Attaching semiconductor dies to substrates with conductive straps

Abstract

This invention provides a method apparatus for electrically connecting a semiconductor die, such as a power MOSFET, to a substrate on which the die is mounted, e.g., a lead frame, with a conductive strap, such that the connection is resistant to the shear stresses incident upon it with changes in temperature of the device. The method includes providing a conductive strap, and in one embodiment thereof, forming a recess in the top surface of the substrate. The bottom surface of a flange portion of the strap is attached to the floor of the recess such that the recess captures the flange and prevents relative horizontal movement of the flange and substrate with variations in the temperature of the device. Other embodiments include attaching the strap to the die and substrate with joints of a resilient conductive elastomer, and forming apertures in the strap and substrate that cooperate with a conductive joint material to reinforce the connection against temperature-induced shear forces.

What is claimed is:  
     
         1 . A method for electrically connecting a semiconductor die to a substrate with a conductive strap, the method comprising: 
 forming a recess in a top surface of the substrate, the recess having a floor disposed below the top surface of the substrate;    attaching a bottom surface of a first portion of the strap to a top surface of the die with a first electrically conductive material; and,    attaching a bottom surface of a second portion of the strap to the floor of the recess with a second electrically conductive material such that the second portion of the strap is mechanically captivated by the recess.    
     
     
         2 . The method of  claim 1 , wherein the substrate comprises a lead frame, and wherein forming a recess in the top surface of the substrate comprises etching, electrical-discharge machining (“EDM”), stamping, punching, coining, or laser-burning the recess therein.  
     
     
         3 . The method of  claim 1 , further comprising forming an aperture through the second portion of the strap.  
     
     
         4 . The method of  claim 3 , wherein the aperture tapers toward the bottom surface of the second portion of the strap.  
     
     
         5 . The method of  claim 3 , wherein the second electrically conductive material flows into the aperture and forms an interlocking key therein.  
     
     
         6 . A method for electrically connecting a semiconductor die to a substrate with a conductive strap, the method comprising: 
 attaching first and second layers of a conductive elastomer to top surfaces of the die and the substrate, respectively;    attaching a bottom surface of a first portion of the strap to a top surface of the first layer of elastomer on the die; and,    attaching a bottom surface of a second portion of the strap to a top surface of the second layer of elastomer on the substrate such that the strap is free to move resiliently relative to the die and the substrate while remaining electrically connected thereto.    
     
     
         7 . The method of  claim 6 , wherein attaching the bottom surface of the die to the top surface of the substrate comprises: 
 attaching a third layer of a conductive elastomer to the top surface of the substrate; and,    attaching the bottom surface of the die to a top surface of the third elastomer layer on the substrate.    
     
     
         8 . The method of  claim 6  wherein attaching the respective bottom surfaces of the first and second portions of the strap to the respective top surfaces of the first and second layers of elastomer comprises: 
 dispensing a layer of fluid, uncured conductive elastomer on respective ones of the die and the substrate;  
 contacting the strap to the top surfaces of the uncured layers of elastomer; and,  
 curing the elastomer layers to solidify them while the strap is in contact with them.  
 
     
     
         9 . The method of  claim 6  wherein attaching the respective bottom surfaces of the first and second portions of the strap to the respective top surfaces of the first and second layers of elastomer comprises: 
 attaching a layer of cured conductive elastomer on respective ones of the die and the substrate;  
 heating the respective top surfaces of the first and second layers of elastomer to melt them;  
 contacting the strap to the melted surfaces of the layers of elastomer; and,  
 cooling the elastomer layers to solidify them while the strap is in contact with them.  
 
     
     
         10 . A method for electrically connecting a semiconductor die to a substrate with a conductive strap, the method comprising: 
 attaching a bottom surface of a first portion of the strap to a top surface of the die with a first electrically conductive material;    forming a first set of corresponding apertures through respective ones of a second portion of the strap and the substrate; and,    attaching a bottom surface of the second portion of the strap to a top surface of the substrate with a second electrically conductive material such that the first set of corresponding apertures are in respective alignment with each other and such that the second conductive material flows into each of the apertures and forms an interlocking key therein.    
     
     
         11 . The method of  claim 10 , wherein respective ones of the first set of corresponding apertures are tapered toward the bottom surface of the second portion of the strap and the top surface of the substrate, respectively.  
     
     
         12 . The method of  claim 10 , wherein attaching the bottom surface of the first portion of the strap to the top surface of the die comprises: 
 forming a second set of apertures through the first portion of the strap; and,    attaching the bottom surface of the first portion of the strap to the top surface of the die such that the first electrically conductive material flows into each of the apertures and forms an interlocking key therein.    
     
     
         13 . The method of  claim 12 , wherein each of the second set of apertures is tapered toward the bottom surface of the cover portion.  
     
     
         14 . The method of  claim 12 , wherein the apertures are formed by an etching process.  
     
     
         15 . The method of  claim 14 , wherein the substrate comprises a lead frame, and further comprising etching the lead frame from a sheet of metal.  
     
     
         16 . The method of  claim 12 , further comprising roughening a surface on the conductive strap and the substrate.  
     
     
         17 . A semiconductor package, comprising: 
 a substrate having a top surface with central die-mounting region and an elongated recess therein adjacent to the die-mounting region;    a semiconductor die having a top surface and an opposite bottom surface mounted on the die-mounting region of the substrate; and,    an electrically conductive strap that includes: 
 a planar cover portion having a bottom surface attached to the top surface of the die with a first electrically conductive material,  
 a down-set portion at an edge of the cover portion that transitions from the cover portion to the substrate, and  
 an elongated flange portion at an edge of the down-set portion disposed within the recess and attached to the substrate with a second electrically conductive material.  
   
     
     
         18 . The semiconductor package of  claim 17 , wherein the recess has a floor disposed below the top surface of the substrate, and the flange portion of the strap has a bottom surface attached to the floor of the recess with the second electrically conductive material.  
     
     
         19 . The semiconductor package of  claim 17 , wherein at least one of the first and second electrically conductive materials comprises solder, an electrically conductive adhesive, or an electrically conductive elastomer.  
     
     
         20 . The semiconductor package of  claim 17 , wherein the flange portion of the strap has a plurality of apertures extending therethrough.  
     
     
         21 . The semiconductor package of  claim 20 , wherein the second conductive material extends into the aperture and forms an interlocking key therein.  
     
     
         22 . The semiconductor package of  claim 21 , wherein each of the apertures tapers toward the bottom surface of the flange portion of the strap.  
     
     
         23 . A semiconductor package, comprising: 
 a substrate having a top surface with central die-mounting region;    a semiconductor die having a top surface and an opposite bottom surface mounted on the die-mounting region of the substrate; and,    an electrically conductive strap that includes: 
 a planar cover portion having a bottom surface attached to the top surface of the die with a first layer of a conductive elastomer,  
 a down-set portion at an edge of the cover portion that transitions from the cover portion to the substrate, and  
 an elongated flange portion at an edge of the down-set portion having a bottom surface attached to the substrate adjacent to the die-mounting region with a second layer of a conductive elastomer.  
   
     
     
         24 . The semiconductor package of  claim 23 , wherein the bottom surface of the die is attached to the die-mounting region of the substrate with a third layer of a conductive elastomer.  
     
     
         25 . A semiconductor package, comprising: 
 a substrate having a top surface with central die-mounting region and a first set of apertures therethrough adjacent to the die-mounting region;    a semiconductor die having a top surface and an opposite bottom surface mounted on the die-mounting region of the substrate; and,    an electrically conductive strap that includes: 
 a planar cover portion having a bottom surface attached to the top surface of the die with a first electrically conductive material,  
 a down-set portion at an edge of the cover portion that transitions from the cover portion to the substrate, and  
 an elongated flange portion at an edge of the down-set portion having a set of apertures therethrough corresponding to the first set of apertures in the substrate and a bottom surface attached to the substrate with a second electrically conductive material such that corresponding apertures in the substrate and the flange portion are in respective alignment with each other, and such that the material of the second electrically conductive material flows into each of the apertures and forms an interlocking key therein.  
   
     
     
         26 . The semiconductor package of  claim 25 , wherein at least one of the first and second electrically conductive materials comprises solder, an electrically conductive adhesive, or an electrically conductive elastomer.  
     
     
         27 . The semiconductor package of  claim 25 , wherein respective ones of the first set of corresponding apertures in the substrate and the flange portion are tapered toward the bottom surface of the second portion of the strap and the top surface of the substrate, respectively.  
     
     
         28 . The semiconductor package of  claim 25 , wherein the cover portion of the strap includes a second set of apertures therethrough, and wherein the first electrically conductive material flows into each of the apertures and forms an interlocking key therein.  
     
     
         29 . The semiconductor package of  claim 28 , wherein each of the second set of apertures is tapered toward the bottom surface of the cover portion.