Abstract:
A heat dissipating element (e.g., a heat sink) is held in an initial position closer to a heat generating structure (e.g., a microprocessor) and in a subsequent position farther from the microprocessor. A thermal interface material (e.g., a thermal grease) spans the gap, but is not held under compression, between the heat sink and the microprocessor.

Description:
Pursuant to 35 U.S.C. § 120, this application is a Divisional of prior U.S. application Ser. No. 09/409,951, filed Sep. 30, 1999 now U.S. Pat. No. 6,472,742. The disclosure of the prior application is considered part of, and is incorporated by reference in, the disclosure of this application. 

   BACKGROUND 
   To prevent microprocessors and other heat generating electronic components from overheating, excess heat is sometimes conducted to a heat sink where it is dissipated. The heat sink may be mounted above the microprocessor with a thermally conductive elastomer held in a thermal gap between the heat sink and the microprocessor. The elastomer is held in compression between the heat sink and the microprocessor to provide a good thermal conduction path that will last for a long period. 
   In such an approach, the distance between the upper surface of the printed circuit board and the upper surface of the microprocessor package may vary from unit-to-unit because of manufacturing tolerances. The spring device provides enough free play to accommodate such changes. 
   SUMMARY 
   In implementations of the invention, a heat dissipating element (e.g., a heat sink) is held in an initial position closer to a heat generating structure (e.g., a microprocessor) and in a subsequent position farther from the heat generating structure. A thermal interface material (e.g., a thermal grease) spans the gap, but is not held under compression, between the heat sink and the microprocessor. 
   Because no compression is applied to the thermal interface material, soft materials that have little or no strength to resist compressive forces may be used as the thermal interface. The thermally conductive path remains good over a long period. Variations in the thermal gap are accommodated without imposing a compressive force on the thermal interface material. Tight control over the size of the thermal gap is maintained. 
   Other advantages and features will become apparent from the following description and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic side view of a mounted microprocessor in accordance with an embodiment of the invention. 
       FIGS. 2 and 3  are cross-sectional schematic side views in more detail, showing two stages in manufacturing. 
       FIG. 4  is a flow chart. 
   

   DESCRIPTION 
   As seen in  FIG. 1 , a microprocessor  10  (or other heat generating device) is held in a socket  12  that is mounted on a printed circuit board  14  (e.g., a computer circuit board such as a motherboard) using a ball grid array  16 . A metal block or plate forming a heat dissipating element (e.g., a heat sink  18 ) is attached to the board  14  using supports  27 ,  29  and is held in a position that defines a thermal gap  20  between the bottom surface  22  of the heat sink and the top surface  24  of the microprocessor  10 . (Although only two supports are shown in the figure, there are actually three or more supports arranged in a triangle or a rectangle.) A thermal grease, gel, or other soft, highly thermally conductive material  26  spans the gap  20  and defines a heat conducting path  28  from the microprocessor to the heat sink. The grease is not held under compression so it remains in place and provides a good thermal path  28  over a long period without begin squeezed from the gap. 
   The height  30  from the top surface  31  of the board to the top surface  24  of the microprocessor varies from unit to unit because of manufacturing variations. It is desirable for the gaps  20  in different units to be of the same height despite of the differences in the heights  30 , in order to provide a consistent adequate thermal path. 
   As seen in  FIG. 2 , the gap  20  is kept at a desired fixed height by an arrangement in which the four supports (pins in this case)  27 ,  29  are securely attached to the heat sink at their upper ends, pass through four corresponding holes  45 ,  46  in the printed circuit board, and are securely attached to the board by locking clips  42 ,  44  held on the lower surface of the board. High-stiffness coiled spacer springs  52 ,  54  surround the pins. 
   Referring to FIG.  3  and to the flow chart of  FIG. 4 , during manufacturing, after the microprocessor has been inserted into the mounted socket, a quantity of the thermally conductive material  26  is placed ( 70 ) on the top surface of the microprocessor or on the bottom surface of the heat sink. The heat sink is then attached ( 72 ) by lowering it over the microprocessor and the pins are inserted through the holes and into the locking clips. The heat sink is pushed down (forcing the pins further into the locking clips) until the bottom surface of the heat sink contacts the top surface. The thermal grease is squeezed from the gap as indicated by arrows  50  but remains temporarily in the vicinity of the perimeter of the gap. 
   Next, referring again to  FIG. 2 , when the downward force on the heat sink is released, the springs force the heat sink upward. The lower ends of the pins are grabbed by the sharp edges of the holes in the locking clips and, as the heat sink moves upward ( 74 ), the pins pull up on the locking clips, distorting them from their original conical shapes to flat shapes that define jammed positions. The vertical distance traveled from the lowest position of the heat sink to the position at which the clips are jammed is relatively constant from unit to unit and defines the gap. As the heat sink is pushed upward, the thermal grease is drawn back ( 76 ) into the gap by viscous forces and suction as indicated by arrows  60 , to fill the gap and provide the desired thermal path. In this final position, no compressive force is applied to thermal grease, which therefore is able to remain in place and provide a good thermal path over a long period of time. 
   In particular implementations, the microprocessor could be a Pentium® II processor or Pentium® III processor or other microprocessor, for example, of a kind that is surface mounted for use in notebook computers. 
   Other arrangements can be used to attach the heat sink to the board, including a leaf spring or tab, a screw spring combination, an external heat sink like clip with a spring between the heat sink and that board.