Abstract:
To achieve optimal thermal contact between opposing surfaces, it is necessary to align such surfaces so that maximum contact is achieved. In a semiconductor package, it is necessary to align the surface of a semiconductor integrated circuit (IC) and a heat sink surface, where the heat sink contains a nano-composite wire structure. By using a self-aligned structure that forces the alignment of the IC surface and the heat sink, maximum thermal contact between the two surfaces is achieved. The self-alignment of a pressure measurement device for same is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to the alignment of surfaces for the purpose of achieving optimal thermal contact. More particularly, the invention relates to the self-alignment of such surfaces, where one of the surfaces is a nano-composite wire structure.  
         [0003]     2. Discussion of the Prior Art  
         [0004]     The transfer of heat, to remove heat from a hot surface to the environment, usually for dissipation purposes, may require a mechanism having a sophisticated configuration to ensure that a sufficient amount of heat is dissipated to ensure proper operation of an underlying device. The efficient dissipation of heat from a semiconductor integrated circuit (IC) is of particular interest because as a significant amount of heat is generated from a relatively small surface. This heat must be dissipated to the environment to avoid adverse effects on the semiconductor chip. Such adverse effects include, but are not limited to, complete damage, rendering the device non-functional or destroyed. Therefore, heat dissipation technology has been developed to allow for such heat to dissipate by a variety of types of heat sinks. However, the need to dissipate ever increasing amounts of heat is rapidly growing as ICs increase in operational frequency and size.  
         [0005]     One of the challenges involved in heat dissipation is the transfer of heat between two heat conducting surfaces. It is well-known in the art that heat transfer between two plates that are pressed against each other, and that are generally somewhat flat and smooth, occurs in a small contact area, perhaps a few percentage points of the total surface area of the opposing plates. This small area represents the effective area available for thermal transfer. The actual effective contact area depends on characteristics of both surfaces and the material from which the respective surfaces are composed. Among others, effective area depends on hardness, roughness, flatness, pressure, parallelism, surface convexity, and more. In the case of solid-to-solid thermal contact, a small contact area presents a serious problem that prevents effective heat transfer between e.g. two plates which are often of dissimilar materials with different thermal conductivity properties.  
         [0006]     To obtain maximum thermal transfer between two opposing surface plates it is necessary to maximize the total area of contact between the two surfaces. Typically the plates are of a hard and non bendable material, made of a crystal or a hard metal, for example, silicon, copper, aluminum, and the like. The plates are pressed against each other by applying a pressure in. the range of, for example, 30-70 pounds per square inch (psi). In a typical case, less than the possible maximum area for thermal contact is obtained. This occurs typically if the opposing surfaces have less than perfect degrees of parallelism, flatness, or micro-roughness, as well as the impact of surface convexity or concavity. To overcome such problems, the existing art of thermal interface contact design teaches the use of grease, such as a thermal interface material (TIM), to achieve a larger thermal contact area. The bond line thickness is in such cases a key parameter to be controlled.  
         [0007]     An existing design  100  is shown in  FIG. 1 , where a heat sink  110  is screwed onto, for example, a printed circuit board (PCB)  150 . The bond line thickness of the thermal interface material  130  is controlled by counting the turns of screws  120 . Notably, such application of pressure is generally very imprecise, causing the bond line thickness to vary at various positions, thereby impacting the thermal conductivity between the heat sink  110  surface and the heat dissipating surface  140 . The area of contact between the heat sink  110  surface and the heat dissipating surface  140  is also shown enlarged in  FIG. 1  for illustration purposes. For a thermal interface contact application that uses a solid TIM, a decrease of the effective contact area is observed when the pressure is applied unevenly, resulting in an increase in the thermal interface resistance.  
         [0008]     In modern heat sink technology use is made of a thermal interface that comprises a carbon nano-tube array (CNTA) or similar material. In such a case, if the surfaces of the CNTA and the heat dissipating surface are not parallel, the heat conducting advantages of the CNTA rapidly decrease. Furthermore, the CNTA can be severely damaged by the force that applies the pressure between the surfaces.  
         [0009]     In view of the limitations of prior-art solutions, it would be advantageous to provide a mechanism for solving the lack of parallelism between surfaces intended for the conduction of heat. It would be further advantageous if such solution is a self-adjusting mechanism that maximizes the surface area contact when two surfaces are brought together for the purpose of thermal transfer from one surface to the other. It would be further advantageous if such a solution would avoid the damages that occur to CNTAs when put in contact between such surfaces.  
       SUMMARY OF THE INVENTION  
       [0010]     To achieve optimal thermal contact between opposing surfaces, it is necessary to align such surfaces so that maximum contact is achieved. In a semiconductor package, it is necessary to align the surface of a semiconductor integrated circuit (IC) and a heat sink surface, and specifically where the heat sink contains a nano-composite wire structure. By using a self-aligned structure that forces the alignment of the IC surface and the heat sink, maximum thermal contact between the two surfaces is achieved. The self-alignment of a pressure measurement device for same is also disclosed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic diagram showing a heat sink being connected to a hot surface mounted on top of a PCB;  
         [0012]      FIG. 2  is a schematic diagram showing a structure for self-adjustment of a heat dissipation surface and a hot surface to ensure a high degree of parallelism between the surfaces;  
         [0013]      FIGS. 3A-3E  show the steps for mounting a heat sink surface on top of a hot surface using the structure for self-adjustment in accordance with the invention; and  
         [0014]      FIG. 4  is a schematic diagram showing a device for self-adjustment of a load cell to a first surface of a structure designed in accordance with the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The invention disclosed herein comprises a self adjusting method and apparatus for providing maximum surface area contact when two surfaces are brought together for purposes of enabling thermal transfer from one surface to the other surface. In one preferred embodiment of the invention, one of the surfaces is the back of a semiconductor integrated circuit (IC). The other surface comprises an array of wire-like nano-structures. Such nano-structures may include, but are not limited to, nano-tubes with uneven lengths which touch and bend when compressed against another surface. An advantage of the self-adjusting apparatus is that both surfaces can have a degree of non-parallelism because the self-adjusting structure compensates for such defect. In the invention, the surfaces self-adjust to the maximum possible degree of parallelism when they are pressed against each other with a prescribed pressure.  
         [0016]      FIG. 2  is a schematic diagram showing a structure  200  for self-adjustment of a heat dissipation surface and a hot surface to ensure a high degree of parallelism between the surfaces. The structure  200  comprises a plurality of rods  230 , for example four rods, a mounting structure  210  and a spring  220 . The rods  230  and the mounting structure  210  create a supporting platform for the spring  220 . The spring  220  is connected to the tip of a top screw  250 , allowing it to pivot at essentially a single point. When the spring  220  is put in contact with a heat sink surface, the tightening of the top screw  250  initiates a self-adjustment process, as described in more detail below with respect to  FIG. 3 .  
         [0017]     A person skilled-in-the-art would note that the structure capable of applying pressure on a first surface, for example the heat dissipation surface, by means of a spring, for example the spring  220 , may be accomplished by different designs of the spring  220  that is mounted at a center point and enabled to apply pressure onto the first surface by means of tightening of a single screw, thereby enabling the self adjustment of the first surface to a second surface, for example, a hot surface. Hence, the spring  220  may have a form of a plurality of prongs, a disk, and the like, all being connected to the structure via a single screw essentially centered in respect of the plurality of rods  230  of the structure  200 . In a preferred embodiment of the invention, there are at least two rods  230 . The mounting structure  210  may be formed from a plurality of prongs, or fingers, for example four, connected at one point, as shown in  FIG. 2 . In another embodiment of the invention, the mounting structure  210  may be a plate of any kind of desired shape. In another embodiment, the spring is designed to conform with the features of the rods  230 . For example, in the case of a single dimension ( 1 D) where only two rods  230  are used, an essentially single dimension spring  220  is used, where each prong extends top screw towards its respective rod  230 . In yet another embodiment of the invention, a plurality of springs  220  may be connected to a single mounting structure  210 . In such a case, the pressure applied by each of the plurality of springs should be essentially equal to ensure the self-alignment properties of the disclosed invention.  
         [0018]      FIGS. 3A-3E  show steps  310  through  350  for mounting a heat sink surface on top of a hot surface using the structure for self-adjustment in accordance with the invention. Specifically, the attachment method is intended to affix structure  200  and a heat sink  270  onto a PCB  240 . As a result of applying pressure on the spring  220  by means of central screw  250 , to cause the self-adjustment of heat sink  270  with the hot surface  260 . The hot surface  260  may be but is not limited to, the hot surface of a semiconductor IC. The invention achieves the best possible parallelism between the contact surfaces, maximizing contact area, avoiding damage to the CNTAs of the heat sink  270  during the initial contact, and causing the CNTAs to perform in the buckling mode. A detailed discussion of the buckling mode may be found in E. Suhir U.S. patent application Ser. No. 11/207,096 titled An Apparatus and Test Device for the Application and Measurement of Prescribed, Predicted and Controlled Contact Pressure on Wires, assigned to a common assignee (the “&#39;096 patent application”), and which is herein incorporated in its entirety by this reference thereto.  
         [0019]     The construction of the structure  200  begins with step  310  where the rods  230  are connected to the top plate  210 . In step  320 , the spring  220  is attached to the top plate  210  by means of, for example, a screw  250 , also referred to herein as the top screw. The spring  220  is mounted to the top plate  210 , such that the spring  210  can pivot, allowing the spring to tilt as may be necessary as it comes into contact with the heat sink  270  (discussed further below). In step  330 , the structure  200  is mounted to the PCB  240  by means of the rods  230 . Preferably, the structure  200  is position above a hot surface to which a heat sink  270  is to be attached in accordance with the invention. In step  340 , the heat sink  270  is inserted between the spring  220  and the hot surface  260 , while the hot surface  260  may be the hot surface of a semiconductor IC. As shown in  FIG. 3C , the steps  330  and onwards, it is possible, and quite common, that the hot surface  260  and the heat sink  270  are not aligned. In step  350 , the top screw  250  is tightened for the purpose of causing the self-adjustment. The spring  220  spreads the pressure applied by the top screw  250  but, because of its spring properties, adjusts so that the pressure causes the heat sink  270  to self-adjust with respect to the hot surface  260 . The top screw  250  is securely tightened to provide the required compressive force to the spring  220 . The application of this force completes the process of self-adjustment, and in the case of the CNTAs, is adjusted to a value that causes the necessary buckling of the nano-tubes, in accordance with the teaching of the &#39;096 patent application.  
         [0020]     While the apparatus for the self-adjusting of a first surface to a second surface is described in detail with respect of the self-adjustment of a heat sink to a hot surface of a semiconductor IC, this should not be viewed as a limitation on the general scope of the invention, and it is specifically noted that other implementations required self-adjustment of a first and second surface using a structure essentially in the spirit disclosed herein are specifically envisioned as part of the invention. It should be further noted that multiple springs  220  may be placed on the mounting structure  210 . In yet another embodiment of the invention, multiple structures  200  may be used in conjunction with a single heat sink  270 .  
         [0021]      FIG. 4  shows a schematic diagram of a device  400  that causes the self-adjustment of a load cell  440  to a first surface  430  in accordance with the invention. A test pressure device  440  may be, for example, a miniature industrial load cell, such as those provided in the LCDG series by Omega Engineering, Inc, the specification sheets of which are hereby incorporated by reference. The structure  400  comprises an upper plate  410 , tightening screws  420 , a CNTA  430 , a load cell  440 , and a lower plate  450 . The screws  420  are enabled to tighten the upper plate  410  towards the lower plate  450 , with the CNTA  430  and test-pressure device  440  sandwiched in between the upper plate  410  and the lower plate  450 . The tightening screws  420  establish and maintain a fully parallel contact. The CNTA  430 , typically a sample to be tested for the pressure to be applied to achieve the desired level of buckling, is glued onto load cell  440 . In the preferred embodiment of the invention the load cell  440  comprises a rounded bottom  445 . Upon application of pressure by the tightening of the screws  420 , the rounded bottom  445  of the load cell  440  causes the self-adjustment required to ensure the necessary parallelism between the CNTA  430  and the upper plate  410 .  
         [0022]     Notably, to achieve the best thermal performance, the following conditions are to be met: the top plate  410  should be parallel to the sample surface, for example the CNTA  430 ; the top plate  410  should not crush, or otherwise damage the carbon nano-tubes of the CNTA  430  when the upper plate  410  comes into initial contact with the CNTA  430 ; and, the CNTA  430  should be in buckling mode. Therefore the first step in the assembly process of the structure  400  is to establish an initial contact between the top plate  410  and the CNTA  430 . The top plate  410  is typically held on a micro-stage that can be moving on a micro scale in the vertical direction. Pressure is measured in real time through a connection from the load cell  440  to an appropriate reading device (not shown). The top plate  410  is then lowered downwards under the control of, for example, the micro-stage (not shown). As soon as the top plate  410  comes into contact with the CNTA  430 , the load cell  440  starts to self-adjust, in accordance with the principles explained above, i.e. due to the round bottom  445  characteristics of the load cell  440 .  
         [0023]     Once contact is established the second step starts when the pressure reaches a desired level, for example 5 psi. At this stage, the load cell  440  balances itself, and the CNTA  430  is in maximum contact with the top plate  410 . This step is intended to transfer the pressure to the CNTA  430 , being the sample to be measure. The pressure is transferred from the micro-stage to the screws  420  and respective springs. By gently tightening the screws  420  and releasing the micro-stage, the load is gradually transferred from the micro-stage to the screws  420  and their respective spring sets, while maintaining full contact between the CNTA  430  and the top plate  410 . In the third step, the required pressure is adjusted by further tightening the screws  420 . As a result, the respective springs are compressed to an extent that provides the pressure designated for a specific load experiment.  
         [0024]     Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.