The present invention relates to an alloy and, in particular, a lead-free solder alloy. The alloy is particularly, though not exclusively, suitable for use in the manufacture of electrical fuses.

Electrical fuses are supplied in many forms including the type where a current is passed along a fine wire. If the threshold current is exceeded, then the wire melts and the fuse fails. The fuse wire, which may be, for example, copper, tinned copper, silvered copper, or other alloy as appropriate, typically runs though a glass or ceramic tube. End caps (for example nickel coated brass caps) are fitted over the ends of the tube to form terminations.

In the bottom of each cap there may be provided a melted slug of cored solder wire, which performs the dual function of sealing the tube and providing a solder joint to the fuse wire. An example of a conventional solder composition is a tin-lead alloy comprising 35 - 50 wt% tin. Such an alloy starts to melt at a temperature of about 183 ⁰ C, but melting is not complete until a higher temperature (about 230 ⁰ C) has been reached. At an intermediate temperature falling between these two limits, the solder is "pasty" and, as a consequence, relatively immobile. This property is important for processes such as fuse manufacture where the joint may have to be made with the cap inverted so that a fluid solder can run under gravity. Alternatively, if the cap and tube are pressed

together so that there is a sudden movement, then a fluid solder may splash up the wire or tube.

For environmental reasons, there is an increasing demand for lead-free replacements for lead-containing conventional alloys. Many lead-free alloys are tin rich, with small additions of elements such as copper, silver, bismuth, indium , antimony and zinc, for example. Lead-free alloys have been developed for use in the electronics industry, although these alloys generally do not exhibit a significant "pasty" range, which, as mentioned above, is an important characteristic for alloys used in the manufacture of electrical fuses. It has proved very difficult to develop an alloy composition which will work well in more demanding fuse manufacturing procedures.

The present invention aims to address at least some of the problems associated with the prior art. ^' Accordingly, the present invention provides a solder alloy for use in the manufacture of electrical fuses, the alloy comprising: Copper - from 0.5 to 4 wt.% Silver - from 0.1 to 1 wt.% Antimony - from 0.2 to 3 wt.% Bismuth - from 0 to 1.5 wt.% Zinc - from 0 to 2 wt.%

Nickel - from 0 to 0.3 wt.% Cobalt - from 0 to 0.3..wt.% Phosphorus - from 0 to 0.01 wt.% Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.1 wt.% and the balance tin, together with unavoidable impurities, provided that if the silver content is 0.5 wt.% or less,

then copper is present in an amount of not less than 0.9 wt.% or more and/or bismuth is present in an amount of not less than 0.1 wt.%.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Preferably, the copper content is from 1.5 to 4 wt.%, more preferably from 1.5 to 3 wt.%, still more preferably from 1.7 to 2.3 wt.%, still more preferably from 1.8 to 2.2 wt.%.

In a first preferred aspect, the alloy comprises: Copper - from 1.5 to 3 wt.%

Silver - from 0.2 to 0.6 Antimony - from 0.5 to 2 wt.% Bismuth - from 0.1 to 1.3 wt.% Zinc - from 0 to 1 wt.% Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.% Phosphorus - from 0 to 0.01 wt.% Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

The zinc content is preferably up to 1 wt. %, more preferably up to 0.5 wt. %.

More preferably in this aspect the alloy comprises: Copper - 1.8 to 2.2 wt. % Silver - 0.3 to 0.5 wt. % Antimony - 0.7 to 1.5 wt. % Bismuth - 0.2 to 1 wt.% Zinc - from 0 to 0.5 wt.% Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.% Phosphorus - from 0 to 0.01 wt.% Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities, and the balance tin, together with unavoidable impurities.

Still more preferably in this aspect the alloy comprises: Copper - approximately 2 wt.%

Silver - approximately 0.4 wt.%

Antimony - approximately 1 wt.%

Bismuth - approximately 0.3 wt.% and the balance tin, together with unavoidable impurities.

In a second preferred aspect, the alloy comprises:

Copper - from 1.5 to 3 wt.%

Silver - from 0.2 to 0.6 wt.%

Antimony - from 0.5 to 2 wt.% Bismuth - from 0 to 0.5 wt.%

Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.%

Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt.%

Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

More preferably in this aspect the alloy comprises:

Copper - 1.8 to 2.2 wt.%

Silver - 0.3 to 0.5 wt.% Antimony - 0.7 to 1.5 wt.%

Bismuth - 0 to 0.5 wt.%

Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.% Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt.%

Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

If present, the bismuth content will typically be from 0.1 to 1 wt.%. The bismuth content preferably does not exceed approximately 0.3 wt.%, more preferably the bismuth content does not exceed approximately 0.2 wt.%.

Still more preferably in this aspect the alloy comprises:

Copper - approximately 2 wt. % Silver - approximately 0.4 wt.% Antimony - approximately 1 wt.% and the balance tin, together with unavoidable impurities.

In a third preferred aspect, the alloy comprises:

Copper - from 1.5 to 3 wt.%

Silver - from 0.3 to 1 wt. %

Antimony - from 0.3 to 1 wt. %

Bismuth - from 0.1 to 0.5 wt.% Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt. %

Cobalt - from 0 to 0.3 wt. %

Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt. % Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities

More preferably in this aspect the alloy comprises:

Copper - 1.8 to 2.2 wt. % Silver - 0.6 to 0.8 wt. %

Antimony - 0.4 to 0.6 wt.%

Bismuth - 0.2 to 0.4 wt.%

Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt.% Cobalt - from 0 to 0.3 wt.%

Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt.%

Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

Still more preferably in this aspect the alloy comprises:

Copper - approximately 2 wt.%

Silver - approximately 0.7 wt.% Antimony - approximately 0.5 wt.%

Bismuth - approximately 0.3 wt.% and the balance tin, together with unavoidable impurities.

In a fourth preferred aspect, the alloy comprises:

Copper - from 0.5 to 2.5 wt.%

Silver - from 0.3 to 0.7 wt.% Antimony - from 0.2 to 0.5 wt.%

Bismuth - from 0.1 to 0.4 wt.%

Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.% Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt.%

Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

More preferably in this aspect the alloy comprises:

Copper - 0.7 to 1.3 wt.%

Silver - 0.4 to 0.6 wt. %

Antimony - 0.2 to 0.5 wt. %

Bismuth - 0.1 to 0.3 wt.% Zinc - from 0 to 0.5 wt.%

Nickel - from 0 to 0.3 wt.%

Cobalt - from 0 to 0.3 wt.%

Phosphorus - from 0 to 0.01 wt.%

Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.03 wt.% and the balance tin, together with unavoidable impurities.

Still more preferably in this aspect the alloy comprises: Copper - approximately 1 wt.%

Silver - approximately 0.5 wt.%

Antimony - approximately 0.2 wt.%

Bismuth - approximately 0.2 wt. % and the balance tin, together with unavoidable impurities.

The alloys according to the present invention are lead- free or essentially lead-free. These alloys offer environmental advantages over conventional alloys used in the manufacture of electrical fuses.

The alloys according to the present invention are suitable for use in the manufacture of electrical fuses. In particular, the alloys are advantageously used for (i) sealing/joining an end cap to a tube for an electrical fuse, and/or (ii) providing a solder joint between an end cap and a fuse wire. In particular, the alloys according to the present invention have the property of being "pasty" or "sluggish" within a sufficiently broad temperature range intermediate between the temperature at which the alloy is fully molten and the temperature at which the alloy is fully solidified. The precise physical characteristics by which this is achieved are not fully understood, but the success in an operating system is believed to depend on the alloy having a sufficient working range at which some liquid and solid exist in equilibrium, together with viscosity and surface tension characteristics that limit the flow of the alloy at the operating temperature. The tin-based alloys according to present invention typically start to melt at temperatures above approximately 200 ⁰ C, more typically above approximately 21O ⁰ C. The liquidus temperature depends primarily on the copper content, and is typically above approximately 260 ⁰ C, and may even extend to approximately 300 ⁰ C. This enables the joint between the cap and the fuse wire, and the co-process of sealing the cap to the fuse

body, to be accomplished without excess metal running away for the cap, along the inside or outside of the tube.

While not wishing to be bound by theory, it is believed that the alloying elements Cu, Sb, Ni (optional) , Co

(optional) , Zn (optional) and Bi (optional) may act to form intermetallics. The presence of intermetallics is believed to have a positive effect on the flow properties of the alloys, particularly in relation to the "pasty" range.

While not wishing to be bound by theory, it is believed that alloying elements Ag, Bi (optional) and Zn (optional) act to widen the temperature difference between liquidus and solidus, and increase the residual liquid phase just above the solidus. This is believed to have a positive effect on the mechanical properties of the alloys, in relation to the "pasty" range. Ag and Bi are preferred over Zn because Zn has the disadvantage that it reduces the spread of the solder during the first melting of the wire onto the cap, so that the complete coverage of the cap may not be achieved. It also makes the wire slightly more difficult to draw.

If present in the alloy, the bismuth content is preferably from 0.1 to 1 wt.%, more preferably from 0.1 to 0.5 wt.%. If present in the alloy, the zinc content is preferably from 0.1 to 2 wt.%, more preferably from 0.1 to 1 wt.%. If present in the alloy, the nickel content is preferably from 0.1 to 0.3 wt.%, more preferably from 0.1 to 0.2 wt.%. If present in the alloy, the cobalt content is preferably from 0.1 to 0.3 wt.%, more preferably from 0.1 to 0.2 wt.%. If present in the alloy, the phosphorus content is preferably from 0.002 to 0.01 wt.%. If present in the

alloy, the indium content is preferably from 0.02 to 0.2 wt.%, more preferably from 0.02 to 0.1 wt.% . If present in the alloy, the germanium content is preferably from 0.005 to 0.1 wt.%

The alloys according to the present invention will typically be provided in the form of a wire, preferably a cored wire, which incorporates a flux. The wire is preferably made by the conventional techniques of extrusion and drawing. Accordingly, the alloys according to the present invention possess mechanical properties that are compatible with extrusion and wire drawing processes.

Bismuth may be present in the alloy according to the present invention in an amount of up to 1.5 wt.%, for example from 0.5 to 1.5 wt.%. However, if the bismuth level is too high, the alloy can become difficult to draw. The bismuth level therefore preferably does not exceed 0.5 wt.%, more preferably the bismuth level does not exceed 0.3 wt.%, still more preferably the bismuth level does not exceed 0.2 wt.%.

If the copper level is too high, the alloy can become more difficult to draw. The copper level is preferably from 1.5 to 3 wt.%, more preferably from 1.7 to 2.3 wt.%, still more preferably from 1.8 to 2.2 wt.%.

Indium may be present in an amount of up to 0.2 wt.%, but preferably does not exceed 0.1% wt.%. The presence of indium can benefit the wetting properties of the alloys. Silver also improves the wetting properties.

Nickel and/or cobalt may be present in an amount of up to 0.3 wt. %. The presence of nickel and/or cobalt has been found to have a beneficial effect on the "pasty" characteristics of the alloys according to the present invention. Furthermore, it has also been found that nickel and/or cobalt reduces the rate of attack of the solder alloy on copper and copper-alloy wires, which are typically used in electrical fuses.

Antimony is believed to form intermetallics with tin and some other constituents of the alloy, and has been found to have a beneficial effect on the flow properties of the alloy, reducing the mobility in the process temperature operating region.

Phosphorus and/or germanium, if present, act to reduce the production of dross on the surface of the molten solder. This is a benefit during the manufacturing process.

The alloys according to the present invention may also be provided in the form of sphere or a preform cut or stamped from a strip or solder. These may be alloy only or coated with a suitable flux as required by the soldering process. The alloys may also be supplied as a powder blended with a flux to produce a solder paste.

The present invention further provides an alloy as herein described in the form of a bar, a stick, an ingot, optionally together with a flux, a solid or flux-cored wire, a foil or strip, or a powder or paste (powder plus flux blend), or solder spheres or other pre-formed solder pieces.

The present invention further provides a solder bath or tank, wherein the solder bath contains an alloy as herein described in the molten state.

The present invention further provides a soldered joint or coating comprising an alloy as herein described.

The alloys will typically comprise at least 90 wt.% tin, preferably from 94 to 99.1 % tin, more preferably from 95 to 99 % tin, still more preferably 96 to 98 % tin.

Accordingly, the present invention further provides an alloy for use in the manufacture of electrical fuses, the alloy comprising:

Copper - from 0.5 to 4 wt.% Silver - from 0.1 to 1 wt.%

Antimony - from 0.2 to 3 wt.% Bismuth - from 0 to 1.5 wt.% Zinc - from 0 to 2 wt.%

Nickel - from 0 to 0.3 wt.% Cobalt - from 0 to 0.3 wt.%

Phosphorus - from 0 to 0.01 wt.% Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.1 wt.% Tin - from 95 to 99 wt.% together with unavoidable impurities, provided that if the silver content is 0.5 wt.% or less, then copper is present in an amount of not less than 0.9 wt.% or more and/or bismuth is present in. an amount of not less than 0.1 wt.%.

It will be appreciated that the alloys according to the present invention may contain unavoidable impurities, although, in total, these are unlikely to exceed 1 wt.% of

the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than 0.5 wt. % of the composition, more preferably not more than 0.3 wt. % of the composition.

The alloys according to the present invention may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory (i.e. Sn, Cu, Ag and Sb) other non- specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence. Accordingly, the present invention still further provides an alloy for use in the manufacture of electrical fuses, the alloy consisting essentially of:

Copper - from 0.5 to 4 wt. % Silver - from 0.1 to 1 wt.% Antimony - from 0.2 to 3 wt.% Bismuth - from 0 to 1.5 wt.% Zinc - from 0 to 2 wt.%

Nickel - from 0 to 0.3 wt.% Cobalt - from 0 to 0.3 wt.% Phosphorus - from 0 to 0.01 wt.% Indium - from 0 to 0.2 wt.% Germanium - from 0 to 0.1 wt.%

Tin - from 95 to 99 wt.% together with unavoidable impurities, provided that if the silver content is 0.5 wt.% or less, then copper is present in an amount of not less than 0.9 wt.% and/or bismuth is present in an amount of not less than 0.1 wt.%.

The present invention also provides an electrical fuse comprising a fuse wire, a tube for the fuse wire, and at least one end cap for the tube, wherein the end cap is at least partially joined and/or sealed to the tube by virtue of a solder alloy as herein described (together with optional flux) . The present invention also provides an electrical fuse comprising a fuse wire, a tube for the fuse wire, and at least one end cap for the tube, wherein the fuse wire is joined to the end cap by a solder alloy (together with optional flux) as herein described.

The present invention also provides a process for making an electrical fuse comprising:

(a) providing a fuse wire, a tube for the fuse wire, and at least one end cap for the tube,

(b) providing a solder alloy as herein described,

(c) placing the solder alloy in the end cap

(d) joining the fuse wire to the end cap by contacting the fuse wire with the solder alloy in the end cap, and heating the solder alloy whereby a solder joint is formed between the end cap and the fuse wire. The process may further involve at least partially sealing the end cap by placing the end cap over one end of the tube, heating the solder alloy in the end cap, and applying pressure to the tube and end cap whereby the solder alloy (together with optional flux) contacts and fills an end portion of the tube and at least partially seals any space between an outer wall of the tube and an inner wall of the cap.

The fuse wire may be formed from, for example, copper or an alloy thereof. Examples include tinned copper, silvered copper, or other alloy as appropriate. The tube is typically a cylindrical tube which may be formed from, for

example, from a glass or a ceramic. The end caps may be formed from any suitable electrically conducting metal or alloy. Examples include brass caps and nickel-coated brass caps.

It will be appreciated that the alloys according to the present invention preferably possess adequate wetting characteristics to enable them to be used for the desired purpose. The alloys according to the present invention should also possess adequate mechanical properties to enable theme to drawn into wires if desired. The alloys according to the present invention have the property of being "pasty" and relatively immobile within a sufficiently broad temperature range intermediate between the temperature at which the alloy is fully molten and the temperature at which the alloy is fully solidified. This characteristic means that the alloys are particularly useful in the manufacture of electrical fuses.

The following are non-limiting examples provided to further exemplify the solder alloys according to the present invention.

Examples

The following alloys (wt.%) were found to exhibit the characteristic of being "pasty" within a sufficiently broad temperature range intermediate between the temperature at which the alloy is fully molten and the temperature at which the alloy is fully solidified. These alloys could be also be formed into a cored wire by conventional techniques and, furthermore, possessed good "wetting" characteristics.

Eg 1: 97.0Sn - 2.0Cu - 1.0Sb - 1.0Bi - 0.4Ag

Eg 2: 97.0Sn - 2.0Cu - 1.0Sb - 0.4Ag

Eg 3: 96.5Sn - 2.0Cu - 0.7Ag - 0.5Sb - 0.3Bi Eg 4: 96.78Sn - 2.2Cu -0.42Ag -0.4Sb- 0.1Bi - 0.1Ni

The properties of these alloys make them suitable for use in sealing/joining an end cap to a tube for an electrical fuse, and providing a solder joint between an end cap and a fuse wire.

The alloy according to Eg 2 above was cast into billets, extruded together with a solder flux (AlphaFry GCl), and drawn into a cored wire. The wire was cut into small pieces (approx. 100 mg) suitable for forming a joint in a solder cap. The solder piece was melted into the cap, by placing the latter onto a hot plate at 335°C. The molten solder covered all or most of the base of the cap. Furthermore, there remained flux residue with sufficient activity for the second stage of the process.

According to the manufacturing technique used, the process of soldering the cap to the fuse body containing the fuse wire may be undertaken one joint at a time or both at once. In either case, the alloy possesses the required lack of fluidity (in other words the alloy is "pasty" or "sluggish") so that a good bond with the fuse wire can be achieved without the solder being pushed out of the cap during the compression phase, in which the cap and the tube are pushed together.

With regard to Eg 4, the addition of nickel reduced the spread very slightly, so that the temperature of the first melting of the solder piece was increased to 345°C.

Eg 5: 96.09Sn-2.8Cu-O.6Sb-O.3Ag-O.15Bi-O .06Ni

The alloy according to Eg 5 above was cast into billets, extruded to make a solid wire, which was then drawn to 3.0 mm diameter. The wire was used, together with "Powerflow flux", to join two 28 mm copper tubes using an end feed copper fitting. Joints in large diameter pipes are more difficult to solder with an alloy which is very fluid at soldering temperatures. This alloy was shown to have beneficial properties such that soldering could be accomplished more easily without excessive run of molten solder away from the joint area. Accordingly, the alloys according to the present invention are not limited solely to applications involving the manufacture of electrical fuses.

The following alloys are provided by way of comparison.

CEg 1: 97.0Sn - 3.0Cu

CEg 2: 98.0Sn - 2.0Cu

CEg 3: 99.0Sn - 0.7Cu - 0.3Ag

Each of these alloys is characterised as being too fluid in a fuse manufacturing process in that loss of solder away from the cap was excessive.

CEg 4: Sn - Zn

Tin-Zinc alloys were found to have poor wetting characteristics.

CEg 5: 94.4Sn - 2Cu - 1.0Sb - 2.0Bi 0.6 - Ag

This alloy proved difficult to draw into a wire.