Abstract:
A flip chip package includes a substrate, a silicon die supported on and electrically connected with the substrate, and a lid attached with the substrate and covering the silicon die. The silicon die has a die surface including a first set of tracks formed therein, and the lid may have a lid surface including a second set of tracks formed therein. A thermal interface material may be disposed between the die surface and lid surface and at least partially fill the first and second sets of tracks.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Provisional Application No. 61/971,948, filed Mar. 28, 2014, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to flip chips and the packaging thereof. 
     BACKGROUND 
     Flip chip assemblies interconnect semiconductor devices, such as integrated circuit chips and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. These bumps are deposited on the chip pads on the top side of the wafer during final wafer processing. To mount the chip to external circuitry, such as a circuit board or another chip, it is inverted so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit. The solder is then re-melted to produce an electrical connection, typically using a Thermosonic bonding or other reflow solder process. This leaves a small space between the chip&#39;s circuitry and the underlying mount. A dielectric adhesive may then be used to fill (underfill) this space to increase the strength of the mechanical connection and reduce stress on the solder joints due to thermal expansion differences between the chip and its surrounding elements: the underfill distributes forces generated during thermal expansion, which hinders stress concentration at the solder joints. 
     Flip chip assemblies are typically smaller than traditional carrier-based systems because the chip sits directly on the circuit board. Associated shorter wires reduce inductance, which allows higher-speed signals and better heat conduction. As such, flip chip assemblies can be well suited for a variety of electronic products. 
     SUMMARY 
     A lid-less flip chip package includes a substrate, a silicon die supported on and electrically connected with the substrate, and one or more support structures supported on the substrate and positioned around the silicon die without covering the silicon die. The silicon die and support structures cooperate to support a heat sink such that the silicon die and support structures distribute compression loads associated with the heat sink to the substrate. 
     The package may further include solder bumps and underfill disposed between the silicon die and substrate. Each of the support structures may comprise silicon, solder bumps and underfill. The support structures may comprise non-silicon materials. The package may further include a thermally conductive layer on the silicon die. The layer may comprise a metal, graphite, thermal epoxy, thermal film or thermal pad. The package may further include a thermally conductive medium on the silicon die. The thermally conductive medium may be a cooling liquid. The silicon die may have a side including one or more tracks formed therein and positioned to be adjacent to the heat sink. The package may further include a thermally conductive layer on the side and at least partially filling the tracks. The package may further include a seed metal coating the tracks. A distance from the substrate to a top of each of the support structures may be equal to or less than a distance from the substrate to a top of the silicon die. 
     A flip chip package includes a substrate and a silicon die supported on and electrically connected with the substrate. The silicon die has a die surface including a first set of tracks formed therein configured to receive a thermal interface material. The package further includes a lid attached with the substrate and covering the silicon die. 
     The lid may have a lid surface including a second set of tracks formed therein configured to receive a thermal interface material. The die surface and lid surface may be adjacent to one another. The package may further include a thermal interface material disposed between the die surface and lid surface and at least partially filling the first and second sets of tracks. The thermal interface material may be a cooling liquid. The package may further include a seed metal coating the first set of tracks. 
     A flip chip package and heat sink assembly includes a substrate, a silicon die supported on and electrically connected with the substrate, a lid attached with the substrate and covering the silicon die, and a heat sink configured to apply a load to the lid, silicon die and substrate. The silicon die has a die surface adjacent to the lid. The lid has a first lid surface adjacent to the silicon die and a second lid surface adjacent to the heat sink. The heat sink has a sink surface adjacent to the lid. At least one of the die, first lid, second lid and sink surfaces has tracks formed therein configured to receive a thermal interface material. 
     The package may further include a thermal interface material disposed within some of the tracks. The package may further include a thermally conductive medium on the silicon die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view, in cross-section, of a prior art lidded flip chip package and heat sink assembly. 
         FIG. 2  is a side view, in cross section, of a lid-less flip chip package and heat sink assembly. 
         FIG. 3  is a plan view of the lid-less flip chip package of  FIG. 2 . 
         FIG. 4  is a side view, in cross-section, of another lidded flip chip package and heat sink assembly. 
         FIG. 5  is a side view, in cross-section, of a portion of the lidded flip chip package and heat sink assembly of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     With reference to  FIG. 1 , a typical prior art lidded flip chip package and heat skink assembly  10  includes a flip chip assembly  12  contained within a socket  14  on a printed circuit board (PCB)  16 . (A ball grid array may also be used instead of the socket  14 , etc.) The assembly  10  further includes a bolster plate  18  on which the PCB  16  is mounted and a heat sink  20 . The heat sink  20  is fastened to the bolster plate  18  via spring-loaded bolt assemblies  22  to compress the flip chip assembly  12 , socket  14  and PCB  16  there between to enhance the thermal path between the flip chip assembly  12  and heat sink  20 , and/or to enhance the electrical connections associated with the socket  14 . 
     The flip chip assembly  12  includes a silicon die  24  mounted to a package substrate  26  via solder bumps  28  and underfill  29 . The flip chip assembly  12  further includes a lid  30  bonded to the package substrate  26  via, for example, an epoxy  31 , and covering the silicon die  24 . A thermal interface material  32  is disposed between the silicon die  24  and lid  30 . And, a thermal interface material  33  is disposed between the heat sink  20  and lid  30 . The thermal interface materials  32 ,  33  may be the same or different depending on application objectives. Heat from the silicon die  24  conducts through the thermal interface material  32 , lid  30 , and thermal interface material  33  to the heat sink  20 . 
     The distance between the spring-loaded bolt assemblies  22  relative to the much narrower width of the lid  30  supporting the heat sink  20  may cause the heat sink  20  to cock relative to the PCB  16  and bolster plate  18 . This condition may be even more likely for lid-less assemblies given that silicon dies are narrower than corresponding lids. Even though the spring-loaded bolt assemblies  22  can accommodate a certain amount of non-coplanarity between the bolster plate  18  and heat sink  20  while sufficiently compressing the flip chip assembly  12  therebetween, such non-coplanarity may affect heat transfer between the flip chip assembly  12  and heat sink  20 , as well as long-term wear and durability. Moreover, increases in silicon power may require increased heat conductivity between the heat sink  20  and silicon die  24  to achieve desired cooling. 
     Certain embodiments disclosed herein may include support arrangements spanning between, for example, a printed circuit board and heat sink to reduce stresses on associated silicon dies and package substrates during manufacturing, testing and operation. In these or other embodiments, patterned cuts, drill trenches, holes, grooves, etc. (“tracks”) may be chemically or mechanically etched or otherwise made on the backside of the silicon die or wafer, lid surfaces (to the extent the package includes a lid), and/or heat sink surface to increase the effective surface area of the thermal interface. In liquid cooling arrangements for example, these increased surface areas that are in contact with the cooling liquid can improve heat transfer. 
     The tracks may form a grid, spiral, parallel line or other pattern. Thermal interface materials, such as alloys, cooling liquids (in liquid cooling arrangements), metals, thermal epoxies, thermal films, thermal pads or thermal preforms may fill the tracks. Some thermal interface materials may be applied to fill the tracks prior to lid or heat sink installation. The tracks may be coated or sputtered with seed metal, such as titanium or tungsten followed by nickel or gold, prior to filling with solder or other metal/metal alloys. 
     With reference to  FIG. 2 , a lid-less flip chip package and heat sink assembly  110  may include a flip chip assembly  112  contained within a socket  114  on a printed circuit board (PCB)  116 . (A ball grid array may also be used instead of the socket  114 , etc.) The assembly  110  may further include a bolster plate  118  on which the PCB  116  is mounted and a heat sink  120 . The heat sink  120  is fastened to the bolster plate  118  via spring-loaded bolt assemblies  122  to compress the flip chip assembly  112 , socket  114  and PCB  116  therebetween. In certain circumstances, up to 400 pounds per square inch of pressure (or more) may be applied to the flip chip assembly  112 . 
     The flip chip assembly  112  includes a silicon die  124  mounted to a package substrate  126  (e.g., a ceramic substrate) via solder bumps  128  and underfill  129 . Other mounting techniques, of course, are also contemplated. An indium or indium-gallium foil, graphite film, thermal epoxy, thermal film, thermal pad, etc.  131 , in this embodiment, is disposed between the heat sink  120  and silicon die  124  to promote heat transfer therebetween. Additionally the foil or film  131 , which may range in thickness from 4 to 8 thousandths of an inch depending on design constraints and objectives (other thicknesses are also possible), may act as a cushion to maintain thermal contact between the heat sink  120  and silicon die  124  even though these two components may be non-coplanar relative to one another. In other embodiments, the heat sink  120  (or a cooling plate in liquid cooling arrangements) may be in direct contact with the silicon die  124 . 
     The heat sink  120  and silicon die  124  have tracks  134 ,  136  respectively formed on adjacent surfaces to increase the effective surface area for heat transfer. The tracks  134 ,  136 , which may take any suitable shape in cross-section, have a width and depth selected, in this embodiment, such that the foil or film  131  flows into and fills the tracks  134 ,  136  upon installation of the heat sink  120 . The width and depth of the tracks  134 ,  136 , however, may be different in other embodiments. The tracks  134  may extend, for example, at least half-way into the heat sink  120 , etc. Other embodiments may include tracks on either of the heat sink  120  or silicon die  124 , or may lack tracks all together depending on expected operating temperatures and target heat transfer rates between these components. 
     With reference to  FIG. 3  (and continued reference to  FIG. 2 ), support arrangements  138  are positioned around a perimeter of the flip chip assembly  112  on the package substrate  126 : the support arrangements  138  are located at the corners of the package substrate  126 . The foil or film  131  as illustrated in  FIG. 2  is of sufficient size to cover the support arrangements  138 . The foil or film  131  in other embodiments, however, may only cover the silicon die  124 . Other scenarios are also possible. Assuming, for example, that the area of the silicon die  124  is 529 square millimeters, the support area of each of the support arrangements  138  may be 25 square millimeters. The shape, location and number of the support arrangements  138  may vary with design. Moreover, the support area defined by the support arrangements  138  may vary with design. 
     The height of the support arrangements  138 , in this example, is the same as the height of the flip chip assembly  112 . Certain silicon dies, however, may exhibit crowning such that the center of the die is located slightly further away from the package substrate as compared with its corners depending on die size. The height of the support arrangements  138 , in these or other embodiments, may be slightly less than that of the flip chip assembly  112  (e.g., less than 1 thousandth of an inch) to ensure that the heat sink  120  achieves and maintains full contact with the silicon die  124 . A typical distance between the heat sink  120  and the package substrate  126  falls within the range of 4 to 9 millimeters. Other distances, however, are possible in different arrangements. 
     Because the support arrangements  138  span the distance between the package substrate  126  and foil or film  131 /heat sink  120 , they assist in keeping a planar mounting surface of the heat sink  120 , which includes the tracks  134 , generally parallel to the package substrate  126  and thus the silicon die  124 . Such even contact between the heat sink  120  and silicon die  124  (and foil or film  131  if present) promotes favorable heat transfer characteristics therebetween and reduces uneven stress distribution within the flip chip assembly  112 . Additionally, the support arrangements  138  may act as a barrier to prevent unintended contact between the flip chip assembly  112  and various tooling during assembly and test as well as provide additional area for part marking and identification. 
     Each of the support arrangements  138  includes a silicon die  140  mounted to the package substrate  126  via solder bumps  142  and underfill  144 . That is, the support arrangements  138  generally mimic the construction of the flip chip assembly  112 . The silicon die  140  may be a “dummy” silicon die or contain active or passive circuitry. Other support structures, however, may be used. Non-silicon (e.g., metal, etc.) rings of sufficient thickness, for example, may be used to similarly support the heat sink  120 . The support arrangements  138  illustrated in  FIG. 2 , however, may offer improved dimensional control relative to alternative constructions: the desired height of the support arrangements  138  may be achieved with greater precision during manufacturing if the support arrangements  138  are constructed similarly to the flip chip assembly  112 . 
     With reference to  FIGS. 4 and 5 , a lidded flip chip package and heat sink assembly  210  includes a flip chip assembly  212 , mounted on on a printed circuit board (PCB)  216  via a ball grid array  217 . The assembly  210  further includes a heat sink  220 . The flip chip assembly  212  includes a silicon die  224  mounted to a package substrate  226  via solder bumps  228  and underfill  229 . The flip chip assembly  212  further includes a lid  230  bonded to the package substrate  226  via, for example, an epoxy  231 , and covering the silicon die  224 . Support structures (not shown) may or may not be included. 
     Mating surfaces of the heat sink  220  and lid  230  include tracks  246 ,  248  respectively formed therein. Likewise, mating surfaces of the silicon die  224  and lid  230  include tracks  250 ,  252  respectively formed therein. Other embodiments may include tracks on any one or more of the mating surfaces. The mating surfaces including the tracks  250 ,  252  are coated with a seed metal  254 . (In other examples, the seed metal  254  may, of course, be absent.) A thermal interface material  232  (e.g., metal) is disposed between the silicon die  224  and lid  230 , and fills the tracks  250 ,  252 . Another thermal interface material  233  (e.g., thermal epoxy) is disposed between the heat sink  220  and lid  230 , and fills the tracks  246 ,  248 . The mating surfaces of the silicon die  224  and/or lid  230 , in certain arrangements, may come in contact with liquid coolant. For example, a cooling liquid from a cooling plate may be supplied to one or more of the mating surfaces. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. As an example, fastening techniques other than spring-loaded bolt assemblies may of course be used to attach the heat sink to the rest of the package: spring loaded clips may be used to attach a printed circuit board having a flip chip assembly mounted thereon with a heat sink. Other arrangements are also contemplated. 
     The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications. Moreover, use of the word “or” in the claims does not imply combinations of the listed elements.