Anti-tombstoning lead free alloys for surface mount reflow soldering

Abstract

A lead-free solder alloy composition comprising tin, silver and copper, and a process for reflow soldering for minimizing tombstoning frequency are disclosed. In one particular exemplary embodiment, the lead-free Sn—Ag—Cu solder alloys for minimizing the tombstoning effect of the present disclosure display high mass fraction during melting and prolonged melting as shown by a widened DSC peaks, that allows for a balanced surface tension on both ends of the chip component to develop. In accordance with further aspects of this exemplary embodiment, the alloys display a mass fraction of solid during melting greater than 20% and a DSC peak width greater than 8° C. using a 5° C./min scan rate. In accordance with further aspects of this exemplary embodiment, the alloy comprises on a weight basis Ag 1-4.5%, Cu 0.3-1% balanced with Sn.

1 . A lead-free anti-tombstoning solder alloy comprising tin, silver and copper which exhibit greater than 20% mass fraction of solid during melting.  
     
     
         2 . A lead-free anti-tombstoning solder alloy of  claim 1 , wherein the heat of absorption peak width, ΔT, is greater than 8° C. on a DSC (Differential Scanning Calorimetry) scan rate of 5° C./min.  
     
     
         3 . A lead-free anti-tombstoning alloy of  claim 2 , wherein the constituent metals comprise from about 1% to about 4.5% by weight of silver, from about 0.3% to 1% by weight of copper balanced with tin.  
     
     
         4 . A lead-free anti-tombstoning alloy of  claim 3 , wherein the constituent metals comprise from about 4% to about 2% by weight of silver, from about 0.5% to about 1% by weight of copper balanced with tin.  
     
     
         5 . A lead-free anti-tombstoning alloy of  claim 4 , wherein the constituent metals comprise from about 3.8% to about 2.5% by weight of silver, from about 0.5% to about 1% by weight of copper balanced with tin.  
     
     
         6 . A lead-free anti-tombstoning alloy compositions as in any of claims  1 - 5 , wherein the alloy includes a mechanical property improving element.  
     
     
         7 . A lead-free anti-tombstoning alloy composition of  claim 6 , wherein the mechanical property improving element comprises at least one or more element selected from the group consisting of Sb, Cu, Ni, Co, Fe, Mn, Cr and Mo in a total amount of at most 1 weight % of the solder alloy.  
     
     
         8 . A lead-free anti-tombstoning alloy compositions as in any of claims  1 - 5 , wherein the alloy includes a melting temperature lowering element.  
     
     
         9 . A lead-free anti-tombstoning alloy composition of  claim 8 , wherein the melting temperature lowering element comprises at least one or more element selected from the group consisting Bi, In and Zn in a total amount of at most 3 weight % of the solder alloy.  
     
     
         10 . A lead-free anti-tombstoning alloy compositions as in any of claims  1 - 5 , wherein the alloy includes an oxidation resistance improving element.  
     
     
         11 . A lead-free anti-tombstoning alloy composition of  claim 10 , wherein the oxidation resistance improving element comprises at least one or more element selected from the group consisting of P, Ga and Ge in a total amount of at most 0.5 weight % of the solder alloy.  
     
     
         12 . A process of reducing tombstoning effect during reflow soldering in electronic assemblies comprising the usage of an anti-tombstoing solder in said assemblies, wherein said solder comprises 1% to about 4.5% by weight of silver, from about 0.3% to 1% by weight of copper balanced with tin.  
     
     
         13 . A process of  claim 12 , wherein said solder exhibits greater than 20% mass fraction of solid during melting.  
     
     
         14 . A process of  claim 13 , wherein said solder comprises from about 4% to about 2% by weight of silver, from about 0.5% to about 1% by weight of copper balanced with tin.  
     
     
         15 . A process of  claim 14 , wherein said solder comprises from about 3.8% to about 2.5% by weight of silver, from about 0.5% to about 1% by weight of copper balanced with tin.  
     
     
         16 . A process as in any of claims  12 - 15 , wherein the alloy includes a mechanical property improving element.  
     
     
         17 . A process of  claim 16 , wherein the mechanical property improving element comprises at least or more element selected from the group consisting of Sb, Cu, Ni, Co, Fe, Mn, Cr and Mo in a total amount of at most 1 weight % of the solder alloy.  
     
     
         18 . A process as in any of claims  12 - 15 , wherein the alloy includes a melting temperature lowering element.  
     
     
         19 . A process of  claim 18 , wherein the melting temperature lowering element comprises at least one or more element selected from the group consisting of Bi, In and Zn in a total amount of at most 3 weight % of the solder alloy.  
     
     
         20 . A process as in any of claims  12 - 15 , wherein the alloy includes an oxidation resistance improving element.  
     
     
         21 . A process of  claim 20 , wherein the oxidation resistance improving element comprises at least one or more element selected from the group consisting of P, Ga and Ge in a total amount of at most 0.5 weight % of the solder alloy.