Lead-free solder balls and method for the production thereof

Abstract

Lead-free solder balls having a good surface appearance with no appreciable surface defects such as seams and shrinkage cavities comprises an alloy having a composition consisting essentially of about 4.0% to about 6.0% by weight of Ag, about 1.0% to about 2.0% by weight of Cu, and a balance of Sn, and they have a diameter of from 0.05 mm to 1.0 mm. The solder balls can be produced by forming a molten alloy having the above-described composition into solidified balls having a diameter of from 0.05 mm to 1.0 mm using the surface tension of the molten alloy.

What is claimed is:  
     
         1 . Lead-free solder balls for forming solder bumps on a substrate, the solder balls comprising an alloy having a composition consisting essentially of about 4.0% to about 6.0% by weight of Ag, about 1.0% to about 2.0% by weight of Cu, and a balance of Sn, the solder balls having a diameter of from about 0.05 mm to about 1.0 mm.  
     
     
         2 . The lead-free solder balls of  claim 1  wherein the alloy has a composition consisting essentially of about 5.7% by weight of Ag, about 1.3% by weight of Cu, and a balance of Sn.  
     
     
         3 . The lead-free solder balls of  claim 1  wherein the solder balls have a good surface appearance which is substantially free of surface defects which may interfere with the use of the solder balls.  
     
     
         4 . A method for producing lead-free solder balls comprising forming a molten alloy having a composition which consists essentially of about 4.0% to about 6.0% by weight of Ag, about 1.0% to about 2.0% by weight of Cu, and a balance of Sn into solidified balls having a diameter of from about 0.05 mm to about 1.0 mm using the surface tension of the molten alloy.  
     
     
         5 . The method of  claim 4  including dropping pieces of a solder alloy into an hot oil bath to allow the pieces to melt and allowing the molten pieces to descend within the bath to form them into solidified balls.  
     
     
         6 . The method of  claim 5  wherein the hot oil bath has a vertical temperature gradient with a temperature in an upper portion thereof which is high enough to melt the solder alloy and a temperature in a lower portion thereof which is low enough to solidify the molten alloy.  
     
     
         7 . The method of  claim 4  wherein forming the molten alloy into solidified balls comprises dripping the molten alloy through an orifice or nozzle.