Aluminum based solderable contact

A method of producing a solderable aluminum contact comprises formulating an ink, applying the ink to an aluminum substrate to form an ink layer on a surface of the aluminum substrate, and melting the ink layer. The ink includes a solderable element that is conductive. The melting of the ink layer forms an alloy on the surface of the aluminum substrate including the solderable element.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date under 35 U.S.C. § 119(a)-(d) of Chinese Patent Application No. 201711103931.4, filed on Nov. 10, 2017.

FIELD OF THE INVENTION

The present invention relates to an electrical contact and, more particularly, to an aluminum based solderable contact.

BACKGROUND

High current battery powered devices commonly use bus bars to electrically couple one or more battery cells to a circuit board. The bus bar may be a copper-clad aluminum strip. Aluminum is used due to its lower overall cost and weight and the copper-cladding is used to facilitate soldering the bus bar to the circuit board. It is difficult to make a reliable electrical and mechanical connection with pure aluminum as, for example, tenacious and fast-recovering oxides, high thermal conductivity, and other properties associated with aluminum make it difficult to solder.

During the cladding process, the copper and aluminum are thoroughly cleaned to remove any oxidation. The materials are then compressed together. The copper and aluminum, for example, may be passed through a pair of rollers under sufficient pressure to bond the layers. The pressure is high enough to deform the copper and aluminum and reduce the combined thickness of the clad material. Unfortunately, the cladding process is relatively time consuming and expensive, which leads to increased cost for devices that use bus bars.

SUMMARY

A method of producing a solderable aluminum contact comprises formulating an ink, applying the ink to an aluminum substrate to form an ink layer on a surface of the aluminum substrate, and melting the ink layer. The ink includes a solderable element that is conductive. The melting of the ink layer forms an alloy on the surface of the aluminum substrate including the solderable element.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

A process for producing an aluminum based solderable contact is shown inFIG. 1. The process will be described with reference to a solderable contact200shown inFIGS. 2A and 2B.

At block100shown inFIG. 1, an ink220shown inFIG. 2B, which will eventually form a solderable surface of the contact200, is formulated. The ink220includes one or more solderable elements held together with a binder and/or solvents. In various embodiments, the elements may be solderable elements such as tin (Sn), silver (Ag), zinc (Zn), copper (Cu), magnesium (Mg), palladium (Pd), nickel (Ni), silver/copper alloy, silver/tin alloy, or a different solderable element and any combination thereof. In exemplary embodiments, the solderable elements are silver/copper alloy or silver/tin alloy.

At block105shown inFIG. 1, an aluminum substrate202, which will eventually correspond to a core of the contact200, is prepared. The aluminum substrate202may be on a reel, as shown inFIG. 2B. In various embodiments, the aluminum substrate202has a thickness in a range of about 0.1 mm to 8 mm, or a range of about 0.2 to 5 mm. Aluminum is chosen due to its electrical and thermal performance. Aluminum has relatively high conductivity, low density, high thermal conductivity, and/or low work function. However, it is challenging to form a permanent aluminum contact due to its relatively poor solderability. Relatively pure aluminum compositions (99% or higher) are the most solderable and aluminum alloys with copper (Cu), manganese (Mn), and zinc (Zn) are reasonably solderable. Aluminum alloys with magnesium (Mg) and silicon (Si) are the least solderable.

At block110shown inFIG. 1, the ink220is applied to a surface of the aluminum substrate202, forming an ink layer204on the surface as shown inFIG. 2A. In an embodiment, the ink220is applied by a printer225shown inFIG. 2B. In various embodiments, the ink layer204may be formed by printing the ink220using a screen printing process, gravure printing process, flexographic printing process, inkjet printing process, stencil printing process, pad printing process, or a different printing process. In some embodiments, the ink220may be applied to the aluminum substrate202within a vacuum chamber or an inert atmosphere230, as shown inFIG. 2B, to reduce the amount of oxide build-up that may occur between processing of the aluminum substrate202and the application of the ink220.

At block115shown inFIG. 1, the ink layer204is melted onto the substrate202via an energetic beam apparatus235shown inFIG. 2Bconfigured to generate a focused beam of energy for melting the ink layer204at a precise location. In various embodiments, the energetic beam apparatus235may be used to apply a continuous energetic beam (for example, from a CO2 laser or electron beam welder), apply a pulsed energetic beam (for example, from a neodymium yttrium aluminum garnet laser), apply a focused beam, apply a defocused beam, or perform any other suitable beam-based technique. The energy of the electrons/beam voltage may be set to ensure a minimum penetration depth equal to a thicknesses of the ink layer204. In an embodiment, the voltage-penetration depth is adjusted according to the following equation:

The beam power (voltage×current) and the beam dwell time are set to ensure that the ink layer204melts uniformly and that the heat dissipation through the aluminum substrate202is overcome. The dwell time corresponds to the amount of time the beam is directed at one spot and is different from the time it takes to entirely melt the ink layer204. The beam power may range between 5-30 W, 25-100 W, 50-250 W, 100-1000 W, 500-2500 W, 1000-5000 W. The dwell time may range between 1-10 μS, 4-100 μs, 50-250 μs, 100-1000 μs, 0.5-10 μs, and 5-50 μs, or a different range.

During the melting, a thermoplastic material in the ink220may be burned away. In addition, a native oxide layer on the aluminum substrate202may be broken down by the energetic beam and/or dissolved by the weld pool (i.e., the melted ink layer204). When broken down, the native oxide layer is no longer a macroscopically continuous layer. Removal of or breaking down the oxidation layer promotes strong adhesion between the material in the ink layer204and the aluminum substrate202. Removal of or breaking down the native aluminum oxide layer also reduces contact resistance when forming a joint. This is different from other conventional processes such as electroplating or dip coating which may not be able to break down the oxide layer.FIG. 3Ashows a cross-section300of the contact after melting. The ink layer305corresponds to a copper/silver (CuAg) ink layer melted on top of an aluminum substrate306.

In an embodiment, although not necessarily required, a flux material may be introduced into the ink220to reduce tenacious native oxide layers such as aluminum oxide from forming between the ink layer204and the aluminum substrate202, to thereby improve adhesion. The flux material may be added to the ink220prior to application to the aluminum substrate202or applied to the aluminum substrate202beforehand, for example as a first layer before printing of the ink220. In various embodiments, the flux is an organic amine based flux, inorganic chloride/fluoride based flux, fluoroaluminate based flux, an acid based flux, or a different flux.

In some embodiments, in addition to the use of flux, the surface of the aluminum substrate202may be processed to remove any excess oxidation by mechanical means either in an inert gas atmosphere or in a vacuum. For example, these mechanical means include but are not limited to grinding, wire brushing, sand blasting, shot peening, and/or by other similar methods.

Subsequent to melting, the surface of the substrate202includes intermetallic elements corresponding to the materials in the ink layer204and the substrate202; the melting forms an alloy on the surface of the substrate202including the one or more solderable elements. These intermetallic elements improve wetting and adhesion of low temperature tin (Sn) based solders.

At block117inFIG. 1, in some embodiments, the surface of the aluminum substrate202with the welded ink layer204is roughened, as shown inFIG. 3B, to provide a textured surface topography that promotes stronger adhesion to soldering materials. For example, as described above, grinding, wire brushing, sand blasting, shot peening and other similar methods may be used to roughen the surface. In some embodiments, the surface may be roughened via energetic beam melting.

At block120shown inFIG. 1, a singulator240shown inFIG. 2Bcuts the roll of aluminum substrate202with the welded ink layer204on top into individual contacts200.

The solderable aluminum contact200can be used to form a joint with itself or with a second substrate. In an embodiment, the second substrate is a material comprising one of: copper (Cu), aluminum (Al), tin (Sn), gold (Au), nickel plated copper, silver plated copper, a silver plated polymeric material, a gold plated polymeric material, and a combination thereof. Any suitable solder pastes can be used to form a joint including the solderable aluminum contact200. In various embodiments, the solder paste can be selected from the list of alloys consisting of: SnPb, SnSb, SnBi, SnCuAg (SAC alloys), SnCuNi (SN100C) SnCu, SnAg, SnZn, SnAgPb, SnAgSb, SnIn, AuGe, and AuIn. In other embodiments, the solderable aluminum contact200enables the use of common solders such as low temperature lead-free solders to form a solder joint.

In other embodiments, any suitable conductive adhesive can be used to form a joint comprising the aluminum contact200. In various embodiments, the conductive adhesive is selected from the list of adhesives consisting of: epoxy, cyanoacrylate, polyurethane, acrylic, or silicone with filler materials comprising silver (Ag), tin (Sn), copper (Cu), gold (Au), nickel (Ni) or a combination thereof.

The conductive adhesive or solder paste is heated to form a joint between the contact200and the second substrate. In an embodiment, the joint is a fluxless joint.

The contact200made according to the process shown inFIG. 1and described above has several advantages over traditionally manufactured contacts. The solderability of the aluminum surface is improved when compared to a solid aluminum contact because the surface of the aluminum is alloyed with solderable materials, which facilitates soldering, for example, with tin (Sn) based solder paste/wire. The strength of a solder joint between the contact200and, for example, a second substrate made of a metal material has a shear strength greater than 3 MPa as measured according to ASTM D-1002.

The contact200also has improved electrical properties. The solderable contact200has a joint resistance less than 1 milliohm, and in some embodiments less than 0.1 milliohm, when soldered to other metal components. As shown in the chart400ofFIG. 4, the joint resistance405between two contacts formed over an area of 7.6×7.6 mm2with a copper/silver alloy layer on an aluminum substrate is less than 0.5 milliohms. The joint resistance410between a contact formed over an area of 7.6×7.6 mm2with a copper/silver alloy layer on an aluminum substrate and a copper contact is also less than 0.5 milliohms. On the other hand, the joint resistance415between two aluminum contacts over an area of 7.6×7.6 mm2is over 1.5 milliohms and the joint resistance420between an aluminum contact and a copper contact over an area of 7.6×7.6 mm2is greater than 1 milliohm. The joint resistance between the aluminum with the surface alloy and the second substrate is generally at least 75 percent less than that between pristine (i.e. uncoated) aluminum and the second substrate. In some embodiments, a copper/silver alloy layer on an aluminum substrate reduced the joint resistance by 90 percent or more. In other embodiments, a copper/silver alloy layer on an aluminum substrate reduced the joint resistance by 75 percent or more.

The aluminum based solderable contact200improves solderability, conductivity, and cost when compared to traditionally manufactured copper clad aluminum contacts. While the aluminum based solderable contact200has been described above with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the claims of the application. Various modifications may be made to adapt a particular situation or material to the teachings disclosed above without departing from the scope of the claims. Therefore, the claims should not be construed as being limited to any one of the particular embodiments disclosed, but to any embodiments that fall within the scope of the claims.