Flux formulations and joining method

Abstract

Flux formulations and joining method A solder flux comprising: one or more amines; a zinc compound; and an organotin compound.

Description

[0001] Flux formulations and joining method

[0002] The present invention relates to a solder flux, a solder preform, a solder joint, an electrical device, an electric vehicle, a method of forming a solder joint and the use of a solder flux.

[0003] Manufacturers of EV batteries use copper busbars to form reliable and compatible electrical connections with electronic packages. It would be beneficial to use an aluminum busbar instead of a copper busbar, since aluminium is lighter and cheaper than copper. Aluminium busbars need to be connected with copper wire or terminals for current conduction. However, the direct attachment of aluminum to copper by mechanical fastening, such as bolting, is not a preferred solution as two metals having different potentials (E°) will result in galvanic corrosion in the presence of air and moisture at the interface.

[0004] Air-free joints between aluminum and copper can be formed by using laser welding. However, a very powerful laser system is required due to the thickness of the metals to be joined and the operation temperature may reach up to 700 °C. This is a practical limitation due to higher operational costs and safety concerns for the operator. An alternative is ultrasonic welding, which has a lower operational temperature, but it results in unacceptable material rugosity.

[0005] One solution to such a problem would be to fix a thin copper slice to the aluminum busbar where the cable will be connected. Air (oxygen) must be prevented from getting in between the two metals to avoid surface oxidation and eventual functional failure of the electrical connection. This can be achieved by adding a thin layer of electrically conductive material, such as a solder alloy, between the aluminum and copper layers using various possible soldering techniques. However, aluminium is a very difficult surface to solder on as the oxide layer present on it is extremely difficult to remove by using commercially available liquid fluxes that are used for soldering electrical and electronic components. Additionally, as very corrosive inorganic fluxing agents are required for this purpose, the ideal liquid flux to solder aluminum to copper to solder alloy (in any order of soldering) needs to have compatible flux residues that can be completely or partially removed, such as to prevent early onset of electrochemical migration.

[0006] There is a need for solder fluxes that are capable of wetting on Sn-Ag-Cu and other Pb-free solders, on aluminum, and on copper surfaces. This would enable the manufacture of an assembly of aluminum-solder-copper layers with no air gap between the layers.

[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 solder flux comprising: one or more amines; a zinc compound; and an organotin compound.

[0009] Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.

[0010] The inventors have surprisingly found that the flux may be effective at joining an aluminium surface and a copper surface with a lead-free solder alloy, such as a tin-based solder alloy. Advantageously, the aluminium surface and the copper surface may be joined with substantially no air gaps, typically no air gaps. In comparison to solder joints formed between an aluminium surface and a copper surface using conventional solder flux, the solder joint may exhibit improved thermo-mechanical reliability and / or improved electrochemical reliability and / or increased tensile strength. For example, solder joints prepared using the solder flux that are aged under standard thermomechanical stress conditions, such as 85 °C I 85 %RH for 1000 hours, or 125 °C for 3000 hours, may show only minimal reduction in tensile strength. Furthermore, reduction of joint strength after 1000 temperature cycles of -40 °C to 125 °C and 15 min dwell time may be very similar to that observed with the same solder alloy in copper-to-copper solder joints.

[0011] The one or more amines may remove the oxide layer from the aluminium surface and the copper surface, typically at temperatures of around 300 °C, which are typical reflow temperatures of lead-free (e.g. tin-based) solder alloys. This may enable effective wetting of the solder alloy on the aluminium surface and the copper surface, and may result in the formation of a strong solder joint. Advantageously, the one or more amines may be effective at removing the oxide layer from the aluminium surface without substantially corroding the copper surface.

[0012] The combination of the zinc compound and the organotin compound may enable better wetting of a lead-free solder alloy, in particular a tin-based solder alloy, to an aluminium surface. In particular, such wetting is improved in comparison to fluxes containing a zinc compound but no organotin compound, or an organotin compound but no zinc compound. In other words, such improved wetting appears to be due to the presence of both the zinc compound and the organotin compound in combination. Without being bound by theory, it is considered that during use tin ions and zinc ions undergo a redox reaction, dissolve the surface of aluminium and form a deposit thereon. Such a deposit may be more easily wetted with the solder alloy in comparison to a bare aluminium surface.

[0013] Advantageously, any flux residue formed during use may be soluble in water, typically at ambient temperature. Since the flux residue may be soluble in water, it may be removed from the solder joint simply by rising the solder joint with water, preferably deionised water. This may prevent chemical corrosion of the assembly during its service life, even at elevated temperature and / or humidity. The rinsing may be carried out at ambient temperature.

[0014] The solder flux is typically in the form of a liquid. In other words, the solder flux is typically a liquid solder flux. The solder flux preferably comprises a solvent, more preferably an aqueous solvent, more preferably water, such as deionised water. In other words, the solder flux may be in the form of a solution, typically an aqueous solution. The one or more amines, the zinc compound and the organotin compound are typically dissolved in the solvent. The solder flux preferably comprises from 1 to 10 wt.% solvent (e.g. water), more preferably from 2 to 6 wt.%, even more preferably from 3 to 5 wt.%.

[0015] The flux may be dispensed by any practical method such as, for example, a brush or spray or by dispensing.

[0016] The zinc compound preferably comprises zinc oxide or zinc salt, more preferably zinc halide salt, even more preferably zinc chloride salt. Such compound may be particularly suitable for forming a solderable deposit on aluminium. In addition, when such compounds are used, any flux residue may be particularly soluble in water. When a zinc halide salt or a zinc chloride salt is used, metals contained in the solder flux my form oxyhalides I oxychlorides, which may be particularly soluble in water. The organotin compound preferably activates at a temperature of from 250 to 350 °C. The term “activates” as used herein may encompass the state in which the organotin compound is reactive with the aluminium of the aluminium surface. Such temperatures are typical temperatures employed in a soldering method using lead-free solder alloys, typically tin-based solder alloy. In other words, such temperatures correspond to typical melting points of typical lead-free solder alloys, such as tin-based solder alloys.

[0017] The organotin compound preferably comprises an organotin salt. An organotin salt may be particularly effective at improving the wetting of an aluminium surface with tin-based solder alloy.

[0018] The organotin compound preferably comprises tin carboxylate, more preferably tin (II) carboxylate. Each carboxylate group preferably comprises from 4 to 12 carbon atoms, more preferably from 6 to 10 carbon atoms, even more preferably from 8 to 10 carbon atoms. The tin carboxylate preferably comprises tin(ll) 2- ethylhexanoate and / or tin(ll) neodecanoate. Such organotin compounds may be particularly effective at improving the wetting of an aluminium surface with lead- free solder alloy, in particular tin-based solder alloy.

[0019] The one or more amines preferably comprise primary, secondary and / or tertiary amine.

[0020] The one or more amines preferably comprise alkanolamine, preferably ethanol amine. The one or more amines preferably comprise one or more of primary ethanol amine, secondary ethanol amine and tertiary ethanol amine, preferably all three. The one or more amines preferably comprise one or more of monoethanol amine, diethanol amine, triethanol amine, monoisopropanolamine, diisopropanolamine and aminoethylethanol amine, more preferably wherein the one or more amines comprise monoethanol amine, triethanol amine and aminoethylethanol amine. The one or more amines preferably comprise one or more of 2-aminoethanol, 2,2',2"-nitrilotriethanol and 2-[(2-aminoethyl)amino]- ethanol, preferably all three. Such compounds may be capable of interacting with surface oxides of aluminum and forming soluble complexes which are easily removed exposing a clean metal surface for solder joint formation. Accordingly, such compounds may contribute to the ability of the flux to improve the wetting of a tin-based solder alloy to an aluminium surface and a copper surface. Such compounds also have high thermal stability, meaning that they can be employed at typical reflow temperatures of tin solder alloys. Furthermore, such compounds may act as surfactants. This may increase the ability to remove solder residue merely by rinsing with water. By including multiple types of amines, the flux may exhibit fluxing activity across a broader temperature range.

[0021] The solder flux preferably comprises less than 0.1 wt.% ethoxylated tallow amine, more preferably less than 0.01 wt.% ethoxylated tallow amine, even more preferably is substantially free of ethoxylated tallow amine. This may be beneficial in view of the high toxicity of ethoxylated tallow amine. Ethoxylated tallow amine used in conventional fluxes acts as a surfactant and helps the flux to spread on the surface evenly.

[0022] The solder flux preferably comprises less than 0.1 wt.% surfactant, more preferably less than 0.01 wt.% surfactant, even more preferably is substantially free of surfactant. This does not include the one or more amines, which may function as surfactants. Surfactants may not evaporate and / or degrade at typical tin-based solder reflow temperatures, meaning that the solder joint may be contaminated with organic residue, thereby degrading the mechanical properties of the solder joint. The use of small amounts of surfactants in conventional fluxes helps the flux spread evenly on the surface. The solder flux preferably comprises less than 0.1 wt.% ethoxylated phenols, more preferably less than 0.01 wt.% ethoxylated phenols, even more preferably is substantially free of ethoxylated phenols. This may be beneficial in view of the high toxicity of ethoxylated phenols and the fact that the use of such species is restricted in several jurisdictions.

[0023] In a preferred embodiment, the solder flux comprises, based on the total weight of the solder flux: from 20 to 35 wt.% weight monoethanol amine, more preferably from 25 to 30 wt.%; and / or from 10 to 30 wt.% triethanol amine, more preferably from 15 to 25 wt.%; and / or from 3 to 20 wt.% aminoethylethanol amine, more preferably from 5 to 15 wt.%. Such a solder flux may exhibit favourable fluxing activity across a broad temperature range.

[0024] In a preferred embodiment, the solder flux comprises, based on the total weight of the solder flux: from 20 to 35 wt.% weight monoethanol amine, preferably from 25 to 30 wt.%; from 10 to 30 wt.% triethanol amine, preferably from 15 to 25 wt.%; and from 3 to 20 wt.% aminoethylethanol amine, preferably from 5 to 15 wt.%. Such a solder flux may exhibit favourable fluxing activity across a broad temperature range.

[0025] The solder flux preferably comprises from 5 to 20 wt.% zinc chloride, more preferably from 10 to 15 wt.% zinc chloride.

[0026] The solder flux preferably further comprises fluoroboric acid. The solder flux preferably comprises from 1 to 20 wt.% fluoroboric acid, more preferably from 3 to 15 wt.% fluoroboric acid (50% aqueous solution). The solder flux preferably comprises from 0.5 to 10 wt.% fluoroboric acid, more preferably from 1 .5 to 7.5 wt.% fluoroboric acid. Fluoroboric acid may react with aluminum oxide and remove it from the aluminum surface leaving a clean metal to be soldered on later. The solder flux preferably further comprises tin fluoroborate. The solder flux preferably comprises from 1 to 15 wt.% tin fluoroborate, preferably from 3 to 10 wt.% fluoroborate (50% aqueous solution). The solder flux preferably further comprises from 0.5 to 7.5 wt.% tin fluoroborate, more preferably from 1.5 to 5 wt.% tin fluoroborate.

[0027] The solder flux preferably further comprises methanesulphonic acid. The solder flux preferably comprises from 3 to 15 wt.% methanesulphonic acid, more preferably from 5 to 12 wt.% methanesulphonic acid. Methanesulphonic acid may react with aluminum oxide and remove it from the aluminum surface leaving a clean metal to be soldered on later.

[0028] The solder flux preferably further comprises ammonium chloride. The solder flux preferably comprises from 1 to 5 wt.% ammonium chloride. The solder flux preferably further comprises ammonium ions and / or chloride ions. The solder flux preferably comprises from 0.4 to 1.7 w.% ammonium ions and / or from 0.6 to 3.3 wt.% chloride ions.

[0029] The solder flux preferably further comprises hydrochloric acid. The solder flux preferably comprises from 1 to 10 wt.% hydrochloric acid, more preferably from 2 to 6 wt.% hydrochloric acid (35% aqueous solution). The solder flux preferably comprises from 0.3 to 4 wt.% hydrochloric acid, more preferably from 0.6 to 2 wt.% hydrochloric acid. Hydrochloric acid may reacts with aluminum oxide and remove it from the aluminum surface leaving a clean metal to be soldered on later.

[0030] The solder flux preferably further comprises water, more preferably a balance of water. In other words, the solder flux is preferably an aqueous solution. The solder flux may comprise unavoidable impurities, for example up to 0.1 wt.% unavoidable impurities based on the total weight of the flux, or up to 0.01 wt.%, or up to 0.001 wt.%.

[0031] In a preferred embodiment, the solder flux comprises or consists of or consists essentially of: from 10 to 15 wt.% 2-[(2-aminoethyl)amino]-ethanol; from 25 to 30 wt.% 2-aminoethanol; from 15 to 20 wt.% 2,2',2"-nitrilotriethanol; from 10 to 15 wt.% zinc chloride; from 1 to 5 wt.% aluminium chloride; from 3 to 10 wt.% tin bis(tetrafluoroborate) (50% aqueous solution, alternatively 1 .5 to 5 wt.% tin bis(tetrafluoroborate)); from 5 to 15 wt.% tin(ll) 2-ethylhexanoate; from 3 to 15 wt.% tetrafluoroboric acid (50% aqueous solution, alternatively 1 .5 to 7.5 wt.% tetrafluoroboric acid); from 8 to 12 wt.% methanesulphonic acid (70% aqueous solution, alternatively 5.6 to 8.4 wt.% methanesulphonic acid) and balance water.

[0032] In a preferred embodiment, the solder flux comprises or consists of or consists essentially of: from 5 to 10 wt.% 2-[(2-aminoethyl)amino]-ethanol; from 25 to 30 wt.% 2-aminoethanol; from 20 to 25 wt.% 2,2',2"-nitrilotriethanol; from 10 to 15 wt.% zinc chloride; from 1 to 5 wt.% aluminium chloride; from 3 to 5 wt.% tin bis(tetrafluoroborate) (50% aqueous solution, alternatively 1 .5 to 2.5 wt.% tin bis(tetrafluoroborate)); from 5 to 15 wt.% tin(ll) 2-ethylhexanoate; from 3 to 15 wt.% tetrafluoroboric acid (50% aqueous solution, alternatively 1 .5 to 7.5 wt.% tetrafluoroboric acid); from 5 to 10 wt.% methanesulphonic acid (70% aqueous solution, alternatively 3.5 to 7 wt.% methanesulphonic acid); from 2 to 6 wt.% hydrochloric acid (35% aqueous solution, alternatively 0.7 to 2.1 wt.% hydrochloric acid) and balance water.

[0033] The term “consisting essentially of” as used herein may allow for the presence of additional components, as long as these components do not materially affect the essential characteristics of the solder flux.

[0034] In a further aspect, the present invention provides a solder flux comprising: one or more amines; zinc ions; and an organotin compound.

[0035] The advantages and preferable features of the first aspect apply equally to this aspect.

[0036] In a further aspect, the present invention provides a solder preform comprising: solder material; and the solder flux described herein.

[0037] The advantages and preferable features of the first aspect apply equally to this aspect.

[0038] In a further aspect, the present invention provides a solder paste comprising: solder material; and the solder flux described herein. The advantages and preferable features of the first aspect apply equally to this aspect.

[0039] The solder material preferably comprises: a tin-based alloy; and / or one or more of: tin, tin-silver alloy, tin-silver-copper alloy, tin-bismuth alloy and tin-silver-bismuth alloy; and / or one or more of Sn-Ag-Cu, Sn-Ag, Sn-Cu, Sn-Ag-Cu-Bi-Sb-Ni, Sn-ln, Sn- Bi, Sn-ln-Ag, Sn-Bi-Ag, Sn-ln-Ag-Cu, Sn-Bi-Ag-Cu, Sn-ln-Ag-Cu-Sb, Sn-Bi-Ag- Cu-Sb, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, Sn-Ag-Cu-ln-Ni-Bi, Sn-Bi-ln, Sn-Bi-ln-Ag, that may optionally include other minor elemental additions up to 1 wt.%; and / or 96.5Sn-3Ag-0.5Cu.

[0040] The solder material is preferably in the form of, for example, a wire or strip. The solder flux is typically disposed on the outer surface of the solder material. In other words, the solder flux may be in the form of a coating on the solder material. The solder flux may coat the entirety of the outer surface of the solder material or part of the outer surface of the solder material.

[0041] In a further aspect, the present invention provides a soldered joint manufactured using the solder flux as described herein or the preform as described herein.

[0042] The advantages and preferable features of the first aspect apply equally to this aspect.

[0043] In a further aspect, the present invention provides an electrical device, preferably a battery, more preferably an electric vehicle (EV) battery, comprising the soldered joint described herein. The advantages and preferable features of the first aspect apply equally to this aspect.

[0044] In a further aspect, the present invention provides an electric vehicle comprising the electric vehicle battery described herein.

[0045] In a further aspect, the present invention provides a method of forming a solder joint between a first surface and a second surface, wherein the first surface comprises aluminium and the second surface comprises copper, the method comprising: disposing the solder preform described herein in the vicinity of the first surface and the second surface; and heating the solder preform to form a solder joint between the first surface and the second surface.

[0046] The advantages and preferable features of the first aspect apply equally to this aspect.

[0047] In the vicinity of the first surface and the second surface typically comprises in contact with the first surface and the second surface, more typically in between the first surface and the second surface.

[0048] Since the flux is capable of removing oxides from the aluminium surface without substantially corroding the copper surface, a single heating step may be employed using a single solder flux. This may speed up the method and / or reduce the cost of the method and / or reduce the environmental impact of the method.

[0049] The heating preferably comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds. The heating may be carried out using, for example, conduction heating of the parts on a hot plate, convection heating in a box or conveyer oven, induction heating, resistance welding or any other heating method.

[0050] In a further aspect, the present invention provides a method of forming a solder joint between a first surface and a second surface, wherein the first surface comprises aluminium and the second surface comprises copper, the method comprising: disposing the solder preform described herein in the vicinity of the first surface; heating the solder preform to form a solder deposit on the first surface; disposing the solder flux described herein in the vicinity of the solder deposit and the second surface; and heating the flux and solder deposit to form a solder joint between the first surface and the second surface.

[0051] In the vicinity of the first surface typically comprises in contact with the first surface. In the vicinity of the solder deposit and the second surface typically comprises in contact with the solder deposit and the second surface.

[0052] Heating the solder preform to form a solder deposit on the first surface preferably comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds; and / or heating the flux and solder deposit to form a solder joint between the first surface and the second surface comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds. The heating may be carried out using, for example, conduction heating of the parts on a hot plate, convection heating in a box or conveyer oven, induction heating, resistance welding or any other heating method. Preferably, the method further comprises rinsing the solder joint, more preferably using water, even more preferably using deionized water. The rinsing is preferably carried out at ambient conditions. The rinsing may remove solder residue. The solder joint may undergo mechanical agitation and / or sonication during the rinsing. This may aid removal of solder residue. Following rinsing, the solder joint may not show any evidence of corrosion after aging at 40 °C 1 90 %RH for 10 days.

[0053] Preferably, the first surface is a surface of an aluminium busbar and the second surface is a surface of a copper wire, copper terminal and / or copper plate.

[0054] Preferably, the aluminium busbar is part of a battery, more preferably an electric vehicle battery; and / or the copper wire, copper terminal and / or copper plate is part of a battery, more preferably an electric vehicle battery.

[0055] Alternatively, preferably the first surface is a surface of an aluminium heat sink and the second surface is a surface of a copper power substrate. This may be beneficial in power electronics applications.

[0056] In a further aspect, the present invention provides use of the solder flux described herein and / or the solder preform described herein in the formation of a solder joint between a copper workpiece and an aluminium workpiece.

[0057] The advantages and preferable features of the first aspect apply equally to this aspect.

[0058] The invention will now be further described with reference to the following nonlimiting examples. Flux formulations were prepared by mixing various constituents at different temperatures (25 to 80 °C) based on raw material characteristics.

[0059] Example 1 :

[0060] Example 2: Example 3:

[0061] Example 4:

[0062] Example 5:

[0063] Example 6: A soldered assembly was prepared using the solder flux of Example 4. A drop of the flux was dispensed on an aluminum coupon and a SAC305 solder preform of dimension 6mm x 6mm x 0.2mm was placed on the fluxed area. One more drop of the flux was dispensed on the solder preform and then a copper piece of dimension 6mm x 6mm x 2.5mm was placed on the fluxed solder preform. The assembly was then heated on a Direct Heat or using induction heating and then cleaned with water to remove flux residue from the assembly.

[0064] Shear testing was carried out under the following conditions:

[0065] Instrument: Dage 4000

[0066] Test Load: 100 KG

[0067] Test Speed: 100 pm / sec Land Speed: 500 pm / sec Shear Height: 100 pm

[0068] The solder joints showed very high joint strength, as indicated in Figure 1 . Assemblies made with both heating processes showed bulk failure after the shear test. The effects of aging at high temperature on the die shear are shown in Figures 2 and 3. SEM images of the solder joints are shown in Figure 4 (direct heat) and Figure 5 (induction). The SEM images show the presence of intermetallic compound Ag2AI at the interface of aluminium and SAC305, which indicates the metallic bond formation between these two metallic species.

[0069] 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 equivalent

Claims

Claims1. A solder flux comprising: one or more amines; a zinc compound; and an organotin compound.

2. The solder flux of claim 1 , wherein the zinc compound comprises zinc oxide or zinc salt, preferably zinc halide salt, more preferably zinc chloride salt.

3. The solder flux of claim 1 or claim 2, wherein the organotin compound activates at a temperature of from 250 to 350 °C.

4. The solder flux of any preceding claim, wherein the organotin compound comprises an organotin salt.

5. The solder flux of any preceding claim, wherein the organotin compound comprises tin carboxylate, preferably tin (II) carboxylate.

6. The solder flux of claim 5, wherein each carboxylate group comprises from 4 to 12 carbon atoms, preferably from 6 to 10 carbon atoms, more preferably from 8 to 10 carbon atoms.

7. The solder flux of claim 5 or claim 6, wherein the tin carboxylate comprises tin(ll) 2-ethylhexanoate and / or tin(ll) neodecanoate.

8. The solder flux of any preceding claim, wherein the one or more amines comprise primary, secondary and / or tertiary amine.

9. The solder flux of any preceding claim, wherein the one or more amines comprise alkanolamine.

10. The solder flux of any preceding claim, wherein the one or more amines comprise one or more of monoethanol amine, diethanol amine, triethanol amine, monoisopropanolamine, diisopropanolamine and aminoethylethanol amine, preferably wherein the one or more amines comprise monoethanol amine, triethanol amine and aminoethylethanol amine.11 . The solder flux of any preceding claim, wherein the one or more amines comprise one or more of primary ethanol amine, secondary ethanol amine and tertiary ethanol amine, preferably all three.

12. The solder flux of any preceding claim, wherein the one or more amines comprise one or more of 2-aminoethanol, 2,2',2"-nitrilotriethanol and 2-[(2- aminoethyl)amino]-ethanol, preferably all three.

13. The solder flux of any preceding claim, wherein the solder flux comprises less than 0.1 wt.% ethoxylated tallow amine, preferably less than 0.01 wt.% ethoxylated tallow amine, more preferably is substantially free of ethoxylated tallow amine.

14. The solder flux of any preceding claim, wherein: the solder flux comprises less than 0.1 wt.% surfactant, preferably less than 0.01 wt.% surfactant, more preferably is substantially free of surfactant; and / or the solder flux comprises less than 0.1 wt.% ethoxylated phenols, preferably less than 0.01 wt.% ethoxylated phenols, more preferably is substantially free of ethoxylated phenols.

15. The solder flux of any preceding claim comprising, based on the total weight of the solder flux: from 20 to 35 wt.% weight monoethanol amine, preferably from 25 to 30 wt.%; and / or from 10 to 30 wt.% triethanol amine, preferably from 15 to 25 wt.%; and / or from 3 to 20 wt.% aminoethylethanol amine, preferably from 5 to 15 wt.%.

16. The solder flux of any preceding claim comprising, based on the total weight of the solder flux: from 20 to 35 wt.% monoethanol amine, preferably from 25 to 30 wt.%; from 10 to 30 wt.% triethanol amine, preferably from 15 to 25 wt.%; and from 3 to 20 wt.% aminoethylethanol amine, preferably from 5 to 15 wt.%.

17. The solder flux of any preceding claim, comprising from 5 to 20 wt.% zinc chloride, preferably from 10 to 15 wt.% zinc chloride.

18. The solder flux of any preceding claim, further comprising fluoroboric acid.

19. The solder flux of claim 18, comprising from 1 to 20 wt.% fluoroboric acid, preferably from 3 to 15 wt.% fluoroboric acid (50% aqueous solution).

20. The solder flux of any preceding claim, further comprising tin fluoroborate.21 . The solder flux of claim 20, comprising from 1 to 15 wt.% tin fluoroborate, preferably from 3 to 10 wt.% fluoroboric acid (50% aqueous solution).

22. The solder flux of any preceding claim, further comprising methanesulphonic acid.

23. The solder flux of claim 22, comprising from 3 to 15 wt.% methanesulphonic acid, preferably from 5 to 12 wt.% methanesulphonic acid.

24. The solder flux of any preceding claim, further comprising ammonium chloride (or ammonium ions and chloride ions).

25. The solder flux of claim 24, comprising from 1 to 5 wt.% ammonium chloride (or from 0.4 to 1 .7 w.% ammonium ions and from 0.6 to 3.3 wt.% chloride ions).

26. The solder flux of any preceding claim, further comprising hydrochloric acid.

27. The solder flux of claim 26, comprising from 1 to 10 wt.% hydrochloric acid, preferably from 2 to 6 wt.% hydrochloric acid (35% aqueous solution).

28. The solder flux of any preceding claim, further comprising water, preferably a balance of water.

29. The solder flux of any preceding claim comprising or consisting of: from 10 to 15 wt.% 2-[(2-aminoethyl)amino]-ethanol; from 25 to 30 wt.% 2-aminoethanol; from 15 to 20 wt.% 2,2',2"-nitrilotriethanol; from 10 to 15 wt.% zinc chloride; from 1 to 5 wt.% aluminium chloride; from 3 to 10 wt.% tin bis(tetrafluoroborate) (50% aqueous solution); from 5 to 15 wt.% tin(ll) 2-ethylhexanoate; from 3 to 15 wt.% tetrafluoroboric acid (50% aqueous solution); from 8 to 12 wt.% methanesulphonic acid (70% aqueous solution) and balance water- 22 -30. The solder flux of any of claims 1 to 27 comprising or consisting of: from 5 to 10 wt.% 2-[(2-aminoethyl)amino]-ethanol; from 25 to 30 wt.% 2-aminoethanol; from 20 to 25 wt.% 2,2',2"-nitrilotriethanol; from 10 to 15 wt.% zinc chloride; from 1 to 5 wt.% aluminium chloride; from 3 to 5 wt.% tin bis(tetrafluoroborate) (50% aqueous solution); from 5 to 15 wt.% tin(ll) 2-ethylhexanoate; from 3 to 15 wt.% tetrafluoroboric acid (50% aqueous solution); from 5 to 10 wt.% methanesulphonic acid (70% aqueous solution); from 2 to 6 wt.% hydrochloric acid (35% aqueous solution) and balance water.

31. A solder flux comprising: one or more amines; zinc ions; and an organotin compound.

32. A solder preform comprising: solder material; and the solder flux of any of claims 1 to 31.

33. The solder preform of claim 32, wherein the solder material comprises: one or more of: tin, tin-silver alloy, tin-silver-copper alloy, tin-bismuth alloy and tin-silver-bismuth alloy; and / or one or more of Sn-Ag-Cu, Sn-Ag, Sn-Cu, Sn-Ag-Cu-Bi-Sb-Ni, Sn-ln, Sn- Bi, Sn-ln-Ag, Sn-Bi-Ag, Sn-ln-Ag-Cu, Sn-Bi-Ag-Cu, Sn-ln-Ag-Cu-Sb, Sn-Bi-Ag- Cu-Sb, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, Sn-Ag-Cu-ln-Ni-Bi, Sn-Bi-ln, Sn-Bi-ln-Ag, that may optionally include other minor elemental additions up to 1 wt.%; and / or- 23 -96.5Sn-3Ag-0.5Cu.

34. A soldered joint manufactured using the solder flux of any of claims 1 to 31 or the preform of claim 32 or 33.

35. An electrical device, preferably a battery, more preferably an electric vehicle (EV) battery, comprising the soldered joint of claim 34.

36. An electric vehicle comprising the electric vehicle battery of claim 35.

37. A method of forming a solder joint between a first surface and a second surface, wherein the first surface comprises aluminium and the second surface comprises copper, the method comprising: disposing the solder preform of claim 32 or 33 in the vicinity of the first surface and the second surface; and heating the solder preform to form a solder joint between the first surface and the second surface.

38. The method of claim 37, wherein the heating comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds.

39. A method of forming a solder joint between a first surface and a second surface, wherein the first surface comprises aluminium and the second surface comprises copper, the method comprising: disposing the solder preform of claim 32 or 33 in the vicinity of the first surface; heating the solder preform to form a solder deposit on the first surface; disposing the solder flux of any of claims 1 to 31 in the vicinity of the solder deposit and the second surface; and- 24 -heating the flux and solder deposit to form a solder joint between the first surface and the second surface.

40. The method of claim 39, wherein: heating the solder preform to form a solder deposit on the first surface comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds; and / or heating the flux and solder deposit to form a solder joint between the first surface and the second surface comprises heating to a temperature of from 250 to 350 °C, preferably for from 5 to 200 seconds.41 . The method of any of claims 37 to 40, further comprising rinsing the solder joint, preferably using water.

42. The method of any of claims 37 to 41 , wherein the first surface is a surface of an aluminium busbar and the second surface is a surface of a copper wire, copper terminal and / or copper plate.

43. The method of claim 42, wherein: the aluminium busbar is part of a battery, preferably an electric vehicle battery; and / or the copper wire, copper terminal and / or copper plate is part of a battery, preferably an electric vehicle battery.

44. Use of the solder flux of any of claims 1 to 31 and / or the solder preform of claim 32 or claim 323 in the formation of a solder joint between a copper workpiece and an aluminium workpiece.- 25 -

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