Lead-on-chip lead frame

Abstract: A multi-level lead frame configuration (114) for an integrated circuit chip (116) comprises a main lead frame (115) having a plurality of lead frame bond fingers (122 and 124) that directly connect to a plurality of bond pads (126) on the integrated circuit chip (116). Associated with the main lead frame (115) is a bus bar lead frame (128 and 130) having a plurality of bus bar lead fingers (118 and 120) that directly connect to a second plurality of inner bond pads (126) on the integrated circuit chip (116). The bus bar bond fingers (118 and 120) associate with the main lead frame (115) and main lead frame bond fingers (122 and 124) to permit a lead-on-chip configuration of the main lead frame and the bus bar lead frame.

Description: TECHNICAL FIELD OF THE INVENTION The present invention generally relates to packaging of electronic devices and more particularly relates to a lead-on-chip inner lead bonding method and apparatus that permits direct inner lead bonding of a lead frame to bonding pads of an integrated circuit chip. BACKGROUND OF THE INVENTION Lead-on-chip semiconductor packaging structures use a lead frame that extends over and is adhesively joined to the surface of the integrated circuit chip. The lead frame has numerous bond fingers to carry signals between the chip and external circuitry. The bond fingers electrically connect via numerous bonding wires to bond pads typically located in a center row of the chip's top surface. In most lead frame configurations, the bond fingers appear on two sides of the bond pads. In lead-on-chip packages, a double-sided adhesive tape is used to attach the bond fingers to the chip and provide insulation between the chip and bond fingers. An advantage of this type of configuration is that power buses for the circuitry of the integrated circuit chip may be located external to the chip. In other words, the power buses may be part of the lead frame design. One merit of this packaging concept is that the power buses of the circuitry can be located external to the chip as part of the lead frame design. This reduces the chip size and improves the plastic-to-silicon ratio in the lead-on-chip package. The buses run alongside the bond pads. Connections between the bond pads and the bond fingers are, therefore, made by routing the bonding wires around the buses. More modern lead-on-chip configurations avoid the use of bonding wires and, instead, connect bond fingers from a lead frame directly to the bond pads on the chip. Techniques that use these configurations are called inner lead bonding or ILB techniques. Power buses that run alongside the bond pads and must be routed around limit the ability to employ ILB techniques. With the power bus obstructing access to the bonding pads of the integrated circuit, it is not possible to easily access the bond pads with ILB lead frames. EP-A-0 447 922 discloses a resin seal type semiconductor device comprising a lead frame having a support and a plurality of leads. A semiconductor chip is mounted on the die support and has a plurality of pads connected to the leads. Furthermore, the device has a wire lead arranged above the semiconductor chip. For example, power source is supplied from the lead supplying power source to one of the pads receiving power source, by connecting the lead and the one pad. Furthermore, power source is supplied from the lead to another pad located far from the one pad by connecting the lead and the another pad through the wire lead. EP-A-0 478 250 discloses a lead over chip centre bond device in which bone pads lie along the central horizontal axis of a semiconductor integrated circuit chip. The chip lies beneath a lead over chip lead frame. The lead over chip lead frame may he formed of conductive metal. The backside of the chip is inactive. Two pieces of double sided adhesive tape are disposed over the top active surface of semiconductor chip and attach lead frame over the top of semiconductor chip. The integrated semiconductor chip and the lead frame form a self supporting structure. Power busses comprise spaced apart adjacent parallel conductive busses that run along the middle of the integrated chip. The adhesives are spaced apart such that the bond pads are exposed for bonding to the conductive lead fingers of the lead frame. High speed thermosonic gold ball wire bonding is accomplished to connect the bond pads to various lead fingers and to the power supply busses. One end of the wire bonds are connected to various of the bond pads. The other end of various wire bonds are connected to the two centrally disposed power supply busses of the lead frame. Multiple wire bond contacts may be made to these busses to more efficiently distribute voltage. The other end of various wire bonds cross over a power supply bus to make contact with the internal ends of the conductive lead fingers. EP-A-0594299 is a European patent application falling within the terms of Article 54(3) EPC and describing a multilayered leadframe assembly. There is a need, therefore, for a method to employ inner lead bonding techniques on an integrated circuit chip that uses lead-on-chip packaging. There is a need for an apparatus that functions as a power bus for an integrated circuit chip and that permits an inner leading bonding adaptation to a lead-on-chip lead frame configuration. The present invention provides a lead-on-chip device, comprising: a semiconductor device having a plurality of bond pads located on a surface thereof; a plurality of bond fingers overlying the semiconductor device having a first end directly connected to a respective one of a first plurality of said bond pads and a second end for connection to external circuitry, the first end being downset with respect to the second end; and at least one bus bar which traverses some of the plurality of bond fingers said bus bar lying in a different plane to said plurality of bond fingers in the region where said bus bar traverses said bond fingers, said bus bar having a plurality of bus bar bond fingers directly connected to a second plurality of said bond pads of the semiconductor device Examples of the invention shown in the accompanying drawings use a lead-on-chip lead frame configuration with inner lead bonding integrated circuit packaging techniques to overcome or reduce disadvantages and limitations associated with prior lead frame design and lead-on-chip circuitry connection methods. The examples of the invention shown in the accompanying drawings employ a multi-level lead frame configuration for an integrated circuit chip that has a main lead frame with a plurality of lead frame bond fingers. In the illustrated examples, the bond fingers directly connect to a first plurality of inner bond pads on the integrated circuit chip. Additionally, at least one bus bar that has a plurality of bus bar bond fingers is used to directly connect to a second plurality of inner bond pads on the integrated circuit chip. The bus bar bond fingers associate with the lead frame bond fingers to permit a lead-on-chip configuration of the main lead frame and the bus bar. A technical advantage of the present invention is that it allows use of a double-level or multi-level lead frame configuration to facilitate direct inner lead bonding of the bond fingers with the bond pads using a half-etched lead frame for a single row of bond pads in the center of the chip. This permits inner lead bonding for both the power bus and the main lead frame. Another technical advantage of the present invention is that it avoids the need for bonding wire by using inner lead bonding techniques, while at the same time employing lead-on-chip center bond connecting methods. Another technical advantage of the examples of the present invention shown in the drawings is that the bus bar pieces are downset at a different location from the main lead frame so that the bus bar may be stacked on the main lead frame. Additionally, in the examples of the present invention the bus bar bond fingers and the lead frame bond finger are on a different plane from that of the bus bar and main lead frame closer to the chip surface plane. This double-level configuration permits connecting the bus bar and the main lead frame with various different attaching methods, including an external lead bonding method and an insulating double-side adhesive tape. Ways of carrying out the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 illustrates a conventional lead frame configuration for lead-on-chip packaging; FIGURE 2 provides a cut-away isometric perspective of the lead frame and the bus bar configuration of the preferred embodiment; FIGUREs 3a and 3b illustrate top and side views, respectively, of certain portions of the preferred embodiment of the present invention; and FIGUREs 4a and 4b show top and side views, respectively, of an alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The preferred embodiment of the present invention is best understood by referring to the FIGUREs wherein like numerals are used for like and corresponding parts of the various drawings. FIGURE 1 is an exploded view of the conventional lead-on-chip configuration for integrated circuit chip package 10. Within package 10 and under protective layer 12, lead frame 14 provides connections to circuitry within integrated circuit chip 16. Lead frame 14 includes two bus bars 18 and 20 and two sets of bond fingers 22 and 24. Connections of bus bars 18 and 20 and bond fingers 22 and 24 to bond pads 26 are made by bond wires 28.Several new techniques now permit inner lead bonding (ILB) of a lead frame to provide for direct connection of lead frame bond fingers to bond pads. The configuration of FIGURE 1, however, prohibits this. This is because bus bars 18 and 20 on lead frame 14 form a barrier to bond pads 26. FIGURE 2 illustrates the preferred embodiment of the present invention and shows how the limitations of the prior art are overcome. In particular, integrated circuit chip package 100 of FIGURE 2 includes dual-level lead frame configuration 114 that is formed around or folded over the sides of integrated circuit chip 116. Dual-level lead frame 114 includes ILB bus bar bond fingers 118 and 120, as well as ILB lead frame fingers 122 and 124. Bus bar bond fingers 118 and 120 and lead frame bond fingers 122 and 124 connect directly to bond pads 126. This avoids the need to use bonding wires and provides to integrated circuit chip package 100 all of the benefits associated with inner lead bonding techniques. Bus bar bond fingers 118 and 120 are integral to bus bar pieces 128 and 130. Bus bar pieces 128 and 130 are separate bars that are attached to both main lead frame fingers 122 and associated support structure 132 and 134 of lead frame 114. FIGUREs 3a and 3b illustrate the construction of the preferred embodiment of the present invention. In particular, FIGURE 3a shows a top view of the double-level lead frame 114 of the preferred embodiment in a fabrication pattern that may be employed using established fabrication techniques. Double-level lead frame 114 is formed from essentially three pieces. They include main lead frame 115, bus bar frame 128 and bus bar frame 130. Main lead frame bond fingers 122 and 124 protrude to centerline 136 for contacting bond pads 126 upon being placed over an integrated circuit chip. For power bus connects, bus bar frames 128 and 130 provide bus bar bond fingers 118 and 120, respectively, that also extend to centerline 136 for connecting to appropriate ones of bond pads 126. In order to stack bus bar ILB frame 128 over main lead frame 115, it is necessary to have the two types of lead frame structures formed in a double-level configuration. FIGURE 3b illustrates a side perspective view of the relationship between bus bar lead frame 128 and main lead frame 122. In particular, bus bar lead frame 128 has an upper level portion 138 that goes to a lower level portion 140 by way of connecting piece 142. Also, main lead frame 115 includes upper portion 144 that connects to lower portion 146 by way of connecting piece 148. Note that at points 150 and 152, bus bar lead frame 128 joins main lead frame 115. This may be accomplished by any one of numerous well-established external lead bonding methods. FIGUREs 4a and 4b illustrate, respectively, top and side views of an alternative embodiment of the present invention. In particular, bus bar ILB lead frames 128' and 130' are stacked over main lead frame 115 using an adhesive tape. FIGURE 4 shows adhesive tape 154 that is placed between top portion 138 of bus bar ILB 128' and bottom portion 146 of main lead frame 115. Tape 154 is a double-sided adhesive tape that is typically used in semiconductor chip manufacturing processes (e.g., comprising a Kapton or polyimide material). In the configuration of 4a and 4b, bus bar lead frame 128' and 130' may extend to external circuitry from main lead frame 115 by connecting and overlying structures 132 and 134 of main lead frame 115. In summary, there has been shown a double-level lead frame configuration that permits direct inner lead bonding or ILB in a lead-on-chip center bond integrated circuit package and that includes a main lead frame having a plurality of lead frame bond fingers that directly connect to a first plurality of inner bond pads on an integrated chip together with at least one bus bar lead frame that has a plurality of bus bar bond fingers to directly connect to a second plurality of inner bond pads on the integrated circuit chip so that the main lead frame and the bus bar bond fingers permit a lead-on-chip configuration of the main lead frame and the bus bar lead frame. Although the invention has been described with reference to the above-specified embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the above description. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the true scope of the invention.

Claims: A lead-on-chip device (100), comprising: a semiconductor device (116) having a plurality of bond pads (126) located on a surface thereof; a plurality of bond fingers (122, 124) overlying the semiconductor device (116) having a first end directly connected to a respective one of a first plurality of said bond pads (126) and a second end for connection to external circuitry, the first end being downset with respect to the second end; and at least one bus bar (128, 130) which traverses some of the plurality of bond fingers (122, 124), said bus bar lying in a different plane to said plurality of bond fingers in the region where said bus bar traverses said plurality of bond fingers, and said bus bar having a plurality of bus bar bond fingers (118) directly connected to a second plurality of said bond pads (126) of the semiconductor device (116). 2. The lead-on-chip device of any of claims 1 to 3, wherein the bond pads (126) are centrally located on the surface of the semiconductor device (116).

Inventors: KARTA TJANDRA NMI, LOW SIU W, TEO BOON C, KARTA, TJANDRA (NMI), LOW, SIU W., TEO, BOON C.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1993
Main classification: H01L23
IPC codes: H01L21, H01L23