Abstract:
A system includes a heat sink, a semiconductor device, and a layer of thermal interface material (TIM) disposed between the heat sink and the semiconductor device. The TIM may facilitate dissipation of heat generated by the semiconductor device via the heat sink. The system also includes a fastener system that couples the semiconductor device to the heat sink about the layer of TIM. The system also includes one or more washers of the fastener system that maintain a coupling force between the semiconductor device and the heat sink after the TIM flows.

Description:
BACKGROUND 
       [0001]    Embodiments of the present disclosure relate generally to systems and methods for improving a manufacturing process used for assembling industrial automation devices. More specifically, the present disclosure is generally related to improved systems and methods for attaching a semiconductor device to a heat sink during the manufacturing process of an industrial automation device. 
         [0002]    Industrial automation systems may employ various types of electronic devices such as an alternating current (AC) drive, a motor, a robot, or the like to perform various industrial processes. Generally, these electronic devices use semiconductor devices to control the power being used by the respective devices. As such, part of the manufacturing process of an industrial automation device includes coupling a semiconductor device to a heat sink, such that the semiconductor device can operate without overheating. It is generally desirable for the semiconductor device to be coupled to the heat sink with a particular force to provide stable coupling and to facilitate efficient heat transfer between the semiconductor device and the heat sink. Accordingly, improved systems and methods for manufacturing these electronic devices and providing couplings between the respective devices and heat sinks are desirable. 
       BRIEF DESCRIPTION 
       [0003]    In one embodiment, a system includes a heat sink, a semiconductor device, and a layer of thermal interface material (TIM) disposed between the heat sink and the semiconductor device. The TIM may facilitate dissipation of heat generated by the semiconductor device via the heat sink. The system also includes a fastener system that couples the semiconductor device to the heat sink about the layer of TIM. The system also includes one or more washers of the fastener system that maintain a coupling force between the semiconductor device and the heat sink after the TIM flows. 
         [0004]    In another embodiment, a non-transitory computer-readable medium includes computer-executable instructions that cause the computer-readable medium to receive a first set of properties associated with a first type of washer of one or more washers that maintain a coupling force between a heat sink and a semiconductor device after a thermal interface material (TIM) disposed between the semiconductor device and the heat sink flows. The computer-readable medium may then determine an expected deflection of the one or more washers after the fastener is torqued to a torque value based on the first set of properties and an expected axial force between the heat sink and the semiconductor device. The computer-readable medium may then determine an expected residual force between the heat sink and the semiconductor device after the TIM between the heat sink and the semiconductor device flows, such that the expected residual force is determined based on the expected deflection and an expected change in thickness of the TIM after the TIM flows. The computer-readable medium may then determine that the first type of washer will adequately maintain a minimum force between the heat sink and the semiconductor device after the TIM flows when the expected residual force is greater than a value. 
         [0005]    In yet another embodiment, an industrial automation drive may include a heat sink-semiconductor assembly. The heat sink-semiconductor assembly may include a thermal interface material (TIM) disposed between a semiconductor device and a heat sink. The semiconductor device may be used to convert an alternating current (AC) voltage into a direct current (DC) voltage or convert the DC voltage into a controllable AC voltage. The heat sink-semiconductor assembly may also include a fastener that couples the semiconductor device to the heat sink and one or more washers that may be inserted into the fastener. Here, the washers may maintain a force between the semiconductor device and the heat sink after the TIM flows. 
     
    
     
       DRAWINGS 
         [0006]    These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0007]      FIG. 1  illustrates a perspective view of an example heat sink-semiconductor assembly, in accordance with embodiments presented herein; 
           [0008]      FIG. 2  illustrates a block diagram of the heat sink-semiconductor assembly of  FIG. 1 , in accordance with embodiments presented herein; 
           [0009]      FIG. 3A  illustrates a top view of a fastener system that may be employed in the heat sink-semiconductor assembly of  FIG. 1 , in accordance with embodiments presented herein; 
           [0010]      FIG. 3B  illustrates a cross-sectional view taken along line A-A of the fastener system of  FIG. 3A , in accordance with embodiments presented herein; 
           [0011]      FIG. 3C  illustrates a side view of the fastener system of  FIG. 3A , in accordance with embodiments presented herein; 
           [0012]      FIG. 3D  illustrates a perspective view of the fastener system of  FIG. 3A , in accordance with embodiments presented herein; 
           [0013]      FIG. 4A  illustrates a top view of a washer that may be employed in the fastener system of  FIGS. 3A-3D ; 
           [0014]      FIG. 4B  illustrates a side view of a washer that may be employed in the fastener system of  FIGS. 3A-3D ; 
           [0015]      FIG. 4C  illustrates a perspective view of a washer that may be employed in the fastener system of  FIGS. 3A-3D ; and 
           [0016]      FIG. 5  illustrates a flow chart of a method for identifying a washer that may be employed in the fastener system of  FIGS. 3A-3D . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0018]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0019]    Embodiments of the present disclosure are generally directed towards improved systems and methods for manufacturing industrial automation devices. Industrial automation devices such as drives, motor starters, rectifiers, and the like may use semiconductor devices to control power used or output by the respective devices. Semiconductor devices, such as insulated-gate bipolar transistors (IGBTs), may be used by industrial automation devices to accommodate efficient and fast switching to alter voltage waveforms received by the devices. As the semiconductor device switches, however, the semiconductor device may generate heat that may reduce the life of the semiconductor device or the industrial automation device in which the semiconductor device is located. As such, a semiconductor device may be coupled to a heat sink, which may dissipate the heat generated by the semiconductor device away from the semiconductor device, the industrial automation device, or both. 
         [0020]    For example, the industrial automation device may be an electric drive that may include a rectifier circuit, such as a Diode Front End (DFE) rectifier an Active Front End (AFE) rectifier that may convert alternating current (AC) voltage into a direct current (DC) voltage. The electric drive may also include an inverter circuit that may convert the DC voltage into a controllable AC voltage that may be provided to a motor that may be coupled to a load. In this example, semiconductor devices may be used in the rectifier circuit and/or the inverter circuit to effectively convert the AC voltage into the DC voltage and to convert the DC voltage into the controllable AC voltage. That is, the semiconductor devices may rapidly switch off and on to generate a DC voltage waveform and a controllable AC voltage waveform. However, as mentioned above, as the semiconductor device switches, the semiconductor devices may generate heat that should be dissipated away from the semiconductor device. As such, a heat sink may be coupled to each semiconductor device to dissipate heat away from the semiconductor device. 
         [0021]    In order to provide a stable attachment and efficient heat transfer via the heat sink, it may be desirable to couple the semiconductor device and the heat sink together with a level of force within a certain range. Additionally, to further enable heat to transfer more efficiently from the semiconductor device to the heat sink, a thermal interface material (TIM) may be applied to a surface of the heat sink and/or to a surface of the semiconductor device where the heat sink and the semiconductor device interface or physically contact each other when coupled together. The TIM may fill in gaps that would otherwise exist at such interfaces such that better heat transfer occurs. A fastener coupling the semiconductor device and heat sink will generally apply force to the TIM positioned between the coupled features. However, it is now recognized that due to heat generated by the semiconductor device while operating, the TIM between the semiconductor and the heat sink may melt or flow. As a result, a portion of the force used to couple the semiconductor device and the heat sink may be lost. 
         [0022]    Generally, the heat sink and the semiconductor device may be coupled together using a fastener system. As such, the thermal transfer properties of the TIM may depend on a pressure or coupling force between the heat sink and the semiconductor device when coupled together via the fastener system. The fastener system may be torqued to a particular torque value, such that a pressure or coupling force between the coupled heat sink and semiconductor device is sufficient to enable the TIM to effectively transfer heat from the semiconductor device to the heat sink. 
         [0023]    When an industrial automation device is placed in service, the device may experience a burn-in process. That is, when the industrial automation device is initially placed in service, heat generated by the semiconductor device in the industrial automation device may cause the TIM to melt or flow. During this burn-in process, the TIM may change from a solid state to a semi-liquid state. When the TIM is in the semi-liquid state, the TIM may spread across the surface of the heat sink and fill in gaps that may exist on the surfaces of the heat sink and the semiconductor device. After the initial change from the solid state to the semi-liquid state, the TIM may change again into the solid state. At this point, the thickness of the TIM may have decreased due to the changes of state by the TIM and due to the TIM filling in the gaps on the surfaces of the heat sink and the semiconductor device. As a result of this thickness change, the pressure or coupling force between the coupled heat sink and semiconductor device may also decrease, thereby reducing the ability of the TIM to transfer heat to the heat sink efficiently. 
         [0024]    To compensate for this loss in pressure, the presently disclosed manufacturing process may include coupling the heat sink to the semiconductor device using the TIM, torqueing a fastener system used to couple the heat sink to the semiconductor device to a particular torque value, heating the coupled heat sink-semiconductor assembly in an industrial oven, and then re-torqueing the coupled heat sink-semiconductor assembly back to the particular torque value. In this manner, the manufacturing process may compensate for the loss of pressure or coupling force between the heat sink-semiconductor assembly due to the TIM melting or flowing by re-torqueing the heat sink-semiconductor assembly after the TIM has been heated. After the TIM has melted or flowed once, the gaps of the surfaces of the heat sink and the semiconductor device may be filled, and any future melting or flowing of the TIM may be limited. That is, after the TIM is heated once, the heat sink-semiconductor assembly may not experience any significant loss in pressure or coupling force between the coupled heat sink and semiconductor device. 
         [0025]    Although the method described above may compensate for the loss of pressure or coupling force between the heat sink and semiconductor device, the use of an oven in the manufacturing process may involve storing and operating a large heating device at a manufacturing facility. Moreover, the process of re-torqueing the heat sink-semiconductor assembly may be an inefficient use of manufacturing time (e.g., man hours). In addition to the time and cost issues discussed above, the above manufacturing process may include a risk of failing to re-torque certain fasteners after the TIM flows, a risk of damaging hardware during the re-torqueing process, a delivery delay associated with the re-torqueing process, and added costs for maintaining and operating an industrial oven. 
         [0026]    Keeping the foregoing in mind, in one embodiment, each of the fasteners used to couple the heat sink to the semiconductor device may include a series of Belleville-style washers, such that each adjacent Belleville-style washer in the series may be arranged in a reverse position. That is, each adjacent Belleville-style washer in the series may be arranged in opposite directions. As a result, the Belleville-style washers may act as springs limiting the loss of pressure or coupling force between the heat sink and the semiconductor device after the semiconductor device moves and/or the thickness of the TIM decreases during the burn-in process. By limiting the loss of pressure or coupling force between the heat sink and the semiconductor device during the burn-in process, the manufacturing process for the industrial automation device may no longer involve heating the heat sink-semiconductor assembly using an oven and re-torqueing the heat sink-semiconductor assembly. 
         [0027]    By way of introduction,  FIG. 1  illustrates a perspective view of an example heat sink-semiconductor assembly  10  that may be employed in various types of industrial automation devices. The heat sink-semiconductor assembly  10  may include a heat sink  12  and a semiconductor device  14 . The heat sink  12  may be a passive heat exchanger that cools the semiconductor device  14  by dissipating heat generated by the semiconductor device  14  away from the semiconductor device  14 . The semiconductor device  14  may include any type of semiconductor switch such as diodes, bipolar transistors, field-effect transistors, insulated-gate bipolar transistors (IGBT), thyristors, and the like, that may be coupled to a heat sink. 
         [0028]    In certain embodiments, the semiconductor device  14  may be coupled to the heat sink  12  using fastener system  16 . The fastener system  16  may include any type of component that may be used to couple two objects (e.g., semiconductor  14  and heat sink  12 ) together. As such, the fastener system  16 , for example, may include screws, nuts, clamps, bolts, or the like. 
         [0029]    Before coupling the semiconductor device  14  to the heat sink  12 , a thermal interface material (TIM) may be applied to surfaces  18  of the semiconductor device  14  and the heat sink  12  where the semiconductor device  14  and the heat sink  12  may physically contact each other when coupled together.  FIG. 2 , for example, depicts a block diagram view  15  of the heat sink-semiconductor assembly  10  illustrating a position of the TIM  19  with respect to the heat sink  12  and the semiconductor device  14 . 
         [0030]    The TIM  19  may be a viscous fluid substance that may increase a thermal conductivity of the interface of the heat sink  12  by filling microscopic air-gaps on the surface  18  of the heat sink  12  or the semiconductor device  14 . That is, since the surfaces  18  of the heat sink  12  and the semiconductor device  14  may be imperfectly flat or smooth, the TIM  19  may fill in those imperfections when the TIM  19  melts. As a result, the TIM  19  may enable the heat sink  12  to more efficiently dissipate the heat generated by the semiconductor device  14 . 
         [0031]    In order to ensure that heat is effectively dissipated by the heat sink  12  and the TIM  19 , a particular pressure or axial force between the heat sink  12  and the semiconductor device  14  should be maintained. As such, the particular pressure between the heat sink  12  and the semiconductor device  14  may be obtained by torqueing the fastener system  16  to a particular torque value when coupling the semiconductor device  14  to the heat sink  12 . The torque value may be specified by the supplier of the semiconductor device  14  as a minimum mounting torque value. Generally, the fastener system  16  should maintain the minimum mounting torque value or a percentage of the minimum mounting torque value after the burn-in process to ensure that the heat from the semiconductor device  14  effectively dissipates via the heat sink  12 . 
         [0032]    In one embodiment, each fastener system  16  may include one or more washers that may be inserted on a fastener used to couple the semiconductor device  14  to the heat sink  12 . As such, the washers may limit the loss of pressure or coupling force between the semiconductor device  14  and the heat sink  12  after the burn-in process.  FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D , for example, depict a top view  20 , a cross-sectional view  22 , a side view  30 , and a perspective view  40  of a washer-fastener system  22  that may be employed when coupling the semiconductor device  14  to the heat sink  12 . 
         [0033]    Referring now to  FIGS. 3A-3D , the fastener system  16  may include a fastener  23  inserted through one or more washers  24  and a flat washer  26 . The washers  24  may be Belleville-type washers, which may also be known as a coned-disc spring, a conical spring washer, a disc spring, a Belleville spring, or a cupped spring washer. In one embodiment, the washers  24  may have a frusto-conical shape that may provide each washer  24  with a spring characteristic. 
         [0034]    As shown in  FIGS. 3B-3D , the washers  24  may be placed in series with each other to provide a certain amount of force between the heat sink  12  and the semiconductor device  14 . In one embodiment, the fastener  23  may include a thread lock patch, as shown with the dashed lines in  FIGS. 3B-3D . Although  FIGS. 3B-3D  illustrate three washers  24  on the fastener  23 , it should be noted that the systems and techniques described herein are not limited to using three washers  24 . Instead, it should be understood that the systems and techniques it should also be understood that the systems and techniques described herein may be employed using any number of washers  24 . Additionally, it should be noted that the systems and techniques described herein may be employed using a variety of different sized fasteners  23 , washers  24 , and flat washers  26 . 
         [0035]    Various views of an example washer  24  are depicted in  FIGS. 4A-4C . As shown in a top view  50 , a side view  60 , and a perspective view  70  of  FIG. 4A ,  4 B, and  4 C, respectively, the washer  24  may have a first side having a diameter  52  and a second side having a diameter  54 . The diameter  52  may be positioned at the apex of the frusto-conical shaped washer  24  and the diameter  54  may be positioned at the base of the frusto-conical shaped washer  24 . Generally, the diameter  52  may be less than the diameter  54 . 
         [0036]    Keeping this in mind and referring back to  FIGS. 3A-3D , in one embodiment, a first washer  24  placed on the fastener  23  (i.e., the washer  24  closest to a head  25  of the fastener  23 ) may be inserted into the fastener  23  in any direction. That is, the first washer  24  may be placed against the head  25  of the fastener  23 , such that a first side of the washer  24  having a larger diameter (i.e., diameter  54 ) may be placed against the head  25  of the fastener  23 . Alternatively, a second side of the washer  24  having a smaller diameter (i.e., diameter  52 ) may be placed against the head  25  of the fastener  23 . In either case, any washer  24  following the first washer  24  may be inserted in an opposite direction as the immediately preceding washer  24 . For example, if the first washer  24  placed against the head  25  of the fastener  23  is arranged such that the side having the larger diameter rests against the head  25  of the fastener  23 , then a second washer  24 , immediately following the first washer  24 , may be placed such that the side of the second washer  24  having the smaller diameter (i.e., diameter  52 ) may be placed against the side of the first washer  24  having the smaller diameter (i.e., diameter  52 ). 
         [0037]    After the series of washers  24  have been inserted on the fastener  23  in an alternating manner, a flat washer  26  may be placed at the end of the series of washers  24 . The flat washer  26  may distribute a force applied by the washers  24  on the head  25  of the fastener  23  and against the heat sink  12  and/or the semiconductor device  14 . Although the foregoing discussion of the fastener system  16  is described with reference to the head  25  of the fastener  23 , it should be noted that in embodiments where the fastener  23  is a clamp, for example, the washer  24  may be place adjacent to the body of the clamp. 
         [0038]    As discussed above, the fastener  23  may be used to couple the semiconductor device  14  to the heat sink  12  and to apply a particular pressure or coupling force between the semiconductor device  14  and the heat sink  12 . However, in order to compensate for the loss of pressure or coupling force due to the TIM  19  flow during the oven heating portion of the manufacturing process or after the burn-in process of the industrial automation device having the heat sink-semiconductor assembly  10 , the washers  24  may provide a spring force or pressure between the semiconductor device  14  and the heat sink  12  after the TIM  19  flows. That is, after the TIM  19  flows and the thickness of the TIM  19  decreases, the initial pressure between the heat sink-semiconductor assembly  10  may decrease. However, this loss of pressure may cause the spring attributes of the washers  24  to expand and compensate for at least a portion of the total pressure loss. 
         [0039]    Generally, the type of washer  24  and the number of the washers  24  used to provide a sufficient amount of force or pressure between the heat sink  12  and the semiconductor device  14  may be determined based on a size of the fastener  23 , an amount of torque being applied to the fastener  23 , and an amount of expected deflection between the heat sink  12  and the semiconductor device  14  due to the TIM  19  flow. Keeping this in mind,  FIG. 5  illustrates a flow chart of a method  80  for determining a type of washer  24  and a number of washers  24  that may be employed in the fastener system  16 . 
         [0040]    In one embodiment, the method  80  may be performed by a computing device that may include a processor, a memory, a storage, and the like. The processor may be any type of computer processor or microprocessor capable of executing computer-executable code. The memory and the storage may be any suitable articles of manufacture that can serve as media to store processor-executable code. These articles of manufacture may represent computer-readable media (i.e., any suitable form of memory or storage) that may store the processor-executable code used by the processor to perform the presently disclosed techniques. The computer-readable media is tangible and non-transitory, which merely means that the computer-readable media is not a signal. 
         [0041]    Referring now to  FIG. 5 , at block  82 , the computing device may receive data related to one or more properties of a type of a washer  24  that may be used with the fastener  23 , as described above. The properties may include a maximum washer compression force and a maximum washer deflection associated with the washer  24 . These properties may be provided by a manufacturer of the washer  24 . The properties may also include a number of washers  24  that may be inserted in series on the fastener  23 . 
         [0042]    At block  84 , the computing device may determine a spring rate (k bw ) for the series of washers  24  based on the properties received at block  82 . The spring rate (k bw ) may correspond to an amount of force provided when deflecting the series of alternating washers  24  one inch. In one embodiment, the spring rate (k bw ) for the series of alternating washers may be determined according to Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     k 
                     bw 
                   
                   = 
                   
                     
                       maximum 
                        
                       
                           
                       
                        
                       washer 
                        
                       
                           
                       
                        
                       compression 
                        
                       
                           
                       
                        
                       force 
                     
                     
                       maximum 
                        
                       
                           
                       
                        
                       washer 
                        
                       
                           
                       
                        
                       deflection 
                       * 
                       number 
                        
                       
                           
                       
                        
                       of 
                        
                       
                           
                       
                        
                       washer 
                        
                       
                           
                       
                        
                       in 
                        
                       
                           
                       
                        
                       series 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0043]    At block  86 , the computing device may calculate an axial force (f t ) applied on the heat sink-semiconductor assembly  10  based on a torque value applied to the fastener  23 . The torque value applied to the fastener  23  may correspond to a torque value specified by the manufacturer of the semiconductor device  14  or the heat sink  12 . In one embodiment, the axial force (f t ) applied on the heat sink-semiconductor assembly  10  may be determined according to Equation 2 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     t 
                   
                   = 
                   
                     Torque 
                     
                       K 
                       × 
                       fastener 
                        
                       
                           
                       
                        
                       diameter 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Torque is the torque value specified by the manufacturer of the semiconductor device  14  or the heat sink  12  for the heat sink-semiconductor assembly  10 , K is a nut factor that is based on the materials, plating, and lubrication of the internal and external threads for the fastener  23 , and fastener diameter is a diameter of a part of the fastener  23  in which the washers  24  may be inserted. 
         [0044]    At block  88 , the computing device may calculate a deflection of the series of the washers  24  after the fastener  23  is torqued at the torque value specified by the manufacturer. The deflection of the series of washers  24  may correspond to a distance in which the series of washers  24  may compress after the fastener  23  is torqued at the torque value. In one embodiment, the deflection of the series of the washers  24  may be determined according to Equation 3 below: 
         [0000]    
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       f 
                       t 
                     
                     
                       k 
                       bw 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0045]    At block  90 , the computing device may determine a percentage of residual force remaining after the TIM  19  flows. In one embodiment, the percentage of residual force may be determined according to Equation 4 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     Percentage 
                      
                     
                         
                     
                      
                     of 
                      
                     
                         
                     
                      
                     residual 
                      
                     
                         
                     
                      
                     force 
                   
                   = 
                   
                     
                       
                         δ 
                         - 
                         
                           Expected 
                            
                           
                               
                           
                            
                           Change 
                            
                           
                               
                           
                            
                           in 
                            
                           
                               
                           
                            
                           Thickness 
                         
                       
                       δ 
                     
                     * 
                     100 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0046]    The Expected Change in Thickness corresponds to an expected change in thickness of the TIM  19  due to the heating of the heat sink-semiconductor assembly  10  during the manufacturing process or the burn-in process. The Expected Change in Thickness may be up to a 25% decrease in thickness. 
         [0047]    Certain factors may be evaluated when determining the Expected Change in Thickness. For example, the surface roughness of the both the semiconductor device  14  and the heat sink  12  and the amount of the TIM  19  may fill in the small crevices in these surfaces may be one factor in determining the Expected Change in Thickness. A second factor may be related to the flatness of the semiconductor device  14  and the heat sink  12 . If one or both of the semiconductor device  14  and the heat sink  12  are concave, the TIM  19  may flow into a cupped area of the concave curve. If one or both of the semiconductor device  14  and the heat sink  12  are convex, the TIM  19  may more readily flow towards the edges and out from under the semiconductor device  14 . A third factor may be related to the amount of perimeter of the semiconductor device  14  as compared to the total area of the semiconductor device  14 . This ratio may be related to an amount of the TIM  19  that may flow out of the edges during the burn-in process. 
         [0048]    At block  92 , the computing device may determine whether the percentage of the residual force determined at block  80  is greater than some assigned threshold value. The assigned threshold value may correspond to an acceptable percentage of force lost due to the TIM  19  flow. In one embodiment, the assigned threshold value may be between 20% and 30% of force lost or between 70% and 80% of the initial force maintained. 
         [0049]    If, at block  92 , the computing device determines that the percentage of the residual force determined at block  90  is not greater than the assigned threshold value, the computing device may return to block  82 . After the computing device returns to block  82 , the computing device may repeat the method  80  using washer properties for a different washer type or a different number of washers for the same washer type. 
         [0050]    If however, at block  92 , the computing device determines that the percentage of the residual force determined at block  90  is greater than the assigned threshold value, the computing device may proceed to block  94 . At block  94 , the computing device may determine that the type of washer  24  and the number of washers  24  that corresponds to the washer properties received at block  82  may adequately compensate for the loss of pressure or axial force between the heat sink  12  and the semiconductor device  14  due to the TIM  19  flow. 
         [0051]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.