Patent Application
20150311173
1. Field
Embodiments of this disclosure relate to packaged semiconductor devices. More particularly, this disclosure relates to devices and methods for reducing corrosion in wire bonds.
2. Related Art
Wire bonding is used in the semiconductor industry to form electrical connections between contact pads on an integrated circuit (IC) die and contact pads on a package substrate. Ball bonds are formed with one end of the wire on the contact pads of the IC die and wedge or stitch bonds are formed with the other end of the wire on the contact pads of the package substrate. Alternatively, wedge bonds can be used on both ends of the wire at the bond pad on the IC die and the bond pad on the package substrate.
In some instances, the wire is coated with one or more layers of a metal, ceramic and/or organic material that exhibit desirable characteristics such as promoting formation of a bond during the wire bonding process and improving reliability in air-to-air temperature cycling and high temperature bake. The coating can also help prevent corrosion of the wire when the packaged semiconductor device during assembly and when used in the field. A problem can arise if portions of the coating are removed during the bonding process thereby exposing the wire to potentially corrosive elements.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
Devices and methods disclosed herein help minimize or reduce corrosion in wire bonds by removing coating to expose a portion of a wire in a selected location. The location selected is generally in the vicinity of areas of the wire bond that were unintentionally exposed during the bonding process. Corrosion of the metal wire may be observed when the package undergoes high temperature bake such as 175° C. for long duration such as 1000 hours. In situations where the coating is a metal more noble than the underlying wire metal, the exposed areas cause the formation of an electrochemical corrosion cell, whereby the metal coating acts as a cathode, and the exposed underlying metal wire portion acts as anode from which portions of the wire metal will be removed, or corrode. When the unintentionally exposed areas are small compared to the area of the remaining portion, then the relatively small anode area causes corrosion to occur quickly. By selectively removing additional portions of the coating material, the anode area is increased relative to the cathode area, and the corrosion rate at each exposed locations is significantly slower. In addition, the selectively exposed area helps attract corrosive agents that would otherwise be attracted to the unintentionally exposed areas, potentially causing a deeper level of corrosion that could impair conductivity or even break the wire. By selectively exposing areas of the wire, corrosion may still occur, but the corrosion level is not likely to be as extensive, thereby promoting reliability and integrity of the wire bond.
Wire 110 is held in a bonding tool referred to as capillary 302 while a free air ball (FAB) (not shown) at the end of wire 110 is formed by electrical flame-off that heats the end of wire 110 to a malleable state. The FAB is then lowered to contact bond pad 106, compressed, and subjected to ultrasonic generation (USG). The vibration of the USG effectively scrubs the FAB against the aluminum bond pad 106, promoting interdiffusion of the metal of the FAB and the metal of the bond pad 106, creating a conductive intermetallic compound 312. At the same time, portions 306, 308, 310 of wire 110 may be exposed where coating 300 is unintentionally removed by a surface 304 of the capillary 302 rubbing against coating 300 during the bonding process. A chamfered area where the capillary 302 contacted the ball bond 116 remains after the wire bonding process.
The device 100 (
In situations where the coating 300 is a metal more noble than the underlying metal in wire 110, the exposed areas of wire 110 cause the formation of an electrochemical corrosion cell, whereby the metal coating 300 acts as a cathode, and the exposed underlying metal wire 110 acts as anode from which portions of the wire 110 will be removed, or corrode.
The location of exposed portion 404 is typically chosen to be close enough to unintentionally exposed portions 306-310, 402 to attract corrosive elements away from exposed portions 306-310, 402, or at least diffuse the corrosive elements over a larger area of exposed portions 306-310, 402 and 404 to prevent the corrosion from extending to a level that would affect the reliability of wire bond 112. Also, the intentionally exposed area 404 increases the total anode area relative to the cathode area, reducing the anodic corrosion rate caused by the overall electrochemical cell. In some embodiments, the intentionally exposed portion 404 can be located to include one or more of the unintentionally exposed portions 306-310, 402, thereby incorporating the one or more unintentionally exposed portions 306-310, 402 in the intentionally exposed portion 404.
Additional intentionally exposed portions having the same or different size and shape may be formed in suitable locations relative to unintentionally exposed portions 306-310, 402. Further, an intentionally exposed portion may be formed on a section of wire 110 that is not part of ball bond 116.
Additional intentionally exposed portions having the same or different size and shape may be formed in suitable locations relative to unintentionally exposed portions 504. Further, an intentionally exposed portion may be formed on a section of wire 110 that is not part of stitch bond 114.
It should be understood that in this description, as in the art generally, materials referred to by their elemental names may contain trace impurities and/or be alloyed with small amounts of other materials. For example, the wire material commonly referred to as “copper” in the art may be alloyed with aluminum or silicon or both in small amounts, such as a few percent by weight or less, to modify certain properties of pure copper.
Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all of the claims.
Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus these terms are not necessarily intended to indicate temporal or other prioritization of such elements.
