Patent Publication Number: US-6700195-B1

Title: Electronic assembly for removing heat from a flip chip

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field Of The Invention 
     The present invention generally relates to thermal management of semiconductor devices. More particularly, this invention relates to an electronic assembly that dissipates heat from a flip chip, and also improves the life expectancy of the chip by reducing induced strains in the chip and its solder connections. 
     (2) Description Of The Related Art 
     Power flip chips and certain other semiconductor devices require thermal management in order to minimize their operating temperatures. A variety of techniques have been developed for dissipating heat generated by power flip chips. One such technique is disclosed in commonly-assigned U.S. Pat. Nos. 6,180,436 and 6,365,964 to Koors et al., and involves conducting heat from a power flip chip with a heat-conductive pedestal brought into thermal contact with the topside of the chip, i.e., the surface opposite the solder connections that attach the chip to its substrate. A thermally-conductive lubricant is placed between the topside of the chip and the pedestal to fill gaps between the chip and pedestal in order to promote thermal contact, as well as decouple lateral mechanical strains that arise as a result of different thermal expansions and movement between the chip, substrate and pedestal. 
     While the approach taught by Koors et al. has been successfully implemented, induced stresses can be sufficiently high under severe conditions to fracture the solder connections and even the chip die. Furthermore, thermally-conductive lubricants suitable for placement between a chip and pedestal typically contain a hard particulate filler that can abrade the chip, and the thermal performance of such lubricants is typically compromised as a result of the compositional requirements necessary to achieve adequate lubricity. Finally, thermal energy could be more efficiently dissipated if the contact interface resistance between the chip and pedestal could be reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to an electronic assembly for conducting heat from a semiconductor device, such as a power flip chip. The assembly is generally constructed to dissipate heat from a chip mounted to a substrate, preferably a laminate such as a printed wiring board (PWB). The substrate has a first region with conductors thereon, a second region surrounding and supporting the first region, and a third region surrounding and supporting the second region, with the second region being more flexible than the first and third regions. The chip is mounted to the first region of the substrate, and has solder connections on a first surface thereof that are registered with the conductors on the first region of the substrate. A heat-conductive member thermally contacts a second surface of the chip oppositely disposed from the first surface. Biasing means contacts the first region of the substrate for biasing the chip into thermal contact with the heat-conductive member. 
     A significant advantage of the electronic assembly of this invention is that the second region of the substrate improves the mechanical decoupling of strains that arise as a result of different thermal expansions and movement between the chip, substrate, and heat-conductive member, thereby reducing the induced stresses that can cause fracturing of the chip and its solder connections. Induced strains are reduced by the second (“flex”) region of the substrate because the chip is located on a separate island of “rigid” substrate, namely, the first region. The size and mass of the first region can be small, adapted to support a single chip. As such, stresses resulting from chip-to-chip stack-up tolerances can be completely eliminated. Furthermore, the flexibility of the substrate is able to tolerate greater variances in the dimensions of the heat-conductive member, such that fabrication tolerances can be relaxed. 
     Another advantage of the invention is that, because of the significantly lower induced stresses, a lubricant property is not required between the chip and heat-conductive member. Elimination of this restriction opens up the possibility for using a variety of other materials having better thermal properties than thermally-conductive lubricants. In particular, highly-conductive materials can be used that reduce the tendency for the chip to be abraded during the life of the assembly. Yet another advantage is that a thicker substrate than Koors et al. becomes practical because the chip-to-chip tolerance that would otherwise be taken up by the flexing of a thin laminate substrate can be fully taken up by the flexible second region of the present invention. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a housing enclosing a power flip chip mounted to a substrate, with a heat-conductive pedestal contacting the topside of the chip. 
     FIGS. 2 and 3 are more detailed cross-sectional and plan views, respectively, of portions of the substrate of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an electronic assembly  10  containing a power flip chip  12  attached to a substrate  14 . The assembly  10  includes a housing formed by two housing members  20  and  22  that enclose the chip  14  and substrate  14 . Mounting of the chip  12  to the substrate  14  can be performed by conventional flip-chip techniques, in which preformed solder bumps on the frontside of the chip  12  (the surface of the chip  12  on which the flip chip microcircuitry is formed) are registered with and reflow soldered to conductors  44  (FIG. 2) on the surface of the substrate  14  to yield solder connections  18 . The chip  12  may be underfilled with a suitable polymeric material (not shown), as is conventionally done in the art to promote the thermal cycle life of the solder connections  18 . While a single chip  12  is depicted in FIG. 1, any number of chips could be attached to the substrate  14  and enclosed within the housing. Furthermore, while a power flip chip is illustrated, the present invention is generally directed to any type of heat-generating device that utilizes solder connections to provide physical and electrical connection to a substrate. 
     According to the invention, the substrate  14  preferably has a laminate construction (e.g., a PWB) that defines multiple discrete regions, such as the three regions  15 ,  16  and  17  represented in FIGS. 1 and 2. A first of the regions  15  is completely surrounded by a second of the regions  16 , which is thinner and, according to the invention, significantly more freely able to move than either the first region  15  or the remaining third region  17 , which completely surrounds the second region  16 . As a result of this construction, the relatively rigid first region  15  of the substrate  14  is physically supported by the flexible second region  16 , which in turn is physically supported by the relatively rigid third region  17 , with the effect that the first region  15  is almost completely mechanically decoupled from the third region  17 . The chip  12  is depicted in FIG. 1 as being attached solely to the first region  15  of the substrate  14 , such that the chip  12  and its solder connections  18  are also mechanically decoupled from the third region  17 . At the same time, the flexible second region  16  does not mechanically decouple the first region  15  and the chip  12  attached thereto from the housing  20 - 22  (if used to support the third region  17 ), which is much stiffer than the third region  17 . 
     A pedestal  26  is shown as projecting into the interior of the housing  20 - 22  from the upper housing member  20  (as viewed in FIG.  1 ), and engages the entire topside of chip  12 . While shown as being integrally formed as a portion of the upper housing member  20 , the pedestal  26  could be separately formed and subsequently attached to the housing member  20 . A number of convection cooling fins  28  are shown as projecting outwardly from the upper housing member  20 . 
     The lower housing member  22  (as viewed in FIG. 1) encloses a biasing member  30  that contacts the first region  15  of the substrate  14  and urges the chip  12  on the first region  15  into engagement with the pedestal  26 . As depicted in FIG. 1, the biasing member  30  contacts an interior wall of the lower housing member  22  so as to be braced for engagement with the first region  15  of the substrate  14 . The biasing member  30  is shown as contacting essentially the entire facing surface of the first region  15  so that, in cooperation with the pedestal  26 , a generally uniform load is applied to the chip  12  and the first region  15  of the substrate  14 . The biasing member  30  is preferably an elastomeric member as depicted in FIG. 1, with a particularly suitable material for the biasing member  30  being a silicone-based polymer that can be preformed or formed in place. However, other types of biasing elements could be used, such as a spring. 
     From the structure described above, it can be seen that the biasing member  30  serves to bias the chip  12  into engagement with the pedestal  26 , so that the pedestal  26  is able to conduct heat away from the chip  12  and into the housing member  20 . For this reason, at least the upper housing member  20  and pedestal  26   28  are preferably formed of a material that readily conducts heat, such as a metal or a metal-filled plastic. To facilitate manufacturing, the entire upper housing member  20 , including integrally-formed pedestal  26  and fins  28 , can be molded, stamped or formed from a conductive material, such as aluminum. While the lower housing member  22  need not be formed of a thermally-conductive material in the embodiment shown, it is foreseeable to do so to provide a larger heat sink. The lower housing member  22  can also be equipped with cooling fins to further promote heat dissipation to the environment. The choice of material for the lower housing member  22  depends in part on the type of biasing member  30  used, since a biasing member  30  formed of a highly conductive material, e.g., metal, would promote conduction of heat back to the chip  12  if the lower housing member  22  is also thermally conductive. 
     The load applied by the biasing member  30  to the chip  12  affects the heat transfer across the interface between the chip  12  and its pedestal  26 , with higher loads promoting conduction. However, the applied load must not be so high as to be structurally detrimental to the chip  12  and substrate  14 . Generally, a load of about three to five pounds (about 13 to about 22 Newtons) should typically be acceptable, though lower and higher loads are foreseeable. To further promote heat transfer between the chip  12  and pedestal  26 , a thermally-conductive interface material  40  is also preferably provided between the topside of the chip  12  and its pedestal  26  to promote heat transfer therebetween. Importantly, a feature of this invention is that the interface material  40  is not required to decouple lateral mechanical strains that arise as a result of different thermal expansions and movement between the chip  12 , substrate  14  and pedestal  26 . Instead, the arrangement of the first, second and third regions  15 ,  16  and  17  of the substrate  14  provides the desired decoupling effect by isolating the rigid first region  15  to which the chip  12  is attached with the second region  16 , which is sufficiently flexible to accommodate differences in thermal expansion as well as variations in pedestal height and flip chip planarities to achieve uniform contact between the chip  12  and pedestal  26 . As a result, a variety of material other than lubricants can be used as the interface material  40 . Notable examples include highly-conductive films, gels and phase-changing materials, including those having fillers with a Mohs hardness of greater than 5, as well as conductive adhesives that have been incompatible with prior isolation techniques. Particular examples include XTIG-7500 available from the Bergquist Company, ATTA available from Browne Technology, Inc. (btechcorp.), M-220 from Thermoset Advanced Electronic Materials, and 4173 from Dow. It is foreseeable that a variety of other heat-conducting materials, both lubricating and nonlubricating, could be used. 
     FIG. 2 represents a particular construction for the substrate  14 , by which the regions  15 ,  16  and  17  acquire their differences in rigidity as a result of the number and type of laminate layers that form them. As represented in FIG. 2, the first and third regions  15  and  17  are essentially identical, containing the same number, type and arrangement of dielectric and conductor layers  42  and  44  built up on a base stock comprising a pair of conductor layers  34  laminated to a dielectric layer  32 . In contrast, the flexible second region  16  is shown as sharing only the dielectric and conductor layers  32  and  34  with the rigid first and third regions  15  and  17 . As a result of having one or more of the additional layers  42  and  44  depicted in FIG. 2, the first and third regions  15  and  17  of the substrate  14  are more rigid than the second region  16 . Suitable dielectric materials for the dielectric layer  32  of the base stock include standard FR 4  laminate (epoxy prepreg), polyimide, polyimide prepreg, acrylic, etc., and suitable dielectric materials for the additional dielectric layers  42  include B-stage epoxy and standard FR 4  laminate. Suitable electrically conductive materials for the conductor layers  34  and  44  include copper foil. The second region  16  is also depicted in FIG. 2 as having outer polyimide layers  36  that protect the conductor layer  34 . All of these materials and associated processing are well known in the art, and therefore will not be discussed in any further detail. 
     FIG. 3 shows the second region  16  as further including through-holes  38  that are arranged and sized to promote flexing of the second region  16  for optimizing isolation of the first region  15 . The through-holes  38  are located at the corners of the second region  16 , between areas of the second region  16  where conductors pass to electrically interconnect the first and third regions  15  and  17 . The through-holes  38  are depicted as generally have a circular or oval shape that avoid the creation of stress risers, though other shapes are foreseeable. If present, the through-holes  38  increase the flexibility of the flexible second region  16 , and therefore the ability of the second region  16  to accommodate tolerance variations, such as between the chip  12  and the pedestal  26 . 
     While circuit substrates having discrete rigid and flexible regions, referred to as “rigid flex constructions,” are known in the art, as exemplified by U.S. Pat. No. 3,409,732 to Dahlgren et al. and U.S. Pat. No. 4,800,461 to Dixon et al., their use has generally been limited to providing a circuit board with a flexible peripheral region through which interconnections are made, to provide board-to-board interconnect functions, e.g., to separate digital and analog circuits, and to assist in processing a circuit board assembly which is then broken and folded for placing in a housing. In contrast, the present invention uses the flexible second region  16  to mechanically isolate a rigid (first) region  15  that is preferably sized to carry a single chip  12  (or a chip  12  and filter capacitor, or two or more closely-spaced chips  12 ) allowing the rigid region  15  (and devices mounted thereto) to move relatively independently with respect to a remaining rigid (third) region  17  of the same substrate  14  in order to compensate for different thermal expansions and movement between the chip  12 , substrate  14 , and pedestal  26 . As a result, the rigid-flexible construction of the substrate  14  serves to reduce the induced stresses that can cause fracturing of the chip  12  and its solder connections  18 . The substrate  14  also allows for a greater tolerance for dimensional variations, including those of the pedestal  26  and chip-to-chip stack-up tolerances. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.