Patent Publication Number: US-6043560-A

Title: Thermal interface thickness control for a microprocessor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the thermal interface between a semiconductor device and a thermal plate, and more particularly, to an apparatus and method for controlling the thickness of the thermal interface layer between a processor die and the thermal plate in a microprocessor device. 
     2. Description of the Related Art 
     In microprocessor devices, especially of the control collapse chip connection (C4) type, it is desirable to control the thickness of the thermal interface between the processor die and thermal plate. A continous and consistent thermal path will not be formed between the processor die and thermal plate if a controlled interface thickness is not maintained. This will impede the cooling of the processor die and other components of the microprocessor. It is therefore desirable that the thickness and thickness tolerance of the thermal interface be tightly controlled. 
     In the semiconductor industry, it is common for original equipment manufacturers (OEMs) to purchase microprocessors that have not been fully packaged. Often, the OEMs complete the packaging steps during the final assembly steps of the end product. As a result, many OEMs are left with the task of applying the thermal interface layer between the processor die and thermal plate. For the tolerance of the thermal interface layer to be tightly controlled, this packaging step is complex and costly for the OEMs. 
     The present invention is directed at reducing the complexity and thus cost associated with precisely and reliably controlling the thickness of the thermal interface layer for the OEMs during final assembly of the end product. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an apparatus for controlling the thickness of a thermal interface between a semiconductor device and thermal plate in a microprocessor assembly is provided. The apparatus includes a thermal top cover having a recessed portion of predetermined depth that at least partially encloses the semiconductor device. 
     In another aspect of the present invention, a method for controlling the thickness of a thermal interface between a semiconductor device and thermal plate in a microprocessor assembly is provided. The method includes securing a thermal top cover having a recessed portion of predetermined depth around the semiconductor device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a cross-sectional view of a partially constructed microprocessor assembly illustrating an embodiment of a thermal top cover according to the present invention; 
     FIG. 2 is a perspective view of the thermal top cover shown in FIG. 1; and 
     FIG. 3 is a cross-sectional view of an embodiment of a completed microprocessor assembly according to the present invention. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, 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 will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Turning now to the drawings and referring initially to FIG. 1, an improved microprocessor assembly is shown generally by reference numeral 10. The microprocessor assembly 10 includes an OLGA (Organic Land Grid Array) or printed circuit board 12 that is mounted to a substrate 14. The microprocessor 10 also includes a processor die 16, e.g., a C4 processor, which is in turn mounted to the OLGA 12. The processor die 16 has a substantially flat top surface 17, as shown in FIG. 1. The processor die 16 is an integrated circuit that is the processing core of the microprocessor assembly 10. 
     The microprocessor assembly 10 further includes an apparatus 18 for controlling the thickness of a thermal interface 20 (shown in FIG. 3). The apparatus 18 includes a thermal top cover 22 that is secured to the substrate layer 14 by four screws (not shown), which are received into holes 23 extending through the top cover 22 and into the substrate 14. The thermal top cover 22 is disposed in contact with the OLGA 12. In one embodiment, the thermal top cover 22 is rectangular in shape, as shown in FIG. 2, and is formed substantially of stainless steel. However, as those of ordinary skill in the art will recognize, other devices may be used to secure the thermal top cover 22 to the substrate 14 and the thermal top cover 22 may be formed of other thermally conductive or non-conductive materials. 
     The thermal top cover 22 is defined by a substantially flat outer periphery 24, as illustrated in FIG. 1. It is further defined by a substantially flat top surface 26 and a recess 27, as shown in FIGS. 1 and 2. The recess 27 is in turn defined by a sloped portion 28, and a substantially horizontal section 29. A pair of channels 30 are formed in the substantially horizontal section 29, as shown in FIG. 1. The channels 30 are adapted to fit around and interface with a corresponding pair of ink swatches 31 of the type known in the art, which are disposed on the OLGA 12. In one embodiment, the channels 30 and corresponding ink swatches 31 are approximately 3.5 mm wide and 15 mm long. A generally rectangular-shaped aperture 34 is formed in the recess 27, as illustrated in FIG. 2. 
     In one embodiment, the thermal top cover 22 is approximately 55.76 mm wide, 59.67 mm long, and 5.5 mm high. In this embodiment, the recess 27 is approximately 32.10 mm wide, 32.10 mm long, and 0.91 mm deep. The thermal top cover 22 is designed so that the substantially flat top surface 26 is disposed above the top surface 17 of the processor die 16, as shown in FIG. 1. This is what enables the thickness of the thermal interface 20 to be accurately and reliably controlled, as further discussed below. 
     The thermal top cover 22 is formed with recesses 36 at each of its four corners, as shown in FIG. 2. The recesses 36 have an arcuate section 38 disposed opposite to each respective corner. Also, each of the recesses 36 is provided with the holes 23 described above, which are adapted to receive the screws (not shown). The recesses 36 facilitate mounting of the thermal top cover 22 to the substrate 14. 
     In one embodiment, the thermal top cover 22 is formed by a stamping process. In an alternate embodiment, the thermal top cover 22 is formed by cold forging, die casting, or other similar high volume manufacturing technique. 
     The assembly shown in FIG. 1 illustrates what may typically be shipped to an OEM customer. The OEM customer may then fill the recess 27 with a thermal interface layer 20, as shown in FIG. 3. In one embodiment, the thermal interface layer 20 is a thermal grease formed of a silicon-based material, e.g., Shin-Etsu G749™ manufactured by Shin-Etsu of Japan, which may be applied by a dispensing machine. In another embodiment, the thermal interface layer 20 is a THERMFLOW™ film, which may be obtained from Chromerics, located in Massachusetts. The recess 27 controls the thickness of the thermal layer 20 so that the thermal layer 20 sufficiently covers the processor die 16 and also interfaces with a thermal plate 40. The depth of the recess 27 is greater than the thickness of the processor die 16. This ensures that the thermal plate 40 does not contact the surface of the processor die 16 and thus cause it to be damaged. 
     The thermal top cover 22 enables the OEM customer to maintain a close tolerance in the thickness of the thermal layer 20 during its formation. It is desirable to maintain such a close tolerance because the resistance of the thermal layer 20 is directly related to its thickness. Thus, the thicker the thermal layer 20 is the greater its thermal resistance. The optimal thickness of the thermal layer is between 0.003 and 0.008 inches. As long as the thickness of the thermal layer is maintained in this range, maximum heat transfer from the processor die 16 to the thermal plate 40 can be achieved. 
     The thermal plate 40 is disposed above the OLGA 12 and processor die 16 and is provided for cooling the various electrical components of the microprocessor assembly 10. The thermal plate 40 performs this function by dissipating heat along its entire surface, which is many times the size of the OLGA 12 and processor die 16. In one embodiment, the thermal plate 40 is approximately the same size as the substrate 14. As shown in FIG. 3, heat is transferred from the processor die 16 to the thermal plate 40 along the thermal path indicated by the arrows A. In this embodiment, the thermal plate 40 is formed of aluminum. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.