SOLDER FLUX

Abstract

There is provided the use of at least one ionic liquid as a soldering/brazing flux. There is also provided a method of soldering a metal comprising applying a solder/braze comprising a flux to a surface of the metal and heating said metal to a desired soldering/brazing temperature, wherein the soldering/brazing flux comprises one or more ionic liquids.

Claims

1. A soldering/brazing flux comprising at least one ionic liquid and at least one additive which improves flux functionality.

2. The soldering/brazing flux according to claim 1 wherein the ionic liquid is a Deep Eutectic Solvent (DES) selected from at least one of the following: (i) metal salt+organic salt (ii) metal salt hydrate+organic salt (iii) organic salt+hydrogen bond donor (iv) metal salt hydrate+hydrogen bond donor.

3. The soldering/brazing flux according to claim 2 wherein the DES comprises: (a) a mixture of two or more compounds of formula (I),
(R.sup.+).sub.n(X.sup.n-)(I) or a hydrate thereof, wherein n is 1, 2 or 3, R.sup.+ is a primary, secondary, tertiary, quaternary or unsubstituted ammonium cation or a quaternary phosphonium cation, X.sup.n- is a monovalent, bivalent or trivalent anion; and (b) one or more compounds of formula (IIIa) and/or one or more compounds of formula (IIIb), ##STR00002## wherein: R.sup.8 represents H, C.sub.1-10 alkyl (which latter group is optionally substituted by one or more F atoms), phenyl (which latter group is optionally substituted by one or more substituents selected from halo, C.sub.1-10 alkyl and C.sub.1-10 alkoxy) or N(R.sup.9)R.sup.10, R.sup.8a represents H or C.sub.1-10 alkyl (which latter group is optionally substituted by one or more F atoms), R.sup.9 and R.sup.10 independently represent H or C.sub.1-10 alkyl (which latter group is optionally substituted by one or more F atoms), Y represents C.sub.2-10 alkylene or C.sub.4-8 cycloalkylene optionally (i) substituted by one or more substituents selected from F, OH, SH, N(R.sup.11)R.sup.12 and C.sub.1-10 alkyl (which latter group is optionally substituted by one or more substituents selected from F and OH), and/or (ii) interrupted by one or more groups selected from O, S and NR.sup.13, and R.sup.11 to R.sup.13 independently represent H or C.sub.1-10 alkyl (which latter group is optionally substituted by one or more substituents selected from F and OH).

4. The soldering/brazing flux according to claim 3 wherein in component (a), R.sup.+ may be a quaternary phosphonium cation or, particularly, a primary, secondary, tertiary, quaternary or unsubstituted ammonium cation.

5. The soldering/brazing flux according to claim 3 wherein in component (a), X.sup.n- is an anion selected from the list comprising halide, chlorate, perchlorate, bromate, nitrate, nitrite, cyanide, cyanate, thiocyanate, hydrogencarbonate, carbonate, sulfate, hydrogensulfate, pyrosulfate, sulfite, hydrogensulfite, phosphate, monohydrogenphosphate, dihydrogenphosphate, metaphosphate, pyrophosphate, hexafluorophosphate, tetrafluoroborate, borate, diborate, triborate, tetraborate, carboxylate and sulfonate.

6. The soldering/brazing flux according to claim 5 wherein the anion selected from the list comprising fluoride, chloride, bromide, iodide, nitrate and acetate.

7. The soldering/brazing flux according to claim 3 wherein the or each compound forming component (b) is an amide or polyol.

8. The soldering/brazing flux according to claim 7 wherein component (b) is one or more compounds selected from the list comprising benzamide, acetamide, N-methylurea, N,N-dimethylurea urea, glycerol, mannitol, xylitol, ethylene glycol and propylene glycol.

9. The soldering/brazing flux according to claim 3 wherein the DES is a mixture of: (a) one or more compounds selected from the list comprising benzyltrimethylammonium halide, tetrabutylammonium halide, ethylmethylimidazolium halide, acetylcholine halide and choline halide; and (b) one or more compounds selected from the list comprising benzamide, acetamide, N-methylurea, N,N-dimethylurea, urea, glycerol, mannitol, xylitol and propylene glycol.

10. The soldering/brazing flux according to claim 2 wherein the DES is selected from one or more of the following DES: zinc chloride: choline chloride mixture, chromium trichloride hexahydrate: choline chloride mixture, copper chloride dihydrate: choline chloride mixture, tin chloride dihydrate: choline chloride mixture, reline, ethaline, propylene, butaline, glyceline, acetaline, maline, choline chloride: glucose mixture, choline chloride: fructose mixture, choline chloride: xylitol mixture, choline chloride: erythritol mixture, aluminium trichloride: acetamide mixture, aluminium trichloride: urea mixture and tin chloride dihydrate: ethylene glycol.

11. The soldering/brazing flux according to claim 1, wherein the additive which comprises flux functionality is selected from at least one of: (i) a wetting agent; (ii) a wetting agent improver; or (iii) a rheology modifier, or combinations thereof.

12. The soldering/brazing flux according to claim 11, wherein the wetting agent is a cationic, anionic or non-ionic surfactant.

13. The soldering/brazing flux according to claim 12, wherein the wetting agent is selected from cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, polyoxyethylene octyl phenyl ether, sodium laureth sulfate, cetylpyridinium chloride, benzalkonium chloride, benzethonium chloride, ammonium laureth sulfate, sodium stearate, dioctyl sodium sulfosuccinate, sodium perfluorobutylsulfonate, polyoxyethylene glycol ethers, polyoxpropylene glycol alklyl ethers and polyoxyethylene glycol alkyl phenol ethers.

14. The soldering/brazing flux according to claim 11, wherein the wetting agent improver is an acid or salt.

15. The soldering/brazing flux according to claim 14 wherein the wetting agent improver is an acid selected from hydrobromic acid, hydroioic acid, sufluric acid and phosphoric acid.

16. The soldering/brazing flux according to claim 14 wherein the wetting agent improver is a fluoride-containing salt, preferably wherein the fluoride-containing salt is selected from sodium tetrafluoroborate, ammonium tetrafluoroborate, potassium fluoride, sodium hexafluorophosphate and tetrafluoroboric acid.

17. The soldering/brazing flux according to claim 11, wherein the additive is a rheology modifier and is selected from one or more of: (i) ethylene glycol, glycerol and mixtures thereof; (ii) a hydrophilic polymer selected from poly (N-isopropylacrylamide), poly (2-oxazoline), polyethylenimine, poly (acrylic acid), poly (vinyl alcohol), poly (vinyl pyrrolidinone), poly (allylamine), poly (vinyl sulfonic acid) and glycerol propoxylate; (iii) a polyol selected from xylitol, erythritol, arabitol, ribitol, sorbital and mannitol.

18. The soldering/brazing flux according to claim 1 wherein the flux is in the form of a liquid or a paste.

19. The soldering/brazing flux according to claim 1 wherein the flux is diluted to 2.5 wt % with a solvent.

20. A composite cored solder comprising a hollow cylinder of solder/braze, said hollow containing a flux, according to claim 1.

21. A method of soldering/brazing a metal comprising applying a solder/braze flux according to claim 1 to a surface of the metal and heating said metal to a desired soldering/brazing temperature.

22. A method of fabricating lightweight mechanical or electrically conducting architectures using a solder/braze flux according to claim 1.

23. The method according to claim 22 wherein the architecture is a printed circuit board.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0124] FIG. 1 shows the soldering of copper using SAC305 solder where (a) is a standard soldered piece, (b) is a 3d microscope image of a solder ball which has bonded to a copper sheet and (c) is the cross section of the solder covered Cu piece (see Example 1).

[0125] FIG. 2 shows solder joints with the substrates (a) brass, (b) nickel, (c) stainless steel and (d) cast iron produced using the DES reline 200 as the solder flux (see Examples 2-5).

[0126] FIG. 3 shows solder wetting balance trace using the DES Reline 200 as a solder flux for (a) uncleaned, untreated Cu wire using SAC 305 solder at 250 C. and (b) stainless steel using SAC 305 solder at 350 C., both at an immersion depth of 5 mm (Examples 7 and 8).

[0127] FIG. 4 shows images of pads and cross sections for soldered PCB's where the surface finish was: (a & b) bare Cu (see Example 9), (c & d) electroless nickel immersion gold (see Example 10), (e & f) organic soldering preservative (see Example 11), (g & h) immersion silver (see Example 12), (i & j) immersion tin (see Example 13), (k & l) lead free hot air solder levelling (see Example 14) and (m & n) leaded hot air solder levelling (see Example 15).

[0128] FIG. 5 shows images of cross section of plated through holes which have been filled with solder during Tri-moore testing using the DES Reline 200 diluted by 50% v/v with water as a solder flux (see Example 16).

[0129] FIG. 6 shows (a) optical image of micro-section and (b) x-ray microscope image of a soldered BGA component that was soldered using the DES Reline 200 diluted 50% v/v with water as the solder flux (see Example 17).

[0130] FIG. 7 shows solder wetting balance traces for the DES Reline 200 as a solder flux for Cu wire using SAC 305 solder at 250 C. and an immersion depth of 5 mm where the DES was diluted with water to (a) 50, (b) 30, (c) 20, (d) 10, (e) 5 and (f) 2.5% v/v (see Examples 18-23).

[0131] FIG. 8 shows solder wetting balance traces for the DES Glyceline 200 as a solder flux for Cu wire using SAC 305 solder at 250 C. and an immersion depth of 5 mm where the DES was diluted with water to (a) 100, (b) 50 and (c) 25% v/v (see Examples 24-26).

[0132] FIG. 9 shows optical microscope images of the surface of a hot air solder levelled surface of (a) bare copper and (b) electroless nickel where flux used was Reline 200 (see Example 27).

[0133] FIG. 10 shows a cross section of solder bond of aluminium with SAC 305 where the flux was 5 wt % KOH in Reline 200 (see Example 28).

[0134] FIG. 11 shows (a) photo's of droplets of Glyceline 200 (left), 10 mM CTAB in Glyceline 200 (centre) and 2 mM Triton X100 in Glyceline (right) on a copper coupon from above and side on view and (b) the same copper coupon that been rotated until one of the droplets had nearly fallen off.

[0135] FIG. 12 shows (a) solder ball formed from melting of reline based solder paste on stainless steel at 250 C.; (b) solder ball formed from melting of reline with added hydrochloric acid based solder paste, (c) and (d) are cross-sections from samples (a) and (b) respectively.

[0136] FIG. 13 shows: (a) Reline 200 which is the active component of the solder paste, (b) this is made into a gel by mixing with mixed molecular weights of polyethyleneglycol followed by (c) mixing with solder powder to form a solder paste.

[0137] FIG. 14 shows PCB production panel that has been HASL coated using (a) Glyceline 200 as a solder flux; or (b) Glyceline 200 with added 20 wt % polyethyleneglycol 400.

[0138] Certain embodiments of the invention are illustrated by way of the following examples.

EXAMPLES

[0139] Examples 1-6: Soldering on a variety of substrates using the DES Reline 200 (see FIGS. 1-3, Table 1).

TABLE-US-00001 TABLE 1 Qualitative investigation of utility of the DES Reline 200 as a solder flux for a variety of different substrates Example Liquid Substrate Outcome 1 Reline Copper Solder wets surface readily 2 Reline Brass Solder wets surface readily 3 Reline Nickel Solder wets surface readily 4 Reline Stainless Steel Solder wets surface readily 5 Reline Cast Iron Solder wets surface after prolonged heating and mechanical agitation 6 Reline Aluminium Solder wets surface sparingly after prolonged heating and mechanical agitation

[0140] Examples 7 and 8: Solder wetting balance studies of Reline for the substrates copper and stainless steel (see FIG. 3; Examples 1 and 4; FIG. 1 and FIG. 2(c) respectively).

[0141] Examples 9-15: Qualitative study of the effect of dilution of the DES Reline 200 by 50% v/v with water on the solderability on different PCB surface finishes (see FIG. 4, Table 2).

TABLE-US-00002 TABLE 2 Qualitative investigation of utility of the DES Reline 200 diluted 50% v/v with water as a solder flux for a variety of common PCB surface finishes Example Liquid Surface finish Outcome 9 Reline Copper Uniform solder bond formed 10 Reline Electroless nickel Uniform solder bond formed immersion gold 11 Reline Organic Soldering Uniform solder bond formed Preservative 12 Reline Immersion Silver Uniform solder bond formed 13 Reline Immersion Tin Uniform solder bond formed 14 Reline Tin Hot Air Solder Uniform solder bond formed Levelling 15 Reline Lead Hot Air Solder Uniform solder bond formed Levelling

[0142] Examples 16 (FIG. 5) and 17 (FIG. 6): Use of the DES Reline 200 for the soldering of a variety of different PCB mounting methodologies (see Table 3).

TABLE-US-00003 TABLE 3 Qualitative investigation of utility of the DES Reline 200 for a variety of common PCB mount methodologies. Example Liquid Mount method Outcome 16 Reline Plated through holes Uniform solder bond formed 17 Reline Ball grid array (BGA) Uniform solder bond formed

[0143] Examples 18-23: Solder wetting balance trials testing the efficacy of the DES Reline 200 as a solder flux for copper wire at a variety of dilutions with water (see FIG. 7 and Table 4).

TABLE-US-00004 TABLE 4 Summation of solder wetting balance results for various dilutions of the DES Reline 200 with water Example Liquid % v/v in water Outcome 7 Reline 200 100 Surface wets rapidly 18 Reline 200 50 Surface wets rapidly 19 Reline 200 30 Surface wets rapidly 20 Reline 200 20 Surface wets rapidly 21 Reline 200 10 Surface wets rapidly 22 Reline 200 5 Some results positive, others show limited wetting of surface 23 Reline 200 2.5 All results show limited wetting of surface

[0144] Examples 24-26: Solder wetting balance trials testing the efficacy of the DES Glyceline 200 as a solder flux for copper wire at a variety of dilutions with water (see Table 5 and FIG. 8).

TABLE-US-00005 TABLE 5 Summation of solder wetting balance results for various dilutions of the DES Reline 200 with water Example Liquid % v/v in water Outcome 24 Glyceline 200 100 Surface wets rapidly 25 Glyceline 200 50 Surface wets but at a slower rate 26 Glyceline 200 25 Limited wetting of substrate

[0145] Example 27: Use of DES as a flux for hot air solder levelling of copper and electroless nickel substrates (see FIG. 9).

[0146] Examples 28: Use of additives to improve solderability of difficult substratesKOH in Reline for soldering of aluminium (see FIG. 10).

[0147] The Applicants have found that a DES electrolyte based on urea and ethylene glycol in combination with choline chloride function as effective solder fluxes and have found that Cu substrate wetting with lead-free SAC305 solder is qualitatively good, indeed better and more rapid with the DES flux than with some commercial Rosin based fluxes.

[0148] This is important in the context of the typically used lead-free soldering for two reasons. First, since lead-free solders are generally operated at higher temperatures, faster wetting times are desirable to minimise heat damage to boards and components. Second, good wetting leads to stronger joins and this is a key issue since lead-free solder joins are generally found to be more susceptible to shock damage and failure. Importantly the DES fluxes of the present invention are inexpensive, have low environmental impact and contain no noxious or toxic materials.

[0149] In addition the same fluxes can be used to solder/braze a range of other metals including aluminium and steel. The Applicant has also found that DES solders can be formulated to solder unconventional materials such as stainless steel, carbon steel and aluminium for light engineering construction as well as electronics. This presents a unique opportunity for Cu/AI heat-sink bonding methods as well as fabrication of light weight mechanical or electrically conducting architectures. Normally, these metals cannot be joined using soldering methods with conventional flux materials without the use of specialist solders.

[0150] PCB cleanliness is a critical requirement of many electronic products. Residues from the flux constituents and their breakdown products can cause in-service failures via corrosion or electro-migration mechanisms and so must be removed. The nature of these residues is such that extensive cleaning is often required and various aqueous-based, organic solvent-based or other cleaning systems are common. The DES fluxes do not breakdown to produce baked-on, hard to remove residues and are readily soluble in water so cleaning, when required, should be easier. This will reduce costs and improve environmental impact. Some fluxes, or formulation variants, can be used to solder join reactive and difficult metals such as aluminium. In the PCB industry bonding technologies are in high demand for the reliable and durable joining of Al heat sink components directly to copper track PCB assemblies for efficient heat transfer.