High power, pluggable tape automated bonding package

Abstract: A TAB package is described which includes a flexible dielectric film (30) with an outer edge, back and front faces and an aperture therein. The front face, as is conventional, is provided with a plurality of beam leads (31), which leads extend into the aperture and connect to a semiconductor chip (34). Thermally conductive body means (10) is provided which has a well (18) formed therein, the well defined by a lip (22) comprising the outer rim (20) of the body means. The back face of the semiconductor chip is thermally connected to the thermally conductive body means (10) and the flexible dielectric film (30) is formed to conform to the surface of the well and to extend over its rim whereby the beam leads also conform to the well and rim structure. The beam leads are thereby made available for electrical connection in the vicinity of the rim. Means (40, 42) are also provided for attaching the outer edge of the film to the thermally conductive body means so as to enable flexure of the film in the event of differential expansion or contraction at connection points.

Description: This invention relates to integrated circuit chip packages and more particularly to a tape automated bonding package adapted for use with high power dissipation semiconductor chips. The prior art is replete with various package configurations for semiconductor chips. In recent years, the cost of such packages has, in many cases, out-distanced the cost of the chips. An inexpensive packaging technique known as "Tape-Automated-Bonding" (TAB) packaging has come into being and has been especially useful with lower cost, low power dissipation integrated circuits. TAB packages employ a web of polymeric material, generally called a carrier tape, to carry beam-lead conductors which provide connections between the chip and the outside world. The carrier tape is incremented past a number of operating stations, one of which places a chip on an inner-cluster of beam-leads, which cluster is then bonded to connecting pads on the chip. The carrier tape is then incremented to a station where the active face of the chip, including the inner lead-bonds is coated with a passivating material. In conventional systems, the tape is then moved to a further station where the outer beam leads are registered with conductive pads on an underlying circuit board and connections are made therebetween. TAB packaging is, by its nature, inexpensive. Its use, however, has been mainly restricted to low power dissipation circuits as it has been difficult to apply heat sinks to such structures while still maintaining the package's low cost characteristics. In addition to its low cost, TAB packaging provides an important temperature compensation feature which is not readily apparent. The conventional TAB package is connected to the underlying circuit board only by its outer lead-bonds. Thus, if there is a difference in the expansion/contraction characteristics between the underlying circuit board and the tape carrier, the flexibility of the package enables it to cope with that phenomenon, thus avoiding bond ruptures which might occur in a more rigid packaging system. One disadvantage of the TAB package is that it is not inherently pluggable (i.e. able to be plugged into a female connecter) without substantial alteration and/or modification of its configuration. In US-A- 3 846 825, a package is disclosed wherein a semiconductor chip is joined to metal patterns on an insulating flexible foil. The combination is mounted on a ceramic wafer which has an opening aligned with the chip. A conductive heat dissipator, on the reverse side of the ceramic wafer, makes contact with the non-active face of the chip - thereby providing a heat sink. In order to make the package "pluggable", the metal conductors on the foil are soldered to corresponding metal conductors extending from pins which pass through the ceramic wafer and are adapted to be inserted into a female connector. Utilization of the pin connections and the metallized support that carries the pins represent an undesirable added cost to the package. In US-A- 4 147 889, another chip carrier arrangement is shown wherein a thin dielectric with beam conductors thereon is formed into a dish shaped receptacle. The chip contact pads are connected to these conductors by "flying" lead bonds. The receptacle has flexible flanges which are used to mount the combination on a circuit board. Since the package is held in place by relatively thin metallic flanges, its heat dissipation characteristics are limited. Furthermore, there is no teaching of any means for making the package pluggable. In US-A- 4 132 856, an integrated circuit package is described wherein a carrier tape/chip assembly is emplaced on a heat sink. The combination is then encapsulated with the outer bonds being available for interconnection. The encapsulation of the package eliminates the flexibility features of the TAB package and makes the outer lead bonds much more susceptible to rupture in the event of differential expansion/contraction with interconnected lands on a circuit board. Accordingly, it is an object of this invention as claimed to provide an improved TAB package which enables high power dissipation from an attached chip. It is a further object to provide a TAB package which is pluggable. It is another object to provide an improved TAB package which is adaptable to differential expansion/contraction at its interconnection points. The present invention as defined in claim 1 provides a TAB package which includes a flexible dielectric film with an outer edge, back and front faces and an aperture. The front face, as is conventional, is provided with a plurality of beam leads, which leads extend into the aperture and connect to a semiconductor chip. Thermally conductive body means is provided which has a well formed therein, the well defined by a lip comprising the outer rim of the body means. The semiconductor chip is thermally connected to the thermally conductive body means and the flexible dielectric film is formed to conform to the surface of the well and to extend over its rim whereby the beam leads also conform to the well and rim structure. The beam leads are thereby made available for electrical connection in the vicinity of the rim. Means are also provided for attaching the outer edge of the film to the thermally conductive body means so as to enable flexure of the film in the event of differential expansion or contraction at connection points. For a better understanding of the present invention, together with further objects, advantages and capabilities thereof, in the following preferred embodiments of the invention are described with reference to the accompanying drawings, in which Fig. 1 is an exploded isometric view of the thermally conducting body means and TAB carrier/chip before assembly; Fig. 2 is a section taken along line 2-2 of Fig. 1 after assembly and placement of the package on a circuit board; Fig. 3 is a section of a pluggable package embodying the invention wherein a unitary heat sink is employed; and Fig. 4 is a section of a portion of the package showing a modification to enable relative movement between the thermally conducting body and the TAB carrier. Referring now to Fig 1, an isometric view is shown of thermally conducting body 10 and tape carrier/chip combination 12 prior to assembly. Fig. 2 is a section taken through thermally conducting body 10 along line 2-2 after it has been assembled with tape carrier/chip assembly 12. Thermally conducting body 10 comprises a finned heat sink 13 which nests in and is supported by insulating base member 14. A portion of base member 14 has been cut away to enable its inner structure to be visualized. Heat sink 13 has a downwardly extending portion 16 which mates with an opening 15 through the central portion of base member 14. Base member 14 has a well area 18 formed into its lower surface such that its lower-most extremity forms a rim 20 about the well area. The upper surface of base member 14 has an opening in which heat sink 13 is seated. A grooved lip 22 is formed about the upper extremity of base member 14. The size of the grooved opening 19 in lip 22 is slightly larger than the thickness of tape carrier 30. At each corner of base member 14, there is provided an opening 23 into well area 18. Openings 23 provide access for easy flux cleaning when the package is soldered to a printed circuit board. A plurality of beam lead conductors 31 are affixed to the underside of tape carrier 30. The inner lead portions extend into opening 32 and are bonded to chip 34 thereby making electrical connection therewith. The outer portions of conductors 31 terminate prior reaching the outer extremities of tape carrier 30. An encapsulating material 36 covers the active face of chip 34 and passivates its connection points. Intermediate areas 35 have been removed from tape carrier 30 as to enable the bending of beam leads 31 about outer rim 20 when thermally conducting body 10 and tape carrier/chip assembly 12 are joined. Prior to assembling tape carrier/chip assembly 12 with thermally conductive body 10, a layer of thermally conductive adhesive 38 is emplaced on the upper surface of chip 34 (see Fig. 2). In addition, an adhesive material 40 is placed around the periphery of tape carrier 30. It should be noted that corner portions of tape carrier 30 have been removed so as to create arms which may be bent or formed independently of each other. To assemble the circuit package, a forming tool (not shown) is raised to contact the bottom surface of tape carrier/chip assembly 12 from the direction shown by arrow 42. As tape carrier/chip assembly 12 moves upwardly, the top surface of chip 34 and thermally conducting adhesive 38 come into contact with downwardly extending portion 16 of heat sink 13. Simultaneously, each of the arms of tape carrier 30 is formed about an associated rim area 20. The outer-most extremity of each arm and its adhesive layer 40 is thereby moved into contact with wall surface 21 of base member 14. Additionally, the edge of each of the arms is inserted into the respective grooved lip 22. Pressure is maintained on the assembly until cement 40 causes the tape carrier 30 to adhere to wall surface 21 of base member 14 and adhesive 38 causes chip 34 to adhere to portion 16 of heat sink 13. As can be seen from Fig. 2, the only points of rigid interconnection between tape carrier/chip assembly 12 and thermally conductive body 10 are thermally conductive adhesive 38 and adhesive 40. Well portion 18 of base member 14 provides an offset 42 so that chip 34 and its associated interconnections are kept well away from any supporting circuit board or conductive land areas. In Fig. 2, the circuit package is shown as being soldered to conductive lands 50 which are, in turn, supported by circuit board 52. Since tape carrier 30 is connected to base member 14 only by adhesive 40, should differential thermal expansion be experienced between circuit board 52 and any portion of the circuit package, tape carrier 30 is adapted to move relative to body 14 to relieve any stresses and avoid ruptures of the circuit interconnections. Referring now to Fig. 3, the structure shown in Fig. 2 has been modified to include a unitary heat sink 60. In this instance, heat sink 60 has an identical external shape to the thermally conductive body 10 shown in Figs. 1 and 2, but rather is comprised of a single metal substrate. As a result, heat sink 60 is somewhat heavier and but provides more efficient thermal dissipation properties. An additional layer of insulating material 55 encompasses rim area 20 and acts to isolate conductors 31 from metal heat sink 60. In addition, in lieu of interconnecting conductors 31 directly to printed circuit board 52, a female socket 61 is shown which is provided with a plurality of contacts 62. When the circuit package assembly is inserted into socket 61, conductors 31 connect to contacts 62 and thereby enable communication with chip 34. It should be noted, that the structures shown in Figs. 2 and 3 both provide pluggable TAB packages. In Fig. 4, a modification is illustrated to the circuit packages shown in Figs. 1-3 which provides for greater relative movement between the tape carrier/chip assembly 12 and thermally conducting body 10. In this case, a small space 100 is deliberately left between beam leads 31 and rim 20 (including insulating material 55) when tape carrier 30 and beam leads 31 are formed thereover. This space provides for greater relative movement between the conductor carrying elements of the circuit package and the heat sink.

Claims: A TAB circuit package comprising: a flexible dielectric film (30) having an outer edge, back and front faces and an aperture (32) therein, said front face provided with a plurality of beam leads (31) affixed thereto, inner lead portions of said beam leads extending into said aperture and outer lead portions of said beam leads terminating prior to reaching the outer edge of said flexible dielectric film; a semiconductor chip (34) positioned in said aperture (32) and having back and front faces, said front face having electrical contacts connected to said beam leads, said chip oriented such that its back face and said back face of said dielectric film are similarly oriented; thermally conductive body means (10) having a well (18) formed in one of its surfaces, said well being defined by an outer rim (20), the back face of said semiconductor chip thermally connected within said well to said thermally conductive body means and said back face of said flexible dielectric film formed to conform to said well and said outer rim, whereby said beam leads also conform to said well and said outer rim and are thereby made available for electrical connection to their outer lead portions in the vicinity of said outer rim; and said outer edge of said flexible dielectric film (30) being attached to said thermally conductive body means (10) at an outer wall surface (21) of said outer rim (20) by means of a thermally conductive adhesive (40) to enable flexure of said beam leads (31) in the event of differential thermal expansion or contraction between a point of electrical connection at the outer lead portions and any portion of the circuit package. The circuit package of claim 1, wherein intermediate areas (35) of the flexible dielectric film (30) have been removed to enable bending of the beam leads (31) about said outer rim (20). The circuit package of one of the preceding claims, wherein said thermally conductive body means (10) comprises a finned heat sink (13, 60) thermally connected to said chip. The circuit package of one of claims 1 and 2 wherein said thermally conductive body means (10) comprises: an insulating base member (14) forming said well (18) and said outer rim (20) and having an opening (15) through its central portion; and a heat sink (13) nesting in said base member (14) and having an extension (16) mating with the opening (15) in the base member (14) and being thermally connected to said chip. The circuit package of claim 4, wherein a grooved lip (22) is formed in the base member (14) about the outer wall surface (21) of said outer rim (20) for receiving the outer edge of said flexible dielectric film (30). The circuit package of claim 2, wherein said thermally conductive body means (10) comprises a unitary heat sink (60) comprised of a single metal substrate and forming said outer rim (20) and said well (18), and wherein an insulating material (55) encompasses said outer rim (20) to isolate said beam leads (31) from said heat sink (60). The circuit package of one of the preceding claims, wherein a space (100) remains between said beam leads (31) and said outer rim (20) to enable relative movement therebetween. The circuit package of one of the preceding claims, wherein said outer lead portions of said beam leads (31) are directly connected to a circuit board (52) by a female socket (61) which is provided with a plurality of contacts (62).

Inventors: BICKFORD HARRY R, MOK LAWRENCE S, PALMER MICHAEL J, BICKFORD, HARRY R., MOK, LAWRENCE S., PALMER, MICHAEL J.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01L23
IPC codes: H01L23, H01L21