Flux paste for soft soldering of aluminum

Abstract

Flux paste for soft soldering aluminum and aluminum alloys, containing 10 to 90% by weight of halide-based flux and 90 to 10% by weight of a paste base material, wherein the flux has a composition (in weight percent based on the flux composition) of:SnCl282.5 to 95.4%,NH4Cl 1.7 to 13% andNaF 2.9 to 4.5%, wherein the flux paste has a melting temperature of below 150°C.

Description

[0001] The present invention relates to a flux paste for soft soldering aluminum.

[0002] It is known to join components made of aluminum or aluminum alloys to each other or to components made of other suitable metallic materials by soldering.

[0003] The solder is melted, spreads in its liquid state over the surface, and diffuses into the surface areas of the metallic components. An intermetallic intermediate layer forms that bonds the components. Soldering processes at temperatures up to approximately 450°C are referred to as soft soldering, with solders that melt below this temperature being used. Above 450°C, hard soldering occurs.

[0004] A key prerequisite for a strong and durable solder joint is good wetting of the surface by the solder. For good wettability, the surfaces must be metallically bright, meaning they must be free of oxide layers.

[0005] Soldering aluminum or aluminum alloys is particularly difficult because aluminum and its alloys have a chemically extremely stable, compact oxide layer that is correspondingly difficult to remove.

[0006] Fluxes are designed to remove oxide layers and prevent new ones from forming during the soldering process.

[0007] The flux is applied to the soldering surfaces before soldering or in conjunction with the solder. The flux melts due to the heat generated during soldering and removes the oxide layer through a chemical reaction. It is essential that the flux's melting temperature is lower than that of the solder used, so that the oxides dissolve before the solder melts. The molten flux should exhibit good flowability and wettability.

[0008] Fluxes therefore have a different composition depending on the type of material of the parts to be soldered and the melting point of the solder used. They can be solid, liquid, or pasty.

[0009] Suitable fluxes for soft soldering of aluminum are generally divided into three classes according to DIN EN 29454: - F-LW-1: Solder-forming tin and / or zinc chlorides, - F-LW-2: Purely organic compounds, for example amines, and - F-LW-3: Organic halogen compounds.

[0010] The letters F stand for flux, L for light metal, and W for soft soldering. Of the classes mentioned, flux class F-LW-1 is the most reactive.

[0011] According to JD Down “Solders, Fluxes, and Techniques for Soldering Aluminium”, Symposium on Solder, ASTM, 1957, of the three flux classes mentioned, zinc and tin chlorides have the best soldering properties, with tin chloride showing better soldering and wetting results at lower soldering temperatures than zinc chloride.

[0012] For additional activation, fluorides or fluorine compounds can be added to the fluxes of all three classes.

[0013] Fluxes and flux compositions for soft soldering of aluminum or aluminum alloys are already known.

[0014] A flux of the most reactive class, F-LW-1, optimized with regard to reaction temperature and required soldering time is described in US Pat. No. 3,988,175. It consists of 10 to 14 parts by weight of tin chloride (84 to 98 percent by weight), 0.25 to 1.5 parts by weight of ammonium chloride (1.7 to 13.0 percent by weight), and 0.05 to 0.3 parts by weight of sodium fluoride (0.3 to 2.84 percent by weight) and can be applied as a powder or liquid mixed with an alcohol. However, the melting point of the fluxes described here is well above 150°C. At these high temperatures, wetting and flowability can be impaired, especially with rapid heating. Furthermore, it must be assumed that the alcohol must be completely anhydrous and the mixture must be prepared immediately before application. Therefore, it cannot be stored.

[0015] Furthermore, US 2,238,069 describes a flux that is an addition compound of a boron fluoride with an aliphatic polyamine. The flux can be solid, liquid, or pasty and can contain a paste base, as required.

[0016] GB 642,869, filed on April 23, 1940, concerns a flux based on a halide of a metal such as zinc or tin, an aluminum halide, and optionally a small amount of alkali fluoride. For use, the flux is incorporated as a core into a wire made of solder material. The reason for this is to avoid the use of a paste material. It is stated that paste materials make the composition more complex and can leave undesirable carbon residues on the solder material.

[0017] The flux composition according to US 4,204,889 comprises 60 to 90% of a zinc or tin chloride or a mixture thereof, 2 to 25% of an ammonium or alkali metal halide, 1 to 20% of a compound containing ammonium or alkali metal fluoride, and 0.5 to 20% of a silver chloride, silver bromide, or silver iodide. The flux can be used in liquid or solid form; preferably, the flux is present in conjunction with the solder. For example, it can be incorporated into the solder material.

[0018] Compared to solid or liquid fluxes, pastes have the advantage that they are easier to apply due to their high viscosity and stickiness, remain on the application site and do not drip.

[0019] When attempting to produce a paste, it was found that the highly reactive substances of class F-LW-1 react in an undesirable manner with numerous typical solvents used in paste production to adjust the viscosity, such as water, ethanol, isopropanol and glycol, which results in their loss of reactivity as fluxes for removing the oxide layers.

[0020] It was therefore an object of the present invention to provide a flux paste based on the most reactive flux class F-LW-1, wherein the flux paste according to the invention has a melting temperature, in particular of the active components, which is below 150°C.

[0021] This allows rapid heating and rapid wetting of the soldering surface.

[0022] According to the invention, this object is achieved by a flux paste for soft soldering aluminum and aluminum alloys, wherein the flux paste comprises from 10 to 90 percent by weight of a halide-based flux and from 90 to 10 percent by weight of a paste base material, wherein the flux has a composition (in percent by weight based on the flux composition) of: - SnCl2 82.5 to 95.4%, - NH4Cl 1.7 to 13% and - NaF 2.9 to 4.5% and - the flux paste has a melting temperature of less than 150° C.

[0023] According to a further embodiment, 4 to 12 weight percent ZnCl2 can be added to the flux, wherein the ratio of the ternary system SnCl2-NH4Cl-NaF remains unchanged.

[0024] Fluxes of the three-component or four-component systems according to the invention have a melting temperature of below 150°C and preferably below 130°C.

[0025] For example, a flux according to the invention containing 86.75% SnCl2, 10% NH4Cl and 3.25% NaF shows a solidus temperature of 146°C and 128°C with the addition of 8% ZnCl2.

[0026] A concrete example of a flux paste according to the invention comprises tin chloride 52.0%, ammonium chloride 5.3%, sodium fluoride 2.2%, zinc chloride 5.3% and petrolatum 35.2%.

[0027] The paste base material is a material that is inert to the flux. An example of a suitable paste base material is a semi-solid paraffin such as petroleum jelly.

[0028] However, it is understood that in principle any paste base material can be used as is used for the production of flux pastes for soft soldering, as long as the base material is inert towards the flux system according to the invention.

[0029] Examples include other paraffins, waxes and other lipogels other than Vaseline, as well as fats.

[0030] The flux paste according to the invention based on fluxes of the most reactive flux class F-LW-1 is extremely efficient in removing chemically very resistant oxide layers, such as those formed by aluminum.

[0031] It is understood that the flux paste according to the invention is not limited to the soft soldering of aluminum and its alloys, but can equally be used for the soldering of other metals and metal alloys, in particular metals and metal alloys which, similar to aluminum, form chemically very resistant oxide layers, such as chromium, titanium or rust and acid-resistant steels.

[0032] In addition, the flux paste according to the invention or the flux composition used therein has a comparatively low melting point of below 150°C.

Claims

[1] Flux paste for soft soldering of aluminium and aluminium alloys, containing 10 to 90% by weight of halide-based flux and 90 to 10% by weight of a paste base material, the flux having a composition (in weight percent based on the flux composition) of: SnCl2 82.5 to 95.4%, NH4Cl 1.7 to 13% and NaF 2.9 to 4.5%, whereby the flux paste has a melting temperature of below 150°C. [2] Flux paste according to claim 1, wherein the flux contains 4 to 12 weight percent ZnCl2 (based on the flux composition). [3] Flux paste according to one of the preceding claims, wherein the paste base is a semi-solid paraffin. [4] Flux paste according to claim 3, wherein the semi-solid paraffin is petrolatum. [5] Use of a flux paste according to any one of claims 1 to 4 for the soft soldering of aluminium and aluminium alloys.