling so that the solder re-solidifies to form the solder joint.

[0015] The paste flux preferably comprises an activator. The presence of an activator may improve wetting of the solder paste by removing oxides from the contact surfaces of the workpieces and the solder particles. Accordingly, the strength and mechanical reliability of the resulting solder joint may be improved.

[0016] The activator preferably comprises an organic acid and / or an amine, more preferably an organic acid. Such species are particularly effective at removing surface oxides. In addition, such species tend to be compatible with solder alloys. The activator preferably comprises: one or more first organic acids having a pKa of less than or equal to 4.2 and exhibiting a total weight loss of greater than 80% by the solidus temperature of the solder when subjected to thermogravimetric analysis according to ASTM E1131-20; and one or more second organic acids having a pKa of greater than 4.2 and exhibiting a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20.

[0017] The use of the first and second organic acids may result in an advantageous combination of having flux activity throughout the temperature profile and avoiding electrochemical migration in the solder joint. The use of multiple organic acids having different pKa values and different “out-gassing” profiles may ensure that the activator operates (provides “fluxing activity”) over the entire temperature profile. The first organic acids with lower pKa values may provide better fluxing activity at low temperatures but will need to have decomposed / evaporated by the time the solder melts so as not to be contained in the solder joint (and thereby cause electrochemical migration). The second organic acids may substantially remain undecomposed / unevaporated during the temperature profile so that fluxing activity is maintained over the whole temperature profile. Although the pKas of the second organic acids are high, the higher temperatures will ensure that there is a reasonable level of flux activity. Second organic acids may be present in the final solder joint. However, since they have a pKa of greater than 4.2, they are unlikely to cause any significant electrochemical migration.

[0018] Preferably, the one or more first organic acids exhibit a total weight loss of greater than 90% by the solidus temperature of the solder when subjected to thermogravimetric analysis according to ASTM E1131-20, preferably greater than 95%, more preferably greater than 97%; and / or the one or more second organic acids exhibit a total weight loss of less than 15% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20, preferably less than 10%, more preferably less than 5%.

[0019] The one or more first organic acids preferably have a boiling point of from 100 °C to less than 250°C. Based on the solidus temperature of typical solders, this may ensure substantially complete “out-gassing” before the solder begins to melt. As a result, void forming gases are not generated during reflow and low pKa organic acids, which may cause electrochemical migration, do not remain in the solder joint.

[0020] The one or more first organic acids are preferably selected from the group consisting of benzoic acid, formic acid, methyl succinic acid, 2-hydroxy-3- methylbutyric acid, succinic acid, pyruvic acid and glyoxylic acid. Such organic acids may have suitable pKa values and suitable boiling points. In addition, such organic acids may be compatible with conventional solders.

[0021] The one or more second organic acids preferably have a boiling point of greater than or equal to 250 °C. This may ensure that there is substantially no outgassing during a typical reflow temperature profile.

[0022] The one or more second organic acids are preferably selected from the group consisting of adipic acid, glutaric acid, dimethyl glutaric acid, suberic acid, pivalic acid, sebacic acid, azelaic acid, lauric acid, myristic acid and palmitic acid. Such organic acids may have suitable pKa values and suitable boiling points. In addition, such organic acids may be compatible with conventional solders. The activator preferably comprises from 70 to 90 wt.% of the one or more first organic acids and from 10 to 30 wt.% of the one or more second organic acids. This may result in flux activity over the entire temperature profile without a large volume of organic material being present in the solder joint, which may reduce the mechanical properties and / or mechanical reliability of the solder joint.

[0023] The paste flux preferably comprises one or more solvents. The solvent may dissolve the other components of the paste.

[0024] The one or more solvents preferably have a boiling point of less than 280 °C, more preferably from 150 to 270 °C, even more preferably from 200 to 260 °C. This may ensure that substantially all the solvent has evaporated before the solidus temperature of conventional solders.

[0025] The one or more solvents preferably comprise one or more glycol ethers. Preferably, one or both hydroxyl groups of the glycol ether is substituted with an alkyl group. The one or more solvents are preferably selected from the group consisting of ethylene glycol butyl ether, dipropylene glycol methyl ether, butyl carbitol acetate, ethylene glycol butyl ether acetate, dibutyl carbitol , triporpylene glycol butyl ether, and hexyl carbitol. Such solvents may be effective to dissolve the other components of the solder paste and may exhibit favourable boiling points. In addition, such solvents may be compatible with conventional solders.

[0026] The paste flux preferably comprises rosins. The presence of rosins may provide thermal stability during reflow and may inhibit oxidation of the solder during reflow.

[0027] The paste flux more preferably comprises one or more rosins exhibiting a total weight loss of less than 25% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131- 20. Since a substantial proportion of the one or more rosins remains in the paste during the temperature profile, thermal stability and / or inhibition of solder oxidation may be provided for the entire temperature profile. In addition, rosin remaining in the final solder joint may serve to hinder electrochemical migration.

[0028] The one or more rosins preferably exhibit a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20, preferably less than 15%, more preferably less than 12%, even more preferably less than 10%.

[0029] The one or more rosins are preferably selected from the group consisting of gum rosin, Dimerax rosin, Foral-ax, Foral-axe, Arakawa-KE610, Arakawa-KR604, Arakawa-KE612, Arakawa-CP140, Rosin HX, Staybelite-A, Unirez-7952, RHR301 , AR120, Teckros-HX, Malkyd-33, Malkyd 31 , Polypale resin and Resin HM-100M. Such rosins may be particularly effective at providing thermal stability during reflow, inhibiting oxidation of solder and hindering electrochemical migration. Such rosins may also be compatible with solder.

[0030] The paste flux preferably comprises one or more rheology modifiers. The presence of the rheology modifier may result in more favourable rheological properties, thereby rendering the paste more easily printed or dispensed.

[0031] The rheology modifier preferably comprises an amide-based rheology modifier and / or a castor oil-based rheology modifier.

[0032] The paste flux preferably comprises: one or more polymers; and / or one or more corrosion inhibitors; and / or one or dispersants; and / or one or more defoaming agents. The paste flux preferably comprises, based on the total weight of the paste flux: from 1 to 15 wt.% activator; and / or from 20 to 40 wt.% solvent; and / or from 20 to 30 wt.% rosin; and / or from 1 to 6 wt.% rheology modifier; and / or from 1 to 15 wt.% polymer; and / or from 1 to 5 wt.% corrosion inhibitor from 1 to 15 wt.% dispersant; and / or from 1 to 5 wt.% defoaming agent.

[0033] The solder paste preferably comprises, based on the total weight of the solder paste: from 85 to 90 wt.% solder particles; and from 10 to 15 wt.% paste flux.

[0034] The highest temperature in the temperature profile is preferably less than or equal to 260 °C, more preferably from 200 to 250 °C, even more preferably from 220 to 245 °C. This may result in reduced levels of “out-gassing” during the temperature profile.

[0035] The soldering method preferably comprises surface mount technology (SMT). The issue of out-gassing can be particularly problematic for SMT soldering methods.

[0036] The two or more workpieces to be joined comprise an electrical component and a printed circuit board.

[0037] Considering the sensitivity to voids, Preferably, the electrical component comprises a micro lead frame (MLF) and / or a quad flat no-lead (QFN) package., When one of the two or more work pieces comprises a micro lead frame (MLF) and / or a quad flat no-lead (QFN) package, the issue of void formation is particularly problematic for such components.

[0038] The solder particles preferably have a solidus temperature above 200 °C and / or a liquidus temperature of from 210 °C to 260 °C, preferably a liquidus temperature of from 212 °C to 250 °C, more preferably a liquidus temperature of from 215 °C to 245 °C.

[0039] In a preferred embodiment, the solder particles comprise an alloy comprising from 2.5 to 3.5 wt.% silver, from 0.3 to 0.7 wt.% copper and the balance tin together with any unavoidable impurities (e.g. SAC305). The solder paste flux is compatible with such an alloy. In addition, the solder paste exhibits low levels of outgassing during suitable reflow temperatures (typically at least 13 °C higher than the melting point) of such an alloy.

[0040] In a particularly preferred embodiment, the solder particles comprise a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.01 to 0.5 wt.% nickel; one or more of: up to 7 wt.% antimony (i.e. greater than 0 to 7 wt.%, e.g. 0.001 to 7 wt.%); up to 6 wt.% indium (i.e. greater than 0 to 6 wt.%, e.g. 0.001 to 6 wt.%), up to 0.5 wt.% titanium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% germanium (i.e. greater than 0 to 0.5 wt.%, e.g.

[0041] 0.001 to 0.5 wt.%), up to 0.5 wt.% rare earths (i.e. greater than 0 to 0.5 wt.%, e.g.

[0042] 0.001 to 0.5 wt.%), up to 0.5 wt.% cobalt (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 5.0 wt.% aluminium (i.e. greater than 0 to 5.0 wt.%, e.g. 0.001 to 5.0 wt.%), up to 5.0 wt.% silicon (i.e. greater than 0 to 5.0 wt.%, e.g. 0.001 to 5.0 wt.%), up to 0.5 wt.% manganese (i.e. greater than 0 to 0.5 wt.%, e.g.

[0043] 0.001 to 0.5 wt.%), up to 0.5 wt.% chromium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 t.%), up to 0.5 wt.% iron (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% phosphorus (i.e. greater than 0 to 0.5 wt.%, e.g.

[0044] 0.001 to 0.5 wt.%), up to 0.5 wt.% gold (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 1 wt.% gallium (i.e. greater than 0 to 1 wt.%, e.g. 0.001 to 1 wt.%), up to 0.5 wt.% tellurium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% selenium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% calcium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% vanadium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% molybdenum (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), up to 0.5 wt.% platinum (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%), and up to 0.5 wt.% magnesium (i.e. greater than 0 to 0.5 wt.%, e.g. 0.001 to 0.5 wt.%); and the balance tin together with any unavoidable impurities.

[0045] The inventors have surprisingly found that the paste flux is particularly compatible with such an alloy, i.e. the paste is stable. By stable it is meant that the paste does not phase separate for at least eight hours in use or for at least six months in storage. This is surprising because, without being bound by theory, it is considered that elements such as, for example, bismuth, nickel and antimony, tend to react unfavourably with components of conventional flux pastes (considered most likely to be the activator).

[0046] The term “solder alloy” used herein encompasses a fusible metal alloy melting in the range of 90 to 400 °C. The alloys are lead-free meaning that there is no intentional addition of lead. Thus, the lead content is zero or at no more than accidental impurity levels.

[0047] The solder alloys may exhibit improved high temperature reliability and may be capable of withstanding high operational temperatures of typically at least 150 °C. The solder alloys may exhibit improvement in mechanical properties and high temperature creep resistance compared to the conventional 96.5Sn-3.0Ag0.5Cu alloy.

[0048] The solder alloys may have a high melting point, specifically a solidus temperature higher than 200 °C and / or a liquidus temperature higher than 210 °C and lower than 260 °C. The solder alloys preferably have a liquidus temperature greater than 212 °C, more preferably greater than 215 °C, even more preferably greater than 218 °C, still even more preferably greater than 220 °C. A higher liquidus temperature may allow the alloy to be used in a higher temperature soldering process. The solder alloy preferably has a liquidus temperature below 250 °C, more preferably below 240 °C, even more preferably below 235 °C. Such liquidus temperatures may be advantageous because peak reflow temperatures are typically 25 to 30 °C above the liquidus temperature and reflow temperatures more than about 260 °C may lead to various issues during soldering, including the damage of the most common printed circuit boards and components.

[0049] The solder alloys may have favourable mechanical properties and favourable solderability. The solder alloys may exhibit superior high temperature creep properties and superior thermo-mechanical properties and fatigue life, such as those evaluated during 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 150 °C, and -40 to 125 °C with dwell times of 30 minutes at each temperature.

[0050] The alloying additions are used to modify the alloy microstructure and, hence, its properties due to physical metallurgy mechanisms such as precipitation strengthening, solid solution strengthening, grain refinement and diffusion control. The magnitude of the mechanical properties of the solder alloy, such as hardness, tensile strength, and high temperature creep, may be at least twice that of 96.5Sn- 3.0Ag-0.5Cu (SAC305). Bismuth, antimony, and indium, for example, affect the solidus and liquidus temperatures of the solder alloy, also have high solid solubility in tin and thus may contribute significantly towards solid solution strengthening of the matrix. The variations in solidus or liquidus temperatures do not appear to affect adversely the mechanical properties of the alloys. Time-dependent creep deformation processes depend on the homologous temperature, i.e., ratio of the test temperature to melting temperature of the material in absolute scale. At 150 °C, the homologous temperature of the solder alloys may be in the range of 0.80 to 0.86. The melting temperature of the solder alloy therefore has no major effect on the mechanical properties.

[0051] An optimum combination of solid solution and precipitation strengthening may result in a well-distributed network of precipitate particles in a strong matrix. For example, AgsSn and (Cu,Ni)eSn5 particles may form as precipitates. The movement of grain boundaries and other metallurgical mechanisms during creep deformation, thus enhancing the creep strength.

[0052] The solder alloy comprises from 2.5 to 5.0 wt.% of silver. The solder alloy preferably comprises from 2.8 to 4.5 wt.% silver, more preferably from 2.8 to 4 wt.% silver. The presence of silver in the specified amount may help to improve mechanical properties, for example strength, through the formation of network-like intermetallic compounds such as, for example, AgsSn. Additionally, the presence of silver may improve wetting and spread. Higher levels of silver, particularly levels higher than 4.5 wt.% silver, may increase the liquidus temperature and larger needle-like precipitates of AgsSn formed in the solder matrix act as sites of crack initiation and subsequent failure. Lower silver content may not form enough AgsSn precipitates that may be helpful in improving the strength.

[0053] The solder alloys comprise from 0.01 to 5 wt.% of bismuth. The solder alloys preferably comprise from 1 .0 to 4.0 wt.% bismuth, more preferably from 2.0 to 4.0 wt.% bismuth, even more preferably from 2.5 to 4 wt.% bismuth, still even more preferably from 2.7 to 4 wt.% bismuth, still even more preferably from 2.9 to 4 wt.% bismuth. The presence of bismuth in the specified amount may improve mechanical properties through solid solution strengthening. Bismuth may also serve to improve creep resistance. Bismuth may also help to improve wetting and spread. However, bismuth addition more than the specified amount may result in precipitation of bismuth in tin resulting in a more brittle alloy.

[0054] The solder alloy comprises from 0.01 to 2 wt.% of copper. The solder alloy preferably comprises from 0.3 to 1 .2 wt.% copper, more preferably from 0.4 to 0.8 wt.% copper. The presence of copper in the specified amount may improve mechanical properties, for example strength, through the formation of Cu-Sn intermetallic compounds. Addition of copper in the specified range results in the optimum quantity of intermetallic compound precipitates required for strengthening the alloy.

[0055] The solder alloys comprise from 0.001 to 0.5 wt.% nickel. The solder alloy preferably comprises from 0.001 to 0.4 wt.% nickel, more preferably from 0.01 to 0.3 wt.% nickel, even more preferably from 0.02 to 0.2 wt.% nickel. The presence of nickel in the specified amount may improve mechanical properties through the formation of intermetallic compounds with tin and copper, which can result in precipitation strengthening. Additionally, the presence of nickel may act to reduce the copper dissolution rate. Nickel may also enhance thermal reliability by decreasing IMC growth at the substrate / solder interface.

[0056] The solder alloy preferably comprises: from 1 to 7 wt.% of antimony; and / or from 0.001 to 6 wt.% indium; and / or from 0.01 to 0.5 wt.% titanium; and / or from 0.01 to 0.5 wt.% germanium; and / or from 0.001 to 0.5 wt.% rare earths; and / or from 0.001 to 0.5 wt.% cobalt; and / or from 0.001 to 5 wt.% aluminum; and / or from 0.001 to 5 wt.% silicon; and / or from 0.001 to 0.5 wt. % chromium; and / or from 0.001 to 0.5 wt. % manganese; and / or from 0.01 to 0.5 wt.% of iron; and / or from 0.001 to 0.5 wt.% of phosphorus; and / or from 0.001 to 0.5 wt.% of gold; and / or from 0.01 to 0.9 wt.% gallium; and / or from 0.001 to 0.5 wt.% of tellurium; and / or from 0.001 to 0.5 wt.% of selenium; and / or from 0.001 to 0.5 wt.% of calcium; and / or from 0.001 to 0.5 wt.% of vanadium; and / or from 0.001 to 0.5 wt.% of molybdenum; and / or from 0.001 to 0.5 wt.% of platinum; and / or from 0.001 to 0.5 wt.% of magnesium.

[0057] The solder alloys may optionally comprise up to 7 wt.% antimony (e.g. 0.001 to 7 wt.% antimony), preferably from 1 to 7 wt.% of antimony. The solder alloy preferably comprises from 1 .0 to 6.5 wt.% antimony. In a preferred embodiment, the alloy comprises at least 1 .4 wt.% antimony, preferably at least 3 wt.% antimony. The presence of antimony in the specified amount may improve mechanical properties through solid solution strengthening. Antimony may also improve creep resistance and thermal fatigue resistance. Antimony addition may also increase the liquidus temperature of the alloy. Antimony addition lower than the specified range may not provide the required improvement of mechanical strength and thermal fatigue resistance. Additions of antimony 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, like damaging printed circuit boards and components.

[0058] The solder alloy optionally comprises up to 6 wt.% indium, e.g., from 0.001 to 6 wt.% indium. The solder alloy preferably comprises from 0.001 to 5.5 wt.% indium, more preferably from 0.02 to 4 wt.% indium, even more preferably from 0.5 to 3 wt.% indium. The presence of indium in the specified amount may 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 lead to the formation of low temperature phases that will adversely affect the long-term reliability of the alloys.

[0059] The solder alloy optionally comprises up to 0.5 wt.% titanium, e.g. from 0.001 to 0.5 wt.% titanium. The solder alloy preferably comprises from 0.001 to 0.3 wt.% titanium, more preferably from 0.005 to 0.2 wt.% titanium, even more preferably from 0.007 to 0.05 wt.% titanium. The presence of titanium in the amounts specified may act to improve one or more of strength, solid state interfacial reactions and thermo-mechanical reliability.

[0060] The solder alloy optionally comprises up to 0.5 wt.% germanium, e.g., from 0.001 to 0.5 wt.% germanium. The solder alloy preferably comprises from 0.001 to 0.3 wt.% germanium, more preferably from 0.001 to 0.1 wt.% germanium, even more preferably from 0.001 to 0.02 wt.% germanium. The presence of germanium may improve mechanical properties through particle dispersion. Germanium may also help in deoxidation and may also improve wettability, solder joint strength and appearance. In addition, germanium in combination with nickel and / or titanium, may act to improve the thermo-mechanical fatigue properties.

[0061] The solder alloy optionally comprises up to 0.5 wt.% rare earths, e.g., from 0.001 to 0.5 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. The solder alloy preferably comprises from 0.002 to 0.3 wt.% rare earths, more preferably from 0.003 to 0.05 wt.% rare earths. Preferred rare earth elements include cerium, neodymium and lanthanum. The presence of rare earths may improve mechanical properties by particle dispersion and / or microstructure modification. Rare earths may improve spread and wettability.

[0062] The solder alloy optionally comprises up to 0.5 wt.% cobalt, for example from 0.001 to 0.5 wt.% cobalt. The solder alloy preferably comprises from 0.01 to 0.2 wt.% cobalt, preferably from 0.01 to 0.2 wt.% cobalt, even more preferably from 0.02 to 0.1 wt.% cobalt. The presence of cobalt in the specified amounts may act to improve the strength and / or high temperature properties of the solder. In a preferred embodiment, the alloy is “cobalt-free”. This may be advantageous in view of the high cost and the toxicity of cobalt.

[0063] The solder alloy optionally comprises up to 5 wt.% aluminium, e.g., from 0.001 to 5 wt.% aluminum. The solder preferably comprises from 0.001 to 3 wt.%, more preferably from 0.005 to 2 wt.% of aluminum, even more preferably from 0.01 to 1.5 wt.% aluminum, still even more preferably from 0.015 to 1 wt.% aluminum, still more preferably from 0.02 to 0.08 wt.% aluminum. The presence of aluminum in the specified amounts may improve fatigue life of the solder.

[0064] The solder alloy optionally comprises up to 5 wt.% silicon, e.g., from 0.001 to 5 wt.% silicon. The solder alloy preferably comprises from 0.001 to 3 wt.% silicon, more preferably from 0.005 to 2 wt.% of silicon, even more preferably from 0.01 to 1.5 wt.% silicon, still even more preferably from 0.015 to 1 wt.% silicon, still more preferably from 0.02 to 0.08 wt.% silicon. The presence of silicon in the recited amounts may act to improve mechanical properties, fatigue life, and thermal and electrical conductivity of the solder.

[0065] The solder alloy optionally comprises one or more of: up to 0.5 wt.% chromium, preferably from 0.001 to 0.5 wt. % chromium; up to 0.5 wt.% manganese, preferably from 0.001 to 0.5 wt.% of manganese, more preferably from 0.003 to 0.015 wt.% manganese, even more preferably from 0.005 to 0.01 wt.% manganese; up to 0.5 wt.% iron, preferably from 0.01 to 0.5 wt.% of iron, more preferably from 0.01 to 0.1 wt.% iron, even more preferably from 0.015 to 0.035 wt.% iron; up to 0.5 wt.% phosphorus, preferably from 0.001 to 0.5 wt.% of phosphorus; up to 0.5 wt.% gold, preferably from 0.001 to 0.5 wt.% of gold; up to 1 wt.% gallium, preferably from 0.01 to 0.9 wt.% gallium, more preferably from 0.2 to 0.8 wt.% of gallium, even more preferably 0.4 to 0.6 wt.% gallium; up to 0.5 wt.% tellurium, preferably from 0.001 to 0.5 wt.% of tellurium; up to 0.5 wt.% selenium, preferably from 0.001 to 0.5 wt.% of selenium; up to 0.5 wt.% calcium, preferably from 0.001 to 0.5 wt.% of calcium; up to 0.5 wt.% vanadium, preferably from 0.001 to 0.5 wt.% of vanadium; up to 0.5 wt.% molybdenum, preferably from 0.001 to 0.5 wt.% of molybdenum; up to 0.5 wt.% platinum, preferably from 0.001 to 0.5 wt.% of platinum; and up to 0.5 wt.% magnesium, preferably from 0.001 to 0.5 wt.% of magnesium. Aluminium, calcium, gallium, germanium, magnesium, phosphorus and vanadium may act as deoxidizers and may also improve wettability and solder joint strength. Other elemental additions, such as gold, chromium, iron, manganese, molybdenum, platinum, selenium and tellurium may act to improve strength and interfacial reactions. Aluminium in combination with silicon may improve strength and reliability performance of the alloys. Germanium in combination with silicon may also serve to improve strength and reliability performance of the alloys.

[0066] The solder alloy preferably comprises one of titanium, germanium, indium, manganese, rare earths, cobalt, aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium, preferably one of titanium, germanium, manganese, cobalt and indium, more preferably one of titanium, germanium and manganese. Such an alloy may show favourable mechanical properties. The solder alloy preferably comprises at least 70 wt.% tin, more preferably at least 80 wt.% tin, even more preferably at least 84 wt.% tin.

[0067] Preferably, in the solder alloy the wt.% of antimony is greater than the wt.% of bismuth, and more preferably wherein and the sum of their wt.% is preferably greater than or equal to 6.5. Such an alloy may show particularly favourable mechanical properties.

[0068] In a preferred embodiment, the solder alloy comprises 3.5 to 3.9 wt.% silver, 2.7 to 3.2 wt.% bismuth, 0.5 to 0.8 wt.% copper, 0.008 to 0.02 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

[0069] In a preferred embodiment, the solder alloy comprises 2.6 to 3.2 wt.% silver, 2.9 to 3.3 wt.% bismuth, 1.9 to 2.3 wt.% antimony, 0.5 to 0.9 wt.% copper, 0.1 to 0.3 wt.% nickel, 2.9 to 3.5 wt.% indium, and the balance tin together with unavoidable impurities.

[0070] In a preferred embodiment, the solder alloy comprises 3.5 to 4 wt.% silver, 2.8 to 3.2 wt.% bismuth, 1 to 2 wt.% antimony, 0.5 to 0.8 wt.% copper, 0.08 to 0.2 wt.% nickel, and the balance tin together with unavoidable impurities.

[0071] In a preferred embodiment, the solder alloy comprises 2.8 to 3.2 wt.% silver, 2.8 to 3.2 wt.% bismuth, 5 to 7 wt.% antimony, 0.3 to 0.7 wt.% copper, 0.09 to 0.2 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

[0072] In a preferred embodiment, the solder alloy comprises 3.1 to 3.6 wt.% silver, 3.3 to 3.8 wt.% bismuth, 0.5 to 0.8 wt.% copper, 3.3 to 3.8 wt.% antimony, 0.025 to 0.07 wt.% nickel, 0.03 to 0.07 wt.% cobalt, and the balance tin together with unavoidable impurities. In a preferred embodiment, the solder alloy comprises 3.3 to 3.7 wt.% silver, 3.5 to 3.9 wt.% bismuth, 0.3 to 0.7 wt.% copper, 3.2 to 3.6 wt.% indium, 5.8 to 6.2 wt.% antimony, 0.04 to 0.08 wt.% nickel, 0.003 to 0.007 wt.% neodymium, and the balance tin together with unavoidable impurities.

[0073] In a preferred embodiment, the solder alloy comprises 3.3 to 3.7 wt.% silver, 2.8 to 3.2 wt.% bismuth, 0.5 to 0.8 wt.% copper, 3.8 to 4.2 wt.% antimony, 0.03 to 0.07 wt.% nickel, 0.003 to 0.007 wt.% germanium, 0.01 to 0.04 wt.% silicon, and the balance tin together with unavoidable impurities.

[0074] In a further aspect, the present invention provides a soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein: when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%; the solder paste comprises from 85 to 90 wt.% solder particles and from

[0075] 10 to 15 wt.% paste flux; the paste flux comprises, based on the total weight of the paste flux, from 1 to 15 wt.% activator, from 20 to 40 wt.% solvent, from 20 to 30 wt.% rosin and optionally from 1 to 6 wt.% rheology modifier, and / or from 1 to 15 wt.% polymer, and / or from 1 to 5 wt.% corrosion inhibitor, and / or from 1 to 15 wt.% dispersant and / or from 1 to 5 wt.% defoaming agent; the activator comprises, based on the total weight of the activator, from 70 to 90 wt.% of one or more first organic acids having a pKa of less than or equal to 4.2 and exhibiting a total weight loss of greater than 80% by the solidus temperature of the solder when subjected to thermogravimetric analysis according to ASTM E1131 -20 and from 10 to 30 wt.% of one or more second organic acids having a pKa of greater than 4.2 and exhibiting a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20; and the solder particles comprise a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver, from 0.01 to 5 wt.% bismuth, from 0.01 to 2 wt.% copper, from 0.01 to 0.5 wt.% nickel, one or more of: up to 7 wt.% antimony, up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium, and the balance tin together with any unavoidable impurities.

[0076] The advantages and preferable features of the first aspect apply equally to this aspect.

[0077] In a further aspect, the present invention provides a soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein: the paste flux comprises a solvent and an activator; the activator comprises one or more first organic acids and one or more second organic acids; the one or more first organic acids are selected from the group consisting of benzoic acid, formic acid, methyl succinic acid, 2-hydroxy-3-methylbutyric acid, succinic, pyruvic acid and glyoxylic acid; and the one or more second organic acids are selected from the group consisting of adipic acid, glutaric acid, dimethyl glutaric acid, suberic acid, pivalic acid, sebacic acid, azelaic acid, lauric acid, myristic acid and palmitic acid.

[0078] The advantages and preferable features of the first aspect apply equally to this aspect.

[0079] In a further aspect, the present invention provides a soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, when the paste flux is subjected to thermogravimetric analysis, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%.

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

[0081] The thermogravimetric analysis is preferably according to ASTM E1131-20.

[0082] The paste flux may comprise an activator as per the first aspect. Preferably, the activator comprises: one or more first organic acids having a pKa of less than or equal to 4.2 and exhibiting a total weight loss of greater than 80% by the solidus temperature of the solder when subjected to thermogravimetric analysis; and one or more second organic acids having a pKa of greater than 4.2 and exhibiting a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis.

[0083] As with the first aspect, the paste flux preferably comprises rosins. More preferably, the one or more rosins exhibiting a total weight loss of less than 25% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis.

[0084] In a further aspect, the present invention provides a solder paste comprising solder particles dispersed in a paste flux wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to a temperature 30 °C higher than the solidus temperature of the solder is no more than 25%.

[0085] The advantages and preferable features of the first aspect apply equally to this aspect.

[0086] In this aspect, the solder particles preferably comprise a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.01 to 0.5 wt.% nickel; one or more of: up to 7 wt.% antimony; up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; and the balance tin together with any unavoidable impurities.

[0087] In a further aspect, the present invention provides a paste flux wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131- 20, weight loss occurring in the range of from 196 to 245 °C (for example 220 to 245 °C) is no more than 25%.

[0088] The advantages and preferable features of the first aspect apply equally to this aspect.

[0089] In a further aspect, the present invention provides a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.01 to 0.5 wt.% nickel; one or more of: up to 7 wt.% antimony, up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; and the balance tin together with any unavoidable impurities.

[0090] The advantages and preferable features of the first aspect apply equally to this aspect. In a further aspect, the present invention provides a soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, when the paste flux is subjected to thermogravimetric analysis, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%.

[0091] The advantages and preferable features of the earlier aspects apply equally to this aspect.

[0092] In a further aspect, the present invention provides a soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, weight loss of the paste flux as a result of being subjected to the temperature profile is no more than 25% based on the total weight of the paste flux.

[0093] The advantages and preferable features of the earlier aspects apply equally to this aspect.

[0094] In a further aspect, the present invention provides a soldered joint formed using the method described herein and / or the solder paste described herein and / or the paste flux described herein and / or the alloy described herein. The advantages and preferable features of the earlier aspects apply equally to this aspect.

[0095] The invention will now be further described with reference to the following non- limiting examples and drawing in which:

[0096] Figure 1 shows X-ray images of reflowed solder pastes according to Examples 1 to 6 of the present invention and two reflowed conventional solder pastes. The paste-flux formulations were prepared by mixing various constituents at different temperature zones (25 to 120 °C) based on raw material characteristics.

[0097] A general description of each of the category of the raw materials and compositions are given below.

[0098] Example 1:

[0099] 88.2 to 88.8 weight % of solder powder and 11 .2 to 11 .8 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0100] Example 2: 88.4 to 88.9 weight % of solder powder and 11 .1 to 11 .6 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0101] Example 3:

[0102] 88.3 to 88.9 weight % of solder powder and 11 .1 to 11 .7 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0103] Example 4:

[0104]

[0105] 88.0 to 89 weight % of solder powder and 11 .0 to 12 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0106] Example 5:

[0107]

[0108] 88.4 to 89 weight % of solder powder and 11 .0 to 11 .6 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0109] Example 6:

[0110] 88.0 to 89.0 weight % of solder powder and 11 to 12 weight % of paste-flux was mixed together using a planetary mixer to obtain a homogeneous paste. The viscosity of the paste was found to be in the range of 800 to 2000 poise at 10 rpm and is suitable for the stencil printing application.

[0111] TESTS AND RESULTS:

[0112] Stability:

[0113] The solder paste of each example and comparative solder pastes were taken for various tests and results were analyzed. Paste stability in terms of storage life was tested by following the viscosity profile from time zero and one month at room temperature. Stability was also monitored visually to test if paste appearance changed over the time i.e. flux separation, paste drying, paste hardening, texture change etc. All the paste appearances look smooth, no drying or hardening was seen, and no flux separation was observed. Rheology profiles for pastes kept for 1 month at room temperature remained unchanged suggesting good stability of all the pastes.

[0114] Reliability:

[0115] The pastes of Example 1 and Example 2 were subjected to the reliability test Autorotative Damp Heat (ADH) SIR. Both solder pastes passed the electrical and visual requirements of the ADH test. No evidence of electrochemical migration was observed.

[0116] The solder paste of Example 1 was subjected to the test IPC SIR as per J-STD- 004B test. The solder paste passed the electrical and visual requirements of the SIR test. No evidence of electrochemical migration was observed.

[0117] Stencil printing:

[0118] The paste of Example 1 was subjected to continuous rolling for 8 hours and the paste appearance was monitored throughout the same period. The paste remained the same as time zero in terms of texture and behavior.

[0119] X-ray images:

[0120] To compare the void performance on a BTC component, the pastes of Examples 1 to 6 and two other commercial pastes were printed on a PCB having a MLF100 pad followed by component placement and reflow in a standard reflow profile with 240 °C peak temperature. Void performance of all the MLF100 components were compared by taking X-ray images. All the six pastes from Example 1 to Example 6 show significant decrease in void in comparison with two commercial pastes as presented in Figure 1 (from left to right, top to bottom: Conventional Paste 1 , Conventional Paste 2, Example 1 , Example 2, Example 3, Example 4, Example 5, Example 6).

[0121] 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 soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 25%.

2. The soldering method of claim 1 , wherein when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidus temperature of the solder to the highest temperature in the temperature profile is no more than 20%, preferably no more than 15%, more preferably no more than 10%, even more preferably no more than 5%, still even more preferably no more than 2%.

3. The soldering method of claim 1 or claim 2, wherein the paste flux comprises an activator.

4. The soldering method of claim 3, wherein the activator comprises an organic acid and / or an amine.

5. The soldering method of claim 3 or claim 4, wherein the activator comprises: one or more first organic acids having a pKa of less than or equal to 4.2 and exhibiting a total weight loss of greater than 80% by the solidus temperatureof the solder when subjected to thermogravimetric analysis according toASTM E1131-20; and one or more second organic acids having a pKa of greater than 4.2 and exhibiting a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20.

6. The soldering method of claim 5, wherein: the one or more first organic acids exhibit a total weight loss of greater than 90% by the solidus temperature of the solder when subjected to thermogravimetric analysis according to ASTM E1131-20, preferably greater than 95%, more preferably greater than 97%; and / or the one or more second organic acids exhibit a total weight loss of less than 15% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20, preferably less than 10%, more preferably less than 5%.

7. The soldering method of claim 5 or claim 6, wherein the one or more first organic acids have a boiling point of from 100 °C to less than 250°C.

8. The soldering method of claim 5 or claim 6, wherein the one or more first organic acids are selected from the group consisting of benzoic acid, formic acid, methyl succinic acid, 2-hydroxy-3-methylbutyric acid, succinic, pyruvic acid and glyoxylic acid.

9. The soldering method of any of claims 5 to 8, wherein the one or more second organic acids have a boiling point of greater than or equal to 250 °C.

10. The soldering method of any of claims 5 to 9, wherein the one or more second organic acids are selected from the group consisting of adipic acid,glutaric acid, dimethyl glutaric acid, suberic acid, pivalic acid, sebacic acid, azelaic acid, lauric acid, myristic acid and palmitic acid.11 . The soldering method of any preceding claim, wherein the activator preferably comprises from 70 to 90 wt.% of the one or more first organic acids and from 10 to 30 wt.% of the one or more second organic acids.

12. The soldering method of any preceding claim, wherein the paste flux comprises one or more solvents.

13. The soldering method of claim 12, wherein the one or more solvents have a boiling point of less than 280 °C, preferably from 150 to 270 °C, more preferably from 200 to 260 °C.

14. The soldering method of claim 12 or claim 13, wherein the one or more solvents comprise one or more glycol ethers.

15. The soldering method of claim 14, wherein one or both of the hydroxyl groups of the glycol ether is substituted with an alkyl group.

16. The soldering method of any of claims 12 to 15, wherein the one or more solvents are selected from the group consisting of ethylene glycol butyl ether, dipropylene glycol methyl ether, butyl carbitol acetate, ethylene glycol butyl ether acetate, dibutyl carbitol, triporpylene glycol butyl ether, and hexyl carbitol, preferably wherein the one or more solvents comprises dibutyl carbitol and hexyl carbitol.

17. The soldering method of any preceding claim, wherein the paste flux comprises one or more rosins exhibiting a total weight loss of less than 25% bythe highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20.

18. The soldering method of any preceding claim, wherein the paste flux comprises one or more rheology modifiers.

19. The soldering method of claim 18, wherein the rheology modifier comprises an amide-based rheology modifier and / or a castor oil-based rheology modifier.

20. The soldering method of any preceding claim, wherein the paste flux comprises: one or more polymers; and / or one or more corrosion inhibitors; and / or one or dispersants; and / or one or more defoaming agents.21 . The soldering method of any preceding claim, wherein the paste flux comprises, based on the total weight of the paste flux: from 1 to 15 wt.% activator; and / or from 20 to 40 wt.% solvent; and / or from 20 to 30 wt.% rosin; and / or from 1 to 6 wt.% rheology modifier; and / or from 1 to 15 wt.% polymer; and / or from 1 to 5 wt.% corrosion inhibitor from 1 to 15 wt.% dispersant; and / or from 1 to 5 wt.% defoaming agent.

22. The soldering method of any preceding claim, wherein the solder paste comprises, based on the total weight of the solder paste:from 85 to 90 wt.% solder particles; and from 10 to 15 wt.% paste flux.

23. The soldering method of any preceding claim, wherein the highest temperature in the temperature profile is less than or equal to 260 °C, preferably from 200 to 250 °C, more preferably from 220 to 245 °C.

24. The soldering method of any preceding claim, wherein one of the two or more work pieces comprises a micro lead frame (MLF) and / or a quad flat no-lead (QFN) package.

25. The soldering method of any preceding claim, wherein the solder particles have a liquidus temperature of from 210 °C to 260 °C, preferably from 212 °C to 250 °C, more preferably from 215 °C to 245 °C.

26. The soldering method of any preceding claim, wherein the solder particles comprise an alloy comprising from 2.5 to 3.5 wt.% silver, from 0.3 to 0.7 wt.% copper and the balance tin together with any unavoidable impurities.

27. The soldering method of any preceding claim, wherein the solder particles comprise a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.001 to 0.5 wt.% nickel; one or more of: up to 7 wt.% antimony, up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium,up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; and the balance tin together with any unavoidable impurities.

28. The soldering method of claim 27, wherein the solder alloy comprises from 2.8 to 4.5 wt.% silver, preferably from 2.8 to 4 wt.% silver29. The soldering method of claim 27 or claim 28, wherein the solder alloy comprises from 1 .0 to 4.0 wt.% bismuth, preferably from 2.0 to 4.0 wt.% bismuth, more preferably from 2.5 to 4 wt.% bismuth, even more preferably from 2.7 to 4 wt.% bismuth, still even more preferably from 2.9 to 4 wt.% bismuth.

30. The soldering method of any of claims 27 to 29, wherein the solder alloy comprises from 0.3 to 1 .2 wt.% copper, preferably from 0.4 to 0.8 wt.% copper.31 . The soldering method of any of claims 27 to 30, wherein the solder alloy comprises from 0.001 to 0.4 wt.% nickel, preferably from 0.01 to 0.3 wt.% nickel, more preferably from 0.02 to 0.2 wt.% nickel.

32. The soldering method of any of claims 27 to 31 , wherein the solder alloy comprises: from 1 to 7 wt.% of antimony; and / or from 0.001 to 6 wt.% indium; and / or from 0.01 to 0.5 wt.% titanium; and / or from 0.01 to 0.5 wt.% germanium; and / or from 0.001 to 0.5 wt.% rare earths; and / or from 0.001 to 0.5 wt.% cobalt; and / or from 0.001 to 5 wt.% aluminum; and / or from 0.001 to 5 wt.% silicon; and / or from 0.001 to 0.5 wt. % chromium; and / or from 0.001 to 0.5 wt. % manganese; and / or from 0.01 to 0.5 wt.% of iron; and / or from 0.001 to 0.5 wt.% of phosphorus; and / or from 0.001 to 0.5 wt.% of gold; and / or from 0.01 to 0.9 wt.% gallium; and / or from 0.001 to 0.5 wt.% of tellurium; and / or from 0.001 to 0.5 wt.% of selenium; and / or from 0.001 to 0.5 wt.% of calcium; and / or from 0.001 to 0.5 wt.% of vanadium; and / or from 0.001 to 0.5 wt.% of molybdenum; and / or from 0.001 to 0.5 wt.% of platinum; and / or from 0.001 to 0.5 wt.% of magnesium.

33. The soldering method of any of claims 27 to 32, wherein the solder alloy comprises one of titanium, germanium, indium, manganese, rare earths, cobalt,aluminium, silicon, chromium, iron, phosphorus, gold, gallium, tellurium, selenium, calcium, vanadium, molybdenum, platinum and magnesium, preferably selected from nickel, titanium, germanium, indium, manganese, rare earths, cobalt, silicon, iron and gallium, preferably one of titanium, germanium, manganese, cobalt and indium, more preferably one of titanium, germanium and manganese.

34. The soldering method of any of claims 27 to 33, wherein the solder alloy comprises at least 70 wt.% tin, preferably at least 80 wt.% tin, more preferably at least 84 wt.% tin.

35. The soldering method of any of claims 27 to 34, wherein in the solder alloy the wt.% of antimony is greater than the wt.% of bismuth, and preferably wherein and the sum of their wt.% is preferably greater than or equal to 6.5.

36. The soldering method of any of claims 27 to 35, wherein the solder alloy comprises 3.5 to 3.9 wt.% silver, 2.7 to 3.2 wt.% bismuth, 0.5 to 0.8 wt.% copper, 0.008 to 0.02 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

37. The soldering method of any of claims 27 to 36, wherein the solder alloy comprises 2.6 to 3.2 wt.% silver, 2.9 to 3.3 wt.% bismuth, 1 .9 to 2.3 wt.% antimony, 0.5 to 0.9 wt.% copper, 0.1 to 0.3 wt.% nickel, 2.9 to 3.5 wt.% indium, and the balance tin together with unavoidable impurities.

38. The soldering method of any of claims 27 to 37, wherein the solder alloy comprises 3.5 to 4 wt.% silver, 2.8 to 3.2 wt.% bismuth, 1 to 2 wt.% antimony, 0.5 to 0.8 wt.% copper, 0.08 to 0.2 wt.% nickel, and the balance tin together with unavoidable impurities.

39. The soldering method of any of claims 27 to 38, wherein the solder alloy comprises 2.8 to 3.2 wt.% silver, 2.8 to 3.2 wt.% bismuth, 5 to 7 wt.% antimony, 0.3 to 0.7 wt.% copper, 0.09 to 0.2 wt.% nickel, 0.002 to 0.01 wt.% titanium, and the balance tin together with unavoidable impurities.

40. The soldering method of any of claims 27 to 39, wherein the solder alloy comprises 3.1 to 3.6 wt.% silver, 3.3 to 3.8 wt.% bismuth, 0.5 to 0.8 wt.% copper, 3.3 to 3.8 wt.% antimony, 0.025 to 0.07 wt.% nickel, 0.03 to 0.07 wt.% cobalt, and the balance tin together with unavoidable impurities.41 . The soldering method of any of claims 27 to 40, wherein the solder alloy comprises 3.3 to 3.7 wt.% silver, 3.5 to 3.9 wt.% bismuth, 0.3 to 0.7 wt.% copper, 3.2 to 3.6 wt.% indium, 5.8 to 6.2 wt.% antimony, 0.04 to 0.08 wt.% nickel, 0.003 to 0.007 wt.% neodymium, and the balance tin together with unavoidable impurities.

42. The soldering method of any of claims 27 to 41 , wherein the solder alloy comprises 3.3 to 3.7 wt.% silver, 2.8 to 3.2 wt.% bismuth, 0.5 to 0.8 wt.% copper, 3.8 to 4.2 wt.% antimony, 0.03 to 0.07 wt.% nickel, 0.003 to 0.007 wt.% germanium, 0.01 to 0.04 wt.% silicon, and the balance tin together with unavoidable impurities.

43. A soldering method comprising: providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein: when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131-20, weight loss occurring in the range of from the solidustemperature of the solder to the highest temperature in the temperature profile is no more than 25%; the solder paste comprises from 85 to 90 wt.% solder particles and from 10 to 15 wt.% paste flux; the paste flux comprises, based on the total weight of the paste flux, from 1 to 15 wt.% activator, from 20 to 40 wt.% solvent, from 20 to 30 wt.% rosin and optionally from 1 to 6 wt.% rheology modifier, and / or from 1 to 15 wt.% polymer, and / or from 1 to 5 wt.% corrosion inhibitor, and / or from 1 to 15 wt.% dispersant and / or from 1 to 5 wt.% defoaming agent; the activator comprises, based on the total weight of the activator, from 70 to 90 wt.% of one or more first organic acids having a pKa of less than or equal to 4.2 and exhibiting a total weight loss of greater than 80% by the solidus temperature of the solder when subjected to thermogravimetric analysis according to ASTM E1131 -20 and from 10 to 30 wt.% of one or more second organic acids having a pKa of greater than 4.2 and exhibiting a total weight loss of less than 20% by the highest temperature of the temperature profile when subjected to thermogravimetric analysis according to ASTM E1131-20; and the solder particles comprise a lead-free solder alloy comprising: from 2.5 to 5 wt.% silver, from 0.01 to 5 wt.% bismuth, from 0.01 to 2 wt.% copper, from 0.01 to 0.5 wt.% nickel, one or more of: up to 7 wt.% antimony, up to 6 wt.% indium, up to 0.5 wt.% titanium, up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium, and the balance tin together with any unavoidable impurities.

44. A soldering method comprising:providing a solder paste between two or more work pieces to be joined, the solder paste comprising solder particles dispersed in a paste flux; and subjecting the solder paste to a temperature profile to reflow the solder particles and form a solder joint between the two or more work pieces, wherein: the paste flux comprises a solvent and an activator; the activator comprises one or more first organic acids and one or more second organic acids; the one or more first organic acids are selected from the group consisting of benzoic acid, formic acid, methyl succinic acid, 2-hydroxy-3-methylbutyric acid, succinic, pyruvic acid and glyoxylic acid; and the one or more second organic acids are selected from the group consisting of adipic acid, glutaric acid, dimethyl glutaric acid, suberic acid, pivalic acid, sebacic acid, azelaic acid, lauric acid, myristic acid and palmitic acid.

45. A solder paste comprising solder particles dispersed in a paste flux wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131 -20, weight loss occurring in the range of from the solidus temperature of the solder to a temperature 30 °C higher than the solidus temperature of the solder is no more than 25%.

46. The solder paste of claim 45, wherein the solder particles comprise a lead free solder alloy comprising: from 2.5 to 5 wt.% silver; from 0.01 to 5 wt.% bismuth; from 0.01 to 2 wt.% copper; from 0.01 to 0.5 wt.% nickel; one or more of: up to 7 wt.% antimony; up to 6 wt.% indium, up to 0.5 wt.% titanium,up to 0.5 wt.% germanium, up to 0.5 wt.% rare earths, up to 0.5 wt.% cobalt, up to 5.0 wt.% aluminium, up to 5.0 wt.% silicon, up to 0.5 wt.% manganese, up to 0.5 wt.% chromium, up to 0.5 wt.% iron, up to 0.5 wt.% phosphorus, up to 0.5 wt.% gold, up to 1 wt.% gallium, up to 0.5 wt.% tellurium, up to 0.5 wt.% selenium, up to 0.5 wt.% calcium, up to 0.5 wt.% vanadium, up to 0.5 wt.% molybdenum, up to 0.5 wt.% platinum, and up to 0.5 wt.% magnesium; and the balance tin together with any unavoidable impurities.

47. A paste flux wherein, when the paste flux is subjected to thermogravimetric analysis according to ASTM E1131 -20, weight loss occurring in the range of from 220 to 245 °C is no more than 25%.

48. A soldered joint formed using the method of any of claims 1 to 44 and / or the solder paste of claim 45 or 46 and / or the paste flux of claim 47.

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