Abstract:
An integrated circuit package includes a ceramic ring having an inside cavity for accommodating a semiconductor die. Conductive traces are provided on the ceramic ring so as to serve as power and ground signal busses. Power and ground connection pads on the semiconductor die can be commonly bonded to these conductive traces, which are in turn commonly bonded to selected pins of the lead frame. In addition, an acrylic adhesive is used as a moisture-resistant adhesive.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to a copending patent application (“Copending Application”) entitled “Molded Plastic Package with Heat Sink and Enhanced Electrical Performance,” by M. Karnezos, Edward G. Combs et al., Ser. No. 08/116,305, filed on Sep. 3, 1993, and assigned to ASAT, Limited, which is also the assignee of the present application. The disclosure of the Copending Application is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to integrated circuit packages. In particular, the present invention relates to integrated circuit packages with high electrical and thermal performances. 
     2. Discussion of the Related Art 
     In a plastic package, such as that described in the Copending Application, a heat sink or heat spreader coated with a layer of aluminum oxide for electrical isolation can be provided for heat dissipation. However, while such a package provides good thermal performance, imperfection in the aluminum oxide layer leads to high yield loss. 
     Further, in integrated circuit packages, the semiconductor die is typically attached to a die-attach pad, which may be formed integrally with the lead frame. The semiconductor die can be attached using an adhesive based on a B-stage polyimide resin. However, polyimide resins have weak bonding strength to metals and absorbs moisture. In addition, such an adhesive requires a curing step at an elevated temperature over an extended period of time. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an integrated circuit package is provided which includes: (a) a heat spreader; (b) a semiconductor die attached to the heat spreader by an adhesive; (c) leads each including an internal portion for bonding and an external portion to serve as an external terminal for the integrated circuit package; (d) bond wires each bonding one of the internal portions of the leads to a bonding pad of the semiconductor die; and (e) a resin encapsulation enclosing the heat spreader, the semiconductor die, the internal portions of the leads and the bond wires. 
     In one embodiment, a ceramic ring with conductive traces for bonding provided thereon is attached to the heat spreader using an adhesive. In that embodiment, a first bond wire electrically couples one of the conductive traces of the ceramic ring to the internal portion of one of the leads; and a second bond wire electrically coupling that conductive trace of the ceramic ring to one of the bonding pads. The adhesive attaching the ceramic ring to the heat spreader can include an acrylic adhesive. Such an acrylic adhesive can be impregnated with a thermally conductive material. In one embodiment, the acrylic adhesive is provided between the ceramic ring and the heat spreader in two layers, one of the layers being impregnated with a thermally conductive material. Each conductive trace can serve as either a ground or a power terminal. In one embodiment the conductive traces comprise gold. The ceramic can be provided with perforations into which the resin encapsulation material can flow during molding. The cured encapsulation material can then provide locking structures for properly securing the ceramic ring. 
     In accordance with another aspect of the present invention, a downset interposer ring can be attached to the heat spreader. The downset interposer ring can provide a segment positioned between the internal portion of one of the leads and the semiconductor die. Each segment can function as a power supply or ground signal terminal. 
     In accordance with another aspect of the present invention, an integrated circuit package includes (a) a die-attached pad; (b) a semiconductor die attached to the die-attach pad by an adhesive; (b) leads each including an internal portion for bonding and an external portion to serve as an external terminal for said integrated circuit package; (c) a downset interposer ring attached to the die-attach pad, the downset interposer ring having a segment positioned between the internal portion of one of the leads and the semiconductor die; (d) bond wires each electrically coupling one of the internal portions of the leads to one of the bonding pads of the semiconductor die, and a bond wire electrically coupling the segment of the downset interposer ring to a bonding pad of the semiconductor die; and (e) a resin encapsulation enclosing the die-attach pad, the semiconductor die, the internal portions of the leads and the bond wires. In one embodiment, a heat spreader is attached by adhesive to the die-attach pad. 
    
    
     The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an 
     integrated circuit package  100  in accordance with the present invention. 
     FIG. 2 shows integrated circuit package  100  of FIG. 1, as viewed from an oblique angle, without its resin encapsulation  101 . 
     FIG. 3 shows another embodiment of the present invention in integrated circuit package  300 , provided in the “die-down” configuration. 
     FIG. 4 shows another embodiment of the present invention in integrated circuit package  400 , provided in the “die-up” configuration. 
     FIG. 5 shows integrated circuit package  500 , which is an alternative embodiment of the present invention in which semiconductor die  102  is attached to a die-attach pad  501 , rather than heat sink  104 . 
     FIG. 6 shows integrated circuit package  600 , similar to integrated circuit package  500 , but including heat spreader  601 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Integrated circuit package  100 , shown in cross section and oblique elevation in FIGS. 1 and 2, respectively, is an embodiment of the present invention. In this detailed description, like elements in the various figures are designated like reference numerals. 
     As shown in FIGS. 1 and 2, integrated circuit package  100  encloses a semiconductor die  102  in a molded resin encapsulation  101 . Semiconductor die  102  is attached by a die-attach adhesive  107  (e.g., an epoxy resin) to heat sink  104 , which is typically a oxygen-free high conductivity (OFHC) copper heat sink. (Although FIGS. 1 and 2 show a “die-up” configuration, the present invention can be practiced in a “die-down” configuration also). Heat sink  104  includes mold-locking cones  105 , which provide additional grip onto molded resin encapsulation  101 . The portion of heat sink  104  exposed to the outside of resin encapsulation  101  is plated with a film  106  of nickel or solder. Integrated circuit package  100  further includes formed leads  109  and downset interposer ring  103  which are electrically coupled to various signal lines on semiconductor die  102  through bond wires  110  and bonding pads (not shown) on the semiconductor die. Downset interposer ring  103  and leads  109  can be integrally formed from the same lead frame and separated chemically or mechanically, or both, as is known in the art. Downset interposer ring  103  can provide a ground or power ring to which the ground or power pads of semiconductor die  102  and ground or power pins of integrated circuit package  100  (i.e., ground or power pins in formed leads  109 ) can be electrically coupled using wire bonds. 
     In this embodiment, an annular ceramic ring  112  is provided between heat sink  104  and formed leads  109 . According to the present invention, conductive traces (“bonding planes”)  113   a - 113   d  FIG. can be provided on the surface of ceramic ring  112 . Bonding planes  113   a - 113   d  can be provided as “thick films” (e.g., 100 microns thick) by a silk screening process, or as a “thin films” (e.g., a few microns thick) by an evaporative process, such as sputtering. Each of bonding planes  113   a - 113   d  FIG. is provided on ceramic ring  112  as a conductive segment. However, other patterns of segmentation, including no segmentation (i.e., complete rings) can be provided. Common ground or power planes are achieved by wire-bonding ground or power pads of semiconductor die  102  and ground or power pins of formed leads  109  to bonding planes  113   a - 113   d.    
     Ceramic ring  112  can be provided by a ceramic which includes alumina, tungsten, mica, diamond, beryllia, or other materials to achieve various thermal and electrical performance. In this embodiment, ceramic ring  112  can be provided a thickness of up to 25 mils thick, preferably approximately 10 mils thick for desirable thermal and electrical performance. Perforations can be provided at predetermined positions of the ceramic ring, so as to allow flow of resin encapsulation material into the perforations during molding, thereby securely positioning the ceramic ring within the package. 
     In this embodiment, downset interposer ring  103  and ceramic ring  112  are each attached to heat sink  104  via an acrylic adhesive layer  111 . Unlike a polyimide-based adhesive, an acrylic adhesive does not absorb moisture and thus avoids failure mode relating to moisture and corrosion. Also, unlike a polyimide-based adhesive, which is typically cured at an elevated temperature, an acrylic adhesive can be applied and set using a “cold assembly” technique (i.e., at room temperature) over a time period shorter than is required for curing a polyimide-based adhesive. Application of the acrylic adhesive is preferably performed in a clean room. Further, to provide good heat transfer performance, an acrylic adhesive can be impregnated with aluminum oxide or alumina particles. The amount of aluminum oxide can be adjusted to provide a thermal coefficient of expansion (TCE) that matches the TCE of ceramic ring  112  to avoid cracking due to the mismatch in TCEs. The acrylic adhesive can be made conductive by including conductive metal filings. When downset interposer ring  103  is provided as a ground plane, a conductive acrylic adhesive can be used to ground downset interposer ring  103  to heat sink  104 . Conversely, for good electrically insulation, two layers of acrylic adhesives can be provided in an acrylic adhesive layer: a pure acrylic layer and aluminum oxide-impregnated acrylic adhesive to provide good thermal conduction and good electrical isolation performance. 
     Alternatively, the present invention can be practiced without ceramic ring  112  in small outline integrated circuit (SOIC) packages. FIGS. 3 and 4 show integrated circuit packages  300  and  400 , which are embodiments of the present invention, provided in the “die-down” and “die-up” configurations, respectively, without ceramic ring  112 . 
     FIG. 5 shows integrated circuit package  500 , which is an alternative embodiment of the present invention in which semiconductor die  102  is attached to a die-attach pad  501 , rather than heat sink  104 . Die-attach pad  501  can also be formed integrally in a lead frame with leads  109  and separated by a chemical or mechanical process, or both, as is known in the art. Downset interposer ring  103  can be electrically isolated from die-attach pad  501  using a non-conductive acrylic adhesive. In addition, a commercially available heat spreader (e.g., a suitably formed metal plate with a relatively large surface area) can be provided in a package similar to integrated circuit package  500  to increase thermal performance of the package. For example, FIG. 6 shows such an integrated circuit package  600  including a commercially available heat spreader  601 . Heat spreader  601  can be attached to die-attach pad  501  using an acrylic adhesive with a suitable thermal conductivity, in accordance with the present invention. 
     The above detailed description is provided to illustrate the specific embodiments of the present invention and should not be read as limiting the present invention. Numerous variations and modifications within the scope of the present invention are possible. The present invention is defined by the following claims.