Abstract:
A method for attaching an electronic die to a substrate is disclosed. Preferably, the method includes fixing the die to the substrate, interconnecting the electronic die to an at least one bonding pad on the substrate to form an electrical connection, coating the interconnects and the electronic die with an electrically insulating coating, and covering the electronic die with a low temperature melting metal. Thus, the method of the present invention improves the reliability of the electronic die.

Description:
TECHNICAL FIELD 
     The present invention relates to methods for attaching integrated circuit dies to substrates using low temperature solder. 
     BACKGROUND 
     Silicon dies having integrated circuits created therein are typically attached to a heat spreader or substrate. Wire bond is electrically fixed to the silicon die and bonding pad on the substrate to communicate electrical signals from the integrated circuits within the die to electronic circuits external to the die. The attachment of the silicon die to the substrate or heat spreader is of critical importance. The methods and materials used to attach the die to the substrate have a significant impact on the performance and reliability of the silicon die. 
     One prior art method for attaching the silicon die to the substrate utilizes high temperature solder material. While this method accomplishes its intended purpose, significant problems still exist. For example, attaching the silicon die with high temperature solder induces stresses in the die upon thermal cycling. These stresses are caused by the different coefficients of thermal expansion between the silicon die, solder, and substrate. Consequently, the induced stresses lead to early failure of the die. 
     Other methods for attaching the silicon die to the substrate to overcome the disadvantages stated above have been developed. For example, an improved method for attaching the silicon die utilizes low temperature solder. While this method for attaching the silicon die to the substrate or heat spreader eliminates the stresses induced in the die and thus significantly increases the die&#39;s life, significant problems still exist. For example, the use of this method creates voids and cracks in the solder material. The voids and cracks significantly increases thermal resistance in the solder material. Consequently, the increased thermal resistance raises the junction temperature and ultimately leads to premature failure of the silicon die. 
     Therefore, there is a need for a new and improved method for attaching a silicon die to a substrate. This new and improved method must reduce or eliminate thermal stresses in the silicon die and must increase the life in service of the silicon die. 
     SUMMARY 
     In accordance with an aspect of the present invention, a new and improved method for attaching silicon die to a substrate is provided. The method of the present invention creates a void-free attachment of the silicon die to the substrate. More specifically, in one embodiment a low temperature die attachment material (i.e., solder) is used to affix the silicon die to the substrate and in another embodiment a vacuum is used to hold the die against the die holder until the die is permanently fixed to the substrate. 
     In another embodiment of the present invention, a method for attaching an electronic die to a substrate is disclosed. Preferably, the method includes fixing the die to the substrate, interconnecting the electronic die to an at least one bonding pad on the substrate to form an electrical connection, coating the interconnects and the electronic die with an electrically insulating coating, and covering the electronic die with a low temperature melting metal. Thus, the method of the present invention improves the reliability of the electronic die. 
     In still another embodiment of the present invention, the die is temporarily fixed to the substrate using low temperature solder. 
     In still another embodiment of the present invention, the die is temporarily fixed to the die holder using a vacuum, until the die is permanently fixed to the substrate. 
     In yet another embodiment of the present invention, the substrate is filled with a heat dissipating material substantially covering the die. 
     In yet another embodiment of the present invention, the substrate is sealed with a cover after the heat dissipating material is deposited over the die. 
     Further aspects, features and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a top view of a die and substrate, wherein the die was attached to the substrate using prior art methods; 
     FIG. 1 b  is a cross-sectional view of the die and substrate of FIG. 1 a;    
     FIG. 2 a  is a top view of a die and substrate wherein the die is attached to the substrate using the system and method of the present invention; 
     FIGS. 2 b-e  are a cross-sectional view through the die and substrate as indicated in FIG. 2 a;    
     FIG. 3 a  is a top view of an alternative embodiment of the present invention, in accordance with the present invention; and 
     FIGS. 3 b-d  are cross-sectional views through the die and substrate of FIG. 3 a  as indicated in FIG. 3 a , in accordance with the present invention. 
     FIG. 4 is a flowchart illustrating a method for attaching a die to a substrate, in accordance with an embodiment of the present invention. 
     FIG. 5 is a flowchart illustrating an alternative method for attaching a die to a substrate, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIGS. 1 a  and  1   b , a top and cross-sectional views of a prior art system for attaching an integrated circuit die to a substrate is illustrated. In this prior art method and system, an integrated circuit die  10 , typically made of silicon, is affixed to a substrate or heat spreader  12 . The heat spreader may be copper or copper-Invar-copper. Conventionally, high temperature solder  14  is disposed between die  10  and substrate  12  to secure the die thereto. The high temperature solder is typically, 10% Sn-88% Pb-2% Ag or 95% Sn-5% Pb. Such high temperature solders have about a 300° C. melting temperature and no low creep under normal operating temperatures of 60 to 150° C. 
     After the die has been secured to substrate  12 , wire bond  16  may be applied. Wire bond  16  is welded or similarly affixed at one end to die  10  and at another end to a bond pad  18 . Wire bond  16  communicates electrical signals between die  10  and bond pad  18 . Bond pad  18 , as well known in the art, is in communication with electrical circuits and electronic devices external to die  10 . 
     While high temperature solder works to affix die  10  to substrate  12  problems arise through the use of this material. For example, high temperature solder  14  induces stresses in the die upon thermal cycling. The stresses are the result of the disparity in coefficients of thermal expansion between the die, solder, and substrate. A result of the induced stresses is premature failure of the die, by the creation of cracks in the electronic die module. 
     Another problem the present inventors have observed, arising during the attachment of the die to the substrate is air entrapment resulting in the creation of voids in the solder. The voids or trapped air in the solder increases the thermal resistance of the solder and thus increases the die operating temperature. The present invention, eliminates this problem and other problems. 
     With reference to FIGS. 2 a-e , a plan and cross-sectional view of a die  10 ′ mounted in a substrate  12 ′ are illustrated. Substrate  12 ′ in an embodiment of the present invention, includes an upstanding wall  32  extending from a bottom  34 . Wall  32  which extends around the perimeter of bottom  34  together with bottom  34  define a cavity  36 . 
     A plurality of electrical connectors or tabs  38  are disposed within an upper end wall portion  40  of wall  32 . Die  10 ′ is electrically interconnected with a first tab end  42  by a wire bond  44 . A second tab end  46  electrically interconnects die  10 ′ prime with circuitry exterior of the substrate  12 ′. 
     With continuing reference to FIGS. 2 b  through  2   e  a method for attaching die  10 ′ to substrate  12 ′ is illustrated, in accordance with the present invention. FIG. 2 b  is a cross sectional view through die  10 ′ and substrate  12 ′, as indicated in FIG. 2 a . Once die  12 ′ has been fixedly attached to bottom  34  of substrate  12 ′, wire bond  44  may be welded at a first end  48  to die pads  50 . Pluralities of bond pads  50  are etched on the top surface  52  of die  10 ′. A second end  54  of wire bond  44  is electrically connected to first tab end  42 . After the wire bond  44  have been installed, die  10 ′ and wire bond  44  are coated with an electrically insulating epoxy or other suitable material. Thus, there may be more than one die attached to the substrate  12 ′ and after coating with the epoxy the die will be electrically isolated from each other. 
     Referring now to FIG. 2 d  cavity  36  of substrate  12 ′ is shown after being filled with a low temperature melting metal, such as low temperature solder  60 . in accordance with the present invention. As die  10 ′ is encapsulated with solder  60  air trapped within cavity  36  is forced out eliminating air entrapment or formation of void within the solder. Further, solder  60  acts as heat sink during transient conditions or operation. The mass of solder  60  absorbs the power dissipated by silicon die  10 ′. Solder  60  has a higher heat absorbing capacity as heat is absorbed during the phase change of the solder from solid to solidus is to liquid. Thus, essentially little to no stresses are induced either in solder material  60  or in die  10 ′. Therefore, this system and method of die attachment has a high thermal conductivity and significantly improves thermal dissipation in high power applications. Finally, in FIG. 2 e  a lid  62  is mated with wall  32  of substrate  12 ′ to form an airtight container. Cover or lid  62  may be sealed with a vacuum or inert gas to expel air trapped within cavity  36 . 
     Preferably, the low temperature solder is composed of low melting temperature alloys such as 82.7 percent by weight Bi, 17.3 percent by weight Ga in a Bi—Ga solder system or 57 percent by weight Bi-17 percent by weight Sn, 26 percent by weight In, or 52.5 percent by weight Bi, 15.5 percent by weight Sn and 32 percent by weight Pb. 
     Referring now to FIGS. 3 a - 3   d , an alternate embodiment of the system and method for attaching a silicon die  10 ′ or similar integrated circuit (or semiconductor material) to a substrate  12 ′ is illustrated, in accordance with the present invention. Die  10 ′ is placed in a vacuum hold down fixture  70  and a vacuum is applied through an orifice  72  in hold down fixture  70 . The vacuum forces die  10 ′ against hold down fixture  70  in a direction indicated by arrow d. Wire bond is then intereconnected between die  10 ′ and circuit board connectors or tabs  38 . As illustrated in FIG. 3 b  the die  10 ′, wire bond  44 , circuit tabs  38  assembly is removed from the hold down fixture. An epoxy or similar material is applied to the die and wire bond to create an insulating covering. 
     With reference to FIG. 3 c , a low temperature mounting material  60  is deposited within a cavity  36  of substrate  12 ′. Solder material  60  totally covers and encapsulates die  10 ′ and wire bonds  44 . Thus, the die  10 ′ is suspended within cavity  36  of substrate  12 ′. 
     Referring now to FIG. 3 d , a cross-sectional view of die  10 ′ and substrate  12 ′ is further illustrated. After cavity  36  is filled with solder material  60  such that die  10 ′ and wire bonds  44  are totally covered by the solder material  60  a lid or top  62  is fixedly mounted to walls  32  of substrate  12 ′. 
     Referring now to FIG. 4, a first embodiment of the method for attaching die  10 ′ to substrate  12 ′ is illustrated in flow chart form, in accordance with the present invention. At block  152 , die  10 ′ is fixedly secured to substrate  12 ′ by application of a low temperature melting material  30  (shown in FIGS. 2 b  and  2   c ) such as low temperature solder. Wire bond is then electrically connected from the die  10 ′ to circuit board tabs or connectors, as represented at block  154 . An epoxy coating or other electrically insulative material is applied to the wire bonds and die to electrically isolate the wire bonds and die from the surrounding environment. At block  158 , the cavity containing die  10 ′ is filled with a low temperature solder material until die  10 ′ and wire bonds  44  are completely submerged in the solder. A lid or cover is mated with substrate  12 ′ to seal cavity  36 , as represented by block  160 . Thus, the present invention provides a method for attaching a silicon die to a substrate to reduce stresses on the die. 
     Referring now to FIG. 5, an alternative method for attaching a die to a substrate is illustrated, in accordance with the present invention. Method  250  includes at block  252  placing die  10 ′ on a vacuum holder for stabilizing the die and retaining it. At block  254  wire bonds are electrically connected to the die and to circuit board tabs or connectors to electrically interconnect the die to circuits external of the die. At block  256 , the die and wire bond assembly is removed from the vacuum holder. An epoxy is applied to the wire bond and die assembly to coat the die and wire bond with an electrically insulating layer, as representative of block  258 . At block  260 , the wire bond and die assembly is placed in a cavity  36  of substrate  12 ′. The cavity  36  is then filled with a low temperature solder material until the entire wire bond and die assembly is covered, as represented by block  262 . At block  264 , a lid or cover is mated with substrate  12 ′ to provide an air tight seal. Thus, the present invention provides an alternative method for attaching a die to a substrate to reduce stresses induced on the die. 
     The foregoing discussion discloses and describes a preferred embodiment of the invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that changes and modifications can be made to the invention without departing from the true spirit and fair scope of the invention as defined in the following claims.