METHOD FOR PREPARING A SOLDER SUITABLE FOR JOINING TWO METAL PIECES, SOLDER AND USE OF SOLDER

The invention relates a method for preparing a solder suitable for joining two metal pieces, a solder prepared using the method and use of the solder thus prepared by means of the method. The solder composition is based on a material containing tin and zinc with a flux and possibly a binder mixed thereto for rendering the solder a paste-like consistency. The solder paste according to the invention is suited for soft-soldering aluminum or aluminum alloy pieces to copper or copper alloy pieces or another piece made of aluminum or aluminum alloy.

Soldering is a process, wherein filler (solder) generally under a capillary force fills a gap between the metal pieces being joined and wets the metal surfaces without melting the same. The working temperature is the lowest temperature at which the molten solder fills the capillary gap and wets the surfaces to be soldered. The working temperature of a soft solder is below 450 ⁰ C. In hard- soldering, the working temperature is above 450 ⁰ C. The working temperature is dictated by the melting temperature of the solder. In pure metals, melting takes place at a typical temperature characteristic of a given metal. In alloys, melting occurs over a given temperature span (mushy zone of phase diagram), whose lower limit is called the solidus temperature and the upper limit the liquidus temperature at which the mixture is fully molten. The specific temperature, at which a metal alloy composition melts at the lowest possible temperature characteristic of the metal alloy, is called the eutectic temperature and composition. Melting a mixture of two fine metal powders proceeds so that the powder of the lower melting temperature melts first thereby dissolving the powder of the higher melting point. Due to melting, the melting temperature of the powder of lower melting point may either fall or rise. In the most advantageous case, the melting points of the powders do not differ much from each other and, hence, the composition of the mixture is the same

as that of an eutectic alloy, whereby the first melting powder lowers the melting temperature of the melt during the melting process.

Disregarding a few exceptions, copper and copper alloys are generally readily solderable. Aluminum-containing brass alloys are difficult to solder using conventional fluxes if the aluminum content is greater than 1-1.5 %. The most frequently used solder compositions are tin-lead alloys, possibly also containing a third alloying metal such as silver or cadmium, for instance. The melting point of these solder alloys is in the range of 180-310 °C. Lead and cadmium are disadvantaged by their toxicity, which means that their use will be entirely forbidden in the near future. Silver is unfavorable due to its high price. One cost-effective substitute to lead-containing solder compositions is an alloy of 97 % tin and 3 % copper.

Cleaning and prevention of oxide layer formation on surfaces to be joined is carried out with the help of a flux. The optimum type of flux is selected on the basis of the local soldering process conditions. When it is desirable to remove oxides from surfaces to be soldered in a rapid and complete fashion, as well as to prevent the development of new oxides during heating, a flux containing zinc or ammonium chloride with different additives is the most common choice. Also other metal chlorides are used in commercially available fluxes. Such fluxes are corrosive and, hence, their residues must be carefully removed after soldering by, e.g., rinsing with warm water. When this kind of aggressive flux cannot be used for some reason, a rosin-based flux is selected. Its oxide dissolution capability is much weaker meaning that the soldering speed is substantially lower. A benefit is gained inasmuch as there is no need to remove flux residues after solder. The aggressivity of metal chlorides and effect of rosins may be controlled by different additives, which means that, besides both of these basic flux types, a selection of other fluxes with differently modified properties are available. Conventionally, fluxes are powders, pastes, creams or solutions. However, a commercially viable solder-flux combination for joining copper to aluminum has not been

disclosed. For soldering metals of extremely complex behavior such as aluminum, both the soldering method and the structure of the joint must also be optimized to achieve a flawless joint.

The joint structure most commonly occurring in soldering is a capillary gap. The strength of the soldered joint is dictated by the width of the gap. A suitable capillary gap width is typically in the range of 0.05-0.2 mm. The highest strength value is gained using a gap width narrower than 0.1 mm. The gap surfaces to be soldered must be parallel to each other or slightly tapering such that the gap is wider at the solder entrance side. Furthermore, the surfaces being soldered must be free from grease and oxides. Grease can be removed by a grease-dissolving chemical. The oxidized surface layer of copper is removable by mechanical cleaning or pickling. While the oxide layer of an aluminum object is removable, a problem arises from the immediate reoxidation of the surface under an oxygen-containing atmosphere. The oxidized surface layer of an aluminum object can be removed in conjunction with the soldering process by means of an aggressive flux that also protects the cleaned surface from reoxidation.

Soldering can be accomplished using the following tools/methods: soldering iron, flame, dipping, oven-heating, resistive heating or induction heating. The soldering process may be carried out under air, protective gas (e.g., nitrogen or argon) or vacuum atmosphere. The oven used in oven soldering may be a batch oven, a semi-inline oven or an inline oven. The last one of these oven types has the largest production capacity. After the pieces to be soldered have reached the melting point of the solder, the pieces are allowed to cool, whereby the molten solder solidifies thus joining the pieces. The soldering process temperature is higher than the solder melting point. Conventionally, the excess temperature is 20-50 °C. Soldering is made at the lowest possible process temperature to minimize the soldering cost and time. Generally, the pieces are cleaned after soldering free from flux residues by, e.g., rinsing with water in order to prevent later corrosion of the pieces.

For soldering aluminum, tin-zinc solder alloys have been developed, the most common of them being an eutectic alloy containing 91 % copper and 9 % zinc. This kind of solder mixture is not commercially available as a powder, but only in wire or rod format. For automated mass production, the best choice is a solder paste composed of solder alloy powder, flux and possibly an organic binder. The paste can be sprayed or brushed on the surfaces to be joined. However, no commercial product of viable function has been developed for soldering aluminum to copper or another aluminum piece.

It is an object of the present invention to overcome the disadvantages of the prior art and provide an improved method of preparing a solder paste for joining two metal pieces, a solder paste prepared using the method and use of the solder paste prepared by means of the method. More specifically, the essential features of the invention are disclosed in the appended claims.

The solder according to the invention contains at least one basic composition of tin and zinc having a flux and possibly a binder mixed therewith for preparing the solder into a paste form. The solder paste according to the invention is suited for soft-soldering two metal pieces of, e.g., aluminum or aluminum alloy to copper or copper alloy or another piece made of aluminum or aluminum alloy. The solder according to the invention contains 0.1 - 25 wt-%, advantageously 3 - 15 wt-%, most advantageously 5 - 10 wt-%, of zinc, the rest being tin. According to a preferred embodiment of the invention, tin and zinc in the solder may be complemented with at least one of the following alloying additives: manganese, copper, silver, phosphorus, antimony and bismuth, any one of these being present at a maximum content of 3 wt-%.

In the solder according to the invention, the basic composition of tin and zinc is formed by at least one metal powder, tin-zinc powder or tin and zinc powders mixed with each other. A benefit of tin-zinc powder is a low solder melting point, nontoxicity and low price. Inasmuch as soldering aluminum is

very difficult due to the formation of an oxide layer, special arrangements are necessary, wherein the flux must be of a substantially aggressive type capable of dissolving aluminum oxide. Additionally, the shape of the joint between two metal pieces and the width of the capillary gap must be optimized.

In the solder according to the invention, the tin-zinc powder or separately mixed tin and zinc powders are manufactured using a rapid chill technique, such as a gas or water atomization method. In the atomization method the metal is melted in a ceramic crucible, overheated, sprayed through a die opening and atomized into small droplets of molten metal with the help of gas or water jets. During their free fall, the molten droplets undergo solidification. The particle size of the powder is typically in the range of 5-200 μm at a cooling rate of at least 100 K/s, typically 1000-10,000 K/s, depending on the atomization technique and process parameters used such as the atomization pressure, die opening diameter, degree of overheating of the melt and properties of the melt. Water-atomized powder generally has a coarser particle size than that of a gas-atomized powder. Atomized solder powders are characterized by chemical homogeneity, which gives a narrower mushy zone than a solder composition made by conventional melting. The solder powder according to the invention can be prepared using the above- described methods. Furthermore, soldering may be carried out using separately atomized powders that are mechanically mixed with each other in a ratio comprising 0.1 - 25.0 wt-% zinc while the rest is tin.

In accordance with the invention, soldering two metal pieces with each other takes place using a metal powder containing zinc and tin, whereby the pieces being joined are subjected for soft-soldering to a heat treatment at a temperature of 250 - 310 °C.

The fine atomized solder powder can be mixed with a flux, such as ammonium tetrafluoroborate, and a possible binder, such as propylalcohol, tri-

ethanol amine or aminoethylethanol amine, whereby the thus prepared paste is suitable for use in automated soldering. The surfaces of the copper piece to be joined can be cleaned mechanically using another type of flux such as zinc chloride. First, the surface to be joined on the copper and aluminum pieces can be coated with the solder paste. Next, the pieces are pressed together so that the coated surfaces to be joined become intimately contacted with each other. The pieces are soldered together at once. One such application is the soldering of the cooler portion of a heat exchanger (e.g., of a heat sink). Owing to the metallurgical character of the joint, in a cooler manufactured using the method according to the invention, this kind of cooler provides a higher thermal conductivity and cooling effect than what is attainable using mechanical joints. The poor thermal conductivity of mechanical joints results from the air gap remaining between aluminum and copper surfaces. Furthermore, the air gap tends to increase during the unit's service life due to fatigue from the heating/cooling cycles of the aluminum and copper pieces. Fatigue proceeds rapidly inasmuch as the yield strength of aluminum is low and the thermal expansion coefficients of aluminum and copper are substantially different from each other.

The solder according to the invention and the solder paste prepared therefrom can be advantageously used, e.g., in the following applications: products of electrical/electronics industry (such as heat sinks); air-conditioning, cooling and deep-freezing technology; and car radiators made from aluminum, aluminum alloy and/or copper/copper alloy pieces that need soldering for joining.

EXAMPLE

The invention is next elucidated by virtue of an example, wherein mutually compatible aluminum and copper pieces are soldered together using a method according to the invention. The tensile strength of the soldered pieces was found to be at least 149, 148 and 129 N/mm ² as the fractures

started from the aluminum piece. Tensile strength tests on joints made using commercial solder pastes could not be carried out because the pieces did not form a joint with each other.

As the pieces to be soldered were selected a copper plate made from oxygen-free copper (Cu-OF) and a plate of pure aluminum. Both plates had a thickness of 2 mm. The aluminum plate was bent so as to form square- angled side webs. The surface of the copper plate was mechanically cleaned using abrasive paper. The cleaned surface was coated with a first flux that was rubbed off from the surface after about 1 minute. Subsequent to this treatment, the copper surface had a matte finish. The narrow sides of the square-angled aluminum plate were coated with a paste consisting a second flux and a solder powder. The chemical composition of the solder powder was 91 wt-% tin and 9 wt-% zinc. Onto the copper plate were placed pieces of 0.3 mm dia. copper wire serving to set the capillary gap width of the joint being made to an optimum value. After coating with the solder paste, the aluminum plate was placed on the copper wires resting on the surface of the copper plate so that the wide side webs of the square-angled aluminum plate were oriented upright in regard to the copper plate surface. Onto the aluminum plate was placed a load in order to prevent the pieces being joined from moving relative to each other during welding.

The pieces were heated under an air atmosphere to a temperature of 270 ⁰ C. Heating was continued until boiling and bubble formation in the paste came to an end, whereby the solder powder melted and filled the capillary gap. The pieces were allowed to cool thus permitting the solder to solidify and form a pore-free and strong joint between the pieces. Finally, the flux residues were rinsed away with the help of warm water.