Patent Publication Number: US-6699736-B2

Title: Electrical coupling of a stiffener to a chip carrier

Description:
This application is a divisional of Ser. No. 09/657,194; filed on Sep. 7, 2000 now U.S. Pat. No. 6,534,848. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method and structure for conductively coupling a metallic stiffener to a chip carrier. 
     2. Related Art 
     A metallic stiffener may be placed on a chip carrier to reduce bending and other deformations of the chip carrier. For some applications, it may be desirable to provide electrically conductive coupling between the metallic stiffener and the chip carrier. Thus, there is a need to provide a method and structure for conductively coupling a metallic stiffener to the chip carrier. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electronic structure, comprising: 
     a substrate having a first electrically conductive pad on a surface of the substrate; 
     an electrically conductive stiffener; and 
     an adhesive layer that mechanically couples the stiffener to the surface of the substrate and electrically couples the stiffener to the first electrically conductive pad. 
     The present invention provides an electronic structure, comprising: 
     a substrate having a first electrically conductive pad on a surface of the substrate; and 
     a layer of uncured dry adhesive on the substrate, wherein a hole in the uncured dry adhesive includes an uncured electrically conductive contact, and wherein the electrically conductive contact is electrically coupled to the first electrically conductive pad. 
     The present invention provides a method of forming an electronic structure, comprising: 
     providing a substrate having a first electrically conductive pad on a surface of the substrate; 
     forming an uncured adhesive layer on the surface of the substrate; 
     placing an electrically conductive stiffener on the adhesive layer, wherein the uncured adhesive layer mechanically couples the stiffener to the surface of the substrate and electrically couples the stiffener to the first electrically conductive pad; and 
     curing the uncured adhesive layer. 
     The present invention provides a method and structure for conductively coupling a metallic stiffener to the chip carrier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a front cross-sectional view of an electronic structure, in accordance with first embodiments of the present invention. 
     FIG. 2 depicts a front cross-sectional view of an electronic structure, in accordance with second embodiments of the present invention. 
     FIG. 3 depicts a front cross-sectional view of an electronic structure, in accordance with third embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a front cross-sectional view of an electronic structure  10 , in accordance with first embodiments of the present invention. Fabricating the electronic structure  10  includes providing a substrate  12  having an electrically conductive pad  14  on a surface  13  of the substrate  12 , forming an adhesive layer  21  on the surface  13  of the substrate  12 , placing an electrically conductive stiffener  22  on the adhesive layer  21 , and curing the adhesive layer  21 . The substrate  12  may include, inter alia, a laminate such as a chip carrier. The electrically conductive pad  14  may include, inter alia, a conductive metal such as gold or palladium that does not oxidize at room temperature. 
     The adhesive layer  21  may be formed on the surface  13  of the substrate  12  by applying an electrically conductive adhesive  16  to the pad  14 , and setting a dry adhesive  18  on the surface  13  such that the electrically conductive adhesive  16  is within a hole  19  in the dry adhesive  18 . Within the hole  19 , there may be a gap or space  20  between the electrically conductive adhesive  16  and the dry adhesive  18 . The gap or space  20  provides clearance that enables the electrically conductive adhesive  16  to be free of interference from the dry adhesive  18 , and allows room for accommodating mechanical misalignment between the substrate  12  and the electrically conductive stiffener  22 . The electrically conductive adhesive  16  may be applied to the pad  14  before setting the dry adhesive  18  on the surface  13 . Alternatively, the dry adhesive  18  may be set upon the surface  13  before the electrically conductive adhesive  16  is applied within the hole  19  to the pad  14 . The electrically conductive adhesive  16  may include, inter alia, a thermoset adhesive with included silver particles, such as that produced by ABLESTIK as product number RP-507-30. The dry adhesive  18  may be conductive or nonconductive and may include, inter alia, an internal polyimide layer sandwiched between outer layers of an acrylic, such as that produced by E. I. duPont Nemoirs and Company as PYRALUX® LF-131. Other dry adhesive  18  materials that could be used include an expanded and filled teflon such as that produced by W. L. Gore and Associates as product number GST-5020. 
     The electrically conductive stiffener  22  is placed on the adhesive layer  21  such that: the electrically conductive adhesive  16  conductively contacts an electrically conductive pad  24  that is on a surface  23  of the stiffener  22 ; and the dry adhesive  18  mechanically couples the stiffener  22  to the surface  13  of the substrate  12 . The stiffener  12  comprises a conductive material, such as a metal, that is capable of being adhesively coupled to the electrically conductive adhesive  16  and to the dry adhesive  18  upon subsequent curing of the electrically conductive adhesive  16  and the dry adhesive  18 , respectively. The stiffener  22  may include, inter alia, a metallic material such as stainless steel, copper, nickel, or titanium. 
     Curing the adhesive layer  21  includes curing both the dry adhesive  18  and the electrically conductive adhesive  16 . Curing the adhesive layer  21  may be accomplished by any method known to one of ordinary skill in the art, such as by pressurization at an elevated temperature. The pressurization may be accomplished, inter alia, by placing the electronic structure  10  within a sealed enclosure, depressurizing a space within the enclosure by forming a partial vacuum inside the enclosure, and externally applying an external mechanical pressure (i.e., a compressive stress) on the enclosure from outside the enclosure so as to compressively stress the electronic structure  10 , and particularly the electrically conductive adhesive  16  and the dry adhesive  18 . The externally applied mechanical pressure on the enclosure is accompanied by heating the electronic structure  10  within the enclosure. Many combinations of external pressure upon the enclosure and elevated temperature within the enclosure are within the scope of the present invention. For example, an external pressure of about 300 psi may be employed together with an enclosure temperature of about 150° C. for about two hours. The curing of the dry adhesive  18  causes the dry adhesive  18  to adhesively couple the stiffener  22  to the surface  13  of the substrate  12 . The curing of the electrically conductive adhesive  16  causes the electrically conductive adhesive  16  to adhesively and electrically couple the pad  24  to the pad  14 . The electrically conductive adhesive  16  is an example of an electrically conductive contact that could be used for electrically coupling the stiffener  22  to the pad  14 . 
     After the curing of the adhesive layer  21 , an electronic device  30  may be coupled to the surface  13  of the substrate  12 . Such coupling may be accomplished by any method known to one of ordinary skill in the art, such as by, inter alia, interfacing a conductive interface  32  between the electronic device  30  and an electronically conductive pad  34  that is on the surface  13  of the substrate  12 . As an example, the electronic device  30  may include a semiconductor chip, and the conductive interface  32  may include a controlled collapse chip connection (C4) solder ball. 
     FIG. 2 illustrates a front cross-sectional view of an electronic structure  40 , in accordance with second embodiments of the present invention. Fabricating the electronic structure  40  includes providing a substrate  42  having an electrically conductive pad  44  on a surface  43  of the substrate  42 , forming an adhesive layer  51  on the surface  43  of the substrate  42 , placing an electrically conductive stiffener  52  on the adhesive layer  51 , and curing the adhesive layer  51 . The substrate  42  may include, inter alia, a laminate such as a chip carrier. The electrically conductive pad  44  may include, inter alia, a conductive metal, such as gold or palladium, that does not oxidize at room temperature. Alternatively, the electrically conductive pad  44  may include, inter alia, a conductive metal (e.g., copper) that is solder coated such that the conductive metal would not be exposed to air, and thus could not oxidize, at room temperature. 
     The adhesive layer  51  may be formed on the surface  43  of the substrate  42  by applying a metallic solder  46  (in a form of a solder volume such as a solder sphere) to the pad  44 , and setting a dry adhesive  48  on the surface  43  such that the metallic solder  46  is within a hole  49  in the dry adhesive  48 . The metallic solder  46  may include, inter alia, a eutectic alloy of lead and tin (i.e., a lead/tin ratio of about 63/37 by weight with a melting temperature of about 183° C.). Within the hole  49 , there may be a gap or space  50  between the metallic solder  46  and the dry adhesive  48 . The gap or space  50  provides clearance that enables the metallic solder  46  to move when reflowed by heating. The metallic solder  46  may be applied to the pad  44  before setting the dry adhesive  48  on the surface  43 . Alternatively, the dry adhesive  48  may be set upon the surface  43  before the metallic solder  46  is applied within the hole  49  to the pad  44 . The metallic solder  46  may include, inter alia, a eutectic lead/tin alloy. The dry adhesive  48  may be conductive or nonconductive and may include, inter alia, an internal polyimide layer sandwiched between outer layers of an acrylic, such as that produced by E. I. duPont Nemoirs and Company as PYRALUX® LF-131. Other dry adhesive  48  materials that could be used include an expanded and filled teflon such as that produced by W. L. Gore and Associates as product number GST-5020. 
     The electrically conductive stiffener  52  is placed on the adhesive layer  51  such that: the metallic solder  46  conductively interfaces the stiffener  52  with the pad  44 ; and the dry adhesive  48  mechanically couples the stiffener  52  to the surface  43  of the substrate  42 . The stiffener  52  comprises a conductive material, such as a metal, that is solder wettable to the metallic solder  46  and is capable of being adhesively coupled to the dry adhesive  48  upon subsequent curing of the dry adhesive  48 . The stiffener  52  may include, inter alia, a solder wettable metallic material (e.g., series 400 stainless steel), or a metallic material with a solderable plating surface thereon (e.g., 300 series stainless steel having a gold plating surface thereon, copper having a nickel plating surface thereon). The stiffener  52  serves to provide a flat, rigid surface to diminish a non-planarity of the substrate  12 . 
     Curing the adhesive layer  51  includes curing the dry adhesive  48 . The curing of the adhesive layer  51  may be accomplished by any method known to one of ordinary skill in the art such as by pressurization at an elevated temperature. The pressurization may be accomplished, inter alia, by placing the electronic structure  40  within a sealed enclosure, depressurizing a space within the enclosure by forming a partial vacuum inside the enclosure, and externally applying an external mechanical pressure (i.e., a compressive stress) on the enclosure from outside the enclosure so as to compressively stress the electronic structure  40 , and particularly the dry adhesive  48 . The externally applied mechanical pressure on the enclosure is accompanied by heating the electronic structure  40  within the enclosure. Many combinations of external pressure upon the enclosure and elevated temperature within the enclosure are within the scope of the present invention. For example, an external pressure of about 300 psi may be employed together with an enclosure temperature that is high enough (e.g., 200° C.) to reflow the metallic solder  46  for a sufficient period of time (e.g., about two hours). Thus, a minimum acceptable enclosure temperature depends on a metallic composition of the metallic solder  46 . The curing of the dry adhesive  48  causes the dry adhesive  48  to adhesively couple the stiffener  52  to the surface  43  of the substrate  42 . The heating that occurs during the curing reflows the metallic solder  46 . During the reflow of the metallic solder  46 , the dry adhesive  48  acts as a solder stop and a solder gasket, and the reflowed metallic solder  46  solderably adheres to the stiffener  52  while liquified. Thus, the heating during curing causes the metallic solder  46  to adhesively and electrically couple the stiffener  52  to the pad  44 . The metallic solder  46  is an example of an electrically conductive contact that could be used for electrically coupling the stiffener  52  to the pad  44 . 
     After the curing of the adhesive layer  51 , an electronic device  60  may be coupled to the surface  43  of the substrate  42 . Such coupling may be accomplished by any method known to one of ordinary skill in the art, such as by, inter alia, interfacing a conductive interface  62  between the electronic device  60  and an electronically conductive pad  64  that is on the surface  43  of the substrate  42 . As an example, the electronic device  60  may include a semiconductor chip, and the conductive interface  62  may include a controlled collapse chip connection (C4) solder ball. 
     FIG. 3 illustrates a front cross-sectional view of an electronic structure  70 , in accordance with third embodiments of the present invention. Fabricating the electronic structure  70  includes providing a substrate  72  having an electrically conductive pad  74  on a surface  73  of the substrate  72 , forming an adhesive layer  81  on the surface  73  of the substrate  72 , placing an electrically conductive stiffener  82  on the adhesive layer  81 , and curing the adhesive layer  81 . The substrate  72  may include, inter alia, a laminate such as a chip carrier. The electrically conductive pad  74  may include, inter alia, a conductive metal such as gold or palladium that does not oxidize at room temperature. 
     The adhesive layer  81  may be formed on the surface  73  of the substrate  72  by applying an electrically conductive adhesive  78  to the surface  73  of the substrate  72 . The electrically conductive adhesive  78  may include, inter alia, metallic particles dispersed within an epoxy material (e.g., a dry tape epoxy having dispersed silver particles therein, such as that produced by AI Technology Inc. as product number ZEF8410-FP). 
     The electrically conductive stiffener  82  is placed on the adhesive layer  81  such that: the stiffener  82  conductively contacts the electrically conductive adhesive  78 ; and the electrically conductive adhesive  78  mechanically couples the stiffener  82  to the surface  73  of the substrate  72 . The stiffener  72  comprises a conductive material, such as a metal, that is capable of being adhesively and conductively coupled to the electrically conductive adhesive  78  upon subsequent curing of the electrically conductive adhesive  78 . The stiffener  82  may include, inter alia, a metallic material covered with a gold or palladium. 
     Curing the adhesive layer  81  includes curing the electrically conductive adhesive  78 . The curing of the electrically conductive adhesive  78  may be accomplished by any method known to one of ordinary skill in the art such as by force or pressurization at an elevated temperature. The pressurization may be accomplished, inter alia, by placing the electronic structure  70  within a sealed enclosure, depressurizing a space within the enclosure by forming a partial vacuum inside the enclosure, and externally applying an external mechanical pressure (i.e., a compressive stress) on the enclosure from outside the enclosure so as to compressively stress the electronic structure  70 , and particularly the electrically conductive adhesive  78 . The externally applied mechanical pressure on the enclosure is accompanied by heating the electronic structure  70  within the enclosure. Many combinations of external pressure upon the enclosure and elevated temperature within the enclosure are within the scope of the present invention. For example, an external pressure of about 100 psi may be employed together with an enclosure temperature of about 160° C. for about 1 minute. The curing of the electrically conductive adhesive  78  causes the electrically conductive adhesive  78  to adhesively and electrically couple the stiffener  82  to the surface  73  of the substrate  72 . As another example, an external pressure of about 100 psi may be employed together with heated plates at about 160° C. for about 1 minute, wherein the heated plates are applied to the electronic structure  70 . 
     After the curing of the adhesive layer  81 , an electronic device  90  may be coupled to the surface  73  of the substrate  72 . Such coupling may be accomplished by any method known to one of ordinary skill in the art, such as by, inter alia, interfacing a conductive interface  92  between the electronic device  90  and an electronically conductive pad  94  that is on the surface  73  of the substrate  72 . As an example, the electronic device  90  may include a semiconductor chip, and the conductive interface  92  may include a controlled collapse chip connection (C4) solder ball. 
     While particular 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.