Abstract:
A method and assembly ( 10 ) for conducting heat from a semiconductor device, such as a power flip chip ( 12 ). The assembly ( 10 ) is generally constructed to dissipate heat from the flip chip ( 12 ) when mounted to a flexible or rigid substrate ( 20 ). Heat is conducted from the flip chip ( 12 ) through upper and lower pedestals ( 26, 28 ) each of which includes a pliable pre-cured silicone adhesive pad ( 32, 36 ). The pre-cured silicon adhesive pads ( 32, 36 ) promote thermal contact while also decoupling any lateral mechanical strains that may arise as a result of different thermal expansion and movement between the flip chips ( 12 ). The housing portions ( 16, 18 ) form a housing ( 14 ) when assembled, with each housing portion ( 16, 18 ) including a configured edge ( 37, 41 ) that controls the travel of the pedestals ( 26, 28 ) toward each other, to thereby limit the pressure exerted on the flip chip ( 12 ) disposed therebetween. Silicone adhesive can be applied between the edges ( 37, 41 ) to hold the housing portions ( 16, 18 ) together.

Description:
TECHNICAL BACKGROUND 
     The subject invention relates generally to an electronic assembly containing flip chip components on a laminate circuit board within the electronic assembly and, more particularly, to an apparatus and method of providing a relaxed tolerance assembly for the flip chip components and laminate circuit board with respect to heat dissipating structures of the assembly. 
     BACKGROUND OF THE INVENTION 
     A variety of methods are known for dissipating heat generated by semiconductor devices. In the case of semiconductor devices mounted on a circuit board and mounted within an enclosure, thermal management is usually achieved by dissipating heat primarily in the vertical direction, both above and beneath the semiconductor device. For example, heat-generating semiconductor chips, such as power flip chips, are often mounted to alumina substrates that conduct and dissipate heat in the vertical direction away from the chip. 
     One form of assembly utilizes a housing having a plurality of heat sink devices in the form of pedestals that are adapted to be both above and below the flip chip when the housing is assembled. The flip chips are made to come into contact with the pedestals through contact pressure. Additionally, thermal grease is used between the flip chip and the pedestal. The thermal grease provides a conductive path between the heat sink and the flip chip. The thermal grease also protects the flip chip due to the contact pressure required to maintain the heat sink to the flip chip for proper thermal contact. 
     This type of enclosure and/or heat dissipating method, however, requires fairly close tolerances. Particularly, these systems require fairly precise measurement and control. 
     SUMMARY OF THE INVENTION 
     It is an object of the subject invention to provide a system, method and/or apparatus for conducting heat from a flip chip semiconductor device mounted to a substrate that requires less tolerance in providing thermal contact between the flip chip and the heat dissipating structure or heat sink. 
     It is another object of the subject invention to provide a system, method and/or apparatus for conducting heat from a flip chip assembly that minimizes critical height tolerance requirements between a flip chip and a heatsink pedestal. 
     It is yet another object of the subject invention to provide a system, method and/or apparatus for conducting heat from a flip chip assembly that does not require thermal grease. 
     In accordance with a preferred embodiment of the subject invention, these and other objects and advantages are accomplished as follows. 
     According to the subject invention, there is provided a system, method and/or apparatus or assembly for conducting heat from a flip chip semiconductor device such as a power flip chip. 
     In one form, there is provided a heat-dissipating assembly for removing heat from a flip-chip semiconductor device. The assembly includes a housing having a thermally-conductive first housing portion and a second housing portion, a flexible substrate supported within the housing, the substrate having conductors thereon, a flip chip mounted to the substrate, the flip chip having a first surface and solder bumps on the first surface registered with the conductors on the substrate, the flip chip having a second surface oppositely disposed from the first surface, a first heat sink extending inwardly towards the flip chip from the first housing portion; and a first pre-cured silicone adhesive layer disposed on an end of the first heat sink and in thermal relationship with the second surface of the flip chip. 
     In another aspect of the invention, the first and second housing portions are provided with a joint configured to control the pressure exerted on the flip chip through the heat sink components. In one embodiment, the flip chip is mounted between heat sink pedestals connected to the first and second housings. The joint between the housings controls the travel of the two housings toward each other when the assembly is put together. More particularly, the joint prevents excessive travel of the heat sink pedestals toward each other, which might damage the flip chip entrained between the pedestals. 
     In another form, the subject invention provides a method for conducting heat from a flip chip, the method including the steps of: (a) providing a flexible substrate having conductors thereon, a flip chip having a first surface with solder bumps on the first surface and a second surface oppositely disposed from the first surface, the flip chip being mounted to the substrate such that the solder bumps are registered with the conductors on the substrate; and (b) enclosing the substrate and flip chip within a housing so that a first pre-cured silicone adhesive disposed on a first heat sink contacts the second surface of the flip chip in a first thermal transfer relationship. 
     Other objects and advantages of this invention will be appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a side sectional view of a housing that encloses a pair of flip chips mounted to a substrate with heat conductive features in accordance with the principles of the subject invention; and 
     FIG. 2 is representation of one of the flip chips about to be retained between two heatsink pedestals of the housing and depicting the manner in which the two housing portions join. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a heat-dissipating assembly  10  for a pair of power flip chips  12 . The assembly  10  includes a housing or enclosure  14  that encloses the flip chips  12 . The housing  14  includes a first housing or enclosure portion or half  16  and a second housing or enclosure portion or half  18 . The first and second housing portions  14  and  18  are preferably either die-cast or sheet metal. The first and second portions  16  and  18  are joined together as described more fully below to define the housing  14 . The flip chip  12  is situated on a flexible substrate  20  such as laminate circuit board or thin substrate such as is known in the art. Suitable substrates, for example, include thin laminates, rigid inorganic substrates and printed wiring boards (PWB). 
     Mounting of the flip chips  12  to the substrate  20  may be accomplished by conventional flip chip techniques, such as via preformed solder bumps  22  on the front side of the flip chip  12  (i.e. the surface of the flip chips  12  on which the flip chip microcircuitry is formed). The solder bumps  22  are registered with and reflow soldered to conductors  23  on the surface of the substrate  20  to yield solder connections with the substrate conductors  23 . The flip chips  12  are underfilled with a suitable polymeric material  24 , as is conventionally done in the art to promote the thermal cycle life of the solder connections. 
     As indicated above, the housing  14  is composed of first and second portions or members  16  and  18  each having a respective peripheral flange or edge structure  37  and  41 . As described in greater detail below, the two edge structures  37  and  41  provide a manner of joining the two housing portions  16  and  18  together. 
     A pair of heat sinks in the form of pedestals  26  is shown projecting from an inner surface of the housing portion  16  and into the interior of the housing  14 . The pedestals  26  may be integrally formed with the housing portion  16  or may be formed separately and subsequently attached to the housing portion  16 . Thus, the pedestals  26  may be die-cast with the housing portion  16 , formed as sheet metal with the housing portion  16 , or formed thereafter out of a suitable thermal transfer material. In accordance with an aspect of the subject invention, the pedestals  26  have a depression or concavity  30  formed in the end thereof. The concavity  30  is preferably dimensioned so that the flip chip  12  can be at least partially received within the cavity, as depicted in FIG.  1 . Thus, the perimeter of the concavity is similarly configured to the perimeter of the flip chip, although larger in lateral dimension so the flip chip can sit at least partially within the cavity when the heat sink assembly is filly seated together. 
     A silicone adhesive  32  is disposed in the concavity  30  and is thereafter cured. This forms a pre-cured silicone adhesive pad or layer  32  on the pedestal  26 . The concavity  30  is sized to accommodate the flip chip  12 . Particularly, the concavity  30  is preferably sized to be larger than the size of the flip chip  12  such that the cured silicone adhesive  32  is also preferably larger than the size of the flip chip  12 . This allows the silicone pad  32  to receive the flip chip  12  when the housing portions  16  and  18  are fully seated together. 
     A further pair of heat sinks in the form of pedestals  28  extend or project from the housing portion  18  toward the interior thereof. The pedestals  28  may be integrally formed with the housing portion  16  or may be formed separately and subsequently attached to the housing portion  16 . Thus, the pedestals  28  may be die-cast with the housing portion  16 , formed as sheet metal with the housing portion  16 , or formed thereafter out of a suitable thermal transfer material. The pedestals  28  define an upper or end surface  34  on which is disposed a silicone adhesive layer or pad  36 . The silicone adhesive layer  36  is preferably pre-cured before assembly of the housing portions  16  and  18 . 
     FIG. 1 shows assembly  10  in an assembled state with the housing portions  16  and  18  joined together. In this form, the substrate  20 , and thus the flip chips  12 , are retained by the pedestals  26  and  28 . Particularly, the silicone adhesive pad  36  of the pedestal  28  is caused to contact and provide a slight pressure against the substrate  20 . The silicone adhesive pad  36  is thus slightly compressed against the substrate  20  to provide thermal conductivity between the substrate  20  and the pedestal  28 . Moreover, the silicone adhesive pad  32  of the pedestal  26  is caused to contact and provide a slight pressure against the flip chip  12 . The silicone adhesive pad  32  is thus slightly compressed against the flip chip  12 . In both instances, the pre-cured silicone adhesive pads  32  and  36  will still be pliable or soft after cure and thus conform to the shape of the flip chips  12  (in the case of the pad  32 ) and the substrate  20  (in the case of the pad  36 ). This also allows the flip chip assembly (i.e. flip chips and substrate) to be removed from the housing  14  with little to no damage. 
     It should be appreciated that while only two flip chips  12  and pedestal pairs  26  and  28  (i.e. an upper pedestal  26  and lower pedestal  28  for each flip chip  12 ) are shown within the housing  14 , the housing  14  may have any number of pedestal pairs and thus flip chips. Moreover, it should be appreciated that the height of the pedestals  26  and  28  are such that the flip chips  12  and substrate  20  are slightly compressed against and into the respective silicone adhesive pads  32  and  36  upon assembly of the housing portions  16  and  18 . 
     Referring now to FIG. 2, there is illustrated the assembly of the housing portions  16  and  18 . It should be appreciated that the flip chips  12  have previously been assembled onto the substrate  20 . The two housing portions  16  and  18  are joined as indicated by the arrows. When this is accomplished, the pedestals  26 ,  28  with their respective pre-cured silicone adhesive pads  32 ,  36  contact the flip chips  12  and substrate  20  as indicated above. Moreover, the edge portions  37  and  41  of the housing portions  16  and  18  respectively, join together in a manner as now described. 
     Particularly, the edge portion  37 , which extends about the outer periphery of the housing portion  16 , includes a horizontal shelf or flat  38 . A flange or wall  40  extends from the flat  38  in a perpendicular direction. In FIG. 2, the perpendicular direction is oriented as downward. The edge portion  41 , which also extends about the outer periphery of the housing portion  18 , includes a horizontal shelf or flat  42 . Dual flanges or walls  44  and  46  extend from the flat  42  in a perpendicular direction. In FIG. 2, the perpendicular direction is oriented as upward. The dual walls  44  and  46  are spaced from each other to form an inner trough, channel, groove or the like  48 . The channel  48  is dimensioned to receive the wall  40  that is dimensioned slightly smaller than the channel  48 . The channel  48  and wall  40  can thus form a “tongue and groove” type joint. A silicone adhesive is applied in the channel  48  before joining of the housing portions  16  and  18 . 
     The channel  48  and the wall  40  are also dimensioned so that the wall  40  bottoms out in the channel  48  when the silicone pads  32  and  36  contact and provide adequate contact pressure on the flip chips  12  and the substrate  20  respectively. The channel  48  and wall  40  provide the maximum travel of the housing portions  16  and  18  (and thus the pressure of the pads  32  and  36  into the flip chips  12  and the substrate  20 ) allowed in order not to crush the flip chips  12  into the adhesive pads or beyond. Additionally, a silicone adhesive is provided in the channel  48  that will bond with the wall  40 . In this manner, an adhesive joint is provided in addition to the tongue and groove joint. Thus, it is contemplated that no fasteners will be used to secure the housing portions  16  and  18  together. 
     It should be appreciated that the subject invention minimizes any critical tolerance requirements for the assembly, and specifically the height tolerance for the heatsink pedestals  26  and  28 . Furthermore, the subject invention protects the flip chips from damage at assembly and during use. Thus, the dimensions of the joint between the channel  48  and the wall  40  can be calibrated to limit the travel of the pedestals  26 ,  28  toward each other, thereby controlling the pressure exerted on the stacked components, substrate  20 , pads  32 ,  36  and flip chips  12 . Alternatively, the flat  38  can limit the housing travel when it contacts the walls  44 ,  46 , where the height of the wall  40  is significantly less than the depth of the channel  48 . However, it is preferred that the channel/wall joint provide the control feature of the present invention, as described above. 
     While the subject invention may be used in many applications, the subject invention may be used, for example, in powertrain gasoline or diesel engine modules (ECMs), powertrain engine and transmission control modules (PCMs), powertrain transmission control modules (TCMs), and powertrain non-automotive control modules. 
     While this 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. For example, various components of the assembly  10  could be configured differently from that shown in the figures. Moreover, appropriate materials could be substituted for those noted. Accordingly, the scope of the invention is to be limited only by the claims.