Patent Application
20130319726
The present invention relates generally to wires that conduct electrical current, and more particularly, to a multi-core wire used in a wire bonding step in the assembly or packaging of a semiconductor device.
Wires for conducting electrical current such as electrical signals, power and ground are well known. In the semiconductor industry, wires made of copper or gold typically are used to connect the bond pads on a semiconductor die to the lead fingers of a lead frame. However, gold wire is expensive and gold has been increasing in price. On the other hand, copper is not so expensive but it is harder to work with. For example, the interface between a copper bond wire and an aluminum bond pad can be subject to mechanical failure and increased potential contact resistance.
Further, the size of a semiconductor die has been decreasing and the processing capability increasing, so more inputs and outputs are needed for communication with the integrated circuit. Thus, bond pads are placed closer together (pitch) so thinner wires are needed. However, such thin wires must also have the strength to resist bending and breakage caused by external forces, such as when a mold compound flows over the wires during encapsulation. It is well known that the forces exerted on the wires by the mold compound can cause the wires to contact one another. This is known as wire sweep. The mold compound also can break brittle wires or weak bonds.
Thus, it would be advantageous to have a very thin yet strong wire. It would also be advantageous to have a wire that is less expensive in terms of the amount of copper or gold required to form the wires. It further would be advantageous to have a wire that is less subject to IMC (intermetallic compound) induced failure.
The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. In the drawings, like numerals are used for like elements throughout.
Those of skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements.
In one embodiment, the present invention provides a multi-core wire for conducting an electrical current, including a first conductive core and a second conductive core that surrounds and is in contact with the first conductive core, where the first and second conductive cores are formed of different conductive metals. In one embodiment, a plating layer is formed over the second conductive core, the plating layer being of the same metal as the first conductive core.
In another embodiment, the present invention provides a wire used in a wire bonding operation during the assembly of a semiconductor device, the wire consisting essentially of a first conductive core and a second conductive core that surrounds and is in contact with the first conductive core. The first and second conductive cores both conduct electrical current, and during the wire bonding operation, when an end of the wire is attached to a bonding pad of the semiconductor device, the first conductive core melts onto the free air ball (FAB)surface and protects the bonded ball from intermetallic corrosion.
Referring now to
Referring now to
The multi-core wire 20 also has layer 26 that surrounds the second conductive core 24. In a preferred embodiment, the layer 26 is a conductive metal that is plated (the layer 26 also is referred to as a third conductive core) or otherwise formed over the second conductive core 24, and in a preferred embodiment, the plating layer 26 comprises Palladium. Thus, in one embodiment, the present invention comprises a Palladium coated Copper wire that also has a Palladium core.
In one embodiment, the multi-core wire 20 is used for in a wire bonding process in which the wire 20 is used to electrically connect a lead of a lead frame or electrical contact pad of a substrate with a bonding pad of a semiconductor integrated circuit. Copper wire is currently very popular for wire bonding, however, as discussed above, it has drawbacks. The present invention essentially is an improved Copper wire used for wire bonding. Since the wire 20, as shown in
Referring to
In an alternative embodiment, the layer 26 may comprise a non-conductive coating that is formed over and around the second conductive core 24. In this alternative embodiment, the non-conductive coating comprises a polymer and its purpose is to prevent the first and second conductive cores 22, 24 from contact with the conductive cores of adjacent wires.
As can be seen, the first core 22 has a substantially uniform circular cross-section, as does the wire 20 overall. The particular diameter of the first core 22 will vary depending on the material from which the core is constructed, but may have a diameter that ranges from between about 3 um and 200 um. The second conductive core 24, which comprises a metal like Copper, may be plated or otherwise formed over the first conductive core 22 and has a diameter in a range of 13 um to about 250 um. The plating layer or third conductive core 26 may be plated over the second conductive core 24, by methods that are known in the art, and will have a thickness of between about 2 um to about 25 um, so that the wire 20 can range in thickness from about 15 um to about 275 um. In one embodiment, the first conductive core has a diameter of 7 um, the second conductive core 24 has a diameter of about 17 um, and the layer 26 has a thickness of about 3 um, such that the wire 20 has an overall thickness of 20 um.
The wire 20 is particularly suitable for conducting signals between an integrated circuit and external connection terminals therefor. For example, one end of the wire 20 may be bonded to a bonding pad of the integrated circuit and the other end of the wire 20 may be bonded to a lead finger of a lead frame or a bond pad of a substrate. For such uses, the wire 20 is connected to the integrated circuit bonding pad and the lead frame or substrate using commercially available wire bonding equipment. The heat or flame from the wire bonder melts the coating layer such that the coating layer will be bonded to either the IC bond pad, the lead finger or the substrate contact pad, as the case may be.
In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art will appreciate that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, although the present invention is particularly well suited as a bond wire, it will be understood by those of skill in the art that the principles discussed herein may be appled to larger diameter wires for carrying larger currents. Accordingly, the specification and figures are to be regarded in an illustrative rather than restrictive sense, and all such modifications are intended to be included within the scope of the present invention.
Further, relative terms such as “front”, “back”, “top”, “bottom”, “over”, “under” and the like in the description and claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The terms “a” or “an”, as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more.
Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims.
