Method and means for bonding of lead wires for an integrated circuit device

Abstract: Bonding of lead wires (14) between electrical contact points (15) of an integrated circuit and the conductive elements of a flexible tape-like structure (10) on which the integrated circuit (13) is seated is accomplished by means of a vacuum chuck (18) having distributed recesses through which a partial vacuum is applied to the flexible structure. Support elements are provided with the vacuum recesses to ensure that the flexible tape-like structure presents a planar orientation to a bonding tool. The tape-like structure is maintained in a substantially rigid position during the bonding process enabling precision bonding of lead wires.

Description: CROSS-REFERENCE TO COPENDING U.S. PATENT APPLICATION In co-pending U.S. Patent Application (Attorney Docket No. M-423) entitled "Support Assembly for Integrated Circuits" filed on or about January 28, 1987, on behalf of V. K. Sahakian et al. and assigned to the same assignee, an integrated circuit support assembly is disclosed. The patent application describes an integrated circuit package which includes a composite support assembly formed with a rigid lead frame and a thin flexible tape-like structure having inner and outer lead fingers for connection to the IC bonding pads and to external circuitry. The present invention is applicable for use with the structure disclosed in the cited patent application, which is incorporated herein by reference. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the production of integrated circuit (IC) devices and in particular to a method of bonding lead wires to the IC for providing electrical signal paths. Description of the Prior Art In the typical process of producing IC devices, an IC die which has IC logic circuits formed on its surface, is attached to a die attach pad of a lead frame. The lead frame has a number of electrically conductive elements or fingers that are connected to the IC circuits by lead wires which are bonded to contact points or bonding pads of the IC circuits. A large number of wires need to be bonded to the IC die surface and to the attach pad, which are relatively small in dimension. With the present approach to mass production of IC devices at high speeds and low cost, automated bonding apparatus which is under control of a microcomputer is employed. To implement the wire bonding process successfully, it is necessary that the wire bonding points be fixed precisely in position so that there will not be any electrical circuit shorting or erroneous connections to the IC elements. When the IC die is attached to a flexible tape-like structure, such as disclosed in the aforementioned patent application, the flexible structure has a tendency to float and vibrate so that during the wire bonding process accurate fixed positioning of the conductive elements formed with the structure is difficult. Prior art approaches to wire bonding to a lead frame utilize a window clamp to fix the frame and associated conductive elements in a rigid position. However, a window clamp which attaches to the periphery of a flexible structure does not effectively prevent the inner portions of the structure, from vibrating or being displaced from a reference plane. Therefore, bonding of wire leads to an IC assembly, where space is limited and speed of production is highly desirable, poses problems that require effective solutions. The publication US-A-3 617 045 discloses a chuck comprising a base member having a cavity including support pads on the periphery for positioning a package, and a space for receiving the package leads. The chuck further includes additional space for holding a support plate in position. A resilient mat which is impervious to air and which has a hole through the middle portion, fits over the plate and the edge portions of the base member and the edge portions of the package. The hole in the mat exposes the inner ends of the package leads and the bonding pads on a chip to allow work to be done on those areas. However, if an integrated circuit die is attached to a flexible tape-like structure as described above, the peripheral supports as disclosed by this publication fail to ensure a substantially planar orientation of the tape-like structure parallel to the surface of the vacuum chuck. In fact, under certain conditions, the flexible tape-like structure is actually drawn into the cavity defined by this publication. SUMMARY OF THE INVENTION An object of this invention is to provide a method and means for maintaining an IC device and its associated conductive elements in a fixed position during the wire bonding process. The apparatus of the invention in claimed in claim 1, and the method of the invention is claimed in claim 6. In accordance with this invention, a process for bonding lead wires to an IC device that incorporates a flexible tape-like structure serving as the IC die attach pad employs a vacuum means to effectively hold the tape-like structure in a fixed position. The vacuum means provides a substantially uniform vacuum pressure across the surface of the flexible tape, so that the tape is held in a substantially rigid position on a defined plane. In the preferred embodiment, the vacuum means comprises a vacuum chuck formed with a multiplicity of uniformly distributed vacuum cavities that are positioned adjacent to the flexible tape. In this way, automated bonding of lead wires can be effectuated at high speeds at low cost and with virtually no connection errors or electrical shorting problems. DESCRIPTION OF THE DRAWING The invention will be described in greater detail with reference to the drawing in which: FIG. 1 is a schematic representation of a flexible tape-like structure on which an integrated circuit is positioned; and FIG. 2 depicts a vacuum means, such as employed in this invention, to secure the flexible tape-like structure in order to effectuate proper wire bonding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawing, a flexible tape-like structure 10 supports a die attach pad 12 on which an integrated circuit (IC) 13 is formed. Bond wires 14 are electrically connected to bond pads 15 which are formed on the exposed surface of the IC device, and to inner lead fingers 16 that are formed on the tape-like structure. The inner lead fingers 16 are connected to outer lead fingers that extend to make contact with coupling lead bonds, thereby providing a continuous conductive path from the integrated circuit to external package leads. During the bonding of the connecting lead wires 14, which are very closely spaced, the flexible structure needs to be maintained substantially rigid and stationary. In keeping with this invention, a vacuum assembly comprising a vacuum chuck 18, as illustrated in FIG. 2, is positioned adjacent to the lower tape surface and opposite to the surface on which the bond pads are located. The vacuum chuck has a centrally located cavity encompassed by an outer ring that acts as a supporting bond shelf 30. The shelf support 30 is surrounded by a channel 24. A vacuum source 22 is connected through the body of the vacuum chuck to the central cavity 20 and channel 24 so that when the flexible tape-like structure is positioned against the chuck, the structure is held firmly in place. To ensure that the tape-like structure is fixed in a substantially planar orientation parallel to the surface of the vacuum chuck, a plurality of supporting posts 28 are provided within the cavity 20. The posts 28 prevent the flexible tape from being drawn into the cavity 20. The posts 28, the shelf 30 and the face or surface of the vacuum chuck 18 are substantially in the sane plane, so that the flexible tape lies flat across the surface of the vacuum chuck. During the bonding process, the vacuum source 22 provides a partial vacuum, 74,5 kPa (22 inches mercury), for example, through the cavity 20 and channel 24, thereby effectively providing a suction force that holds the tape rigidly in position. A heater 26 that is disposed adjacent to the vacuum chuck is used to heat the metallic chuck 18, so that heat is conducted to enable welding of the wire to the bond pads and lead fingers. The heat is provided if gold wire is used for bonding, whereas if aluminum wire is used, heat is not required. The heating element is preferably disposed within a common housing with the vacuum chuck, but may be separately assembled adjacent to the vacuum chuck for efficient heat exchange. A bonding tool 32 which is preferably automated and controlled by a microcomputer, acts to bond the wires 14 at precise predetermined positions on the IC die and on the tape-like structure during the bonding process when the vacuum is applied to the flexible tape-like structure. The bonding tool is moved relative to the bond contact points to bond the ends of the wires 14 to the lead fingers and to the IC device. Upon completion of the wire bonding, the vacuum is removed allowing further processing and packaging of the tape and IC assembly. The vacuum apparatus disclosed herein ensures that the tape-like structure on which the IC bonding pads are located will not move or shift during wire bonding, as was experienced with the window clamp of the prior art. The clamp did not provide a uniformly distributed holding force to the tape structure, and the structure tended to flutter and move during the bonding process. Also, the vacuum apparatus eliminates the need for manual assembly and disassembly, such as is required with the clamp of the prior art, thereby saving labor cost. For mass production of IC assemblies of this type, the yield is significantly improved with this approach to wire bonding.

Claims: An apparatus for bonding lead wires (14) to a flexible tape (10) and an integrated circuit device (13), said apparatus including a vacuum chuck structure (18) having a planar face and a central cavity (20) therein, characterized by: an annular support shelf (30) disposed within said vacuum chuck structure and encompassing said central cavity (20); a multiplicity of spaced support posts (28) positioned within said central cavity (20) and encompassed by said annular support shelf (30); the top surfaces of said vacuum chuck structure (18), said annular support shelf (30) and said support posts (28) being substantially flat and coplanar; vacuum means comprising first and second apertured elements for providing a source of vacuum, said first apertured element being disposed within said central cavity (20) between said support posts (28) and an inner corner of said annular support shelf (30), said second apertured element being disposed between an outer corner of said annular support shelf (30) and said vacuum chuck structure (18) so that said flexible tape (10) and said integrated circuit device (13) are maintained firmly in place directly on said annular support shelf (30) and said support posts (28) in a substantially planar orientation parallel to the planar face of said vacuum chuck structure (18) during a wire bonding procedure. The apparatus of claim 1, including bonding means (15) for joining said lead wires to said contact points whereby said lead wires are bonded precisely at defined locations. The apparatus of claim 1, further including a heater (26) disposed adjacent to said vacuum chuck structure (18) for heating said vacuum chuck structure, wherein said vacuum chuck structure (18) is made of a metallic material. The apparatus of claim 1, wherein said vacuum means comprises a vacuum source (22) connected to said vacuum chuck structure (18), and wherein said vacuum source provides a partial vacuum of about 74,5 kPa (22 inches mercury) during the bonding process. The apparatus of claim 1, including a bonding means for bonding wires to said integrated circuit device (13), and a microprocessor for controlling said bonding means. A method of bonding lead wires (14) of an integrated circuit device (13) supported on a flexible tape-like structure (10), of the type wherein said tape-like structure (10) is positioned adjacent to a vacuum chuck (18) having a planar face and a central cavity therein, characterized by positioning said tape-like structure (10) on a annular support shelf (30) disposed within said vacuum chuck (18) and encompassing said central cavity (20); further positioning said tape-like structure (10) on a multiplicity of spaced support posts (28) positioned within said central cavity (20) and encompassed by said support shelf (30), wherein the top surfaces of said vacuum chuck (18), said annular support shelf (30) and said support posts (28) are substantially flat and coplanar; and applying a partial vacuum to said vacuum chuck (18) for generating a uniform suction force across the surface of said tape-like structure (10) so that said tape-like structure (10) is held fixedly in a substantially planar orientation. The method of claim 6, including the step of heating said vacuum chuck (18) for bonding gold wire to said integrated circuit device (13).

Inventors: LONG JON, LONG, JON
Applicants: LSI LOGIC CORP, LSI LOGIC CORPORATION
Application year: 1988
Main classification: H01L21
IPC codes: H01L21