Abstract:
A chip package is provided with multiple ways of attaching a heat sink directly to the chip carrier. Corner post are mounted to the surface of the chip carrier. A heat spreading plate, with a surface area substantially the same size as the surface area of the chip carrier, is positioned in thermal contact with the surface of a flip chip, for example. The heat spreading plate has corner cuts to accommodate the corner posts of the chip carrier and notches cut into at least two opposing sides. A heat sink plate with holes extending therethrough at each of its four corners is positioned to allow the corner posts of said chip carrier to extend therethrough. Notches cut in two opposing sides of said heat sink plate are aligned with the notches in said heat spreading plate to create slots for a flexible clip to clamp the assembly together. Alternatively, nuts may also be threaded onto the posts to clamp the assembly together.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic packaging arrangement. More particularly, the present invention relates to an electronic packaging arrangement employing a heat spreader designed to facilitate multiple and reworkable heat sink attachment directly to chip carrier. 
     2. Background and Related Art 
     As the integrated circuit art moves toward smaller and smaller component sizes leading to increased density of circuits and more and more input/output (I/O) connections, the heat generated by such increased density increases correspondingly. Moreover, with such increased density, the cost of integrated circuit packages, such as flip chip packages, also increases. 
     Thus, efficient cooling and reworkability of such packages is highly desirable. Accordingly, where electronic device failure of the package occurs, efficient heat dissipating structure should be such as to be readily removed in non-destructive manner. 
     Typically, heat sink arrangements employing heat fins are positioned in thermal contact with the integrated circuit package mounted on a printed circuit board (PCB). The heat sink is, in turn, removably attached to the printed circuit board. An elastomeric thermal pad or thermal grease may be interposed between heat sink and integrated circuit package to compensate for any mismatch in physical characteristics and thermal expansion between heat sink and integrated circuit package, as well as reduce interface resistance. An example of such an arrangement may be found in U.S. Pat. No. 6,125,037. One of the difficulties in cooling integrated circuit packages is thermally contacting a sufficiently large heat dissipating surface to the integrated circuit to effectively spread heat generated therefrom. Packaging constraints often limit the area size of the heat dissipating surface, particularly where such packages are designed to be reworkable. Moreover, integrated circuit products, such as, single chip flip chip modules, often employ different cooling arrangements for the same product. For example, flip chips may be employed in a single chip module using some form of clip to hold the heat sink in thermal contact with the chip on the PCB. Alternatively, the heat sink may be attached by screws or posts to the PCB. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the present invention, a packaging arrangement is provided wherein a heat sink and heat spreading lid are directly attached to the chip carrier. The heat spreading lid is sufficiently large to accommodate effective spreading of heat from the chip and yet is designed to allow heat sink attachment by either posts positioned directly on the chip carrier at its corners, or by some form of chip arrangement clamped to the underside of the lid or chip carrier at the mid-section of the heat sink. 
     It is, therefore, an object of the present invention to provide an improved integrated circuit package. 
     It is a further object of the present invention to provide an integrated circuit package with improved cooling. 
     It is yet a further object of the present invention to provide an improved reworkable flip chip package. 
     It is yet still a further object of the present invention to provide an improved heat sink attachment arrangement wherein the heat sink is attached directly to the chip carrier. 
     It is another object of the present invention to provide a removable heat sink and heat spreading lid that provides effective chip cooling, with said heat spreading lid arranged to facilitate heat sink attachment to the chip carrier by posts arranged on the carrier or by clipping to the lid or carrier. 
     It is yet another object of the present invention to provide a chip module having attachment posts on the chip carrier for attaching a heat sink directly to the chip carrier, and a heat spreading lid interposing the heat sink and chip or chips with the surface area of the lid being approximately the size of the surface area of the chip carrier so as to effectively spread heat, protect the chip(s) and facilitate heat sink attachment by either the attachment posts or a heat sink clip acting to clamp the heat sink to the chip carrier. 
     These foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, wherein like reference members represent like parts of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows an exploded perspective view of the chip package, in accordance with the present invention. 
     FIG. 2 shows a partially exploded perspective view of the chip package, in accordance with the present invention. 
     FIG. 3 shows a side view of an assembled chip package. 
     FIG. 3A shows a further clip configuration that may be used to hold the heat sink and heat spreading lid on the chip carrier. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, there is shown an exploded view of the integrated circuit package, in accordance with the present invention. Chip carrier  1  is shown with four retention corner posts  3  mounted thereon. Chip carrier  1  may be made of any of a variety of chip carrier materials, such as a laminate chip carrier material. 
     Corner retention posts  3  may be made of any of a variety of materials such as, metal or plastic, although metal would be more effective in aiding heat dissipation. The post may be affixed to chip carrier  1  by glue or an epoxy adhesive. If the posts are metallized, they may be attached by soldering, welding or the like. In this regard, using an epoxy, solder, or the like, avoids the necessity of having to modify the chip carrier by forming holes in the carrier for screws, post insertion, or the like. In addition, attaching the posts directly to the chip carrier also avoids the necessity of having to modify the PCB, upon which the chip carrier sets, by forming holes for posts or screws to hold the heat sink thereon. Avoiding the necessity of having to form holes in the PCB for attaching the heat sink saves much needed wiring area. 
     As further shown in FIG. 1, chip  7  is mounted in the central portion of laminate chip carrier  1 . As is known to those skilled in the art, an array of solder ball connections on the down side of flip chip  7  are attached to a corresponding array of pads on chip carrier  1 . The pads, in turn, are connected to metallurgy which extends to an array of pads on the underside of chip carrier  1 . The latter are then attached to contact pads on the PCB. 
     When the arrangement shown in FIG. 1 is fully assembled, heat spreading lid  5  is positioned in thermal contact with chip  7 . The lid may be in the form of a plate or a cap. Heat spreading lid  5  is made of a high thermal conductivity material, such as, a metal. For example, aluminum, copper or AlSiC may be used. The heat spreading lid, in accordance with the present invention, can be formed with a surface area approximately that of the chip carrier. In this regard, the edges of the heat spreading lid may be coextensive with the edges of the carrier except for cutouts  9  at its four corners and cutouts  11  at the midsection of each side. These cutouts minimally reduce the heat dissipating areas of the lid. 
     As is clear from FIG. 1, cutouts  9  at the four corners of heat spreading lid  5  act to accommodate each of the four posts  3 , used for attachment of the lid and sink to chip carrier  1 . Cutouts or notches  11  at the mid-section of each side of heat spreading lid  5  act to accommodate attachment of the heat sink and lid to chip carrier  1  using a clip or spring, as shown more specifically in FIG.  2 . Although the thickness of the lid can be optimized for heat spreading and height requirements, a typical flip chip arrangement would operate effectively with a lid thickness in the range of 0.5 mm to 4 mm. In this regard, it should be noted that the relative sizes of the various parts shown in FIGS. 1-3A are not to scale, nor scaled proportionately, but are merely shown to facilitate an understanding of the invention. 
     Heat spreading lid  5  not only acts to effectively dissipate heat from chip  7 , it also acts to protect the chip, and its design acts to accommodate different modes of heat sink attachment. Lid  5  is attached to chip  7  using a thermally conductive epoxy, which epoxy allows for effective heat dissipation and provides sufficient compliance for any mismatch in physical and thermal expansion characteristics between lid-heat sink and chip. 
     Heat sink  13 , comprising plate  14  and heat dissipating elements  16 , may be attached to chip carrier  1  by positioning posts  3  in holes  15  and clamping in place with nuts  17 , with lid  5  interposed between plate  14  and chip carrier. Alternatively, heat sink  13  may also be attached to chip carrier  1  by positioning posts  3  in holes  15  and clamping in place with a clip arranged to fit into notch  11  in heat spreading lid  5  and notch  19  in heat sink  13  and then extend to the underside of lid  5 . Alternatively, the clip may be designed to extend beyond the lid to clamp the heat sink to the underside of chip carrier  1 . 
     Since corner posts  3  provide structural support, any of a variety of thermally conductive materials, such as a thermally conductive grease, may be used between heat spreading lid  5  and heat sink  13  to enhance thermal performance. This allows for easy removal of the heat sink for rework. However, it is also possible to use a thermally conductive adhesive between lid  5  and heat sink  13  and, in such case, the use of nuts or a clamp to hold the heat sink in place may not be necessary. 
     With reference to FIG. 2, there is shown a partially exploded perspective view of the view of FIG. 1, showing a clip arrangement that may be used to clamp the heat sink and lid to chip carrier. Thus, when heat sink  13  is positioned on heat spreading lid  5  so that posts  3  extend up through holes  15  in the heat sink, clip  21  may be positioned to engage both notches  19  in the heat sink and notches  11  in lid  5 , and then may expanded by force to clear chip carrier  1  so that, upon release of the force, holding segments  23  of the clip act to engage the underside of the chip carrier, locking the heat sink and lid to the chip carrier. Alternatively, where lid  5  is securely attached to chip  7  with thermally conductive adhesive, for example, the clip may be designed so that upon release of the expanding force, holding segments  23  of the clip act to engage the underside of the lid. It is noted that heat spreading lid  5  has notches on all four sides so that it may be positioned, without regard to orientation, for alignment to notch  19 . As can be seen, for maximum heat spreading capability, heat spreading lid  15  is made to extend to the edges of chip carrier  1 . 
     FIG. 3 shows an end view of the assembled package  25  with like reference characters identifying like elements of FIGS. 1 and 2. As in FIGS. 1 and 2, heat spreading lid  5  is attached to flip chip  7  by a thermally conductive epoxy leaving a gap around the chip between lid and chip carrier  1 . Alternatively, the gap may be filled with thermally conductive epoxy  18  or other like material. Similar to FIG. 2, the assembled package of FIG. 3 uses a clip to attach heat sink  13  and lid  5  to chip carrier  1 . The design structure of clip  27  is, however, different than the structure of clip  21  of FIG.  2 . Although clip  27  is shown in FIG. 3 extending to clamp to the underside of chip carrier  1 , as previously described with respect to FIGS. 1 and 2, the clip may also be arranged to extend and clamp to the underside of lid  5 . 
     Clip  27  in FIG. 3A is designed so that the bowed midpoint  29  of its lateral element  31  contacts surface  33  of heat sink  13 . Clip  27  is engaged by sliding clip holding segments  35  and clip leg segments  37  into respective notches  19  and  11  of the heat sink and lid at opposing edges, and then pressing downwardly so holding segments  35  slide over the edges of chip carrier  1 , thereby spreading the clip. After passing over the edges of the chip carrier, the clip returns to its relaxed position whereby holding segments  35  engage the underside of chip carrier  1 , thereby clamping the heat sink and lid to the carrier. Alternatively, rather than pressing downwardly to the point of engaging the underside of chip carrier  1 , the clip may be pressed to the point of allowing holding segments  35  to slide over and engage the underside of lid  5 . 
     As is understood by those skilled in the art, any of a variety of flexible resilient materials having a high spring rate may be employed to fabricate the clips. In this regard, it is clear that the clip must have the ability to return to its original form, i.e., relaxed state, after being forced out of shape. For example, clips  21  and  27  may be formed from hard type stainless steel or, more generally steel, although other materials such as beryllium copper may be used. Typically, clips  21  and  27  would be integral, i.e., formed from a single piece and are fabricated using well known techniques such as stamping and bending a single piece of metal. The clips may be wire shape or ribbon shape. Also, as is understood by those skilled in the art, the dimensions of the clips must be tailored to the dimensions of the combined heat sink, lid, chip and chip carrier and designed to provide adequate clamping force. It is clear, that other structural design configurations may as well be employed to fabricate a clip to fit over the heat sink and lid in the notches provided therefor to thereby clamp same to the chip carrier. 
     Rather than employ a clip to attack heat sink  13  to chip carrier  1 , nuts may be used on threaded post  3  to hold the assembly together. In this regard, the posts may be designed to have a slightly larger circumference at its base to facilitate good mechanical attachment to the chip carrier. 
     Regardless of whether the method of attachment is through a clip, attached nuts or adhesive, heat spreading lid  5 , as designed, may be used. Thus, a single configuration of heat spreading lid may be used for each method of heat sink attachment. It is clear that the heat spreading lid may also be attached by the above methods without the heat sink. Such an arrangement may be employed where the lid would provide sufficient heat dissipation. 
     Direct attachment of the heat sink and lid to the chip carrier provides a number of advantages. The resultant package provides a removable heat sink for rework which is capable of assembly without modification to the circuit board upon which it is to be mounted. In this regard, the owner of the circuit board may be the customer while the chip package may be provided by the supplier, and attaching the heat sink to the chip carrier rather than PCB simplifies the process, and reduces the cost and complexity of assembly. In addition, attachment of the heat sink may be undertaken either before or after mounting on the PCB. Moreover, rework to remove the heat sink, lid and chip after mounting is simplified. Since a single heat spreading lid configuration acts to facilitate different modes of heat sink attachment, the mode of heat sink attachment may be changed upon rework. 
     Although description of the chip package, in accordance with the present invention, has been directed to a single chip, it is clear that more than one chip may be attached to chip carrier  1 . Attachment of the chip to the chip carrier may be through a flip chip interconnection by solder balls, or may be by solder or pins, or the like. 
     It should be understood that the chip package, in accordance with the present invention, is designed to be further attached to some form of substrate, such as, a PCB. Accordingly, although not shown in the chip package of FIG. 3, the underside surface of chip carrier  1 , i.e. the side opposing the chip-mounted side, contains an array of contact pads for connection to a PCB, for example. Such pads may be connected to pads on a PCB by solder, solder balls, pins, or the like. 
     It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.