Lead-free solder pastes with increased reliability

Abstract

The introduction of undesired intermetallic phases (IMP) is reduced in lead-free solder joints. For this purpose, solder pastes are prepared, which have a metal powder mixture that generates a melting range of at least about 5° Kelvin and preferably less than about 30° Kelvin. Suitable for this purpose are metal powder mixtures, in which the melting points of the metal powder components lie apart from each other more than about 5° Kelvin, particularly more than about 10° Kelvin, preferably more than about 15° K, and optionally less than about 30° Kelvin, particularly up to about 25° Kelvin. However, metal powder mixtures are also possible, in which the melting point spread can equal about 100° Kelvin and more, if the higher melting component dissolves in the melt below the melting point of this component, so that the melting range can be held under about 30° Kelvin. With these solder pastes undesired intermetallic phases can be held nearly arbitrarily low.

1 . A lead-free solder paste, comprising at least two metal powders, whose melting points or melting ranges are at least about 5° Kelvin apart from each other.  
   
   
       2 . The lead-free solder paste according to  claim 1 , wherein the melting points or melting ranges are at least about 10° Kelvin apart.  
   
   
       3 . The lead-free solder paste according to  claim 1 , wherein the sum of the metal powders is at least a ternary metal system.  
   
   
       4 . The lead-free solder paste according to  claim 3 , wherein the solder paste is a tin-silver-copper solder paste.  
   
   
       5 . The tin-silver-copper solder paste according to  claim 4 , wherein the powder mixture in total comprises about 2 to 5% silver and about 0.1 to 1% copper.  
   
   
       6 . The tin-silver-copper solder paste according to  claim 4 , wherein the powder mixture in total comprises about 3 to 4% silver and about 0.3 to 0.8% copper.  
   
   
       7 . The lead-free solder paste according to  claim 1 , wherein the melting points or melting ranges lie less than about 30° Kelvin apart from each other.  
   
   
       8 . The lead-free solder paste according to  claim 7 , wherein the melting points or melting ranges lie less than about 25° Kelvin apart.  
   
   
       9 . A method for producing a stable solder joint with reduced introduction of intermetallic phases into the solder joint, the method comprising providing components for alloying, and first generating a final alloy from the components during a soldering process for forming the solder joint, wherein the components provided for alloying contain fewer intermetallic phases than the final alloy of the older joint.  
   
   
       10 . A method for reducing intermetallic phases in a solder joint, the method comprising subjecting metal powder to a re-melting process to form a solder joint, wherein the metal powder is a mixture of at least two powders with a melting point or melting range differing by at least about 5° K.  
   
   
       11 . A method for manufacturing a solder paste, the method comprising mixing at least two metal powders having a melting point or melting range differing by at least about 5° K with a fluxing agent into a paste.  
   
   
       12 . The method according to  claim 9 , wherein the solder joint comprises at least two different metal powders for wafer bumping or printed circuit board assembly.  
   
   
       13 . The method according to  claim 9 , wherein the solder joint comprises at least two different metal powders on a printed circuit board assembly