Abstract:
An apparatus and method attaching a heatsink to a surface of an electronic package comprising a substrate, an integrated circuit chip attached to the surface of the substrate, an encapsulant encapsulating the integrated circuit chip and contacting at least a portion of the surface of the substrate, and an orifice formed in the top portion of the encapsulant to attach the heatsink to the surface of the electronic package. The heatsink may be attached and removed as desired to allow for package identification or rework.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/306,486, filed on May 6, 1999, now abandoned, which is a division of application Ser. No. 08/991,903, filed on Dec. 17, 1997, which has issued as U.S. Pat. No. 5,969,947 on Oct. 19, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to electronic packages and, more particularly to an apparatus and method for providing attachment of a heatsink to a surface of an electronic package. 
     BACKGROUND OF THE INVENTION 
     Advances in microelectronics technology tend to develop chips which occupy less physical space while performing more electronic functions. Conventionally, the chips are packaged for use in housings which protect the chip from its environment and provide input/output communication between the chip and external circuitry through sockets or solder connections to a circuit board or the like. Miniaturization results in the generation of more heat in less physical space and with less structure for transferring heat from the package. 
     It is generally desirable to optimize an electronic assembly by providing a maximum number of packages in a minimum amount of space. Similarly, the development of electronic circuits using compound semiconductors further expands the packaging requirements to control device temperatures by heat dissipation for devices which operate at higher temperatures. 
     One type of semiconductor chip package includes one or more semiconductor chips mounted on a circuitized surface of a substrate, e.g., a ceramic substrate or a plastic substrate. Such a semiconductor chip package, conventionally termed a chip carrier, is usually intended for mounting on a printed circuit card or printed circuit board. In the case of a Ball Grid Array (BGA) package, the chip carrier will include a second circuitized surface opposite the surface to which the chip is attached, which is in turn connected to the printed circuit card or printed circuit board. 
     One way to obtain a relatively high density of chip connections is readily achieved by mounting one or more semiconductor chips on the circuitized surface of a chip carrier substrate in the so-called flip chip configuration. In this configuration, the chip or chips are mounted active side-down on solderable metal pads on the substrate using solder balls, a controlled collapse chip connection (C 4 ), a gold bump, or a conductive epoxy. Unfortunately, the coefficient of thermal expansion (CTE) of, for example, a silicon chip is significantly different from the CTE of a plastic substrate. As a consequence, if a chip carrier is subjected to thermal fluctuations, then the solder ball connections will be subjected to significant stresses, which tend to weaken, and reduce the fatigue life of, the solder ball connections. 
     Another way to mount a chip to a substrate is to use a wirebond attachment. Cost is one of the primary considerations when choosing a wirebond chip carrier package. Plastic flatpacks and plastic ball grid array (PBGA) chip overmolded packages are often chosen as possible chip carrier solutions because of their low cost. One major problem with these chip carriers is, however, that they are inherently poor thermal performers because they are plastic. With the common trend in electronic packaging of increasing chip powers, compounded with competitive pricing, packaging engineers are pushing the thermal threshold of these packages. These higher power chips are beginning to require enhanced thermal solutions, but the cost of these thermal solutions adds significant development and manufacturing costs and, thus, increases the overall price of the product. 
     In order to conduct heat from the chip to the exterior of the package, many device packages include a high thermal conductivity transfer medium which is in thermal communication with the chip and has a dissipation surface adjacent to the surface of the package. Other packages merely conduct the heat through the material of the package itself. In order to further dissipate heat from the package, an external heatsink may be attached to the device package. Typically, the heatsink is a body of material such as metal which has a relatively high thermal conductivity. The heatsink ordinarily has at least one flat face for positioning adjacent to a face of the device package and may include fins, pins, or other structures for dissipating thermal energy into the surrounding atmosphere. 
     FIGS. 1A and 1B illustrate a prior art method for attaching a heatsink  100  to plastic package  102  (comprising laminate  106  and overmold  108 ). The prior art consists of epoxy attach  104  as shown in FIG. 1A (which tends to be expensive and adds extra processing steps) or a clip  110  (as shown in FIG. 1B) around the edge of laminate  106  which causes laminate  106  to separate or warp resulting in intermittent contact with the circuit board as a result of the force exerted on plastic package  102 . 
     U.S. Pat. No. 5,510,956 issued to Suzuki discloses a device for attaching a heatsink to an integrated circuit chip. As shown in FIG. 1C, circuit chip  120  is attached to substrate  122 . Resin  124  insulates circuit chip  120  from metal encapsulant  126 . Heatsink  128  is then attached to metal encapsulant  126  by soldering heatsink  128  to metal encapsulant  126 . This is a labor-intensive process and does not allow simple detachment of heatsink  128 . 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an electronic package assembly that can mount a heatsink to the electronic package using a clip which is attached to the top surface of the electronic package. 
     The electronic package is provided with integral features for low cost heatsink attachment in electronic chip carriers. Because they are integral, the features require no new process steps during chip carrier manufacturing and add minimal cost to the finished product. These features can be implemented into the normal process flow of manufacturing for both heatsink and non-heatsink parts. Therefore, if a customer later decides that it needs thermal enhancement, a thermal solution can be added using the existing features. These features also allow the customer to use cost-effective, off-the-shelf, extruded heatsinks (available from a variety of heatsink vendors). 
     To solve the aforementioned disadvantages of the conventional heatsink attachments and methods, the present invention provides an apparatus and method for attaching a heatsink to a surface of an electronic package. The apparatus comprises a substrate, an integrated circuit chip attached to the substrate, a member encapsulating the integrated circuit chip and contacting the substrate, and attaching structure formed in the top portion of the encapsulating means. 
     The present invention also relates to an apparatus for attaching a heatsink to an electronic package where the attaching structure is formed in the top portion of the substrate, along an edge of the encapsulating member, or through the substrate. The present invention also relates to a method for attaching a heatsink to a surface of an electronic package by attaching an integrated circuit chip to a substrate, encapsulating the integrated circuit chip with an encapsulant, and forming an attachment in the top of the encapsulant. These features have low cost, can be implemented in the early design stages of the module, provide a heatsink option for customers, and are removable for module identification and rework. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1A is an exploded view of a prior art heat dissipation attachment using an adhesive; 
     FIG. 1B is a side view of an assembled prior art heat dissipation attachment using a spring clip; 
     FIG. 1C is side view of another prior art heat dissipation attachment; 
     FIG. 2A is a perspective view of a first exemplary embodiment of the present invention; 
     FIGS. 2B-2E are side views of the exemplary embodiment of FIG. 2A; 
     FIG. 3A is a perspective view of a second exemplary embodiment of the present invention; 
     FIGS. 3B-3E are side views of the exemplary embodiment of FIG. 3A; 
     FIGS. 4A-4C are side views of a third exemplary embodiment of the present invention; 
     FIGS. 5A and 5B are side views of a fourth exemplary embodiment of the present invention; and 
     FIGS. 6A and 6B are side views of a fifth exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2A, a perspective view of an exemplary embodiment of the present invention is shown. In FIG. 2A, electronic package  200  is comprised of substrate  202 , integrated circuit chip  204  (see FIG.  2 B), and encapsulant  206 . Within encapsulant  206 , apertures  208  are formed. Apertures  208  allow for attachment of heatsink  214  to the top surface of encapsulant  206  using pins  210  and holder  212 . 
     Referring now to FIG. 2B, a side view of the first exemplary embodiment is shown. In FIG. 2B, integrated circuit chip  204  is shown attached to the top of substrate  202  by an adhesive, such as epoxy. Substrate  202  may be a multilayer laminated substrate, for example, and may be made from a polymer or any other suitable material such as a ceramic greensheet. After integrated circuit chip  204  is attached to substrate  202 , using bonding agent  216 , such as epoxy, encapsulant  206  is applied. Encapsulant  206  may be an overmold formed from a polymer or other suitable material. Aperture  208  is formed in the surface of encapsulant  206  and, as shown in FIG. 2C, accommodates pin  210  when inserted therein if a thermal solution, such as heatsink  214 , is desired. If a heatsink is desired, pins  210  are inserted into apertures  208 , heatsink  214  is placed on encapsulant  206 , and holder  212  is placed across heatsink  214  and coupled to the top of pins  210  to hold heatsink  214  in place. Pins  210  may be a push pin, a threaded post, a solder pin, or any other suitable device. Pins  210  may have retainers  222  if necessary to maintain holder  212  in place. Holder  212  may be released from pins  210 , however, if it becomes necessary to remove heatsink  214  from electronic package  200 . 
     Optionally, a heat transfer medium  218 , such as thermal grease, may be used between heatsink  214  and encapsulant  206 . As a further option, heat transfer medium  218  may be an adhesive compound which provides heat transfer features such as epoxies, acrylics, conductive pads, and thermal tapes. 
     Electronic package  200  may be attached to a circuit board (not shown), for example, using a ball grid array (BGA)  220 . Attachment to the circuit board is not affected by the attachment of heatsink  214  to electronic package  200 ; heatsink  214  may be attached before or after electronic package  200  is attached to the circuit board. 
     FIG. 2C shows the completed assembly according to the first exemplary embodiment of the present invention. Pins  210  may be removed from aperture  208  without damaging encapsulant  206  or electronic package  200  thereby allowing for rework or module identification. Alternatively, holder  212  may be de-coupled from pins  210  in order to remove heatsink  214  from electronic package  200 . 
     As shown in the exemplary embodiment of FIGS. 2B and 2C, integrated circuit chip  204  is a wirebond chip. The invention is not limited to this exemplary embodiment and, as shown in FIG. 2D, integrated circuit chip  204  may be a flip-chip. In this case, however, it is not necessary for encapsulant  206  to cover the upper surface of integrated circuit chip  204 . This is illustrated in FIG. 2E, where encapsulant  226  is disposed over substrate  202  but does not encroach upon upper surface  224  of integrated circuit  204 . In this exemplary embodiment, encapsulant  226  may be level with, lower than, or higher than upper surface  224 . 
     Referring to FIGS. 3A-3E, a second exemplary embodiment of the present invention is shown. FIG. 3A shows that electronic package  300  is comprised of substrate  202 , integrated circuit chip  204  (see FIG.  3 B), encapsulant  306 , and orifice  308 . In this exemplary embodiment, orifice  308  is formed through the surface of substrate  202  rather than in the surface of encapsulant  306 . 
     Referring to FIGS. 3B and 3C, side views of the second exemplary embodiment of FIG. 3A are shown. FIG. 3B depicts the interaction of elements during assembly and FIG. 3C depicts the completed assembly according to this embodiment. 
     As shown in FIG. 3B, elements similar to those of the first exemplary embodiment are shown with identical designations. After integrated circuit chip  204  is mounted to substrate  202 , encapsulant  306  is applied to integrated circuit chip  204  and substrate  202 . In this case, however, encapsulant  306  is applied to less of the surface of substrate  202  than in the first exemplary embodiment and encapsulant  306  is not applied to a portion  314  of substrate  202  to provide an unobstructed surface for the formation of orifice  308 . As in the first embodiment, encapsulant  306  may be an overmold formed from a polymer or other suitable material. Orifice  308  is formed through substrate  202  in this case to accommodate pins  210  if a thermal solution is desired. Orifice  308  may be a simple through hole or may be a through hole plated with a suitable material. 
     As shown in FIG. 3C, pin  210  mates with orifice  308  to hold pin  210  in place between retainer ring  310  and clip portion  312 . This allows pin  210  to be easily removed if desired by compressing clip portion  312  and extracting pin  210  from orifice  308 . As shown in FIG. 3C, integrated circuit chip  204  is a wirebond chip. The invention is not limited to this exemplary embodiment and, as shown in FIG. 3D, integrated circuit chip  204  may be a flip-chip, or any other suitable device. In this case, however, it is not necessary for encapsulant  306  to cover the upper surface of integrated circuit chip  204 . This is illustrated in FIG. 3E, where encapsulant  326  is disposed over substrate  202  but does not encroach upon upper surface  324  of integrated circuit  204 . In this exemplary embodiment, encapsulant  326  may be level with, lower than, or higher than upper surface  324 . 
     It is also contemplated that in any of the above exemplary embodiments more than one holder  212  may be used to couple heatsink  214  to the encapsulant  206 ,  306 . If more than one holder  212  is used, however, it is necessary to use additional pins  210  and apertures  208 ,  308  accordingly. 
     Referring to FIGS. 4A-4C a third exemplary embodiment of the present invention is shown. FIG. 4A shows that electronic package  400  is comprised of substrate  202 , integrated circuit chip  204 , encapsulant  406 , and groove  408 . In this exemplary embodiment groove  408 , is formed along an edge of encapsulant  406  rather than in the surface of encapsulant  206  of FIG.  2 B. As shown in FIG. 4A, elements similar to those of the first exemplary embodiment are shown with identical designations. 
     After integrated circuit chip  204  is mounted to substrate  202 , encapsulant  406  is applied to integrated circuit chip  204  and substrate  202  similar to the first exemplary embodiment. As in the first embodiment, encapsulant  406  may be an overmold formed from a polymer or other suitable material. Groove  408  is formed along the edge of encapsulant  406  in this case to accommodate holder  412  if a thermal solution is desired. Groove  408  may be a V-shaped groove or any other shape suitable to mate with holder  412 . Grooves  408  are formed on opposite sides of encapsulant  406  so that holder  412  may snap into place and maintain heatsink  214  in contact with the top surface of electronic package  400  as shown in FIG.  4 B. Optionally, heat transfer medium  218  may be used between encapsulant  406  and heatsink  214  as in the first exemplary embodiment. Holder  412  may be a unitary resilient member formed from a metal, a polymer, or any other suitable material. In addition, holder  412  may be a single holder or more than one holder as desired. FIG. 4C is similar to FIG. 4A except that in place of groove  408  protrusion  414  is formed in encapsulant  406  to couple with holder  412 . 
     Referring to FIGS. 5A and 5B, a fourth exemplary embodiment of the present invention is shown. FIG. 5A shows that electronic package  500  is comprised of substrate  202 , integrated circuit chip  204 , encapsulant  506 , and slot  508 . In this exemplary embodiment, slot  508  is formed in the surface of encapsulant  506  rather than along the edge of encapsulant  406  as shown in FIG.  4 A. Elements similar to those of the first exemplary embodiment are shown with identical designations. 
     As shown in FIG. 5A, slot  508  is formed in the surface of encapsulant  506  in this case to accommodate holder  512  if a thermal solution is desired. Slot  508  may be a U-shaped slot or any other shape suitable to mate with and engage holder  512 . Holder  512  may be a unitary resilient member formed from a metal, a polymer, or any other suitable material. 
     Slots  508  are formed on opposite sides of encapsulant  506  so that holder  512  may snap into place and maintain heatsink  214  in contact with the top surface of electronic package  500  as shown in FIG.  5 B. Heatsink  214  may be applied to electronic package  500  by placing heatsink  214  on the surface of electronic package  500 , placing holder  512  over heatsink  214 , and compressing holder  512  to insert holder  512  into slots  508 . Optionally, heat transfer medium  218  may be used between encapsulant  506  and heatsink  214  as in the first exemplary embodiment. Heatsink  214  may be removed by compressing holder  512  to release holder  512  from slots  508 . 
     Referring to FIGS. 6A and 6B, a fifth exemplary embodiment of the present invention is shown. FIG. 6A shows that electronic package  600  is comprised of substrate  202 , integrated circuit chip  204 , encapsulant  606 , and rib  608 . In this exemplary embodiment, rib  608  is formed along the surface of encapsulant  606 . Elements similar to those of the first exemplary embodiment are shown with identical designations. 
     As shown in FIG. 6A, rib  608  is formed on the surface of encapsulant  606  in this case to accommodate holder  612  if a thermal solution is desired. Rib  608  may be a an inverted U-shaped rib or any other shape suitable to mate with and engage holder  612 . Holder  612  may be a unitary resilient member formed from a metal, a polymer, or any other suitable material. Ribs  608  are formed on opposite sides of encapsulant  606  so that holder  612  may snap into place and maintain heatsink  214  in contact with the top surface of electronic package  600  as shown in FIG.  6 B. Heatsink  214  may be applied to electronic package  600  by placing heatsink  214  on the surface of electronic package  600 , placing holder  612  over heatsink  214 , and compressing holder  612  to engage holder  612  with ribs  608 . Optionally, heat transfer medium  218  may be used between encapsulant  606  and heatsink  214  as in the first exemplary embodiment. Heatsink  214  may be removed by compressing holder  612  to release holder  612  from ribs  608 . 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.