Reduced capacitance lead frame for lead on chip package

Abstract: In a lead on chip, LOC, integrated circuit packaging arrangement (10), the conductors (46) terminate in fingers (50) that receive the bond wires (20). Adjacent the fingers (50), the conductors (46) have arm parts (52) extending over the major face of the integrated circuit (12). These arm parts (52) are formed by stamping, rolling or otherwise to present an upwardly opening channel with at least the bottom lateral margins (54, 56) of the arm part (52) raised above the plane of the bottom surface (58) of the arm part (52). This reduces sagging of the arm part (52) and capacitive interaction with the integrated circuit.

Description: This invention relates to lead frames used for connecting electrical signals to packaged integrated circuits and particularly relates to where the lead frames extend over the integrated circuits, also known as lead on chip or LOC. DESCRIPTION OF THE RELATED ART Present packaging techniques for megabit dynamic random access memory DRAM devices mount the ends or fingers of the lead frame conductors, which extend over a major face of the integrated circuit chip, to the chip with a small piece of double sided adhesive tape. The bond pads on the chip extend along the central longitudinal axis of the chip. The lead frame presents two conductors, used as power rails or busses, extending the length of the chip and adhesively mounted proximate the bond pads, one on each side of the bond pads. These two conductors carry power to the chip and conduct heat from the chip. Other lead frame conductors present arms bent and extending inwardly from opposed sides of the package or chip. The arms terminate in the fingers mounted proximate their related bond pad on a side of the power rails opposite the bond pads. Some of the bond wires then extend from certain of the bond pads to the respective power rails. The rest of the bond wires loop over the power rails to connect respective bond pads to lead frame conductor fingers. U.S. 5,068,712 to Murakami, et al., U.S. 4,916,519 to Ward and U.S. 4,862,245 to Pashby, et al. disclose this packaging technique. The arms extending over the chip and terminating in the fingers have been long, flat and narrow metal formations. By extending over the chip, some capacitive interaction occurs between the signals on the chip, or the voltage applied to the chip substrate, and the signal carried by the arm of each conductor. Some efforts at reducing this capacitive interaction center around reducing the width of the conductor arm, because capacitance relates generally to the area of the arm exposed to the underlying chip. An undesired result of narrowing the arm is the arm bowing or sagging toward and possibly touching the chip. U.S. 5,068,712 particularly overcomes problems related to stray capacity, signal transmission rate and electrical noises by a certain arrangement of the gap between the leading end portions of the inner leads and the semiconductor chip. Patent Abstracts of Japan 5(51), (E-51) [723]; JP-A-56-4260 discloses an electronic device of the type where the conductors do not extend over the integrated circuit chip. The lead frame is composed of a tab, a tab hanger lead, an electrode lead, etc. The electrode lead is connected through a coupling wire to the pad of a semiconductor pellet secured on the tab. Flat flanges are provided on the top of each of the leads. The tab and the bottom of each lead are located at the same height, whereas the flanges are located higher than the tab. The cross section of each lead is shaped as U or V to increase the effective cross-sectional area and to thereby improve the transfer of generated heat. SUMMARY OF THE INVENTION According to the present invention an integrated circuit device as defined in claim 1 is provided. A lead on chip, LOC, packaging arrangement includes lead frame conductors having long, narrow and flat arms that terminate in fingers. The fingers adhesively mount over a major face of the integrated circuit and receive bond wires extending from bond pads centrally located on the integrated circuit. The conductor arms have parts adjacent the fingers that are bent or formed into longitudinal formations different from the cross-sectional shape of the rest of the arm to increase the strength of the arm part against bowing or sagging. Particularly the arm parts are formed into upwardly opening channels. Along the length of these channels, at least the lower margins of the conductor material are raised to be above the plane of the lower surface of the arm part. This formation not only strengthens the conductor arm parts against bending or sagging, but also reduces the capacitive interaction between the conductor arm parts and the underlying integrated circuit chip. The arm parts can be formed by stamping, rolling or otherwise to effect the channel structure. Typically the material of the lateral margins of each arm part will be displaced upwardly from the remaining central material of the arm part. This increases the distance between the lateral margins of the arm part and the underlying chip to reduce the capacitive interaction, while obtaining the structural strength of a channel formation. This also effectively reduces the area of the arm part that lies close to the chip to substantially only the central material of the arm part to reduce the capacitance. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a perspective view of an integrated circuit chip packaged in a LOC arrangement, with part of the encapsulating material removed to expose the chip and lead frame parts; Figure 2 is a transverse cross-sectional view of the packaged integrated circuit of Figure 1; Figure 3 is an enlarged idealized perspective view of a lead frame adhesively mounted over a chip and including a representative bond wire; and Figure 4 is an enlarged idealized perspective view of a lead frame conductor arm part cut in transverse section to exhibit the channel feature of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In Figure 1, packaged integrated circuit device 10 comprises integrated circuit chip 12, conductors 14 forming a lead frame, adhesive tapes 16 adhesively mounting the conductors 14 on the integrated circuit chip 12, soft packaging material 18 overlying the area of the device containing the bond wires 20 and hard packaging material 22 encapsulating the entire device. The packaging arrangement depicted in Figure 1 forms a small outline J lead, or SOJ, package. Chip 12 can be any type of integrated circuit desired and in the preferred embodiment is a dynamic random access memory, DRAM, chip of the so-called 16 megabit size having 2 24 or 16,777,216 memory cells. Two of the conductors 14 have parts 24 and 26 that extend the length of the chip and are used as power rails or buses. The bond pads 28 on the chip extend along the central longitudinal axis of the chip and the power rail or bus parts 24 and 26 mount proximate to the bond pads, one on each side. These two conductor parts carry power to the chip and conduct heat away from the chip. The other lead frame conductors 14 such as conductor 30 present a flat body portion 32, a long and narrow depending leg 34 that is bent under the package in the shape of a "J" and is adapted to be connected to an electrical connector on a printed circuit board. Conductor 30 also presents a long arm 36 bent inwardly from a side of the package and extending over the chip 12 to terminate in finger 38. Arm 36 and finger 38 extend across and lie over a major face 40 of the chip 12. The fingers of all the conductors 14 mount on the adhesive tapes 16 proximate the power rails or buses 24 and 26 and on a side of the power rails or buses opposite the bond pads 28. Some of the bond wires 20 such as bond wires 42 extend from the bond pads 28 to the power rail or bus part 24. Others of the bond wires 20 such as bond wire 44 loop over the top of the conductor part 26 and attach to the top surface of the finger 38. This arrangement of the lead frame conductors, the adhesive tapes 16 and the lead frame conductors 14 is known as a lead over chip or LOC arrangement. In Figure 2, the conductor arm 36 extends over the major face 40 of the integrated circuit chip 12 and terminates in finger 38. Whenever a lead frame conductor is routed over the surface of an integrated circuit chip, the operation of the chip and conductor is hampered by the resulting capacitance between the two. Designers prefer a minimum amount of capacitance to exist between these conductors and chips and a guideline of a two picofarad (2pF) capacitance maximum has been established. Due to the large size of chips required to provide the over 16 million bits of information, the lengths of the conductor arms and fingers overlying the chips has become longer. In order to reduce the effective capacitance, these long runs of conductive material are being designed to be narrower. This causes a problem in making the conductor very flimsy and subject to bowing or sagging under its own weight, which occurs when reaching the limit on the minimum material width. In Figure 3, lead frame conductor 46 includes a rectangular cross-section arm 48 terminating in finger 50. A part 52 of arm 48 adjacent finger 50 and overlying chip 12 is specially formed to have a cross-sectional shape different from the rectangular shape of arm 48 to strengthen arm 48 and to reduce the capacitive interaction between the arm part 52 and the integrated circuit chip 12. Referring also to Figure 4, arm part 52 is formed generally in a channel or an upwardly opening "U" configuration. The lower lateral margins 54 and 56 of the part 52 are raised above the plane of the bottom surface 58 of the arm part to strengthen the arm from bending. Because the width of bottom surface 58 is less than the width of the rectangular arm 48, there is less material proximate the major face 40 of the integrated circuit chip 12, and there is less capacitive interaction between the arm 48 and the integrated circuit chip 12. The embodiment depicted in Figures 3 and 4 displaces the material of both lateral margins 60 and 62 upwardly relative to the central material 64 of the arm part 52. In one embodiment, the material of arm 48 has a thickness of about 0.008" (1"=2.54x10 -2 cm). The lower lateral margins 54 and 56 are raised about 0.003" above the bottom surface 58. The central material 64 has a width of about 0.008" and the arm 48 has a width of about 0.023". Typically the lead frame conductor 46 is made of a material known as alloy 42 comprising iron and nickel. With this channel formation in the arm part 52, the conductor 46 attains an I-beam effect to strengthen the arm part against any bowing or sagging. Also the elevated lower lateral margins present less capacitance to the chip than the middle section. This obtains a better handling of the long arm portion mechanically while remaining within electrical capacitance parameters. Modifications and variations of the invention are possible in light of the above teachings while remaining within the scope of the appended claims. The lead frame material can vary as well as the specific dimensions identified. Other cross-sectional shapes also can be used to obtain strength and reduce capacitance. Moreover, the arm part 52 can be formed by any process desired such as by stamping, rolling or otherwise. Additionally, only one of the lateral margins may be formed to be displaced from the other lateral margin to result in an L or V shape.

Claims: An integrated circuit device comprising: an integrated circuit chip (12); at least one lead (46) comprising a flat and elongate leg section extending from the chip (12) and adapted to be connected to an electrical conductor on a printed circuit board, and a flat and elongate arm section (48) contiguous to said leg section and having a certain cross-sectional shape, said arm section (48) terminating in a finger (50) overlying a first major surface of the chip and adapted to be connected to one end of a bond wire (20), the arm section comprising a first part (52) adjacent the finger (50) with a lower surface defining a first plane overlying and proximate the major surface of the chip (12) and subject to capacitive interaction therewith, and a second arm part contiguous to the first part (52) and adjacent to the periphery of the chip, and having a first cross-sectional width at a lower surface parallel to the first plane, the first arm part (52) being formed with a cross-section shape different from that of the second arm part to strengthen the arm from bending and reduce capacitive interaction with the integrated circuit, wherein the cross-sectional width of the first arm part at the lower surface is less than the first cross-sectional width, and the first arm part is shaped in cross section to include at least one lateral margin (54, 56) comprising a surface contiguous with the lower surface that is opposed to and raised above the first plane and the first major surface of the chip. The integrated circuit device of claim 1 in which the first arm part (52) is formed to present an upwardly opening channel. The integrated circuit device of claim 1 including a plurality of said leads.

Inventors: RUSSELL ERNEST J, RUSSELL, ERNEST J.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1993
Main classification: H01L23
IPC codes: H01L23, H01L21