Abstract:
An electronic package has a cover or lid mounted onto a substrate to enclose an electronic device, and a liquid thermal interface material is subsequently inserted (through dispensing, injection molding or printing through apertures in the cover or lid) between the surface of the electronic device and the cover, and cured to a solid state.

Description:
FIELD AND BACKGROUND OF INVENTION 
       [0001]    This invention is directed to a problem in uniformly applying a thermally conductive interface material (TIM) between an electronic device and an enclosing metal lid or cover. Additionally, the method of application of the thermal material taught here creates a low stress bond between the device and lid, reducing potential of damage to the device. 
         [0002]    Known solutions include attaching and bonding a TIM between a lid and an electronic device, typically in a limited coverage area. The TIM is applied either to the inside of the lid or to the surface of the device, and then the lid and electronic device substrate are mated together at the edges of the lid sides. Force is applied typically to the top of the lid to make contact between the lid, the substrate, the TIM and the electronic device simultaneously. 
         [0003]    This creates a challenge to manage all surface contacts due to non-planarity on any given surface (lid, TIM, substrate, electronic device), resulting in non-attached areas either between the lid sides and the substrate, the inner side of the lid and TIM, or between the TIM and the electronic device surface. An inadequate attachment can create either environmental or structural damage to the device(s) being packaged, or functional damage to the device due to poor thermal conductivity between the heat source (electronic device) and the lid. 
       SUMMARY OF THE INVENTION 
       [0004]    One purpose of this invention is to provide a metallic cover designed and constructed with four sides and a top or lid and a pattern of aperture openings (holes) in the cover located in the lid surface and/or sides of the cover. 
         [0005]    An other purpose is to provide a method of manufacturing an electronic package that consists of mounting a cover or lid onto a substrate, and subsequently inserting a liquid thermal interface material (through dispensing, injection molding or printing through apertures in the cover or lid) between the surface of the electronic device and the cover, and curing the TIM to a solid state. 
         [0006]    The advantage of using this invention instead of known solutions is that this invention provides a way to create a uniform, low stress, thermal interface between a heat source (electronic device) and dispersion source (cover or lid) with no loss of thermal conduction caused by air gaps in the TIM or between interfaces and/or non-uniform TIM coverage of the electronic device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    Some of the purposes of the invention having been stated, others will appear as the description proceeds, when taken in connection with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a representation of a first form of cover in accordance with this invention, shown in exploded elevation view with other elements; 
           [0009]      FIG. 2  is a representation of the apparatus of  FIG. 1 , shown in perspective; 
           [0010]      FIG. 3  is a representation of a second form of cover in accordance with this invention, shown in exploded elevation view with other elements; 
           [0011]      FIG. 4  is a sectional view through an assembled apparatus using the covers of  FIGS. 1 and 2  or  3 ; and 
           [0012]      FIG. 5  is a representation of the process of assembling the cover with a substrate and device and completing the installation of a thermally conductive interface material. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0013]    While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the present invention is shown, it is to be understood at the outset of the description which follows that persons of skill in the appropriate arts may modify the invention here described while still achieving the favorable results of the invention. Accordingly, the description which follows is to be understood as being a broad, teaching disclosure directed to persons of skill in the appropriate arts, and not as limiting upon the present invention. 
         [0014]    As briefly mentioned above, the invention has two aspects. The first addressed here is the mechanical structure involved. 
         [0015]    Referring now to  FIGS. 1 through 3 , a device cover or lid  10 ,  20  is designed and manufactured with a top surface and four side walls. As will later appear, the cover may be a unitary stamping ( FIGS. 1 and 2 ) or an assembly of components ( FIG. 3 ). Apertures or openings are patterned on the upper surface  11  and/or the  12  sides of the cover  10 . These holes are patterned and located to allow liquid thermal interface materials to be inserted through the holes. The exact pattern, location and size of holes are determined through modeling and process optimization. Modeling of the lid holes, through finite element analysis for example, will provide an understanding of the interaction between lid strength, TIM insertion process, post-dispense curing and outgassing. 
         [0016]    As an alternative, the device cover (here, indicated at  20  in  FIG. 3 ) may be fabricated with separate side walls  21  and a separate top or lid  22 . When so manufactured, the top  22  will preferably be a snap fit or the like onto the sidewalls  21 , facilitating assembly with the substrate. 
         [0017]    The second aspect is the process of assembly using the covers  10 ,  20  disclosed ( FIG. 5 ). In that process, an electronic device  11 ,  24  is attached to an interconnect substrate  13 ,  25  through normal means (either epoxy attach and wirebond connection, or flip chip solder attach and epoxy underfill). The cover  10 ,  20  is attached to the substrate  12 ,  25  by conventional means, such as by using a bonding material. The inside of the lid may not contact the electronic device (or any other components that may be on the substrate). As will be understood, the cover  10 ,  20  encloses the electronic device and defines with the substrate  12 ,  25  a volume around the electronic device  11 ,  24 , the cover being spaced from the electronic device and having at least one aperture formed therein. After the cover is attached and bonded to the substrate, a liquid thermally conductive material is then inserted (through dispensing, injection or screen printing) through the holes in the lid that are aligned over the surface area of the electronic device or component. As part of this process, excess liquid material may be inserted to ensure complete surface coverage. 
         [0018]    The package (module) is then sent through a normal curing process to solidify the thermal interface material. As part of this process, the module may be in a fixture that applies pressure between the lid and the package substrate, causing a uniform cure and forcing any air that might be trapped in the liquid material to outgas. Alternatively, the module may go through a curing process without need for a fixture. Any excess material can be cleaned from surface of the cover if necessary. This can be done either chemically or mechanically. 
         [0019]    By attaching the lid to the substrate and curing first, any warpage between surfaces of the inside of the lid and the electronic device will determined. Subsequently inserting the thermal interface material between the device (heat source) and the lid will provide maximum material uniformity and improved thermal conductivity between source and lid can be established. See  FIG. 4 . 
         [0020]    In the drawings and specifications there has been set forth a preferred embodiment of the invention and, although specific terms are used, the description thus given uses terminology in a generic and descriptive sense only and not for purposes of limitation.