Abstract:
A method and apparatus improve the thermal interface between a heat sink and a semiconductor. A support member is attached to a bias plate to facilitate a fixed connection with a surface. The bias plate has a beam for alignment with a heat sink that is attached to the semiconductor. The bias plate is attached to the support member such that the support member forces the beam against the heat sink to improve the thermal interface between the heat sink and the semiconductor. 
     In the method, a heat sink is attached to a semiconductor, and a pivoting beam is biased against the heat sink such that the thermal interface with the semiconductor is improved.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     Generally, this invention relates to the removal of heat from the surface of a semiconductor. More particularly, this invention relates to a method and apparatus to improve the thermal interface between a heat sink and a semiconductor. 
     BACKGROUND OF THE INVENTION 
     As semiconductors continue to decrease in size while power densities increase, heat dissipation problems must continue to be resolved. Typically, heat sinks are used to cool semiconductors. Semiconductors can use ceramic or plastic packages. Heat sinks are very difficult to attach to plastic packages. Adhesives do not stick very well, and mechanical attachment is expensive. Some mechanical attachment methods can damage the semiconductor package. 
     One prior art method uses double sided thermal adhesive tape to attach heat sinks to the plastic packages. An external device provides additional support and protection to the heat sinks. This device is a piece of molded plastic that fits over the top of the heat sinks and is attached to the circuit board with screws. A piece of thin foam rubber is attached to the device over the location of the heat sinks. The foam rubber applies even pressure to the top of the heat sinks and compensates for height tolerance problems. 
     However, this approach is time consuming and expensive. Labor is needed to attach the foam rubber to the device and the foam rubber adds to the cost. 
     Therefore, it would be highly desirable to reduce the cost and labor associated with installing heat dissipation devices for semiconductor devices. 
     In addition, the double sided tape may weaken with age. If the heat sinks are not handled properly, then the loss of the mechanical strength of the tape will increase the thermal impedance at the thermal interface between the heat sink and the semiconductor package. 
     Therefore, it would be highly desirable to improve the thermal interface of the heat sink and semiconductor package by improving the mechanical strength at the thermal interface. 
     SUMMARY OF THE INVENTION 
     An apparatus to improve the thermal interface between a heat sink and a semiconductor comprises a support member attached to a bias plate. The support member facilitates a fixed connection with a surface. The bias plate has a beam for alignment with a heat sink that is attached to the semiconductor. The bias plate is attached to the support member such that the support member forces the beam against the heat sink to improve the thermal interface between the heat sink and the semiconductor. 
     The bias plate of the present invention is used in a computer. The computer has a semiconductor attached to a circuit board. A heat sink is attached to the semiconductor. The bias plate includes a beam for alignment with the heat sink. The bias plate is positioned such that the beam is forced against the heat sink to improve the thermal interface between the heat sink and the semiconductor. 
     A method improves the thermal performance of a semiconductor by attaching a heat sink to a semiconductor, and biasing a pivoting beam against the heat sink such that the heat sink achieves an improved thermal interface with the semiconductor. 
     This invention is more cost effective because it eliminates the foam rubber and the labor of attaching the foam. The function of the foam rubber is now supplied by a beam that is molded into the bias plate to apply the appropriate pressure to the heat sinks. The bias plate can be manufactured at a very low cost, for example, by injection molding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a front view of a computer suitable for use with the present invention. 
     FIG. 2 is a perspective view of the interior of the computer of FIG. 1 showing circuit boards using the heat sink support of the present invention. 
     FIG. 3 is a top view of a bias plate in accordance with a first embodiment of the invention. 
     FIG. 4 is a side, end view of a bias plate and support member in accordance with an embodiment of the invention. 
     FIG. 5 is a side, axial view of the bias plate and support member in accordance with an embodiment of the invention. 
     FIG. 6 is a side, end view of the bias plate and support member of FIG. 4 when used with a semiconductor and heat sink. 
     FIG. 7 is a side, axial view of the bias plate and support member of FIG. 5 when used with a semiconductor and heat sink. 
     FIG. 8 is a perspective view of the bottom of a bias plate with a single beam, in accordance with an embodiment of the invention. 
     FIG. 9 is a bottom view of a bias plate in accordance with a second embodiment of the invention. 
     FIG. 10 is a side, axial view of the bias plate and support member of FIG. 9 along line A—A. 
     FIG. 11 is a side, end view of the bias plate and support member of FIG.  9 . 
     FIG. 12 is a perspective view of the beam used with the bias plate of FIGS. 9,  10  and  11 . 
    
    
     Like reference numerals refer to corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a computer system  100  suitable for use with the present invention. The computer system  100  has the following components: a computer housing  110 , display  120 , keyboard  130 , mouse  140 , hard disk drive  150 , and floppy disk drive  160 . 
     FIG. 2 is the interior of the computer housing  110  of FIG. 1 showing circuit boards using the heat sink support of the present invention. The computer housing  110  comprises a mother board  170  with connectors  172  for supporting and connecting to circuit boards  180 - 188 . 
     FIG. 3 is a top view of a bias plate  310  in accordance with a first embodiment of the invention. The bias plate  310  has a set of beams  320 . The bias plate  310  is integrally formed with or connected to a support member  330 . The support member  330  facilitates a fixed connection with a surface. 
     As used herein, the term “bias” means to apply a force. The term “bias plate” is used because beam members of the bias plate are used to apply a force to the heat sink against the semiconductor. 
     FIG. 4 is a side, end view of the bias plate  310  and support member  330 . In this view, the structure of the beams  320  can be seen. A first beam end  332  attaches to the bias plate  310  at a bias plate pivot point  334 . A second beam end  336  is unattached. Referring to FIGS. 3 and 4, a gap  338  is shown between the second beam end  336  and the bias plate  310 . When engaged with a heat sink, the beam  320  can flex or pivot at the bias plate pivot point  334 , allowing the beam  320  to pass through the gap  338 . Thus, the beam  320  is flexible. The flexing action of the beam  320  exerts a force on a heat sink positioned beneath it, as described below. 
     In a preferred embodiment, the beam  320  has two components: a sloped member  342  attached to a contact member  344 . The sloped member  342  attaches the beam  320  to the bias plate  310 . 
     FIG. 5 is a side, axial view of a bias plate  310  and support member  330  in accordance with an embodiment of the invention. The support member  330  is a pillar and has a cylindrical opening  346  at its base for accepting a fastener  352 . The fastener  352  can be a snap-fit fastener or a screw. 
     FIG. 6 is a side, end view of the bias plate  310  and support member  330  of FIG. 4 when used with a semiconductor and heat sink. Semiconductors  354  are attached to a circuit board  356 . As used herein, the term semiconductor refers to a semiconductor and/or the housing in which it is positioned. The heat sinks  360  have a lower contact surface  362  that is attached to the semiconductor  354 , for example, with double sided thermal adhesive tape (not shown). The opening  346  in the support member  330  is aligned with an opening  364  in the circuit board  356 . The beam&#39;s  320  contact member  344  also has a contact surface  366  for engaging the heat sink  360 . The beam&#39;s  320  contact surface  366  applies substantially uniform pressure to the heat sink  360  and therefore to the semiconductor  354 . 
     FIG. 7 is a side, axial view of the heat sink support of FIG. 5 when used with a semiconductor and heat sink. The heat sinks  360  have fins  368  and the bias plate  310  fits over the fins  368 . A fastener  352  is inserted through the openings  364  and  346  to attach the bias plate  310  to the circuit board  356 . The beams  320  are arranged on the bias plate  310  to engage the fins  368  such that a substantially uniform pressure is applied to the surface of the semiconductor  354 . 
     When engaged, the beam  320  is biased against the heat sink such that the heat sink improves the thermal interface with the semiconductor. To engage the heat sinks, when properly inserted the fastener  352  applies a sufficient force to properly bias the beam  320 . 
     FIG. 8 is a perspective view of the bottom of a bias plate with a single beam, in accordance with an embodiment of the invention. FIG. 8 shows the gap  338  between bias plate  310  and the beam  320  extending along the back and side of the beam  320 . 
     FIG. 9 is a bottom view of a bias plate  410  in a second embodiment of the invention. The bias plate  410  has a set of beams  420  and support members  430 . 
     FIG. 10 is a side, axial view of the bias plate  410  and support member  430  of FIG. 9 along line A—A. The bias plate  410  has support members  430 . In this embodiment, the support members  430  are rails disposed on opposite sides of the bias plate  410 . 
     FIG. 11 is a side, end view of the bias plate and support member of FIG.  9 . The rails have clips  431  for attaching the bias plate  410  to a circuit board. 
     FIG. 12 is a perspective view of the beam  420  used with the bias plate  410  of FIGS. 9,  10  and  11 . In this second embodiment, the beam  420  has an arm  432  and a contacting member  434 . The arm  432  has a first end  436  and a second end  438 . Referring also to FIG. 10, the first end  436  of the arm  432  is attached to and is substantially perpendicular to the bias plate  410 . The second end  438  of the arm  432  is attached to the contacting member  434 . A lateral support member  440  is attached to the beam  420  and the bias plate  410 . The lateral support member  440  attaches to a side of the arm  432  and has a wedge shape—extending from a pivot point  442  at the second end  438  of the arm  432  to a distant point  444  on the bias plate  410 . The wedge-shape has lower air resistance. Alternately, the lateral support member  440  can use other shapes. 
     Preferably, the distant point  444  on the bias plate  410  is aligned with a central axis  446  extending through the beam and arm. Therefore the wedge is symmetrical along the central axis  446 . The lateral support member  440  provides additional support to the beam  420  to ensure that the arm  436  does not move in a direction away from the semiconductor when engaged with the heat sink. In other words, the lateral support member  440  ensures that the beam  420  is properly positioned when engaged with the heat sink. 
     When engaged with the heat sink, the contacting member  434  is substantially parallel to the bias plate  410 . The contacting member  434  flexes about the pivot point  442 . In other words, the beam  430  acts like a spring to apply pressure to the heat sink. 
     When the clips  431  (shown in FIG. 11) are engaged with a circuit board, the beam  420  is biased against a heat sink. The beams  420  apply a sufficient additional force to the fins of the heat sink to improve the thermal interface between the heat sink and the semiconductor. 
     Preferably, for both embodiments, the bias plate, support members and beams are formed of injection molded plastic. Alternately, for both embodiments, the bias plate, support members and beams are a stamped metal plate. 
     Referring back to FIG. 2, both embodiments of the present invention are shown attached to circuit boards  180 ,  182 ,  184 ,  186 , and  188  in vertical slots  172  in the processing unit  110 . The bias plate of the first embodiment  310  is attached to a circuit board  180  that is in one vertical slot  172 . The bias plate of the second embodiment  410  is attached to another circuit board  188  that is in another vertical slot  172 . Another bias plate of the first embodiment  310  is attached to the mother board  170 . Alternatively the bias plate the second embodiment  410  can be attached to the mother board  170 . 
     ALTERNATIVE EMBODIMENTS 
     In an alternative embodiment, the beam  320  of the first embodiment is used with the support  430  and clips  431  of the second embodiment. In another alternative embodiment, the beam  420  of the second embodiment is used with the pillar support member  330  of the first embodiment. 
     Although the invention was described using a single pillar  330 , in alternate embodiments two or more pillars can be used. 
     Preferably, the beams  320 ,  420  have a smooth surface that contacts the fins. In alternative embodiments, the beams have a rough or textured surface. 
     To accommodate for height tolerance problems of a heat sink, multiple beams, of either embodiment  320 ,  420 , can be used to apply pressure to the heat sink. In other words, multiple beams of the first embodiment  320  can be used to apply pressure to a heat sink. Alternatively, multiple beams of the second embodiment  420  can be used to apply pressure to a heat sink. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following Claims and their equivalents.