GB2110249A - Fluxes - Google Patents

Abstract

A flux composition for soldering comprises a vehicle, for example polyethylene glycol, and one or more metallic iodides. The use of such fluxes in soldering, for example plumbing joints, overcomes corrosion problems associated with the use of conventional fluxes.

Description

SPECIFICATION Fluxes This invention relates to inorganic fluxes for soldering and in particular, but not solely, to fluxes such as are suitable for use in plumbing applications.

Soldering fluxes serve the function of removing or preventing the formation of oxides on metal surfaces to be soldered. Certain reactive fluxes may also participate in other reactions which improve the spreading properties of the solder, e.g. as a result of the formation of intermetallic compounds on the surface(s) of the metal(s) to be soldered. Even where the metal surfaces are cleaned by abrasion and/or degreasing solvents prior to soldering, the heat required to melt the solder will usually cause at least some oxidation of the metal surfaces and the oxides so formed may seriously interfere with the formation of a sound soldered joint unless a flux is present. The nature of the flux depends on the nature and surroundings of the materials being soldered.Thus in soldering individual electronic components resin fluxes are commonly used and many proprietary brands of wire solder have resin cores. In plumbing the flux has to satisfy public health standards as well as ensuring sound soldered joints.

Conventional plumbing fluxes are typically based on zinc chloride or a mixture of zinc and ammonium chlorides. These materials are commonly sold commercially in the form of pastes based on a petroleum or other similar jelly. One significant disadvantage of zinc chloride based soldering fluxes is that they are corrosive in particular towards the copper or copper based alloys which are commonly used in domestic and industrial water systems, e.g. in domestic cold water supply systems, secondary hot water systems and heating systems. The chemical nature of zinc chloride can engender pitting corrosion which, once started, is self perpetuating under some conditions and can lead to penetration of the metal adjacent to the soldered joint.This is particularly important in modern plumbing practice especially with regard to original plumbing in new dwellings where the water system may be left empty of water for weeks or months after the joints in it are soldered. These conditions are particularly favourable to the production of corrosion pits.

A further consideration which is becoming particularly important with the introduction of new regulations is that the flux should not give rise to residues which support algal or fungal growth. This means that residual flux or its vehicle should not decompose to produce growth media under soldering conditions.

Some of the organic media used in prior products are unsuitable when judged on this basis.

The present invention accordingly provides a method of soldering one or more metal surfaces which method comprises providing an inorganic flux comprising one or more metallic iodides in fluxing proximity to the metal surface and the solder and heating the metal surface(s) and/or the solder whereby the solder is melted and wets the metal surface(s).

As used herein the term "inorganic flux" means a flux the active fluxing components of which are inorganic compounds. In particular the term distinguishes fluxes of the present invention from rosin-based fluxes. Of course, as will become apparent from the following description, organic compounds may be included in a flux composition useful in the invention but such organic materials will not be present as materials having a fluxing function. Thus organic materials may be included as or in a vehicle and in this function may but need not be volatile, or they maybe present in relatively small quantities, e.g. for thickeners and surface active agents. Reference herein to "metalic iodides" includes ammonium iodide and mixed metal-ammonium iodides.For convenience herein we use the term "flux" to refer to the materials which contribute to fluxing activity and the term "flux composition" to refer to practical forms of the soldering flux which may include vehicles, thickeners, wetting agents, etc.

The invention also includes a flux composition comprising a vehicle and as active fluxing constituents one or more metallic iodides.

We do not fully understand why the invention works. There are a number of mechanisms which may contribute to successful soldering with iodide-based fluxes. The simplest mechanism involves the physical or chemical dissolution of oxide films on the surface of the substrate. However, it seems that other mechanisms may be involved. Thus we have observed that, at soldering temperatures, some metallic iodides dissociate or decompose liberating iodine. Particularly where the substrate surface(s) are of copper or copper-containing alloy, such liberated iodine may react with the surface of the metal or any oxide film on the surface to give a thin coating of cuprous iodide which can aid wetting of the substrate surface and thus spreading of the solder.Other possible mechanisms include the chemical or electrochemical deposition of metal films, in particular tin or lead films. Such mechanisms have been postulated for chloride-based fluxes and we believe that analogous reactions may take place with the iodide fluxes used in the invention. In particular the formation of tin films and, on copper-containing substrates, tin-copper intermetallic compound films have been observed. The tin can be derived from the solder but the effect can be promoted by the inclusion of suitable materials, e.g. especially tin compounds, in the flux. In many cases more than one of these mechanisms may operate and, indeed, other fluxing mechanisms may be involved.

The mechanism of formation of corrosion pits in copper-containing metals is believed to be based on a redox cycle involving both Cul and Cull in solution. With chloride as anion both of these species can be present in solution, CUCIP being readily soluble in water and, although CuCI is sparsely soluble in water, the complex ion CuCI2- is water soluble. Also the redox half cell potential Cu2+(Cl#)/CuCl is about +0.54V.

However, with iodide as anion, Cul2 is unstable in the presence of water (decomposing to Cul and iodine), the redox half cell potential Cu2+(1-)/Cul is about +0.86V thus substantially favouring the formation of Cu' rather than Cu", and Cul is very insoluble (solubility product ~ 1 o-12)10-12) and tends to form a barrier layer on the copper surface. These factors appear to act to diminish corrosion by flux residues after soldering by the method of the invention and in particular to the prevention of the formation of corrosion pits.

Whatever the mechanisms involved in fluxing and avoidance of corrosion by flux residues whether or not the above suggestions are correct, the method of the invention is effective in particular in plumbing or similar applications and especially in soldering copper-containing metals.

The composition of the flux selected for a particular situation will depend largely on the nature of the substrate and on the soldering temperature. The following description includes information on the composition of fluxes suitable for use in the present invention. Thus, as is usual with soldering fluxes, the flux will itself be or will contain a component which at soldering temperature is fluid (liquid or gaseous) in order to ensure that the metal surface(s) to be soldered come into intimate contact with the flux. This is a general requirement of fluxes as at present used although we believe that it does not represent a fundamental limitation of the present invention.

The flux may be a single iodide. The iodides of Bi, Cu, Fe, NH4, Ni, Sn and Zn can be used. However, the temperatures at which most of these compounds melt, dissociate or decompose are rather higher than it is generally convenient to use. It is generally more convenient to use a mixture of iodides which form a co-solution having a lowest temperature of complete crystallisation less than the temperature at which the soldering is performed. Suitable co-solution mixtures can be made from the iodides of Bi, Cu, Fe, K, Li, Mg, Na, NH4, Ni, Sn and Zn. It is preferable, particularly in binary mixtures, to use eutectic or near eutectic, e.g.

within 1 20% by weight of the eutectic composition, mixtures. One particularly preferred mixture is the eutectic between Znl2 and NH41 which occurs at about 48 mol % NH41.

We have found that the activity of a flux can be increased and/or that the temperature at which it becomes active can be reduced by the inclusion therein of a minor proportion, e.g. up to 20 mol % and typically 0.1 to 10, particularly 0.5 to 5, mol %, of a different iodide. The iodides of Ag, NH4 and Sn (both Sn" and Sn'V) are useful in this role. These activating iodides can be effective in combination with single iodides or mixtures.

Where single iodides or mixtures of iodides which are molten at soldering temperature are used the activator can be present as a solid dispersion in the melt or as a fluid component of the melt. Particularly where such activated fluxes are entirely molten at soldering temperature it is convenient that the composition approach that of the ternary eutectic (if any).

A particularly preferred example of the activated type offlux is the ternary system Znl2:NH41:Snl2 or SnI4.

The inclusion of small quantities of Sn" or Sniv iodide, e.g. 0.5 to 5 mol %, in the eutectic mixture of Znl and NH41 does not appear to have a very great effect on the eutectic composition.

The fluxes used in the method and composition of the invention are preferably substantially free of halogen other than iodine. In particular and very preferably they contain substantially no chlorine or chloride (ionic or covalent) because chlorine and chloride containing residues can promote pitting corrosion even when present in relatively small amounts. The fluxes also are preferably and can readily be made free of other corrosive anions, such as sulphate and the various forms of phosphate.

As has been briefly indicated above, flux compositions will usually contain other components. Thus the flux composition will usually include a vehicle or carrier and may optionally include further additives as discussed below. The vehicle or carrier should satisfy the requirement set out above of not leaving bio-degradable residues. Typically suitable vehicles include volatile organic and inorganic solvents known as flux vehicles, e.g. water, alcohols including isopropanol, dimethyl formamide and ethylene glycol. Other materials, in particular polymeric polyhydroxy alcohols especially glycols or solvents of metallic iodides, which have not been previously proposed as flux vehicles such as propylene carbonate can also be used. Of course, approved gel carriers, e.g. petroleum jelly, can be used if desired.

Where simple (non-gel) vehicles are used the simple flux/vehicle composition may take the form of a paste or solution. For relatively thin pastes and solutions it may be convenient to thicken them. Suitable inert thickeners can be used, and in particular thixotropic additives. However, many thixotropic clays contain aluminium as complex oxides or silicates and the presence of these forms of aluminium may cause difficulties in soldering if added in other than very small quantities (i.e. about 2%). Also, if an organic thixotropic additive is used then care should be taken in selecting it that it will not decompose particularly be carbonization unduly at soldering temperature or leave behind substantial amounts of bio-degradable residue.

The use of polymeric polyhydroxy alcohols is referred to breifly above. In particular the polyoxyethylene polyols and especially the polyethylene glycols (PEGs) are preferred. The polyethylene glycols are dihydroxy polyether polyols of the general formula HO(CH2.CH2.O)nH. Clearly the nature of the materials depends on the average degree of polymerization (i.e. the average value of n) which is reflected in the average molecular weight. We have found that a wide range of commercially available polyethylene glycols can be used as or in vehicles for flux compositions of the present invention. PEG's having average molecular weights of from 200 to 9000 are commercially available. Diethylene, triethylene and tetraethylene glycols are considered for present purposes as PEGs having molecular weights of less than 200; they are available as individual compounds rather than the mixtures of compounds of the commercially available PEGs identified by their average molecular weights. Polyethylene glycols having molecular weights up to about 600 are generally liquid at ambient temperature. Those of higher molecular weight are waxy solids with their melting point (ranges) increasing with increasing molecular weight. At present the highest molecular weight available commercially is PEG 9000 (mw 9,000 - 10,000). If and when higher molecular weight PEG's become available then they may well be useful in the present invention.However, since the viscosity of PEG's in the liquid phase increases with increasing molecular weight the viscosity of such higher molecular weight PEG's may be so great at soldering temperatures at to render them less effective.

The PEGs which are liquid at ambient temperature may be used as liquids for vehicles as is described above. However, being much less volatile the organic vehicles mentioned above they will in large part remain around the soldered joint after soldering. This is not disadvantageous because they are not biodegradable and are readily water soluble and are thus removed on washing or flushing out plumbing systems with water. This utility of liquid PEG's is not particularly preferred because they can be used in a much more advantageous fashion.

We have found that by mixing a PEG of relatively high molecular weight in particular PEGs 600 to 9000, with a diluent such as water, ethylene or propylene glycols or a low molecular weight polyethylene glycol, i.e. having a molecular weight of not more than 400 pasty mixtures can be made which form highly suitable vehicles for flux compositions of the invention. In particular we have found that PEG's 1,000 to 6,000 especially PEG 1500 to 4000 are suitable as the higher molecular weight PEG and that PEG 200 or di-, tri- or tetraethylene glycol are suitable as the diluent. The range over which pasty mixtures can be formed is also a function of the flux used. As is further discussed below NH4l is a particularly useful as the flux with PEG vehicles.Using 20% by wt N H41 on the vehicle composition we have found that mixtures which are pasty at 200C can be formed in the range PEG 1500: PEG 200 of 85:15 to 60:40. We do not expect that the man skilled in the art will have any difficulty in making up pasty formulations using PEGs.

Although ethylene glycol is mentioned above as a possible diluent for the vehicle it is not generally as satisfactory as the polymerized materials in that it is somewhat toxic and irritant and it appears to form pasty mixtures only over a rather narrow range of compositions. Since the pasty mixtures tend to set solid on reducing and to melt on increasing the temperature it may be difficult if not impossible to produce a formulation having satisfactory stability in this respect. With the polymerized materials e.g. PEG 200 the relatively wide composition range over which pasty mixtures can be made at ambient temperature reflects a broader range of temperatures over which such pasty mixtures, especially those in the middle of the composition range, are stable.

Water is not as effective a diluent as the low molecular weight PEG's in producing pasty mixtures, particularly in that, as with ethylene glycol, the composition range to form pasty mixtures is relatively narrow. including water in the flux composition can be beneficial because it boils at a temperature below that of normal soldering and in boiling appears to aid cleaning e.g. removing oxide from the surface (particularly if of copper or copper alloy) to be soldered. The most convenient may of at least partially combining the advantages of having water present in the flux composition with a wide pasty composition range seems to be to use as a vehicle a mixture of a relatively high molecular weight PEG e.g. PEG 1500, a relatively low molecular weight PEG e.g. PEG 200 and water.

Above we mentioned that NH41 is a particularly useful flux for use in vehicles using PEGs. This is because NH41 is a highly active flux and is moderately soluble in PEGs of various molecular weights. This makes it relatively simple to make a flux composition which is uniform at ambient temperature and is uniform and fluid at soldering temperature. The capacity of NH41 in PEG based vehicles to form fluid fluxes at soldering temperatures removes the necessity of ensuring that the flux itself is itself fluid at such temperatures. This means that the advantage of using entectic or near entectic compositions for the flux do not really apply.

Indeed since NH41 seems to be the most active metallic iodide flux the inclusion of other metal iodides seems to reduce the activity of NH41 fluxes in PEG based vehicles. The proportion of NH41 used in such compositions will typically be from 0.1 to 25% more usually 0.5 to 20% by weight of the composition, the particular proportion depending on the individual circumstances.

Although we note above that low molecular weight diluents such as water can produce flux compositions having only a narrow pasty range we have found that, particularly with NH41, active flux compositions can be made having low concentrations e.g. from 0.5 to 5% by weight on fthe flux composition, of the inorganic flux. Such low concentrations do not necessarily result in narrow pasty ranges even though higher concentrations of the inorganic flux might give only a very limited (or in extreme cases non-existent) pasty range. This makes the use of water as a diluent more effective and reduce the need to use a low molecular weight PEG in the diluent.

Accordingly the invention includes a flux composition which comprises one or more metallic iodides, preferably ammonium iodide, in a vehicle which comprises one or more polyethylene glycols. Preferably such compositions have a pasty consistency and comprise as the vehicle a polyethylene glycol having a nominal average molecular weight of from 600 to 9000 in combination with a polyethylene glycol having a nominal averge molecular weight of up to 400 and/or water in relative proportions such that the flux composition (including the metallic iodide) has a pasty consistency at ambient temperature. The method of the invention includes a method of soldering in which the flux composition of the invention is employed.

The flux composition of the invention can be made by mixing the components of the composition and, if necessary, warming and/or stirring them. The pasty composition will usually be warmed to a temperature at which they are liquid thus attaining a pasty consistency on cooling. Where not all the material e.g. the flux, dissolves in the mixture it can readily be uniformly incorporated into the pasty flux compositions by stirring it into the composition during cooling.

The literature on polyethylene oxides indicates that they are liable to oxidative degradation at elevated temperatures. Although some samples we have tried have fumed somewhat, it seems that degradation is not extensive, possibly because the commercially available materials contain stabilizers, and that it does not give rise to substantial carbonation or the formation of water insoluble or biodegradable products.

As is discussed above, the invention is particularly applicable to soldering copper and its alloys. The solder used is not particularly critical and those conventionally used for soldering such metals are suitable for use in the invention. Typical of such solders are those set out in B.S. 219 of 1977. The particular flux of the invention selected for any particular situation will, of course, depend on the solder and in particular on the temperature at which it is used.

Some of the fluxes used in the flux compositions of the invention can hydrolyse or produce products which hydrolyse generating free acid. Such acid materials could cause corrosion, e.g. of containers holding the flux. This corrosion can be reduced or avoided by including a corrosion inhibitor e.g. benzotriazole, typically in amounts up to 0.1% by weight on the flux composition. However, iodide fluxes and flux compositions are less susceptible to hydrolysis than chloride based fluxes and it is not expected that corrosion of containers will present any substantially difficulty unless the flux compositions are stored for very extended periods before use.

A further disadvantage of chloride based fluxes is that chloride containing residues can leach heavy metals and in particular lead from the solder. Any lead chloride produced in this way is particularly disadvantageous in domestic water systems because it is dissolved in water flowing past the soldered joint giving rise to undesirably high and potentially toxic levels of lead in the water. The iodide fluxes of the invention appear to be less corrosive towards solder than chloride based fluxes and the corrosion product, lead iodide, is much less soluble than lead chloride and thus would give rise to much lower levels of dissolved lead.

When using flux compositions including a liquid vehicle or carrier, especially with aqueous compositions, it is useful to include in the flux compositions a surface active agent to ensure that the flux composition wets the surface and thus contacts it uniformly. This is particularly important if the surfaces to be soldered are not completely cleaned or degreased. It is typical in plumbing soldering that the joints to be soldered are dirty or greasy and the includion of a surface active agent can reduce or eliminate the necessity for thorough cleaning and degreasing. Suitable surface active agents are detergents including those commercially available under the Registered Trade Marks Teepol and Lissapol. The amount of surface active agent needed depends on the amount of grease and dirt remaining on the substrate surfaces.The minimum practically useful concentration is about 0.1% by volume on the flux composition. Typically amounts of 0.5 to 5% by volume are useful. The use of amounts greater than 10% by volume do not bring any particular added benefit and may especially at concentrations as high as 30% by volume give rise to carbonisation residues which interfere wit the spreading of the solder.

The method of the invention is applicable to a variety of solderable substrates. As has been indicated above the fluxes and flux compositions of the invention are particularly useful in soldering copper or copper-containing alloys such as brass and bronze, particularly gunmetal. However, the method is applicable to other substrates, e.g. steel including mild steel and stainless steel. Steel and in particular stainless steel are well known to be much more difficult to solder than copper or copper alloys, and fluxes for soldering steel have thus to be adequately active to be effective. Chloride fluxes are suitably active but steel and in particular stainless steel are very susceptible to severe pitting corrosion in the presence of chloride ions. Stainless steel pipes have been corroded through in as little as three days after soldering with a chloride based flux.This makes it impractical to use chloride based fluxes on stainless steel. In soldering stainless steel phosphoric acid based fluxes are commonly used. Such fluxes are less corrosive than chloride fluxes but are sensitive to to excessive heat and thus require considerable care in use if sound joints are to be obtained. The fluxes and compositions of the invention have activity which makes them useful in soldering stainless steel and they are much less corrosive than chloride based fluxes. In particular it seems that they are much less liable to give rise to the extreme forms of pitting corrosion on stainless steel which happen with chloride based fluxes.

The solder used is not particularly critical and those conventionally used for soldering the substrate metals set out above are suitable for use in the invention. Typical of lead-tin solders which are useful are those set out in B.S. 219 of 1977. The particular flux or flux composition used in any particular situation will, of course, depend on the solder selected and in particular the temperature at which it is used.

The technique by which the soldering is performed is not critical to the invention. Plumbing soldering is typically carried out using a blowlamp. Industrial soldering and in particular the pre-tinning of, especially electronic, components, e.g. printed circuit boards, is frequently performed by wave soldering. The fluxes and flux compositions of the invention may be used in such techniquies.

The following Examples illustrate the invention. Examples 1 to 7 are the results of tests on the spreading of solder under controlled conditions. Example 8 gives the results of tests carried out to investigate pitting corrosion engendered by flux residues.

In the spreading tests the following technique was employed: 2g of the flux composition to be tested was placed on a 1 mm thick 75 mm square plate of the substrate metal. A solid cylinder of solder 5 mm diameter and 6 mm high and having the composition set out in Table 1 below was placed on the plate in content with the flux. The plate with the solder and flux was heated in a furnace in contact with a copper hot plate at a measured temperature for a measured time. The flux was evaluated by measuring the area over which the solder had spread.

TABLE 1 Composition of solder (wit%) Pb 74 f 0.5 Sn 24 + 0.5 Sb 1.2(2) Fe < 0.001 Cu 0.03(6) Zn < 0.001 Bi 0.02(0) As < 0.001 Ag ~ 0.01 Cd < 0.001 Example 1 Suspensions of cuprous iodide in distilled water were made and tested for their fluxing effect on copper plates as described above. Some of the tests were repeated including various amounts of Laponite a commercial silica-alumina clay type of thixotropic thickener which produces thixotropic gels at concentrations above ca.2% at pHs above ca.5.5. All samples were heated for 3 minutes at 400 C. For comparison a 10% aqueous solution of a 3:1 mixture by weight of ZuCI2 and NH4Cl was used as a flux. This aqueous solution gives results which are approximately comparable with those obtained from commercial ZnCl2/NH4Cl fluxes. The results are set out in Table 2.

TABLE 2 Area of Spread of Solder (mm2) Flux composition Without With Laponite (wt% on flux composition) Laponite 3 5 10 Cul flux. wt % Cul in water 0.3 400 - 1 200 3 700 50 80 50 10 1100 60 50 50 30 1100 50 100 60 60 1000 700 200 50 90 900 606 300 400 10% aqueous solution of 3:1 bywtZnCl2: NH4CI 800 600 50 50 These results show that Cul can adequately flux the solder, producing results comparable with chloride based fluxes although it was noticed that the spreading was somewhat slower with Cul. The compositions containing 60% and 80% Cul left some solid residue mixed with the solder. The results for 1% and 0.3% Cul show that spreading is significantly promoted even at very low Cul concentrations.

Example 2 A series of flux compositions were made up as suspensious/solutions in distilled water of various mixtures of Cul and NHS at an overall concentration of 10% by weight. The test plates were of copper and were treated at 3500C for 2 minutes. The results are set out in Table 3.

TABLE 3 Mol% NH41 in Area of Spread Cul-NH4lflux mm2 3 300 10 250 20 700 40 1100 50 1100 75 1300 100 700 This particularly illustrates the good spreading obtained with compositions close to the Cul - NH41 eutectic which occurs at about 40-50 mol% NH41.

Example 3 Example 2 was repeated but using mixtures of Znl2 and NH41 as the flux and heating the copper plates at 350"C for 2 minutes. The results are given in Table 4 which also lists the proportions of the mixture.

TABLE 4 Mol % NH41 in Area of Spread Znl2- NH41 flux mm2 0 350 22 80 48 800 100 700 Example 4 Example 1 was repeated using solutions of NH41 in water instead of suspensions of Cul in water. No Laponite was included in these tests. The copper plates were treated at 350"C for 2 minutes. The results are set out in Table 5.

TABLE 5 Wt% NH41 in water Area of spread, mm2 0.3 250 1 130 3 80 10 700 30 1100 These results show that, although NH41 promotes spreading at low concentrations the effect is somewhat less than with Cul at equivalent concentrations.

Example 5 A series of flux compositions were made up by dissolving in distilled water a mixture of Znl2 and NH41 containing 48 mol% NH41 at an overall concentration of 10% by weight. To this composition were added varying amounts of tin iodide (as Snl4) and these flux compositions were tested on copper plates at 350"C for 2 minutes. The results are shown in Table 6.

TABLE 6 Amount of Snl4 wt% on Area of Spread solution of Znl2 and NH4l mm2 0 800 1 1300 3 1300 10 500 Example 6 Flux compositions were made by dissolving 1 g of a mixture of Znl2 and NH41 (48 mol% NH41) in 20g of various non-aqueous vehicles. The clear solutions were tested on copper plates at 3500C for 2 minutes. The results are shown in Table 7.

TABLE 7 Vehicle Area of Spread, mm Acetone 700 Isopropanol 700 Propylene Carbonate 800 Ethylene Glycol 700 All four of these organic vehicles gave satisfactory spreading of the solder. c.f. the 10% aqueous solution of this flux (Table 4).

Example 7 A 10% aqueous solution of a Znl2 - NH4l (48 mol% No41) mixture was tested as a flux composition on 18-8 stainless steel. The stainless steel plates were heated at 4000C for 2 minutes. The area of spread obtained was 230 mm2. This reduction in spreading (c.f. Table 4) illustrates the known difficulty in soldering stainless steel as compared with copper.

Example 8 Flux compositions were made up using NH41, Znl2, NH4l-Znl2 (50:50 mol% mixture), and NH4CI (for comparison) as the fluxes in a vehicle of 90% by wt PEG 1000 and 10% bywt. water. The vehicle has a pasty consistency at ambient temperature and the compositions were made by heating the vehicle and flux together and allowing them to cool to ambient temperatures. All the mixtures were pastes except those containing 30% NH4l and 30% NH41-Znl2. The flux compositions were evaluated for solder spread as described above by heating at 400 C for minute. The results are set out in Table 8.

TABLE 8 Area of spread, mm2 Flux wt.% flux in composition 0 1 3 10 30 NH41 200 500 500 800 1400 Znl2 200 400 550 800 800 50mol.% NH4l-Znl2 200 400 400 400 700 NH4Cl 200 300 300 300 300 Example 9 Flux compositions were made up from a vehicle mixture of 75% by wt PEG 1500 and 25% by wt PEG 200 and amounts of NH4l ranging from 0 to 20% by weight of the flux composition by heating a mixture of the components at about 100"C until a uniform solution was formed and cooling to ambient temperature. At ambient temperature the compositions all had a pasty consistency. The results of testing these flux compositions in the area of spread test at 3200C for 1 minute. The results are set out in Table 9.

TABLE 9 Wt% of NH41 in Area of spread flux composition mm2 0 140 5 560 10 580 15 800 20 1400 Example 10 Flux compositions were made up from a vehicle mixture of 75% by wt. PEG 4000 and 25% by wt. water and amounts of NH41 ranging from 0.5 to 10% by wt. of the flux composition. The compositions were made up by dissolving the NH41 in the water of the vehicle mixing in the PEG 4000, warming the mixture to about 60 C stirring to homogenize and then cooling the mixture to ambient temperature. At all the concentrations of NH41 tested the flux composition was of a pasty consistency at ambient temperature. The flux compositions were evaluated for solder spread as described above by heating at 3200C for 1 minute. The results are set out in Table 10.

TABLE 10 Wt%ofNH4l in Area of spread flux composition mm2 0.5 450 1 680 1.5 740 2 730 2.5 680 3 570 4 450 5 400 10 500 For comparison proprietary fluxes tested gave areas of spread of from 600 to 700 mm2. This Example illustrates that NH41 is an active flux even at concentrations of only a few percent.

The results of Examples 1 to 10 are useful particularly for comparative purposes. However, when soldering capilliary joints such as are very commonly used in plumbing the simple solder spread tests will tend to underestimate the spreading effect because capilliary action will aid spreading of the solder and the restricted volume surrounding the soldering site will increase the concentration of gaseous fluxing materials.

Thus, in addition to the spreading tests described above we have also soldered 15 mm copper tube in brass and copper capilliary joints using Cul and Znl2 - NH4l (48 mol% No41) as fluxes. Satisfactory joints were obtained. Further, we have tried similar tests using stainless steel tube, again with satisfactory results.

Example 11 In this Example simulated copper based flux residues were tested to investigate the likelihood that they would engender pitting corrosion. The tests are based on the knowledge that surface films which support pitting corrosion have high electrode potentials and are photonegative.

Cuprous chloride films on copper were prepared by immersing copper specimens (previously cleaned in dilute HNO3) in a 1% solution of CuCI2 in water saturated with CuCI. Cuprous iodide films on copper were prepared by immersing copper specimens in a solution of potassium iodide in dilute sulphuric acid.

Specimens coated with cuprous chloride or cuprous iodide films were immersed in a solution containing 100 mg/l NaHCO3, 50 mg/l CaCI2 and 150 mg/l CaSO4 (with the pH adjusted to 7-7.3 by addition of H2SO4) and the electrode potentials versus the saturated calomel electrode and the change in potential of illumination were measured at intervals after immersion.

After 3 weeks immersion, specimens coated with cuprous chloride films were found to have electrode potentials which were more positive than the electrode potentials of specimens coated with cuprous iodide films. Additionally, specimens coated with cuprous chloride films showed photonegative behaviour (a decrease in potential on illumination), whilst specimens coated with cuprous iodide films showed no change in potential on illumination, or a small photopositive effect. The results obtained suggest that it is unlikely that the presence of residual metallic iodides on copper surfaces will result in pitting corrosion. Accordingly, the fluxes of the present invention are expected not to support pitting corrosion of copper.

Claims (31)

1. A method of soldering one or more metal surfaces which method comprises providing an inorganic flux comprising one or more metallic iodides in fluxing proximity to the metal surface and the solder and heating the metal surface(s) and/or solder whereby the solder is melted and wets the metal surfac(s).

2. A method as claimed in claim 1 wherein the metallic iodide an iodide of Bi, Cu, Fe, NH4, Sn or Zn.

3. A method as claimed in claim 1 wherein the metallic iodide is a co-solution forming mixture of at least two of the iodides of Bi, Cu, Fe, K, Li, Mg, Na, NH4, Ni, Sn and Zn.

4. A method as claimed in claim 2 wherein the mixture is a mixture of Znl2 and No41.

5. A method as claimed in any one of claims 1 to 4 wherein a minor proportion of a different iodide selected from the iodides of Ag, NH4 and Sn is included to activate the iodide.

6. A method as claimed in claims 4 and 5 wherein the mixture is a mixture of Znl2, NH41 and Snl2 or Snl4.

7. A method as claimed in any one of claims 1 to 6 wherein the flux is used in the form of a flux composition comprising the flux in combination with a suitable vehicle.

8. A method as claimed in claim 7 wherein the vehicle is one or more of water, an alcohol, dimethyl formamide, polymeric polyhydroxy alcohols, propylene carbonate or a conventional gel carrier.

9. A method as claimed in claim 7 wherein the vehicle comprises one or more polyethylene glycols.

10. A method as claimed in claim 9 wherein the vehicle comprises a polyethylene glycol having a nominal average molecular weight of from 600 to 9000 in combination with a diluent.

11. A method as claimed in claim 10 wherein the diluent is one or more of water, ethylene glycol, propylene glycol, or a polyethylene glycol having an average molecular weight nor exceeding 400.

12. A method as claimed in claim 10 wherein the diluent is diethylene glycol, triethylene glycol, teteraethylene glycol, polyethylene glycol 200 or a mixture thereof.

13. A method as claimed in claim 10 wherein the polyethylene glycol has a nominal average molecular weight of from 1000 to 6000.

14. A method as claimed in any one of claims 10 to 13 wherein the vehicle comprises polyethylene glycol 1500 and, as diluent, polyethylene glycol 200.

15. A method as claimed in any one of claims 10 to 14 wherein the relative proportions of the components of the vehicle and the flux are selected so that the flux composition has a pasty consistency.

16. A method as claimed in any one of claims 10 to 15 wherein the flux is No41.

17. A method as claimed in any one of claims 7 to 16 wherein the amount of flux is from 0.1 to 25% by weight of the composition.

18. A method as claimed in claim 17 wherein the amount of flux is from 0.5 to 20% by weight of the composition.

19. A method as claimed in any one of claims 1 to 18 wherein the metal surface(s) are of copper or copper containing alloy.

20. A method as claimed in any one of claims 1 to 19 and substantially as hereinbefore described.

21. A flux composition comprising a vehicle and as active fluxing constituents one or more metallic iodides.

22. A composition as claimed in claim 21 wherein the vehicle is one or more polyethylene glycols.

23. A composiftion as claimed in claim 22 wherein the vehicle is one or more polyethylene glycols having a nominal average molecular weight of from 600 to 9000 in combination with a diluent.

24. A composition as claimed in claim 23 wherein the polyethylene glycol(s) have a nominal average molecular weight of from 1000 to 6000.

25. A composition as claimed in either claim 23 or claim 24 wherein the diluent is water, ethylene glycol, propylene glycol or a polyethylene glycol having a nominal average molecular weight of not more than 400.

26. A composition as claimed in claim 25 wherein the diluent is diethylene glycol, triethylene glycol, tetraethylene glycol or polyethylene glycol 200.

27. A composition as claimed in any one of claims 23 to 26 having a pasty consistency.

28. A composition as claimed in any one of claims 22 to 27 wherein the metallic iodide is NH41.

29. A composition as claimed in any one of claims 21 to 28 and substantially as hereinbefore described.

30. A method as claimed in any one of claims 1 to 20 wherein the flux is provided by a composition as claimed in any one of claims 21 to 29.

31. Metal surfaces and joints between metal surfaces whenever soldered by the method claimed in any one of claims 1 to 20 and 30.