Structure and method for lead free solder electronic package interconnections

Abstract

An electronic package having a solder interconnect liquidus temperature hierarchy to limit the extent of the melting of the C4 solder interconnect during subsequent second level join/assembly and rework operations. The solder hierarchy employs the use of off-eutectic solder alloys of Sn/Ag and Sn/Cu with a higher liquidus temperature for the C4 first level solder interconnections, and a lower liquidus temperature alloy for the second level interconnections. When the second level chip carrier to PCB join/assembly operations occur, the chip to chip carrier C4 interconnections do not melt completely. They continue to have a certain fraction of solids, and a lower fraction of liquids, than a fully molten alloy. This provides reduced expansion of the solder join and consequently lower stresses on the C4 interconnect.

1 . An off-eutectic lead free solder composition consisting essentially of: 
 between 52.0-95.0 weight % Sn;    between 48.0-5.0 weight % Ag; and    having inter-metallics with a melting temperature greater than 250° C.    
     
     
         2 . The off-eutectic lead free solder composition of  claim 1 , wherein said composition is about: 
 72.0 weight % Sn;    28.0 weight % Ag; and    having dispersed grains of SnAg inter-metallic phase structure.    
     
     
         3 . The off-eutectic lead free solder composition of  claim 1 , wherein said composition is about: 
 82.0 weight % Sn;    18.0 weight % Ag; and    having dispersed grains of SnAg inter-metallic phase structure.    
     
     
         4 . The off-eutectic lead free solder composition of  claim 1 , wherein said composition is about: 
 88.0 weight % Sn;    12.0 weight % Ag; and    having dispersed grains of SnAg inter-metallic phase structure.    
     
     
         5 . The off-eutectic lead free solder composition of  claim 1 , wherein said composition is about: 
 52.0 weight % Sn;    48.0 weight % Ag; and    having dispersed grains of SnAg inter-metallic phase structure.    
     
     
         6 . An off-eutectic lead free solder composition consisting essentially of: 
 between 84.0-99.3 weight % Sn;    between 16.0-0.7 weight % Cu; and    having inter-metallics with a melting temperature greater than 250° C.    
     
     
         7 . The off-eutectic lead free solder composition of  claim 6 , wherein said composition is about: 
 84.0 weight % Sn;    16.0 weight % Cu; and    having dispersed grains of SnCu inter-metallic phase structure.    
     
     
         8 . The off-eutectic lead free solder composition of  claim 6 , wherein said composition is about: 
 93.0 weight % Sn;    7.0 weight % Cu; and    having dispersed grains of SnCu inter-metallic phase structure.    
     
     
         9 . A lead free solder hierarchy structure for electronic package interconnections comprising: 
 an electronic circuit chip attached to a top side of a chip carrier with a first lead free off-eutectic solder composition;    an array of lead free solder connections having a first end attached to a bottom side of said chip carrier with a second lead free off-eutectic solder composition, said second lead free off-eutectic solder composition having a lower liquidus temperature than said first lead free off-eutectic solder composition; and    a printed circuit board having a top side attached to a second side of said array of lead free solder connections by a third lead free solder composition, said third lead free solder composition having a lower liquidus temperature than said second off-eutectic lead free solder composition thereby creating a lead free hierarchy for electronic packaging interconnections.    
     
     
         10 . The lead free solder hierarchy of  claim 9  wherein said first lead free off-eutectic solder composition is about 72.0 weight % Sn and 28.0 weight % Ag, and having dispersed grains of SnAg inter-metallic phase structure and a liquidus temperature of approximately 400° C.; 
 wherein said second lead free off-eutectic solder composition is about 82.0 weight % Sn and 18 weight % Ag, and having dispersed grains of SnAg inter-metallic phase structure and a liquidus temperature of approximately 355° C.; and    wherein said third lead free solder composition is about 95.5 weight % Sn and 3.8 weight % Ag and 0.7 weight % Cu and has a liquidus temperature of approximately 217° C.    
     
     
         11 - 24 . (canceled)  
     
     
         25 . The off-eutectic lead free solder composition of  claim 1  wherein said composition further comprises approximately 0.01 weight % Bi to approximately 0.5 weight % Bi and approximately 0.01 weight % Sb to approximately 0.5 weight % Sb.  
     
     
         26 . The off-eutectic lead free solder composition of  claim 1  wherein said composition further comprises approximately 0.01 weight % to approximately 0.5 weight % of an element selected from the group consisting of Bi, Sb, In, Zn and Pd.  
     
     
         27 . The off-eutectic lead free solder composition of  claim 6  wherein said composition further comprises approximately 0.01 weight % Bi to approximately 0.5 weight % Bi and approximately 0.01 weight % Sb to approximately 0.5 weight % Sb.  
     
     
         28 . The off-eutectic lead free solder composition of  claim 6  wherein said composition further comprises approximately 0.01 weight % to approximately 0.5 weight % of an element selected from the group consisting of Bi, Sb, In, Zn and Pd.