Abstract:
A method and structure to adhesively couple a cover plate to a semiconductor device. A semiconductor device is electrically coupled to a substrate. A stiffener ring surrounding the semiconductor device is adhesively coupled to the substrate. A cover plate is adhesively coupled to both a top surface of the semiconductor device and a top surface of the stiffener ring using a first and second adhesive, respectively. The modulus of the first adhesive is less than the modulus of the second adhesive.

Description:
This application is a divisional of Ser. No. 10/058,999; filed on Jan. 29, 2002 U.S. Pat. No 6,744,132. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to an electronic structure and associated method to adhesively couple a cover plate to a semiconductor device. 
   2. Related Art 
   A chip coupled to a substrate within an electronic structure may experience thermal strain from thermal cycling operations, in light of coefficients of thermal expansions (CTE) differential within the electronic structure. Such thermal strains may result in mechanical failure of the substrate. Thus, there is a need to inhibit such thermal strains. 
   SUMMARY OF THE INVENTION 
   The present invention provides an electronic structure, comprising: 
   a substrate; 
   a semiconductor device electrically coupled to the substrate; 
   a stiffener ring adhesively coupled to the substrate, wherein the stiffener ring surrounds the semiconductor; and 
   a cover plate on a top surface of the semiconductor and on a top surface of the stiffener ring, wherein the cover is adhesively coupled to the top surface of the semiconductor by a first adhesive, wherein the cover plate is adhesively coupled to a top surface of the stiffener ring by a second adhesive, and wherein an elastic modulus of the first adhesive is less than an elastic modulus of a second adhesive. 
   The present invention provides an electronic structure, comprising: 
   a substrate; 
   a semiconductor electrically coupled to the substrate; and 
   a cover plate on a top surface of the semiconductor, wherein the cover plate is adhesively coupled to the top surface of the semiconductor by a first adhesive, and wherein the first adhesive has an elastic modulus less than about 500 psi. 
   The invention provides a method for forming an electronic structure, comprising: 
   providing a semiconductor device; 
   electrically coupling the semiconductor device to a substrate; 
   adhesively coupling a stiffener ring to the substrate, wherein the stiffener ring surrounds the semiconductor device, and 
   adhesively coupling a cover plate to a top surface of the semiconductor device with a first adhesive and to a top surface of the stiffener ring with a second adhesive, wherein and elastic modulus the first adhesive is less than and elastic modulus of a second adhesive. 
   The present invention advantageously inhibits thermal strains in a substrate within an electronic structure, wherein such thermal strains result from thermal cycling operations on the electronic structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a cross-sectional view of an electronic structure having a cover plate coupled to a semiconductor device by an adhesive, in accordance with embodiments of the present invention. 
       FIG. 2  depicts a cross-sectional view an electronic structure having a cover plate coupled to a semiconductor device by an ultra low modulus adhesive, in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a front cross-sectional view of an electronic structure  10  using the same adhesive  21  in three different locations, in accordance with embodiments of the present invention. The adhesive may include, inter alia, silicone or epoxy. The electronic structure  10  comprises a semiconductor device  37 , a substrate  27 , a stiffener ring  24 , a thermally conductive cover plate  18 , a heat sink  12 , and the thermally conductive adhesive  21 . The semiconductor device may include, inter alia, a semiconductor chip. The cover plate  18  may include, inter alia, nickel plated copper, and the heat sink  12  may include, inter alia, aluminum. The cover plate may have a thickness of at least about 20 mils. The substrate  27  has a compliance range of 10 4  psi to 3×10 6  psi. The semiconductor device  37  is electronically coupled to a substrate  27  using a Controlled Collapse Chip Connection (C 4 ) solder ball  42 . The space surrounding the solder balls  42  may include and underfill  50 . The substrate  27  may comprise, inter alia, a chip carrier or a printed circuit board. Input/Output (I/O) connections  29  may be attached to the substrate  27  such as when the substrate  27  is a chip carrier. The substrate  27  may include organic material such as, inter alia, TEFLON. A bottom surface  26  of the stiffener ring  24  is adhesively bonded to a top surface  34  of the substrate  27  such as by a tacky film adhesive. A bottom surface  22  of the cover plate  18  is adhesively coupled to both a top surface  23  of the semiconductor device  37  and a top surface  25  of the stiffener ring  24  by the adhesive  21 . A bottom surface  14  of the heat sink  12  is adhesively coupled to a top surface  16  of the cover plate  18  by the adhesive  21 . The adhesive  21  has a modulus of at least about 1000 psi (e.g., 1000-1200 psi.) The coefficient of thermal expansion (CTE) of the heat sink  12  (e.g., 10 ppm/° C. to 24 ppm/° C.). is greater than the CTE of the cover plate  18  (e.g., 10 ppm/° C. to 24 ppm/° C.) The CTE of the cover plate  18  is greater than the CTE of the semiconductor device  37  (e.g., 2 ppm/° C. to 5 ppm/° C.). The CTE of the semiconductor device  37  is less than the CTE of the substrate  27  (e.g., 8 ppm/° C. to 50 ppm/° C.). The difference between the aforementioned CTE&#39;s create stress on the substrate  27  during thermal cycling. If the adhesive  21  having a modulus of at least 1000 psi is used in the previously mentioned three locations, thermal cycling will cause the heat sink  12  to expand and express itself through the following: the adhesive  21  below the heat sink  12 , the cover plate  18 , and the next layer of adhesive  21  below the cover plate  18 , ultimately stressing and constraining the semiconductor device  37 . The thermal cycling will cause the substrate  27  to be stressed under the constrained semiconductor device  37 . Since the semiconductor device  37  is constrained by the structure above as described supra, internal strain within the substrate  27  is caused fatigue with sufficient thermal cycling. This internal strain within the substrate  27  is primarily located at corners of the semiconductor device  37  footprint, the underfill  50 , the C 4  solder balls  42 , and the substrate  27 . As stated supra, the preceding problem is observed during thermal cycling. A solution to the previously mentioned problem is illustrated in FIG.  2 . 
     FIG. 2  illustrates a modification of  FIG. 1  using a combination three adhesives, in accordance with embodiments of the present invention. In  FIG. 2 , the electronic structure  10  uses an ultra low modulus adhesive  32  having a modulus of less than about 500 psi and a thermal conductivity of at least about one watt per meter degree K to adhesively coupled the top surface  23  of the semiconductor device  37  to the bottom surface  22  of the cover plate  18 . The ultra low modulus of the ultra low modulus adhesive  32  inhibits constraining of the semiconductor device and corresponding formation of strain on the substrate  27  during thermal cycling. A thermoset adhesive can be used, inter alia, for the ultra low modulus adhesive  32 . For structural integrity, the adhesive  21 , used to adhesively couple the top surface  25  of the stiffener ring  24  to the bottom surface  22  of the cover plate  18  has a higher modulus than the ultra low modulus adhesive  32  used to adhesively couple the top surface  23  of the semiconductor device  37  to the bottom surface  22  of the cover plate  18 . An adhesive  15  is used to adhesively couple the bottom surface  14  of the heat sink  12  to the top surface  16  of the cover plate  18 . An advantage of using the ultra low modulus adhesive  32  to adhesively couple the top surface  23  of the semiconductor device  37  to the bottom surface  22  of the cover plate  18  is that the range of modulus for the adhesive  15  increases from 1000-1200 psi up to a value of about 1,000,000 psi. The adhesive  15  normally has a modulus that is less than the modulus of the adhesive  21 . However, the modulus of the adhesive  15  may of may not be equal to the modulus of the adhesive  32 , and still the advantage of the ultra-low modulus adhesive can be obtained. 
   While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.