Abstract:
A bottom loaded assembly for securing heat sinks to printed circuit boards may use a preloaded spring. The preloaded spring may be positioned on one side of the printed circuit board and the heat sink may be positioned on the opposite side.

Description:
BACKGROUND  
       [0001]     This invention relates generally to securing heat sinks to printed circuit boards.  
         [0002]     An integrated circuit may develop sufficient heat during operation that it needs to be cooled. To this end, finned heat sinks may be secured to integrated circuits for cooling purposes. Because the heat sinks tend to be large, they may be mounted directly to the printed circuit board on which the integrated circuit is also mounted.  
         [0003]     The performance of integrated circuits, such as processors, is increasing. As their performance increases, the amount of heat integrated circuits generate may increase and this may result in the need for heat sinks of increasing size. Conventionally, a heat sink is mounted on the motherboard and the enabling load is applied from the heat sink side towards the chassis in what is called “top loading.” 
         [0004]     In a bottom loading design, the heat sink is mounted on the printed circuit board and the enabling load is applied from below, from the chassis toward the heat sink.  
         [0005]     One problem with bottom loading designs is that the printed circuit board may be bent upwardly by the enabling load. Excessive board deflection may be undesirable because it may result in solder joint cracking between board and board mounted components. In addition, in some designs, there may be relatively limited room between the printed circuit board and the chassis on which the printed circuit board is mounted. The bottom loading configuration must fit into whatever available space is provided.  
         [0006]     Thus, there is a need for improved, bottom loading heat sink attachment solutions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an enlarged, cross-sectional view of one embodiment of the present invention;  
         [0008]      FIG. 2  is a perspective view of a preloaded spring assembly in accordance with one embodiment of the present invention;  
         [0009]      FIG. 3  is a cross-sectional view taken generally along the line  3 - 3  in  FIG. 2  in accordance with one embodiment of the present invention; and  
         [0010]      FIG. 4  is an enlarged, top plan view of the embodiment shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0011]     Referring to  FIG. 1 , an electronic device  10  may include a printed circuit board  30 . The printed circuit board  30  may be a motherboard that receives a processor. A socket  22  couples an integrated circuit  23  electrically and mechanically to the printed circuit board  30 . A heat spreader  16  may distribute heat from the integrated circuit  23  to the heat sink base  12  and, ultimately, to heat sink fins  14  to reduce the temperature of the integrated circuit  23 .  
         [0012]     The heat sink base  12  may be secured to the printed circuit board  30  by pins  18 . The pins  18  may be secured above the heat sink base  12  by way of threaded fasteners  20 . The fasteners  20  may be threaded downwardly from above. The pins  18  may pass freely through the board  30  and are secured to a spring assembly  26  below the board  30 .  
         [0013]     The enabling load is applied to the heat sink base  12  via the loaded spring assembly  26  positioned underneath the printed circuit board  30 . The assembly  26  may include a plurality of cantilevered leaf spring arms  34  mounted on a base  36 .  
         [0014]     In one embodiment, the spring assembly  26  may be formed of two stamped metal sheets. The sheets may be made of different materials. For example, the spring arms  34  may be made of more resilient metal and the base  36  may be made of more rigid material.  
         [0015]     Referring to  FIG. 2 , the preloaded spring assembly  26  may include a plurality of cantilevered spring arms  34  that are mounted onto a base  36 . The base  36  may include a generally rectangular configuration, punctuated by openings  38  to receive the pins  18 .  
         [0016]     A cross bar  54  may extend across the center, in the length direction, of the base  36 . The cross bar  54  may have upwardly offset or raised sections  44  near each edge. Each raised section  44  may have opposed, outwardly extending prongs  42 . The prongs  42  guide the up and down movement of the spring arms  34  in one embodiment. In particular, the spring arms  34  have a pair of downwardly directed spaced apart fingers  48 . A prong  42  extends between the fingers  48  to prevent side-to-side displacement of the deflected spring arms  34 .  
         [0017]     Referring to  FIG. 3 , the spring arms  34  may be formed of a separate metal sheet secured to the base  36  by wrapping portions  50 , each integral with a spring arm  34 , about the base  36 . The portions  50  may then be crimped, soldered, or adhesively secured to the base  36 , to mention a few examples. A tab  60  on the base  36  may extend through an opening  62  in each sheet  64  to align the base  36  with the spring arms  34 . The apex  46  of a spring arm  34  may include the downwardly extending fingers  48 . As shown in  FIG. 3 , each finger  48  may be guided by a prong  42  of the raised section  44  of the base  36 . Each spring arm  34  may also have a turned away end  28 .  
         [0018]     Moving to  FIG. 4 , the engagement between the fingers  48  and the prongs  42  is indicated for one embodiment. Thus, in  FIG. 4 , the spring arms  34  move up and down, but side-to-side motion is prevented by the prongs  42  which guide the motion of the cantilevered leaf spring arms  34  and, particularly, their fingers  48 .  
         [0019]     The spring arms  34  are preloaded. The preloading is achieved by spacing those spring arms from the board  30  by a distance less than the vertical extent of the arms  30  in their tree state. If the arms were released, they would spring upwardly, in one embodiment until their upturned ends  28  extended above the raised section  44 . Instead, the spring arms  34  are pre-stressed or preloaded so that they have advantageous characteristics in operation.  
         [0020]     It is desirable that the spring assembly  26  fit between the printed circuit board  30  and a chassis  24 , in one embodiment, with a gap as small as five millimeters or less to provide the desired bottom loading. That bottom loading may be as high as 150 pounds, in some embodiments, to compress the integrated circuit  23  and socket  22  against the heat sink base  12 . The board  30  deflection may be limited as much as possible during assembly to prevent failures such as via or solder ball cracking. The need to position the spring assembly  26  in a small space and limit board deflection suggests that the spring arms  34  be relatively stiff.  
         [0021]     On the other hand, it is also desirable to reduce the load variation under all dimensional stack up conditions. Regardless of spring installed height variation, the spring load advantageously is as close to its designed value as possible. Excessive load may cause package failure, while an insufficient load does not guarantee full mating with socket contacts. In addition, compression contact sockets, such as land grid array sockets, require that the load be maintained throughout the life of the product. Loss of load due to plastic components and solder ball failure should be reduced if possible. Thus, the need to account for spring installed height variations and to maintain contact over the life of the spring, suggests that the spring arms  34  should be made as soft as possible.  
         [0022]     These conflicting goals can be accommodated by using relative soft spring arms and then preloading the spring arms  34  to a load level that is close to the design load value. For example, if the design load value is 150 lbs, the spring arms can be preloaded to 140 lbs. Upon assembly the printed circuit board  30  only needs to deflect the spring arms by a small displacement which corresponds to the difference between the design load and the preload (which is 10 lbf difference in the example above).  
         [0023]     A non-preloaded spring may exhibit a wide variation of load over the thickness tolerance stack-up. The preloaded spring may maintain a minimum load, have less load variation due to thickness tolerance stack, and require minimum board deflection during installation, in some embodiments.  
         [0024]     In another embodiment, the base  36  may be part of the chassis  24 . Then the spring arms  34  can be preloaded directly onto the chassis  24 .  
         [0025]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.