Abstract:
The present invention relates to enhanced protection of the active surface and the bond wires or ball array of a microelectronic device, and to thermal management of the microelectronic device as it is packaged with a printed circuit board (PCB) or other substrate. The enhanced protection and thermal management are accomplished with a high-temperature thermal grease that is glob topped or encapsulated over the bond wires or ball array, and the active surface of the microelectronic device. The high-temperature thermal grease exchanges heat, particularly by conduction, away from the active surface of the microelectronic device as well as away from the bond wires.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of application Ser. No. 10/898,618, filed Jul. 22, 2004, pending, which is a divisional of application Ser. No. 10/083,034, filed on Feb. 26, 2002, now U.S. Pat. No. 6,794,749, issued Sep. 21, 2004, which is a continuation of application Ser. No. 09/387,640, filed on Aug. 31, 1999, now U.S. Pat. No. 6,424,033, issued Jul. 23, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the packaging of microelectronic devices. More particularly, the present invention relates to heat management for packaged microelectronic devices. Specifically, the present invention relates to the placement of a thermal grease heat transfer medium within an integrated circuit (IC) chip package for heat transfer away from the microchip. The grease acts as a heat sink to assist in the management of heat that is generated by an IC chip in the IC chip package.  
         [0004]     2. Relevant Technology  
         [0005]     Miniaturization is the process of crowding an increasing number of microelectronic circuits onto a single chip. Additionally, miniaturization involves the reduction of the overall chip package size so as to achieve smaller and more compact devices such as hand-held computers, personal data assistants (PDA), portable telecommunication devices, and the like. Ideally, the chip package size would be no larger than the chip itself. Miniaturization has the counter-productive effect upon chip packaging of an increased heat load upon a smaller chip package. Heat management is, therefore, an important aspect of producing a reliable microelectronic device. A heat sink for a chip package allows for enhanced performance of the microelectronics.  
         [0006]     In the packaging of microelectronic devices, protection of the microelectronic device and its connections to the outside world is critical during packaging and field use. A prior art solution to packaging of microelectronic devices was to cover the integrated circuit chip with a plastic or ceramic material after a manner that both the highly sensitive active surface of the chip as well as the electrical connections were protected. Plastic packaging such as an epoxy material is useful to protect the active surface as well as the electrical connections. Plastic packaging has the disadvantage of being a poor conductor of heat compared to ceramic packaging. Where a plastic material is used, its effect as a poor heat conductor often leads to additional measures that must be taken to extract generated heat from the chip package to allow proper functioning of the microelectronic device. Ceramic packaging has an advantage of a higher thermal conductivity compared to plastic, but it is often costly and bulky, as well as potentially brittle. Where the chip package receives a physical blow, the ceramic package may shatter.  
         [0007]     What is needed in the art is a means of transferring heat away from a microelectronic device that overcomes the heat management problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention relates to an integrated circuit chip package having an IC chip with an active surface, where the active surface has extending therefrom an electrical connector in electrical communication with the IC chip. The IC chip is mounted upon a substrate such as a printed circuit board (PCB). A grease is in contact with the active surface of the IC chip and a container is disposed upon the substrate. The grease is enclosed within the container and the substrate.  
         [0009]     The present invention relates to the use of the grease as a protective substance to protect both the active surface of the IC chip and simultaneously as a heat transfer medium to transfer heat away from the IC chip. The present invention also relates to a method of heat transfer away from an IC chip using grease, a substrate upon which the IC chip is mounted, and a container.  
         [0010]     In one embodiment of the present invention, an IC chip is configured as a board-on-chip (BOC) package and a thermal grease is disposed upon the exposed active surface of the chip, as well as over the electrical connectors such as bond wires or solder balls if present. A protective shell covers the thermal grease to prevent disturbance of the grease during both assembly thereof and during field use. Alternatively, a dam structure may be disposed upon the board and the protective shell to hold the protective shell in place. Additionally, at least one vent hole may be disposed in the protective shell to allow for thermal expansion and contraction of the grease. The BOC configuration lends itself to a stacked BOC package where multiple occurrences of BOC may be enclosed within a single protective shell.  
         [0011]     In another embodiment of the present invention, a chip-on-board (COB) chip package is configured with the grease disposed upon the active surface of the IC chip where the grease also covers the bond wires. The protective shell is disposed upon the grease and is secured against the substrate on the same surface onto which the IC chip is disposed. In a variation of this embodiment, the protective shell is configured to make direct contact with the active surface of the IC chip.  
         [0012]     Another embodiment of the present invention includes an IC chip mounted directly upon a heat sink. A substrate is also mounted directly upon the heat sink, and grease covers both the active surface of the IC chip and the bond wires. Additionally, a protective shell is mounted upon the substrate, where the grease is enclosed by the protective shell and the substrate.  
         [0013]     Another embodiment of the present invention comprises a flip-chip configuration wherein the grease is disposed both upon the active surface of the flip-chip and upon the balls of a flip-chip ball array that provides electrical connections to the flip-chip. A dam structure may be disposed upon both the flip-chip substrate and the flip-chip itself to assist in containing the grease. In a variation of the foregoing involving a flip-chip upon a flexible substrate, a protective shell is disposed upon the flex substrate and grease substantially encompasses the entire flip-chip as well as the flip-chip ball array. In a still further variation, the protective shell is in direct contact with the inactive surface of the flip-chip, the protective shell thereby simultaneously acts as a die attach and heat sink, and the flex substrate with the protective shell provide an enclosure for the grease.  
         [0014]     Another embodiment of the present invention includes flip-chip-on-die (FCOD) wherein the flip-chip is disposed against a COB die. In a first configuration of this embodiment, the flip-chip ball array is in contact with a grease and the bond wires from the die are enclosed in a second protective material that is typically a thermoplastic or thermoset resin.  
         [0015]     An alternative embodiment of the FCOD configuration provides for grease to be in contact with both the flip-chip ball array and the bond wires from the die. A protective shell is disposed upon the substrate supporting the die such that the protective shell and the substrate enclose therein both the flip-chip and the die.  
         [0016]     Another alternative embodiment of the FCOD configuration provides for a two-piece protective shell that may allow the inactive surface of the flip-chip to be exposed. This alternative embodiment provides for the flip-chip ball array and the bond wire to be encompassed by grease while allowing the inactive surface to radiate heat away from the flip-chip.  
         [0017]     These and other features of the present invention will become more fully apparent from the following description and appended claims or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     In order that the manner in which the above-recited and other advantages of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not, therefore, to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0019]      FIG. 1  is an elevational cross-section view of a board-on-chip package according to the present invention;  
         [0020]      FIG. 2  is an elevational cross-section view of a stacked board-on-chip configuration according to the present invention;  
         [0021]      FIG. 3  is an elevational cross-section view of a stacked board-on-chip configuration, wherein the board and chip orientation is vertically inverted in comparison to the configuration depicted in  FIG. 2 ;  
         [0022]      FIG. 4  is an elevational cross-section view of a chip-on-board configuration according to the present invention;  
         [0023]      FIG. 5  is an elevational cross-section view of an alternative embodiment of the chip-on-board configuration depicted in  FIG. 4  wherein the protective shell acts as a direct-contact heat sink to the active surface of the chip;  
         [0024]      FIG. 6  is an elevational cross-section view of a chip-on-heat-sink configuration according to the present invention;  
         [0025]      FIG. 7  is an elevational cross-section view of a flip-chip-on-flex configuration according to the present invention;  
         [0026]      FIG. 8  is an elevational cross-section view of an alternative embodiment of a flip-chip-on-flex configuration according to the present invention;  
         [0027]      FIG. 9  is an elevational cross-section view of another alternative embodiment of the flip-chip-on-flex configuration;  
         [0028]      FIG. 10  is an elevational cross-section view of a flip-chip-on-die configuration according to the present invention;  
         [0029]      FIG. 11  is an elevational cross-section view of an alternative embodiment of the flip-chip-on-die-configuration; and  
         [0030]      FIG. 12  is an elevational cross-section view of another alternative embodiment of the flip-chip-on-die configuration. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The present invention relates to an IC chip package that overcomes the problems of the prior art. The IC chip package has a heat sink that comprises a grease that aids heat dissipation and that protects the active surface of the IC chip and/or the electrical connectors such as bond wires or solder balls.  
         [0032]     The present invention may include a fine pitch ball array, typically disposed upon a printed circuit board (PCB). The PCB is typically attached to an IC chip. Disposed upon the active surface of the IC chip is the grease. Simultaneously, the grease may also be in direct contact with the electrical connectors such as bond wires or balls in a ball array. A protective shell is placed over the grease.  
         [0033]     Reference will now be made to figures wherein like structures will be provided with like reference designations. It is to be understood that the drawings are diagrammatic and schematic representations of embodiments of the present invention and are not limiting of the present invention nor are they necessarily drawn to scale.  
         [0034]      FIG. 1  is an elevational cross-section view of an IC chip package  10  with a board-on-chip (BOC) configuration.  FIG. 1  illustrates an IC chip  12  disposed upon a substrate  14  such as a flexible PCB. The active surface  16  of IC chip  12  is disposed against a first side  50  of substrate  14 . Emerging from the active surface  16  of IC chip  12 , are bond wires  18  that act as electrical connectors between active surface  16  of IC chip  12  and substrate  14 .  
         [0035]     For chip package  10 , in the BOC configuration, a ball array  20  is disposed upon a second side  48  of substrate  14 . Second side  48  is opposite and parallel with first side  50  upon which IC chip  12  is disposed.  
         [0036]     A grease  22  is disposed upon active surface  16  of IC chip  12  as well as in direct contact with bond wires  18 . Grease  22  thus provides a heat sink having a first thermal conductivity that is in direct contact with both active surface  16  and bond wires  18 . Preferably, a protective shell  24  is disposed over grease  22  in order to prevent the disturbance and/or flow of grease  22  during ordinary handling incidental to the assembly of chip package  19  and incidental to ordinary field use. The protective shell  24  is preferably composed of a thin metal or other highly thermally conductive material that allows for good thermal coupling to thermal grease  22 . In some cases, such a protective shell may not be necessary.  
         [0037]     Grease  22  may be any high thermal conductivity grease known in the art. Preferably, grease  22  is a high thermal conductivity grease that will flow at a minimum temperature that is in a range from about 190° C. to about 230° C., and preferably will flow at no less than a temperature of about 220° C. An example of preferred high thermal conductivity greases is GELEASE™ manufactured by Thermoset Plastics, Inc. of Indianapolis, Ind. A preferred class of protective materials is described in “High Thermal Conductivity Greases” by Ron Hunadi and Rich Wells (advanced packaging, Apr. 19, 1999, pp. 28-31), the disclosure of which is incorporated herein by reference.  
         [0038]     The present invention contemplates a dielectric grease that has a thermal conductivity in a range from about 0.5 Watts/m.K to about 5 Watts/m.K, preferably from about 2 Watts/m.K to about 4 Watts/m.K. Additionally, the grease will preferably have a dielectric constant that is in a range from about 1.2 to about 10, preferably from about 4 to about 9.5, and most preferably less than about 6. Because of high temperature operation of chip packages, the dielectric grease will preferably have a melting point that is in a range from about 100° C. to about 230° C., and preferably from about 190° C. to about 220° C. Another property that is preferred for the grease  22  is a minimum weight loss at chip package operating temperatures for the conceivable lifetime of the chip package. Preferably, the grease has a weight loss at a sustained temperature of 100° C. over a period of 30 days of less than about 0.15%. It is preferred that, under these conditions, the grease  22  will have a weight loss over a period of about 20 years of less than about 0.25%.  
         [0039]     Vent holes  26  may be provided in protective shell  24  in order to allow the expansion of grease  22  under high temperature cycling conditions. Vent hole  26  may be a single vent hole or a plurality of vent holes. Vent hole  26  allows for the expansion of an excess amount of grease  22  during such high temperature applications as burn-in testing. The size of vent hole  26  may be such so as to allow for excess grease  22  to exude from within the enclosure formed by protective shell  24  and substrate  14 . Multiple vent holes can also be employed.  
         [0040]     A dam structure  28  may be placed in contact with protective shell  24  and second side  48  of substrate  14  to hold protective shell  24  in place. Where the stickiness and viscosity of grease  22  is sufficient to hold protective shell  24  in place, dam structure  28  may be omitted. Alternatively, protective shell  24  can be directly attached to substrate  14  by use of suitable adhesives.  
         [0041]     Protective shell  24  is preferably made of a metallic or ceramic material that has a thermal conductivity that is greater than the thermal conductivity of grease  22 . Thereby, protective shell  24  acts as a second heat sink that facilitates the transfer of heat through grease  22  away from IC chip  12 . Preferred metals for protective shell  24  include Al, Cu, Au or alloys of such metal, and Ag. Most preferably, protective shell  24  is composed of Cu or an alloy thereof.  
         [0042]     The BOC configuration lends itself well to multiple BOC packages that use grease  22  as a heat transfer medium and as a protective substance.  FIG. 2  illustrates a multiple BOC chip package  110  wherein substrate  14  has its own IC chip  12  and ball array  20  along with protective shell  24  that contains grease  22 . Over first side  50  of substrate  14  is disposed a substrate  114  and an enclosed ball array  132 . Substrate  114  supports an IC chip  112  to comprise a second BOC configuration that is stacked upon substrate  14 .  FIG. 2  also illustrates a third BOC configuration such that three BOC configurations comprise chip package  110 .  
         [0043]     A second protective shell  34  encloses the major portion of chip package  110 . Disposed in the interstices of chip package  110  is grease  22 . Alternatively, a dam structure  128  may also be provided upon first side  50  of substrate  14  and against second protective shell  34  in order to hold second protective shell  34  against substrate  14 . Although not pictured, one or multiple vent holes may be provided as illustrated in  FIG. 1 . The vent holes may be provided both for protective shell  24  and for protective shell  34 .  
         [0044]     Another alternative embodiment of multiple, stacked BOC configurations is illustrated in  FIG. 3  as a chip package  210 . The configuration of each BOC substructure is vertically inverted in comparison to the configuration of each BOC substructure depicted in  FIG. 2 . The embodiment depicted in  FIG. 3  includes substrate  14  and IC chip  12  disposed upon first side  50  of substrate  14 . In this embodiment, ball array  20  is also disposed upon first side  50 .  FIG. 3  depicts that each active surface  16  and  216  of IC chips  12  and  212 , and all bond wires  18  and  218 , as well as substrates  214  and connective elements  232 , are enclosed in a single space formed principally by protective shell  224  and substrate  14 . Thereby, two protective shells are not required and chip package  210  is enclosed substantially in a common space with all active surfaces and electrical connectors being in contact with grease  22  contained therein. A vent hole (not pictured) may also be present.  
         [0045]     One of the advantages in relation to heat management that exists in the present invention is the rapid flow of generated heat through grease  22  due to its higher coefficients of thermal conductivity compared to thermoplastics and thermoset resins of the prior art. A particular advantage in the stacked BOC configurations depicted in  FIGS. 2 and 3  is that a chip in the stack that generates more heat than others will be cooled by the presence of other chips, particularly through the conductive heat transfer medium provided by grease  22 .  
         [0046]     The presence of grease  22  in every embodiment of the present invention has an advantage over plastics in that the preferred grease has a greater thermal conductivity than the plastics. The flowability of grease permits direct contact with active surfaces of IC chips and electrical connectors, whereas ceramic housings do not permit this type of intimate contact with hot surfaces. Likewise, with the intimate contact there is a continuum of thermal conductivity between the hot surface, the grease, the substrate, and the protective shell.  
         [0047]     In a chip-on-board (COB) configuration of the present invention,  FIG. 4  illustrates a chip package  310  that includes an IC chip  312  disposed upon a substrate  314 . IC chip  312  has its active surface  16  and bond wires  318  on a first side  350  of substrate  314 . Opposite and parallel to first side  350 , a ball array  320  is disposed upon a second side  348  of substrate  314 . Grease  22  is enclosed by a combination of a protective shell  324 , first side  350  of substrate  314 , and portions of IC chip  312 .  FIG. 4  also illustrates the positioning of an optional vent hole  26  through the wall of protective shell  324 .  
         [0048]      FIG. 5  illustrates an alternative to the embodiment of chip package  310  depicted in  FIG. 4 . A chip package  410  illustrated in  FIG. 5  depicts a section of a protective shell  424  that makes contact with upper surface  16  of IC chip  312 . In this configuration, direct contact of protective shell  424  with upper surface  16  comprises a die-attach heat sink. Where the thermal conductivity of protective shell  424  is greater than the thermal conductivity of grease  22  and where direct contact by protective shell  424  is made onto IC chip  312 , heat transfer away from IC chip  312  is facilitated to a greater degree than the embodiment depicted in  FIG. 4 . It is noted that protective shell  424  can also be attached to chip  312  at active surface  16  through a conductive adhesive or an epoxy such as those used for die-attach applications and are known in the art.  
         [0049]      FIG. 6  is another embodiment of the present invention, wherein a chip package  510  is depicted that includes an IC chip  512  disposed against a heat sink  30 . A substrate  514  of bearing ball array  520  is disposed upon heat sink  30  and active surface  16  is in electrical connection with a first side  550  of substrate  514  through bond wires  518 . According to the present invention, grease  22  is in contact with active surface  16  of IC chip  512  and with bond wires  518 . Further, grease  22  is enclosed by a protective shell  524  that also is disposed upon substrate  514 . According to this embodiment of the present invention, chip package  510  allows for a significant amount of heat transfer into heat sink  30 , while also allowing a significant amount of beat transfer from active surface  16  and bond wires  518  into grease  22 . As in all other embodiments set forth in the present invention, a vent hole is optional. Further, a dam structure is also optional.  
         [0050]      FIG. 7  illustrates another embodiment of the present invention wherein a chip package  610  comprises flip-chip-on-flex (FCOF) technology. A flip-chip  612  has a ball array  620  disposed upon active surface  16  thereof. Ball array  620  is disposed upon a substrate  614  that is typically a flexible PCB. Non-flexible substrates can also be employed. Grease  22  is disposed both against active surface  16  and in contact with each individual ball of ball array  620 . Typically, dam structure  28  is an epoxy material or glob top material. Grease  22  is, therefore, containerized by the combination of active surface  16  of flip-chip  612 , dam structure  28  that acts as a container, and the first surface  650  of substrate  614 . As is typical with FCOF, a second ball array  36  is disposed upon the second side  648  of substrate  614 . It is notable that  FIG. 7  discloses no vent hole to allow for the expansion and contraction of grease  22 . A vent hole, however, may be formed by placing a hole in substrate  614  at a location that opens up to first side  650  without any obstruction from an electrical connection disposed upon first side  650 .  
         [0051]      FIG. 8  is another embodiment of FCOF technology according to the present invention. An FCOF package  710  is depicted as comprising flip-chip  612  with ball array  620  disposed upon active surface  16  thereof. In place of dam structure  28  to act as the container, a protective shell  624  is displayed as being disposed upon substrate  614 . Protective shell  624  is used for enclosing grease  22  along with a combination of protective shell  624 , and first side  650  of substrate  614 . Grease  22  thus substantially contacts all exposed surfaces of flip-chip  612  and also contacts all exposed electrical connectors that comprise ball array  620 .  
         [0052]     A particular advantage of the embodiment depicted in  FIG. 8  is that it allows for a shared heat load by all portions of flip-chip  612  through the medium of grease  22  as a heat transfer material. Where one portion of flip-chip  612  may be more microelectronically active than any other portion, grease  22  will heat in that region and allow for heat to be drawn away therefrom to other portions of flip-chip  612  that are not as active.  
         [0053]     Another embodiment of the FCOF configuration is depicted in  FIG. 9 , wherein a chip package  810  includes flip-chip  612  and ball array  620  disposed upon substrate  614  at its first side  650 . Additionally, a protective shell  824  is disposed upon substrate  614  but it also makes direct contact with flip-chip  612  at its inactive surface  52 . Thus, protective shell  824  acts as a die-attach for flip-chip  612 . Simultaneously, protective shell  824  is both a heat sink and a container for holding grease  22  against active surface  16  of flip-chip  612  and against the electrical connectors that make up ball array  620 .  
         [0054]     Another application of the present invention is directed toward flip-chip-on-die (FCOD) technology as depicted in  FIG. 10 . An FCOD package  910  includes an IC chip  912  that acts as the die in the FCOD configuration. IC chip  912 , referred to hereafter as die  912 , is disposed upon a substrate  914  and also has bond wires  318  that make electrical connection between active surface  16  and first side  950  of substrate  914 . A ball array  920  acts as the electrical connector between a flip-chip  40  and die  912 . Grease  22  is depicted as filling the interstices between individual balls of ball array  920 , between flip-chip  40  and die  912 .  FIG. 10  also illustrates the presence of a second protective material  38  that is preferably a thermoplastic or thermoset resin. Second protective material  38  acts as both a container that is disposed upon substrate  914  and as a protective cover for bond wires  318 .  
         [0055]      FIG. 11  is another embodiment of an FCOD configuration, wherein a chip package  1010  includes die  912  with a ball array  920  disposed upon active surface  16  of die  912 . A flip-chip  40  is disposed upon ball array  920 . A protective shell  924  is disposed upon substrate  914 . Contained within protective shell  924  and substrate  914  is grease  22 .  FIG. 11  illustrates direct contact of protective shell  924  against flip-chip  40 . Accordingly, protective shell  924  acts as a conductive heat sink for flip-chip  40 . Where die  912  produces a major portion of heat during ordinary use of chip package  1010 , flip-chip  40  itself acts as a heat sink for die  912  in addition to protective shell  924  as protective shell  924  makes direct contact with flip-chip  40 . Grease  22  operates to moderate extreme temperature fluctuation due to its ability to conduct heat more efficiently than the thermoplastic and thermoset materials of the prior art.  
         [0056]     Another embodiment of FCOD technology is depicted in  FIG. 12 , wherein a chip package  1110  is configured with both die  912  and flip-chip  40  disposed with ball array  920  therebetween. A protective shell  1124  is depicted as being disposed upon substrate  914 . Optionally, dam structure  28  assists in securing protective shell  1124  to substrate  914 . A second dam structure  44  is also optionally present in order to assist in securing protective shell  1124  to flip-chip  40 . In the embodiment depicted in  FIG. 12 , heat conduction that may occur principally in die  912  is dissipated by the presence of flip-chip  40  as a heat sink therefor.  
         [0057]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.