Abstract:
Shock absorbers or other dampening mechanisms are added to an assembly including a heatsink, a semiconductor device, and a board, to reduce shock and/or vibration induced relative motion between the heatsink and the board.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to mounting of heatsinks and the like to semiconductor devices, motherboards, and the like. More specifically, it relates to dampening measures for such mounting. 
     2. Background Art 
     FIG. 1 illustrates a known technique for mounting a heatsink  10  to a chip assembly on a motherboard  12 . The chip assembly may, for example, include a microelectronic die  14  (such as a flip-chip die) connected by bumps  16  to a card  18  which is connected by solder balls  20  to the motherboard. Typically, thermal grease  22  is used to provide good thermal mating of the die and the heatsink. 
     In applications where the heatsink is quite heavy, springs  24  are used to support the weight of the heatsink, taking the weight off of the die and card. The heatsink is attached, and the springs are drawn into compression, by bolts  26  and nuts  28 . 
     While the springs may do an adequate job of supporting the heatsink under stationary conditions, they have been found inadequate in applications where the assembly is subject to significant shock or vibration. 
     FIG. 2 illustrates one problem that exists in this prior art. When subjected to shock or vibration, the heatsink travels downward, further compressing the spring. If the shock or vibration is severe enough in amplitude or duration, the heatsink may eventually impact the die with sufficient force to break the die or at least some of the interconnects. In the prior art, the solution has been to use stiffer springs in such applications, to reduce the tendency of the heatsink to impact the die. Unfortunately, this introduces another problem. 
     FIG. 3 illustrates what can happen if the springs are too strong and/or are drawn down with excessive preload and/or if the heatsink is subjected to shock or vibration with a significant upward component. In severe cases, the motherboard itself may fracture or suffer failure of its electrical traces or other devices. Even if the springs are not the problem, the motherboard may fail on its own, due to vibration or oscillation. 
     It is desirable to dampen the movement of the heatsink and motherboard relative to each other and relative to the other components of the assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
     FIG. 1 shows, in cross-section, a heatsink attach system according to the prior art, in which springs are used to support the heatsink. 
     FIG. 2 shows, in cross-section, the heatsink breaking the semiconductor die under shock or vibration. 
     FIG. 3 shows, in cross-section, the motherboard fracturing under shock or vibration or excessive spring preload. 
     FIG. 4 shows, in cross-section, one embodiment of the invention, in which shock absorbers are added to the assembly to dampen shock and vibration. 
     FIG. 5 shows, in cross-section, another embodiment in which the springs and shock absorbers are coaxially mounted. 
     FIG. 6 shows, in cross-section, another embodiment in which the springs are under tension rather than compression, obviating the need for hold-down bolts. 
     FIG. 7 shows, in cross-section, another embodiment in which the springs are integrated within the shock absorbers. 
     FIG. 8 shows, in cross-section, one embodiment of a shock absorber which is adapted for being affixed to the heatsink and motherboard. 
     FIG. 9 shows, in cross-section, another embodiment of a shock absorber. 
     FIG.10 shows, in cross-section, another embodiment of a shock absorber, with an integral spring. 
     FIG. 11 shows, in top view, one embodiment of a placement of the shock absorbers. 
     FIGS. 12 and 13 show, in top view, one embodiment of an attachment mechanism for the shock absorber. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are shown in this patent in an application involving a flip-chip die assembly, but the skilled reader will appreciate that the invention is not limited to such applications. 
     FIG. 4 illustrates one embodiment of the invention, in which the shock absorbers  30  are mounted in parallel with the springs, bolts, or other heatsink supporting means. In some such embodiments, it may be desirable to place the shock absorbers as near the springs and hold-down bolts as possible. In others, it may be desirable to place the shock absorbers elsewhere. For example, in some applications it may be desirable to place the shock absorbers as near the ends of the heatsink as practicable, to maximize their effect in preventing the heatsink from levering about a fulcrum (the die). 
     FIG. 5 illustrates another embodiment of the invention, in which the shock absorbers are mounted serially, or coaxially, with the springs. In some such embodiments, the hold-down bolts may be omitted, and the shock absorbers may perform the hold-down function. 
     FIG. 6 illustrates yet another embodiment, in which the springs perform the hold-down function. In some such embodiments, the springs may under tension rather than compression. In such embodiments, the die will bear not only the weight of the heatsink, but also the tension of the springs. Thus, this embodiment may not be suitable for all applications. The reader will appreciate that the springs could readily be mounted coaxially with the shock absorbers. 
     FIG. 7 illustrates yet another embodiment, utilizing shock absorbers which have integral springs. In various such embodiments, the springs may be under compression or tension. If under tension, the springs will, of course, need to be affixed to the shock absorbers rather than merely disposed within them in a free-floating manner. 
     FIG. 8 illustrates details of one exemplary shock absorber, such as those shown in FIG.  4 . The shock absorber includes a cylinder component  40  and a piston component  42 . The piston moves axially within the cylinder. The piston component  42  of the shock absorber includes an operative piston segment  44  sized appropriately to fit within an operative cylinder segment  46  of the cylinder component  40 . In some embodiments, the dampening orifice may comprise a gap between the walls of the cylinder and the piston. In other embodiments, the dampening orifice may comprise one or more holes (not shown) through the piston or cylinder. In some embodiments, it may be desirable to employ pneumatic dampening, while in others it may be desirable to utilize hydraulic dampening. The choice of dampening mechanism, the selection of orifice sizes, fluid viscosities, and such are application dependent, and within the abilities of an ordinary skilled workman. In some embodiments, it may even be acceptable to use a frictional dampening mechanism, such as one in which two or more parts rub against each other to dampen motion in the direction of their overall assembly length. In some embodiments, it may be suitable to use a shock absorber with no moving parts, such as a rubber or plastic foam having a suitable “memory speed” and “memory pressure”. The reader should appreciate that the various drawings, while illustrating piston and cylinder style shock absorbers, may also be interpreted as teaching the use of such foam or frictional dampening mechanisms. 
     FIG. 9 illustrates another embodiment of the shock absorber, in which the end  68  of the cylinder component and the end  70  of the piston component are threaded. In this embodiment, they may be retained to the heatsink and the motherboard by threaded nuts (not shown). In some applications, it may be suitable to thread the piston or cylinder component directly to the heatsink. 
     Other retention mechanisms will be appreciated by the reader as being within the grasp of those of ordinary skill in this field. For example, the piston and/or cylinder components might include integral bolt heads. Or, they may be welded, glued, or otherwise affixed. FIG. 5 also illustrates that it is not necessarily required that the piston and the cylinder be affixed with the same mechanism. 
     FIG. 9 also illustrates what was mentioned previously with reference to FIG. 5, in that in some embodiments the shock absorber itself may provide the hold-down functionality that was done by separate bolts in the prior art and in other embodiments. In this embodiment, the piston and cylinder will, when installed, be topped out against one another at the inner lip of the end of the cylinder and the bottom edge of the piston, denoted as location  72 . This configuration may be utilized in drawing the heatsink support spring into preload, as shown in FIG.  5 . 
     FIG. 10 illustrates another embodiment of the shock absorber, such as that used in FIG.  7 . In this embodiment, the shock absorber includes the spring  74  within the cylinder chamber. In applications where the spring is to be under tension, the spring is affixed to the cylinder portion  40  and the piston portion  42  of the shock absorber. 
     In some applications, it may be desirable to utilize both the main support springs external to the shock absorber, and also the internal shock absorber springs  74 . 
     FIG. 11 illustrates that, in some embodiments, it is not necessarily desirable that the shock absorbers and the hold-down bolts be located near each other. FIG. 11 illustrates one such embodiment, in which the hold-down bolts or other such mechanism, denoted by the circles labeled B, are located at the corners of the heatsink, while the shock absorbers, denoted by the circles labeled S, are located in the middle of each side. The small squares stylistically represent multiple fins on the heatsink. The motherboard  12  is shown, and the outline of the chip  14  is shown as a dotted line. 
     FIGS. 12 and 13 illustrate one embodiment of an attachment mechanism for the shock absorbers, such as utilized in the applications shown in FIGS. 4,  6 , and  7 . Please refer to FIGS. 8,  12 , and  13 . The piston component includes an end cap  48 , a middle cap  50 , and a segment  52  having a diameter and length suitable for engaging a keyed slot  54  on the motherboard. The keyed slot includes a portion  56  sized sufficiently large to pass the end cap  48 . The piston is inserted through the motherboard until the segment  52  is aligned with the motherboard, then the piston is slid into the keyhole, where the end cap and middle cap will mechanically grip the two sides of the motherboard, preventing axial movement of the piston relative to the motherboard, and the segment  52  will prevent lateral movement. The piston may be retained in this position by any suitable mechanism (not shown). For example, the end portion of the slot could be sized slightly larger than the rest of the slot, so the piston would be retained in a snap fit. 
     Similarly, in this embodiment, the cylinder portion  40  includes an end cap  58  and a middle cap  60  with a segment  62  between them, and the cylinder portion may be fitted to the heatsink in the same manner as the piston is fitted to the motherboard. The segment  64  of the piston component between the piston and the middle cap, and the segment  66  of the cylinder component between the cylinder and the middle cap, may be of any suitable length and diameter. 
     The reader will appreciate that the positions of the piston and cylinder may be reversed, in some embodiments. The reader will further appreciate that the cylinder end may be open, to permit removal of the piston component, or it may be substantially closed, to prevent removal of the piston component. 
     The reader will appreciate that the invention may readily be used in applications where the microelectronic die is coupled directly to the motherboard, or those employing an interposer, or those in which the die is socketed, and so forth. 
     The skilled reader will appreciate that the utilization of this invention may permit the removal or reduction of board stiffeners on the motherboard, the use of larger form factor and higher mass heat sinks, and/or thinner motherboards. The skilled reader will further appreciate that the utilization of this invention may also enable the heatsink attachment to withstand more extreme usage environments that result in higher shock or vibration. 
     The reader will appreciate that the term “motherboard” should not be interpreted as meaning only the primary or main board of an electronic system, but that this invention may be utilized in conjunction with a wide variety of boards and the like. The skilled reader will also appreciate that the term “shock absorber” refers to any suitable dampening mechanism, and is not limited to the coaxial piston and cylinder embodiment illustrated above by way of simplicity and teaching. 
     Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
     If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.