Patent Publication Number: US-6219905-B1

Title: Heat sink clip tool

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the packaging of electronic components in electronic devices. More particularly, the present invention relates to a tool for securing a heat sink with a heat sink retainer. 
     BACKGROUND OF THE INVENTION 
     As the art moves towards smaller higher power integrated circuits such as SRAMs, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. As used herein, the term “IC package” includes the heat generating integrated circuit as well as the packaging surrounding the integrated circuit. 
     One conventional technique to remove heat from an IC package is to employ a finned heat sink which is placed in thermal contact with the IC package. In this manner, heat generated by the IC package is conducted to the heat sink and then dissipated to the ambient environment. 
     Of importance, the heat sink must be reliably attached to the IC package in a manner which does not undesirably stress or damage the IC package or the circuit board to which the IC package is connected. One conventional technique is to employ a thermally conductive adhesive which bonds the heat sink to the IC package. However, thermally conductive adhesives do not adhere well to plastic IC packages resulting in an unacceptably high incidence of bond failure between the plastic IC package and the heat sink. Further, once the heat sink is bonded with the thermally conductive adhesive, it is difficult to remove the heat sink from the IC package without causing damage to the IC package, the heat sink or the circuit board. Yet, it is desirable to have a removable heat sink to readily allow chip repair, rework and/or replacement. Accordingly, it is desirable to avoid the use of thermally conductive adhesives altogether. 
     One prior art removable heat sink uses clips and/or fasteners to attach the heat sink directly to the IC package. However, when attached in this manner, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package. To avoid this problem, other removable heat sinks are attached directly to the circuit board to which the IC package is connected. 
     FIG. 1 is a side view of an electronic device  8  which includes a heat sink  10  directly attached by post type fastening members  12  A to a circuit board  14  in accordance with the prior art. Located between heat sink  10  and circuit board  14  is an IC package  16  which generates heat during use. IC package  16  is typically electrically connected to circuit board  14  by one or more circuit interconnections, e.g., solder, which are not illustrated in FIG. 1 for purposes of clarity. Fastening members  12  A urge heat sink  10  towards circuit board  14  and down onto IC package  16  to make the thermal contact between heat sink  10  and IC package  16 . 
     Although providing the force necessary to make the thermal contact between heat sink  10  and IC package  16 , fastening members  12  A cause heat sink  10  to press unevenly on IC package  16 . In particular, IC package  16  acts as a pivot between heat sink  10  and circuit board  14  so that end  10 A of heat sink  10  is urged away from end  14 A of circuit board  14  as indicated by arrows  18 . This causes the force exerted by heat sink  10  on to IC package  16  to be greater at side  16 A of IC package  16  than at side  16 B. This uneven force distribution can damage and even crack IC package  16 . Further, this uneven force distribution can create a gap between side  16 B and heat sink  10  resulting in poor heat transfer between IC package  16  and heat sink  10 . Alternatively, or in addition to, this uneven force distribution can cause circuit interconnection failure near side  16 B of IC package  16 . As those skilled in the art understand, these conditions can ultimately cause failure of device  8 . 
     To avoid these drawbacks, it has become known in the art to attach both sides of heat sink  10  to circuit board  14 . As an example, second post type fastening members  12 B illustrated in dashed lines in FIG. 1 can be employed. This tends to equalize the force exerted by heat sink  10  on both sides  16 A and  16 B of IC package  16 . However, this also causes the ends  14 A,  14 B of circuit board  14  to be pulled up by fastening members  12 B,  12 A, respectively, relative to the die attach region  14 C of circuit board  14  to which IC package  16  is attached. This bending force, indicated by arrows  20 , causes warpage of circuit board  14  such that circuit board  14  is displaced to a position  22 . Over time, this warpage can cause device  8  to fail, e.g., from failure of circuit interconnections between IC package  16  and circuit board  14 . 
     Accordingly, the art needs a heat sink assembly which results in even force application to the IC package and avoids circuit board warpage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a heat sink assembly for an electronic device is presented. The assembly includes a retainer having a body with a first body edge and a second body edge, the second body edge being opposite and removed from the first body edge, a finger extending from the body along the first body edge and at least one leg extending from the body along the second body edge. The assembly further includes a heat sink having a base and fins extending from the base, where slits in the fins define a trench. In the finished assembly, an upper package which includes an electronic device is located between a substrate such as a circuit board and the heat sink, and the retainer holds the heat sink in place. 
     When attached to the heat sink, tension in the retainer causes the finger to apply a downward vertical force on the heat sink. The heat sink, in turn, presses downwards towards the upper package. Of importance, by locating a finger tip of the finger at the center of the heat sink, the heat sink presses down uniformly on the upper package. 
     In one embodiment, the upper package is attached to an upper surface of the circuit board, and a lower package which includes an electronic device is attached to a lower surface of the circuit board directly opposite the upper package. In accordance with this embodiment, the retainer includes two legs which are symmetric with respect to the finger, and each leg applies an equal upward force on the lower package. In this manner, a heat sink assembly in accordance with the present invention avoids the application of uneven force distribution on the upper and lower packages and the associated damage and ultimate device failure caused by use of conventional heat sink assemblies. 
     Further, the vertical upward force is transferred from the legs directly back to the heat sink through the lower package, the circuit board and the upper package without imparting any bending force on the circuit board. In this manner, a heat sink assembly in accordance with the present invention avoids circuit board warpage and the associated ultimate device failure associated with conventional heat sink assemblies. 
     In one embodiment, the body of the retainer is adjacent an edge of the circuit board so that the retainer passes around the circuit board instead of through the circuit board. The allows greater utilization of the circuit board, e.g., allows more traces and/or vias to be located on or in the circuit board, compared to the case where a retainer passes through the circuit board. 
     Also in accordance with the present invention, a method of assembling a heat sink assembly includes locating an upper package having an electronic device between a substrate such as a circuit board and a heat sink. A retainer is then positioned so that a finger of the retainer is aligned with a trench of the heat sink. The finger is then slid into the trench to secure the heat sink in place with the retainer. 
     While aligning the finger of the retainer with the trench of the heat sink, legs of the retainer are also aligned to slide around the circuit board. Thus, when the finger is slid into the trench, the legs also slide around the circuit board. Typically, the legs slide on a lower package attached to a lower surface of the circuit board, the lower package including an electronic device. 
     In some applications, installation of the retainer by the method described above may damage the assembly. For example, sliding of the legs along the lower package can damage the lower package or cause circuit interconnection failure between the lower package and the circuit board. To avoid potential damage to the assembly, the finger is spread apart from the legs. The retainer is then positioned without sliding of the retainer on the heat sink or on the lower package. The finger is then released thus securing the retainer to the heat sink. 
     In accordance with the present invention, a clip assembly tool for grasping the finger and legs of the retainer and spreading them apart from one another is presented. The tool includes a lower arm having a first clasp and an upper arm having a second clasp. The lower arm is pivotally attached to the upper arm. The tool further includes a wedge arm having a nose, where the wedge arm is pivotally attached to the upper arm. 
     A method of using the tool includes inserting the retainer between the first clasp and the second clasp, where the first and second clasps include first and second catches, respectively. The lower arm includes a first handle and the upper arm includes a second handle. The second handle is moved towards the first handle which causes the second clasp to move away from the first clasp allowing the retainer to be inserted between the first clasp and the second clasp. After the retainer is inserted, the second clasp is moved back towards the first clasp by moving the second handle away from the first handle. 
     The retainer is then wedged into the first catch and the second catch. To wedge the retainer, a third handle of the wedge arm is moved towards the second handle of the upper arm. This causes the nose of the wedge arm to press against the retainer and wedge the legs and finger of the retainer into the first and second catchs, respectively. 
     Once securely wedged, the retainer prevents the third handle of the wedge arm from being moved closer to the second handle of the upper arm. Thus, further motion of the third handle towards the first handle of the lower arm produces an equal downward motion of the second handle of the upper arm. As a result, the second clasp again moves away from the first clasp. Since the legs and the finger are securely fastened to the first and second catches, respectively, the finger is spread from the legs. 
     The finger is then inserted into the trench of the heat sink and the legs are positioned below the circuit board and the lower package attached to the lower surface of the circuit board. The tool is then operated by reversing the above described procedure to release the finger and to secure the retainer to the heat sink. 
     Of importance, the retainer is positioned and secured without sliding the finger along the heat sink and without sliding the legs along the lower package. Thus, use of the tool in accordance with the present invention avoids the possibility of damaging the heat sink assembly during installation of the retainer. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an electronic device which includes a heat sink directly attached by post type fastening members to a circuit board in accordance with the prior art. 
     FIG. 2 is a side view of an electronic device employing a heat sink assembly in accordance with the present invention. 
     FIG. 3 is a perspective view of a heat sink retainer in accordance with the present invention. 
     FIG. 4 is a side plan view of the retainer taken from the line IV of FIG.  3 . 
     FIG. 5 is a top plan view of the retainer of FIG.  3 . 
     FIG. 6 is a front plan view of the retainer of FIG.  3 . 
     FIG. 7 is a perspective view of a heat sink in accordance with the present invention. 
     FIG. 8 is a front plan view of the heat sink of FIG.  7 . 
     FIG. 9 is a plan view of the heat sink of FIG.  7 . 
     FIG. 10 is a side plan view of a clip assembly tool in accordance with the present invention. 
     FIG. 11 is a side plan view of a retainer being placed in clasps of the tool of FIG.  10 . 
     FIG. 12 is a side plan view of the retainer being secured in the clasps of the tool of FIG.  10 . 
     FIG. 13 is a side plan view of the retainer being spread by tool of FIG.  10 . 
     FIG. 14 is a top plan view of a wedge arm of the tool of FIG.  10 . 
     FIG. 15 is a side plan view of the wedge arm of FIG.  14 . 
     FIG. 16 is, a top plan view of an upper arm of the tool of FIG.  10 . 
     FIG. 17 is a side plan view of the upper arm of FIG.  16 . 
     FIG. 18 is a top plan view of a lower arm of the tool of FIG.  10 . 
     FIG. 19 is a side plan view of the lower arm of FIG.  18 . 
     In the following description, similar elements are labeled with similar reference numbers. 
    
    
     DETAILED DESCRIPTION 
     This application is related to Bollesen, co-filed U.S. patent application Ser. No. 09/385,151 herein incorporated by reference in its entirety. 
     In accordance with the present invention, a heat sink assembly  100  (FIG. 2) for an electronic device is presented. The assembly includes a retainer  118  having a body  126  with an upper (e.g., first) body edge  121 U and a lower (e.g., second) body edge  121 L, lower body edge  121 L being opposite and removed from upper body edge  121 U. A finger  120  extends from body  126  along upper body edge  121 U and at least one leg  124  extends from body  126  along lower body edge  121 L. Assembly  100  further includes a heat sink  112  having a base  115  and fins  114  extending from base  115 , where slits  136  (FIG. 7) in fins  114  define a trench  138 . Referring again to FIG. 2, IC packages  104  which include electronic devices are located between a substrate  102 , such as a circuit board, and heat sink  112 . Retainer  118  holds heat sink  112  in place. A thermal pad  116  is located between heat sink  112  and IC packages  104 . 
     When attached to heat sink  112 , tension in retainer  118  causes finger  120  to apply downward vertical force on heat sink  112 . Heat sink  112 , in turn, presses downwards towards IC packages  104 . Of importance, by locating a finger tip  122  of finger  120  at the center of heat sink  112 , heat sink  112  presses down uniformly on IC packages  104 . 
     IC packages  104  are attached to an upper surface  102 U of substrate  102 , and IC packages  105  which include electronic devices are attached to a lower surface  102 L of substrate  102  directly opposite IC packages  104 . In accordance with this embodiment, retainer  118  includes two legs  124  which are symmetric with respect to finger  120 , and each leg  124  applies an equal upward force on IC packages  105 . In this manner, a heat sink assembly in accordance with the present invention avoids the application of uneven force distribution on IC packages  104 ,  105  and the associated damage and ultimate device failure caused by use of conventional heat sink assemblies. 
     Further, the vertical upward force is transferred from legs  124  directly back to heat sink  112  through IC packages  105 , substrate  102 , IC packages  104  and thermal pad  116  without imparting any bending force or torque on substrate  102 . In this manner, a heat sink assembly in accordance with the present invention avoids substrate (e.g., circuit board) warpage and the ultimate device failure associated with conventional heat sink assemblies. 
     In one embodiment, body  126  of retainer  118  is adjacent an edge  102 E of substrate  102  so that retainer  118  passes around substrate  102  instead of through substrate  102 . The allows greater utilization of substrate  102 , e.g., allows more traces and/or vias to be located on or in substrate  102 , compared to the case where a retainer passes through substrate  102 . 
     Also in accordance with the present invention, a method of assembling a heat sink assembly includes locating IC packages  104  between substrate  102  and heat sink  112 . Retainer  118  is then positioned so that finger  120  of retainer  118  is aligned with trench  138  of heat sink  112 . Finger  120  is then slid into trench  138  to secure heat sink  112  in place with retainer  118 . 
     While aligning finger  120  of retainer  118  with trench  138  of heat sink  112 , legs  124  of retainer  118  are also aligned to slide around substrate  102 . Thus, when finger  120  is slid into trench  138 , legs  124  also slide around substrate  102 . Typically, legs  124  slide on IC packages  105 . 
     In some applications, installation of retainer  118  by the method described above may damage the assembly. For example, sliding of legs  124  along IC packages  105  can damage IC packages  105  or cause circuit interconnection failure between IC packages  105  and substrate  102 . To avoid potential damage to the assembly, finger  120  is spread apart from legs  124 . Retainer  118  is then positioned without sliding of retainer  118  on heat sink  112  or on IC packages  105 . Finger  120  is then released thus securing retainer  118  to heat sink  112 . 
     In accordance with the present invention, a clip assembly tool  200  (FIG. 10) for grasping finger  120  and legs  124  of retainer  118  and spreading them apart from one another is presented. Tool  200  includes a lower arm  210  having a first clasp  204  and an upper arm  206  having a second clasp  202 . Lower arm  210  is pivotally attached to upper arm  206 . Tool  200  further includes a wedge arm  216  having a nose  220 , where wedge arm  216  is pivotally attached to upper arm  206 . 
     Referring now to FIG. 11, a method of using tool  200  includes inserting a retainer  118 A between first clasp  204  and second clasp  202 , where first and second clasps  204 ,  202  include first and second catches  226 ,  224 , respectively. Lower arm  210  includes a first handle  212  and upper arm  206  includes a second handle  208 . Second handle  208  is moved towards first handle  212  which causes second clasp  202  to move away from first clasp  204  allowing retainer  118 A to be inserted between first clasp  204  and second clasp  202 . After retainer  118 A is inserted, second clasp  202  is moved back towards first clasp  204  by moving second handle  208  away from first handle  212 . 
     Referring to FIG. 12, retainer  118 A is then wedged into first catch  226  and second catch  224 . To wedge retainer  118 A, a third handle  218  of wedge arm  216  is moved towards second handle  208  of upper arm  206 . This causes nose  220  of wedge arm  206  to press against retainer  118 A and wedge legs  124  and finger  120  of retainer  118 A into first and second catchs  226 ,  224 , respectively. 
     Referring now to FIG. 13, once securely wedged, retainer  118 A prevents third handle  218  of wedge arm  216  from being moved closer to second handle  208  of upper arm  206 . Thus, further motion of third handle  218  towards first handle  212  of lower arm  210  produces an equal downward motion of second handle  208  of upper arm  206 . As a result, second clasp  202  again moves away from first clasp  204 . Since legs  124  and finger  120  are securely fastened to first and second catches  204 ,  202 , respectively, finger  120  is spread from legs  124 . 
     Referring to FIG. 2, finger  120  is then inserted into trench  138  of heat sink  112  and legs  124  are positioned below substrate  102  and IC packages  105 . Tool  200  is then operated by reversing the above described procedure to release finger  120  and to secure the retainer to heat sink  112 . 
     Of importance, the retainer is positioned and secured without sliding finger  120  along heat sink  112  and without sliding legs  124  along IC packages  105 . Thus, use of tool  200  in accordance with the present invention avoids the possibility of damaging the heat sink assembly during installation of the retainer. 
     More particularly, FIG. 2 is a side view of device  100  employing a heat sink assembly in accordance with the present invention. Device  100  includes substrate  102  such as a printed circuit board (hereinafter referred to as circuit board  102 ). Attached to upper surface  102 U , e.g., a first surface, of circuit board  102  are IC packages  104  which include heat generating electronic components such as integrated circuits. Also attached to lower surface  102 L, e.g., a second surface opposite and removed from the first surface, of circuit board  102  are two IC packages  105  which include heat generating electronic components such as integrated circuits. IC packages  104 ,  105  may be the same type of IC package, for example, SRAMs. 
     As shown in FIG. 2, IC packages  104  are attached to an upper die attach area  108 U of upper surface  102 U of circuit board  102 . Similarly, IC packages  105  are attached to a lower die attach area  108 L of lower surface  102 L of circuit board  102 . Generally, die attach areas  108 U,  108 L include the circuit board areas to which IC packages  104 ,  105  are attached and also the immediate circuit board area around IC packages  104 ,  105  including the circuit board area between IC packages  104 ,  105 , respectively. IC packages  104 ,  105  are attached typically by one or more circuit interconnections such as solder, solder balls, pins, or by adhesive. 
     Although two IC packages  104  and two IC packages  105  are illustrated in FIG. 2, for a total of four packages, a different number of packages can be used, e.g., one, two, four or more. 
     In thermal contact with IC packages  104  is heat sink  112  having a base  115  and fins  114  extending from base  115 . Heat sink  112  is made of a material having a high thermal conductivity, e.g., is made of a metal such as aluminum or copper. Although fins  114  are illustrated in FIG. 2, other heat sink designs can also be used, e.g., solid heat sinks, plate type heat sinks, tower type heat sinks, pipe type heat sinks and disk shaped heat sinks. 
     Typically, a thermal pad  116  such as an elastomer having a thermally conductive filler, e.g., aluminum oxide and/or boron nitride, is interposed between heat sink  112  and IC packages  104  although other materials such as thermally conductive grease can be used. Illustratively, thermal pad  116  is 0.040 inches thick. In one embodiment, thermal pad  116  is compliant thus compensating for any mismatch in physical characteristics and thermal expansion between heat sink  112  and IC packages  104 . Further, in one embodiment, thermal pad  116  has a high thermal conductivity thus improving heat transfer between IC packages  104  and heat sink  112 . However, in an alternative embodiment, thermal pad  116  is not used and heat sink  112  directly contacts IC packages  104 . 
     Heat sink  112  is held in place and pressed into thermal contact with IC packages  104  by retainer  118 . In this embodiment, retainer  118  is a clip having finger  120 , where finger  120  includes finger tip  122  which presses downwards (e.g., in a first direction) on base  115  and holds heat sink  112  in place. Retainer  118  further has legs  124  which press upwards (e.g., in a second direction opposite the first direction) on IC packages  105 . Finger  120  and legs  124  are integrally connected to and extend from body  126  of retainer  118 . Retainer  118  passes around edge  102 E of circuit board  102  such that body  126  of retainer  118  is located adjacent edge  102 E. 
     In FIG. 2, legs  124  directly contact IC packages  105 . However, a protective pad (not shown) can be located between legs  124  and IC packages  105 , for example an electrically insulative pad can be located between legs  124  and IC packages  105 . As a further alternative, device  100  can be fabricated without IC packages  105 . In this embodiment, legs  124  directly contact and press on lower surface  102 L of circuit board  102  or on a protective pad located between legs  124  and lower surface  102 L of circuit board  102 . 
     Illustrative specifications for the various characteristics shown in FIG. 2 are set forth below in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A3 
                 0.265 
                 Inches 
               
               
                 B3 
                 0.770 
                 Inches 
               
               
                 C3 
                 0.54 
                 Inches 
               
               
                 D3 
                 0.415 
                 inches 
               
               
                   
               
            
           
         
       
     
     FIG. 3 is a perspective view of retainer  118  of FIG.  2 . As shown in FIG. 3, finger  120  is integrally attached to a first body edge  121 U of body  126  and legs  124  are integrally attached to a second body edge  121 L of body  126 , where body edge  121 L is opposite and removed from body edge  121 U. Retainer  118  includes two legs  124  and a single finger  120 . However, in alternative embodiments, a different number of fingers  120  and/or legs  124  can be used. Generally, retainer  118  includes at least one finger  120  and at least one leg  124 . In the case where more than one finger  120  is used, heat sink  112  is modified to accommodate the additional fingers  120 . 
     Retainer  118  is made of a flexible resilient material having a high spring rate. Thus retainer  118  has the ability to return to its original form, i.e., relaxed state, after being forced out of shape. For example, retainer  118  is full hard type  302  stainless steel or, more generally, is steel although other materials such as beryllium copper can be used. Retainer  118  is typically integral, i.e., is a single piece, and is manufactured using well known techniques such as by stamping and bending a single sheet of metal, composites, plastic derivatives, etc. 
     FIG. 4 is a side plan view of retainer  118  taken from the line IV of FIG.  3 . As shown in FIG. 4, finger tip  122  curves downwards (towards legs  124  ) from the main length of finger  120  to an apex  128 . From apex  128 , finger tip  122  curves upwards (away from legs  124 ) to an angled end  130  which extends at an angle away from legs  124 . Further, legs  124  include angled ends  132  which extend at an angle away from finger  120 . Angled ends  130 ,  132  allow retainer  118  to be slid into place. Alternatively, angled ends  130 ,  132  are gripping features which enable a clip assembly tool to grasp retainer  118  as discussed in detail below. 
     Illustrative specifications for the various characteristics shown in FIG. 4 are set forth below in Table 2. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                 A4 
                  0.032 (0.032*) 
                 Inches 
               
               
                 B4 
                 0.85 (0.96) 
                 Inches 
               
               
                 C4 
                 0.77 (0.77) 
                 Inches 
               
               
                 D4 
                 0.54 (0.54) 
                 Inches 
               
               
                 E4 
                 1.41 (1.41) 
                 Inches 
               
               
                 F4 
                 0.06 (0.06) 
                 Inches 
               
               
                 G4 
                 0.29 (0.34) 
                 Inches 
               
               
                 H4 
                 0.12 (0.12) 
                 Inches 
               
               
                 I4 
                 R0.015 (R0.015) 
                 Inches 
               
               
                   
               
               
                 *Note: Values in parenthesis are for an alternative embodiment.  
               
            
           
         
       
     
     FIG. 5 is a top plan view of retainer  118  of FIG.  3 . As shown in FIG. 5, finger  120  is laterally located between legs  124  such that retainer  118  is symmetric about a center line C L . In this manner, force exerted on finger  120  is evenly distributed to legs  124 . 
     Illustrative specifications for the various characteristics shown in FIG. 5 are set forth below in Table 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                 A5 
                 2 × 0.236 
                 Inches 
               
               
                 B5 
                 0.374 
                 Inches 
               
               
                 C5 
                 0.846 
                 Inches 
               
               
                 D5 
                 0.135 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 6 is a front plan view of retainer  118  of FIG.  3 . An illustrative specification for the various characteristics shown in FIG. 6 is set forth below in Table 4. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                 A6 
                 0.18 (0.351*) 
                 Inches 
               
               
                   
               
               
                 *Note: Value in parentheses is for an alternative embodiment.  
               
            
           
         
       
     
     FIG. 7 is a perspective view of heat sink  112  of FIG.  2 . As shown in FIG. 7, fins  114  are parallel with one another. Each fin  114  includes a slit  136  extending from the top  114 T of each fin  114  downwards in the vertical direction (along the y axis) towards base  115 . Collectively, slits  136  form a trench  138  which extends along the longitudinal axis (along the z axis) perpendicular to a plane (x-y plane) defined by fins  114 . 
     Extending parallel to a plane defined by fins  114  (along the x axis) between each fin  114  is a slot  140 . Each slot  140  extends from the tops  114 T of adjacent fins  114  downwards to base  115 . 
     FIG. 8 is a front plan view of heat sink  112  of FIG.  7 . Illustrative specifications for the various characteristics shown in FIG. 8 are set forth below in Table 5. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A8 
                 0.158 
                 Inches 
               
               
                 B8 
                 0.15 
                 Inches 
               
               
                 C8 
                 0.846 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 9 is a side plan view of heat sink  112  of FIG.  7 . Illustrative specifications for the various characteristics shown in FIG. 9 are set forth below in Table 6. 
     
       
         
           
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                 A9 
                 8 × 0.042 
                 Inches 
               
               
                 B9 
                 1.260 
                 Inches 
               
               
                 C9 
                 7 × 0.174 
                 Inches 
               
               
                 D9 
                 1.0  
                 Inches 
               
               
                 E9 
                 0.100 
                 Inches 
               
               
                 F9 
                 7 × 0.118 
                 Inches 
               
               
                   
               
            
           
         
       
     
     Referring now to FIGS. 8 and 9 together, the distance between lower surface  115 L of base  115  and the bottom  138 B of trench  138  (i.e., characteristic B8 of FIG. 8) is greater than the distance between lower surface  115 L of base  115  and the bottoms  140 B of slots  140  (i.e., characteristic E9 of FIG.  9 ). For example, characteristic B8 is 0.15 inches and characteristic E9 is 0.100 inches as set forth above in Tables  5  and  6 , respectively. Stated a different way, slots  140  are deeper than trench  138 . 
     Referring again to FIG. 2, by forming slots  140  with a greater depth than, and perpendicular to, trench  138 , finger tip  122  locks heat sink  112  securely in place. In particular, apex  128  of finger tip  122  extends below trench  138  (trench bottom  138 B is indicated by the dashed line) and into the center slot  140 C of slots  140 . In this manner, side to side motion of heat sink  112  is prevented. Further, by sizing finger  120  and/or finger tip  122  to have a width (i.e., characteristic D5 of FIG. 5) slightly less than and approximately equal to the width of trench  138  (i.e., characteristic A8 of FIG.  8 ), motion of heat sink  112  in the direction perpendicular to the plane of the page of FIG.  2 . is also prevented. As an illustration, characteristic D5 is 0.135 inches and characteristic A8 is 0.158 inches as set forth above in Tables  3  and  5 , respectively. 
     Referring still to FIG. 2, tension in retainer  118  causes finger tip  122  to apply downward vertical force on heat sink  112 . This tension is created because the distance between finger tip  122  and legs  124  is greater when retainer  118  is secured to heat sink  112  than when retainer  118  is in its relaxed state. Heat sink  112 , in turn, presses downwards towards IC packages  104 . More particularly, heat sink  112  presses down on thermal pad  116  which presses on IC packages  104  thus forming the thermal contact between heat sink  112  and IC packages  104 . 
     Of importance, heat sink  112  presses down uniformly on IC packages  104 . This is because finger tip  122  is located in the center slot  140 C of slots  140  and accordingly presses down directly at the center of heat sink  112 . If multiple fingers  120  are used, preferably fingers  120  are symmetric about a centerline of retainer  118 . This helps to assure that the force distribution created by retainer  118  on heat sink  112  is substantially uniform. 
     The vertical downward force (e.g., first force) applied on IC packages  104  by heat sink  112  is countered by an equal vertical upward force (e.g., a second force equal and opposite the first force) applied by legs  124  of retainer  118  on circuit board  102  (via IC packages  105  ) directly opposite IC packages  104 . Since legs  124  are symmetric with respect to finger  120 , each leg  124  applies an equal upward force on IC packages  105 . Accordingly, legs  124  press uniformly on IC packages  105 . Thus, a heat sink assembly in accordance with the present invention avoids the application of uneven force distribution on IC packages  104 ,  105  and the associated damage and ultimate device failure caused by use of conventional heat sink assemblies. 
     As best seen in FIG. 2, legs  124  are located adjacent lower die attach area  108 L of circuit board  102  directly opposite from upper die attach area  108 U. Thus, the vertical upward force is transferred from legs  124  directly back to heat sink  112  through IC packages  105 , circuit board  102 , IC packages  104  and thermal pad  116  without imparting any bending force on circuit board  102 . In this manner, a heat sink assembly in accordance with the present invention avoids circuit board warpage and the associated ultimate device failure associated with conventional heat sink assemblies. 
     Of further importance, retainer  118  passes around edge  102 E of circuit board  102 . This allows greater utilization of circuit board  102 , e.g., allows more traces and/or vias to be located on or in circuit board  102 , compared to having a retainer pass through circuit board  102 . 
     Heat sink  112  is also readily attached by retainer  118 . To attach heat sink  112 , heat sink is positioned such that IC packages  104  and thermal pad  116  are located between heat sink  112  and circuit board  102 . While holding heat sink  112  in place, retainer  118  is positioned so that finger  120  is aligned to slid into trench  138  and legs  124  are aligned to slid under and around circuit board  102  including IC packages  105 . Retainer  118 , typically body  126 , is then pressed towards edge  102 E of circuit board  102  which slides legs  124  around circuit board  102  and on IC packages  105  and also slides finger  120  into trench  138 . Retainer  118  is pressed until finger tip  122  is located in central slot  140 C. Angled ends  130 ,  132  of finger tip  122 , legs  124  prevent finger tip  122 , legs  124  from getting caught on heat sink  112 , IC packages  105 , respectively. 
     Heat sink  112  is also readily removed. To remove heat sink  112 , retainer  118 , typically body  126 , is pulled so that finger  120  slides out of trench  138 . Heat sink  112  is then removed. 
     In some applications, installation of retainer  118  by the method described above may damage the assembly. For example, sliding of legs  124  along IC packages  105  can damage IC packages  105  or cause circuit interconnection failure between IC packages  105  and circuit board  102 , e.g., can cause solder ball cracking or failure. To avoid any potential damage to the assembly, finger  120  is spread apart from legs  124 . Retainer  118  is then positioned such that finger tip  122  is located in central slot  140 C and legs  124  are located below IC packages  105 . Finger  120  is then released securing retainer  118  to heat sink  112 . 
     While retainer  118  can be installed by hand, care must be exercised to prevent sliding finger tip  122  along heat sink  112  and legs  124  along IC packages  105 . However, as set forth above, an alternative method of installation avoids the possibility of this sliding. 
     FIG. 10 is a side plan view of a clip assembly tool  200  in accordance with the present invention. Tool  200  includes clasps  202 ,  204  for grasping finger  120  and legs  124 , respectively, and spreading them apart from one another. In this manner, tool  200  facilitates attachment of retainer  118  to heat sink  112  without sliding of retainer  118 . 
     Tool  200  includes an upper arm  206  which includes a handle  208  at one end and clasp  202  at the opposite end. Tool  200  further includes a lower arm  210  having a handle  212  at one end and clasp  204  at the opposite end. Upper and lower arms  206 ,  210  are pivotally attached to one another by a pivot  214 . Thus, motion of handle  208  towards handle  212  causes clasp  202  to move away from clasp  204  and vice versa. 
     Tool  200  further includes a wedge arm  216 . Wedge arm  216  includes a handle  218  at one end and a nose  220  at the opposite end. Wedge arm  216  is pivotally attached to upper arm  206  by a pivot  222 . Thus, motion of handle  218  towards handle  208  causes nose  220  to become extended towards clasp  202  and vice versa. As discussed in more detail below, nose  220  wedges finger  120  and legs  124  of retainer  118  into catches  224 ,  226  of clasps  202 ,  204 , respectively. Catches  224 ,  226  are pockets so that, after being wedged, finger  120  and legs  124  are securely held by catches  224 ,  226 , respectively. 
     A spring  228  indicated in phantom lines wraps around pivots  214 ,  222  and presses against handles  212 ,  218 . By pressing against handles  212 ,  218 , spring  228  operates to urge handle  218  away from handle  212  keeping nose  220  retracted away from clasp  202  when tool  200  is not in use. 
     In accordance with the present invention, a method of using tool  200  to attach a retainer  118 A to heat sink  112  is presented. FIG. 11 is a side plan view of retainer  118 A being placed in clasps  202 ,  204  of tool  200 . 
     To allow retainer  118 A to be placed between clasps  202 ,  204 , handle  208  of upper arm  206  is moved from its original position  208 A downwards and towards handle  212  to a position  208 B. This pivots upper arm  206  and wedge arm  216  about pivot  214 . Accordingly, clasp  202  moves from its original position  202 A upwards and away from clasp  204  to a position  202 B. At position  202 B, the vertical distance between clasps  202 ,  204 , including the vertical distance between catches  224 ,  226 , is greater than the height of retainer  118 A, i.e., is greater than the distance between leg  124  and finger  120 . 
     After clasp  202  has been moved to position  202 B, retainer  118 A is inserted, body  126  first, in between clasps  202 ,  204 . Retainer  118 A is inserted so that body  126  is adjacent, or is in abutting contact, with nose  220 . Of importance, the lateral distance between catch  224  and nose  220  is greater than the length of finger  120 . Similarly, the lateral distance between catch  226  and nose  220  is greater than the length of legs  124 . Since nose  220  remains retracted due to the action of spring  228  (see FIG.  10 ), locating body  126  adjacent nose  220  allows ends  132  of legs  124  to clear catch  226 . 
     After retainer  118 A has been placed between clasps  202 ,  204 , handle  208  is moved from position  208 B away from handle  212  back to its original position  208 A. This pivots upper arm  206  and wedge arm  216  back about pivot  214 . Accordingly, clasp  202  moves from position  202 B downwards and towards clasp  204  back to its original position  202 A. Since nose  220  remains retracted, locating body  126  adjacent nose  220  allows catch  226  to clear end  130 . 
     At position  202 A, the vertical distance between catches  224 ,  226  is less than the distance between end  130  of finger tip  122  and ends  132  of legs  124 . 
     FIG. 12 is a side plan view of retainer  118 A being secured in clasps  202 ,  204  of tool  200 . After retainer  118 A is placed between clasps  202 ,  204 , handle  218  of wedge arm  216  is moved from its original position  218 A downwards towards upper arm  206  and lower arm  210 . This causes wedge arm  216  to pivot about pivot  222  which causes nose  220  to swing towards clasp  202 . As a result, nose  220  presses on body  126  of retainer  118 A and moves retainer  118 A towards catches  224 ,  226 . This causes ends  130 ,  132  to be forced into and held securely by catches  224 ,  226 , respectively. By forming catches  224 ,  226  as pockets corresponding in shape to ends  130 ,  132  of finger  120 , legs  124 , respectively, tool  200  can be used with a variety of different ends and retainers. For example, end  130  of FIG. 12 is at an angle away from legs  124  and catch  224  is pie shaped to accommodate end  130 . Further, legs  124  include ends  132  which are straight and catch  226  is groove shaped to accommodate ends  132 . 
     Regardless of the shape of catches  224 ,  226 , once ends  130 ,  132  are securely wedged, handle  218  of wedge arm  216  is located at position  218 B. At position  218 B, retainer  118 A prevents wedge arm  216  from further pivoting around pivot  222 . Thus, at position  218 B, retainer  118 A prevents handle  218  from being moved closer to handle  208  of upper arm  206 . 
     FIG. 13 is a side plan view of retainer  118 A having finger  120  spread from legs  124  by tool  200 . As set forth above, when handle  218  is at position  218 B, retainer  118 A prevents handle  218  from being moved closer to handle  208  of upper arm  206 . Thus, further motion of handle  218  from position  218 B downwards towards handle  212  to a position  218 C produces an equal downward motion of handle  208  of upper arm  206 . Thus, handle  208  of upper arm  206  is once again moved from its original position  208 A (see FIG. 11) towards handle  212  to position  208 B. This pivots upper arm  206 , including wedge arm  216 , about pivot  214 . Accordingly, clasp  202  moves from its original position  202 A (see FIG. 11) upwards and away from clasp  204  to position  202 B. Since finger  120 , legs  124  are securely fastened to catchs  224 ,  226 , respectively, finger  120  is spread from legs  124 . Thus, finger  120  moves from its original position  120 A away from legs  124  to position  120 B. 
     Referring again to FIG. 2, after spreading, the distance between finger  120  and legs  124  is greater than the characteristic D3, where characteristic D3 is the distance from the lower surface of device  100  (the lower surfaces of IC packages  105  in this embodiment) and bottom  138 B of trench  138 . Illustratively, characteristic D3 is 0.415 inches as set forth above in Table 1. Finger  120  is then inserted into trench  138  and legs  124  are positioned about and below circuit board  102  including IC packages  105 . Retainer  118 A is moved until finger tip  122  is located in central slot  140 C and legs  124  are located below IC packages  105 . Of importance, since the distance between finger  120  and legs  124  is greater than characteristic D3, retainer  118 A is positioned without sliding finger tip  122  along heat sink  112  and without sliding legs  124  along IC packages  105 . Thus, use of tool  200  in accordance with the invention avoids the possibility of damaging device  100 , e.g., avoids potential damage to IC packages  105  and to the circuit interconnections between IC packages  105  and circuit board  102 . 
     Tool  200  is then operated by reversing the above described steps to release finger  120  and secure retainer  118 A to heat sink  112 . Referring again to FIG. 13, handle  218  is moved from position  218 C away from handle  212  to position  218 B. This moves handle  208  from position  208 B away from handle  212  and back to its original position  208 A (see FIG.  11 ). This pivots upper arm  206  about pivot  214  which moves clasp  202  from position  202 B downwards and towards clasp  204  to its original position  202 A (see FIG.  11 ). As a result, finger  120  is moved from position  120 B downwards and toward legs  124  until finger  120  presses on heat sink  112  (see FIG.  2 ). At this point, tension in retainer  118 A securely holds retainer  118 A in place. 
     Referring now to FIG. 12, handle  218  of wedge arm  216  is moved from position  218 B upwards away from upper arm  206  and lower arm  210 . This causes wedge arm  216  to pivot about pivot  222  which causes nose  220  to swing away from clasp  202 . As a result, nose  220  releases retainer  118 A and in particular, releases ends  130 ,  132  from catches  224 ,  226  respectively. It may be necessary to move tool  200  such that nose  220  contacts body  126  of retainer  118 A to fully release ends  130 ,  132  from catches  224 ,  226 , respectively. 
     Referring to FIG. 11, handle  208  of upper arm  206  is then moved from its original position  208 A towards handle  212  of lower arm  210  to position  208 B. This pivots upper arm  206  and wedge arm  216  about pivot  214 . Accordingly, clasp  202  moves from its original position  202 A upwards and away from clasp  204  to position  202 B. At this point, tool  200  is free and clear of retainer  118 A. 
     Retainer  118  is also readily removed by tool  200  by simply reversing the above described procedure. 
     FIG. 14 is a top plan view of wedge arm  216  of tool  200 . Illustrative specifications for the various characteristics shown in FIG. 14 are set forth below in Table 7. 
     
       
         
           
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                 A14 
                 0.530 
                 Inches 
               
               
                 B14 
                 0.340 
                 Inches 
               
               
                 C14 
                 0.140 
                 Inches 
               
               
                 D14 
                 0.090 
                 Inches 
               
               
                 E14 
                 0.440 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 15 is a side plan view of wedge arm  216  of tool  200 . Illustrative specifications for the various characteristics shown in FIG. 15 are set forth below in Table 8. 
     
       
         
           
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A15 
                 0.270 
                 Inches 
               
               
                 B15 
                 0.625 
                 Inches 
               
               
                 C15 
                 1.825 
                 Inches 
               
               
                 D15 
                 1.62 
                 Inches 
               
               
                 E15 
                 0.93 
                 Inches 
               
               
                 F15 
                 0.300 
                 Inches 
               
               
                 G15 
                 6.72 
                 Inches 
               
               
                 H15 
                 1.05 
                 Inches 
               
               
                 I15 
                 0.500 
                 Inches 
               
               
                 J15 
                 0.435 
                 Inches 
               
               
                 K15 
                 0.205 
                 Inches 
               
               
                 L15 
                 0.685 
                 Inches 
               
               
                 M15 
                 0.990 
                 Inches 
               
               
                 N15 
                 1.350 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 16 is a top plan view of upper arm  206  of tool  200 . An illustrative specification for the various characteristics shown in FIG. 16 is set forth below in Table 9. 
     
       
         
           
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A16 
                 0.125 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 17 is a side plan view of upper arm  206  of tool  200 . Illustrative specifications for the various characteristics shown in FIG. 17 are set forth below in Table 10. 
     
       
         
           
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A17 
                 0.500 
                 Inches 
               
               
                 B17 
                 0.325 
                 Inches 
               
               
                 C17 
                 0.135 
                 Inches 
               
               
                 D17 
                 0.105 
                 Inches 
               
               
                 E17 
                 0.010 
                 Inches 
               
               
                 F17 
                 Rad 0.125 
                 Inches 
               
               
                 G17 
                 1.000 
                 Inches 
               
               
                 H17 
                 0.675 
                 Inches 
               
               
                 I17 
                 0.505 
                 Inches 
               
               
                 J17 
                 0.330 
                 Inches 
               
               
                 K17 
                 0.190 
                 Inches 
               
               
                 L17 
                 0.110 
                 Inches 
               
               
                 M17 
                 0.44 
                 Inches 
               
               
                 N17 
                 0.500 
                 Inches 
               
               
                 O17 
                 5.49 
                 Inches 
               
               
                 P17 
                 1.67 
                 Inches 
               
               
                 Q17 
                 1.6 
                 Inches 
               
               
                 R17 
                 1.545 
                 Inches 
               
               
                 S17 
                 1.455 
                 Inches 
               
               
                 T17 
                 0.295 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 18 is a top plan view of lower arm  210  of tool  200 . Illustrative specifications for the various characteristics shown in FIG. 18 are set forth below in Table 11. 
     
       
         
           
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A18 
                 0.850 
                 Inches 
               
               
                 B18 
                 0.150 
                 Inches 
               
               
                 C18 
                 0.250 
                 Inches 
               
               
                 D18 
                 0.600 
                 Inches 
               
               
                 E18 
                 0.700 
                 Inches 
               
               
                   
               
            
           
         
       
     
     FIG. 19 is a side plan view of lower arm  210  of tool  200 . Illustrative specifications for the various characteristics shown in FIG. 19 are set forth below in Table 12. 
     
       
         
           
               
               
               
             
               
                 TABLE 12 
               
               
                   
               
               
                 Characteristic 
                 Specification 
                 Units 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 A18 
                 2.245 
                 Inches 
               
               
                 B19 
                 2.12 
                 Inches 
               
               
                 C19 
                 2.055 
                 Inches 
               
               
                 D19 
                 2.005 
                 Inches 
               
               
                 E19 
                 1.26 
                 Inches 
               
               
                 F19 
                 Rad 0.250 
                 Inches 
               
               
                 G19 
                 0.250 
                 Inches 
               
               
                 H19 
                 Dia 0.125 
                 Inches 
               
               
                 I19 
                 8.575 
                 Inches 
               
               
                 J19 
                 0.32 
                 Inches 
               
               
                 K19 
                 0.52 
                 Inches 
               
               
                 L19 
                 0.765 
                 Inches 
               
               
                 M19 
                 0.045 
                 Inches 
               
               
                 N19 
                 Rad 0.0625 
                 Inches 
               
               
                 O19 
                 0.095 
                 Inches 
               
               
                   
               
            
           
         
       
     
     The drawings and the forgoing description gave examples of the present invention. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of the invention is at least as broad as given by the following claims.