Abstract:
A chip mounting assembly is provided which includes a dielectric substrate having at least one integrated circuit (I/C) chip mounted thereon. An electrically conductive cover plate is in contact with all the chips with an electrically non-conducting thermally conducting adhesive. A stiffener member is provided which is mounted ante substrate and laterally spaced from the integrated circuit chip. At least one electrically conductive ground pad is formed an the substrate. The stiffener member has at least one through opening therein and electrically conductive adhesive extending through each opening and contacting the cover plate and each ground pad. The invention also provides a method of forming such an I/C chip assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/727,271, filed Nov. 30, 2000, now U.S. Pat. No. 6,407,334 B2. 
    
    
     BACKGROUND INFORMATION 
     1. Field of the Invention 
     This invention relates generally to I/C chip mounting structures which include a substrate and an electrically and thermally conducting cover plate and a method of manufacturing the same. In even more particular aspects, this invention relates to an I/C chip assembly which electrically insulates the chip from the cover plate but provides grounding of the substrate to the cover plate. 
     2. Background Information 
     In the packaging of I/C chips, there has developed a need for a chip package that includes a cover plate for the assembly, which cover plate is thermally conducting for heat transfer, and also electrically conducting for grounding the substrate, while preventing the chip itself from being electrically grounded to the cover plate, so that the cover plate can act as both a heat sink for the chip and also an electrical ground for the substrate. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a chip mounting assembly is provided which includes a dielectric substrate having at least one integrated circuit (I/C) chip mounted thereon. An electrically conductive cover plate is in contact with said at least one chip by an electrically non-conducting thermally conducting material. A stiffener member is provided which is mounted on the substrate and laterally spaced from the integrated circuit chip. At least one electrically conductive ground pad is formed on the substrate. The stiffener member has at least one through opening therein and electrically conductive material extending through said at least one opening and contacting said cover plate and said at least one ground pad. The invention also provides a method of forming such an I/C chip assembly. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of a substrate, a stiffener member and a thermal adhesive in position for lamination as a first step in the process of forming the device of the present invention; 
     FIG. 2 is a perspective view showing the substrate stiffener member and a film of adhesive material laminated together to form the basis for forming the present invention; 
     FIGS. 3-10 are longitudinal, sectional views, somewhat diagrammatic, showing the steps in forming an I/C chip assembly according to this invention, and 
     FIG. 11 is a flow chart of the steps involved in preparing the chip assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides an I/C chip assembly which includes a substrate mounting one or more I/C circuit chips and, optionally, other devices, wherein the chip assembly includes a cover plate which acts as a heat sink by being connected by an electrically non-conductive, thermally conductive adhesive to the chip, and which cover plate also is connected by an electrically conductive adhesive to ground pads on the substrate to provide a ground for the substrate. 
     As seen in FIG. 1, a generally conventional dielectric substrate  10  is provided which has a series of mounting pads formed thereon (other circuitry and features have been omitted for clarity). The mounting pads include a group of mounting-pads  12  which are for mounting an I/C chip; a group of mounting pads  14  for mounting capacitors, and a group of mounting pads  16  for providing electrical connections for grounding purposes to a cover plate. The substrate  10  can be made of any conventional dielectric material, such as FR4, polyimide, polytetrafluoroethylene or other dielectric materials, 
     A dry film of adhesive material  18  is provided. The film of material  18  has a central opening  20 , a plurality of side openings  22  and a plurality of corner openings  24  formed therein. The opening  20  corresponds to and is in alignment with the set of mounting pads  12 ; the openings  22  correspond to and are in alignment with the mounting pads  14 , and the openings  24  correspond to and are in alignment with the ground pads  16 . Preferably, the film of material  18  is a dry film adhesive, such as a polyimide/acrylic adhesive of the type sold under the trademark Pyralux by duPont. However, any film adhesive which can have openings formed therein can be used. 
     A stiffener member  26  is provided which preferably is formed of stainless steel, preferably  400  Series, and, more particularly,  420 . The stiffener  26  member has a central opening  28  which corresponds to and aligns with the opening  22  in the film material  18 , side openings  30  which correspond to and align with the openings  22  in the film  18  and corner openings  32  which correspond to and align with openings  24  the film  18 . 
     The substrate  10 , film  18  and stiffener member  26  are laid up in lamination press and heated to about 150° C. at 300 psi gauge for about an hour and half to form a laminate of these three items, as shown in FIGS. 2 and 3. (The preferred sequence for forming the chip assembly is shown in FIG.  11 ). 
     Alternatively, instead of a separate dry film, adhesive  18  can be replaced with a liquid adhesive which can be applied to the stiffener member  26 , and this will be used as the adhesive to secure this stiffener to the substrate. The liquid adhesive cat be any one of several adhesives; a particularly useful adhesive is GE 3281 manufactured by General Electric Corporation. 
     Following the lamination process, an I/C chip  34  is mounted on the pads  12 , and capacitors  36  are mounted on the pads  14  by conventional solder reflow techniques, as shown in FIG.  4 . Following the attachment of the I/C chip  34  and capacitors  36 , conventional underfill material  38  is applied around the solder connections of the chip  34  to the substrate and the capacitors  36  to the substrate, as shown in FIG.  5 . Conventional underfill material preferably is an epoxy with silica filler, such as Namics 8437-2 sold by the Namics Corporation, although other materials can be used. The Namics 8437-2 underfill  38  is cured at about 150° for about one hour at atmospheric pressure. 
     Following the application of the underfill  38 , a conductive material such as a conductive adhesive  40  is applied over each set of the ground pads  16  and in the openings  24  and  32  of the dielectric film  18  and stiffener member  26 , respectively, as shown in FIG.  6 . Any type of conductive adhesive can be used. A preferred adhesive is Able Stik 965-1L, which is an epoxy with silver flakes therein, sold by Able Stik Chemical Corporation. Other conductive materials such as conductive inks may be used. 
     Following the deposition of the conductive epoxy  40  on the ground pads  16 , a generally circumferential layer of electrically conductive material  42 , preferably the same material as the material  40 , is provided which is disposed over both the material  40  and the capacitors  36  and in the openings  22  and  30  in the adhesive film  18  and the stiffener member  26 , respectively. Care must be taken that this conductive material  42  does not extend over and into contact with the I/C chip  34  since the conductive material  42  eventually will serve to provide the electrical connection between the ground pads  16  and the cover plate, which will be described presently, and which would short out the chip  34  and cause the assembly to be inoperable. This step in the process is shown in FIG.  7 . 
     Following the deposition of the conductive material  42 , a non-electrically conductive, thermally conductive material, preferably an adhesive  44  is applied over the I/C chip  34  and in the openings  20  and  28  in the adhesive film  18  and  26 , respectively. The adhesive material  44  is preferably a silicone material having a filler therein. A particularly useful material is sold by General Electric Corporation under the trademark GE 3281. However, other thermally conductive, non-electrically conducting materials can be used. This step is shown in FIG.  8 . 
     A cover plate  46  is placed overlying the adhesives  42  and  44 . The cover plate  46  functions both as a grounding device and as a heat sink and, thus, a good electrically and thermally conducting material is preferred for the cover plate  46 . A particularly useful material is copper, although other metal plates could be used. As can be seen in FIG. 9, the cover plate  46  is electrically connected through adhesives  42  and  40  to the ground pads  16  on the substrate  10 , and the I/C chip  34  is connected through electrically non-conducting, thermally conductive adhesive  44  to the cover plate  46 . Thus, the cover plate  46  operates both as a heat sink with respect to the I/C chip  34  and a ground connection with respect to the substrate  10 . The assembly is cured for approximately one hour at about 150° C. at atmospheric pressure to provide the final heat chip assembly. 
     A plurality of ball grid array solder ball connectors  48  are applied to the bottom surface  50  of the substrate  10 , as shown in FIG.  10 . These provide the necessary connections to attach the I/C chip assembly to a printed wiring board. 
     The various steps of forming the I/C chip assembly are shown in FIG.  11 . 
     While the invention has been described using but a single I/C chip and various capacitors, it is to be understood that several I/C chips could be mounted on the substrate as well as other devices besides or in addition to capacitors. The important thing is that the conductive adhesive not come in contact with any of the I/C chips or any other device which could be unintentionally electrically grounded. Therefore, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.