Method for low temperature bonding of electronic components

Abstract

A method for bonding an LED assembly 71 or other electronic package 31 to a substrate PCB containing a heat-sink 52 , which utilizes layers of reactive multilayer foil 51 disposed between contacts 32, 34 of the electronic package 31 and the associated contact pads 55 on the supporting substrate PCB. By initiating an exothermic reaction in the reactive multilayer foil 51 , together with an application of pressure, sufficient heat is generated between the contacts 32, 34 and the associated contact pads 55 to melt adjacent bonding material 54 to obtain good electrically and thermally conductive bonds between the contacts 32, 34 and contact pads 55 without thermally damaging the electronic package 31 , heat-sensitive components 35 associated with the electronic package 31 , or other the supporting substrate PCB.

1 . A method for bonding an electronic package having at least one electrically or thermally conductive contact to a supporting substrate having at least one associated electrically or thermally conductive contact disposed in a bonding region, comprising:
 coating each of said contacts with a layer of bonding material;   disposing a reactive multilayer foil between each of said contacts of said electronic package and said associated contacts of said supporting substrate;   applying a pressure to said contacts in the bonding region; and   initiating an exothermic reaction in said reactive multilayer foil, said exothermic reaction resulting in the formation of a bond containing said bonding material and the remains of said reactive multilayer foil between said electronic package contact and said associated substrate contact.   
     
     
         2 . The method of  claim 1  wherein said applied pressure is at least 1 MPa. 
     
     
         3 . The method of  claim 2  wherein said applied pressure is between 1 MPa and 9 MPa, and is applied to urge said electronic package towards said substrate. 
     
     
         4 . The method of  claim 2  wherein said applied pressure is between 10 MPa and 100 MPa, and is applied to urge said conductive contacts of said electronic package towards said associated contacts of said supporting structure. 
     
     
         5 . The method of  claim 1  wherein said layer of bonding material is a tin alloy. 
     
     
         6 . The method of  claim 1  wherein the reactive multilayer foil is coated with a metal or alloy selected from the group consisting of: tin, lead, indium, silver, Georo®, or Incusil®. 
     
     
         7 . The method of  claim 1  wherein said at least one layer of bonding material is a metal or alloy selected from the group comprising: gold, palladium, indium, Incusil® or silver. 
     
     
         8 . The method of  claim 1  wherein said layer of reactive multilayer foil disposed between each of said contacts has an area smaller than the area of the associated substrate contact defined by a solder mask surrounding the associated substrate contact. 
     
     
         9 . The method of  claim 1  wherein at least one of the electronic package contacts is of a type selected from the group consisting of: “gull-wing”, J-lead, flat lead, “leadless”, and ball grid array. 
     
     
         10 . The method of  claim 1  wherein said pressure is applied by an external pressure source comprising a spring or an elastomer. 
     
     
         11 . The method of  claim 1  wherein each of said layers of bonding material has a thickness between 4 μm and 30 μm. 
     
     
         12 . The method of  claim 1  wherein said reactive multilayer foil has a thickness less than 100 μm. 
     
     
         13 . The method of  claim 12  wherein said reactive multilayer foil has a thickness less than 50 μm. 
     
     
         14 . The method of  claim 1  wherein each of said layers of bonding material is a layer of tin having a thickness between 1 μm and 30 μm. 
     
     
         15 . The method of  claim 1  wherein said applied pressure is selected in response to the size of the contact area in said bonding region and to a desired shear strength of said resulting bond. 
     
     
         16 . A bonded electronic assembly, comprising:
 an electronic package having at least one electrically or thermally conductive contact lead coated with a first bonding material;   a supporting substrate comprising at least one electrically or thermally conductive contact pad coated with a second bonding material, and   the remains of a reactive multilayer foil disposed within at least one bond formed between said contact lead and said contact pad by said first and second bonding materials.   
     
     
         17 . The bonded electronic assembly of  claim 16  wherein said first and second bonding materials are tin or a tin alloy. 
     
     
         18 . The bonded assembly of  claim 16  wherein at least one of said first or second bonding materials is selected from either gold, silver, palladium, Georo®, Incusil® or an indium alloy. 
     
     
         19 . The bonded assembly of  claim 16  wherein the electronic package contains at least one light emitting diode. 
     
     
         20 . The bonded assembly of  claim 16  wherein the electronic package contains at least one photovoltaic device. 
     
     
         21 . The bonded assembly of  claim 16  wherein the supporting substrate is selected from the group consisting of a metal core printed circuit board, a heat sink, a polymer-based printed circuit board, and a flex-circuit material. 
     
     
         22 . The bonded assembly of  claim 16  wherein the type of electronic package is selected from the group consisting of: flip chip, chip on board, bare die, chip scale package, package on package, ball grid array, fine-pitch no-lead, leadless chip carrier, quad flat pack, plastic leaded chip carrier, flat lead, small outline package, DPAKs, and D2PAKs. 
     
     
         23 . The assembly of  claim 16  wherein at least one of the electronic package contacts is of a type selected from the group consisting of: “gull-wing”, J-lead, flat lead, “leadless”, and ball grid array. 
     
     
         24 . The assembly of  claim 16  wherein said electronic package includes one or more temperature sensitive components. 
     
     
         25 . A method for bonding a temperature-sensitive electronic package having at least one contact to a supporting substrate receiving contact disposed in a bonding region, comprising:
 disposing a reactive multilayer foil between said contact of said electronic package and said associated receiving contact of said supporting substrate;   applying a controlled pressure to said layer of reactive multilayer foil through said contacts; and   initiating an exothermic reaction in said reactive multilayer foil, said exothermic reaction resulting in the formation of a bond containing the remains of said reactive multilayer foil between said electronic package contact and said associated substrate receiving contact.   
     
     
         26 . The method of  claim 25  wherein said step of applying pressure maintains said applied pressure during said exothermic reaction and bond formation. 
     
     
         27 . The method of  claim 25  wherein said exothermic reaction generates sufficient thermal energy to raise the temperature and flow one or more bonding layers disposed between said contact and said receiving contact, said one or more bonding layers subsequently solidifying during formation of said bond. 
     
     
         28 . The method of  claim 27  wherein the temperature rise is substantially limited to said bonding region.