Reflow of low melt solder tip C4's

Abstract

An array of C4 solder bumps and a method for making is described incorporating an array of conductive areas on an electrical device, each conductive area having a layer of ball limited metalurgy at the device surface and two layers of solder having respective melting temperatures to form the C4 structure. The method includes melting the second layer of solder in the down position or towards earth to form a C4 solder ball or bump. The invention overcomes the problem of low temperature solder from wicking over the sidewall surfaces of the high melt solder of the C4 structure and attacking the edges of the underlying seed layers of the ball limited metalurgy.

Having thus described our invention, what we claim as new and desire to secure as Letters Patent is:  
     
         1 . A method for forming an array of C4 bumps on a first substrate of an electrical device comprising the steps of: 
 forming an array of conductive areas on said first substrate, each conductive area including a ball limited metalurgy to provide adhesion to said first substrate and a diffusion barrier,    forming a first layer of solder having a first melting temperature on said conductive areas, forming a second layer of solder having a second melting temperature lower than said first melting temperature on said first layer of solder,    applying a first force on said first and second layer of solder away from said ball limited metalurgy, and    heating said second layer of solder above said second melting temperature to cause said molten second layer to flow into a shape determined by said first force and a second force generated by said molten second layer of solder to wick to the edges of said first layer of solder,    said first force adjusted to prevent said melted second layer from wicking over exposed sidewall surfaces of said ball limited metalurgy.    
     
     
         2 . An array of C4 solder balls on a substrate of an electrical device comprising: 
 an array of conductive areas on said first substrate,    a first layer of solder having a first melting temperature on said respective conductive areas,    a second layer of solder having a second melting temperature lower than said first melting temperature on said first layer of solder on said respective conductive areas.    
     
     
         3 . The array of C4 solder balls according to  claim 2  wherein said first solder alloy is selected from the group consisting of 63% tin 37% lead, 60% indium 40% lead, 63% tin 36% lead 1% silver, 70% indium 30% lead, 43% lead 43% tin 14% bismuth 80% indium 15% lead 5% silver, 51% tin 31% lead 18% cadmium, 97% indium 3% silver, 58% bismuth 42% tin, 52% indium 48% tin, 46% bismuth 34% tin 20% lead and 44% indium 42% tin 14% cadmium (by weight).  
     
     
         4 . The array of C4 solder balls according to  claim 2  wherein said second solder alloy is selected from the group consisting of 97% lead 3% tin, 95% lead 5% tin, 92% lead 5% indium 3% silver and 92% lead 5% tin 3% silver (by weight).  
     
     
         5 . A reflow method for an array of C4 bumps on a first substrate wherein said C4 bumps includes a first layer of solder having a first melting temperature on a second layer of solder having a second melting temperature greater than said first melting temperature on said first substrate comprising the steps of: 
 positioning said array of C4 bumps wherein said first layer of solder is facing down with respect to said second layer of solder, and    reflowing said first layer of solder.    
     
     
         6 . The reflow method according to  claim 5 , further including the step of heating said C4 bumps to a temperature 20-40 degrees C higher than said first melting temperature but below said second melting temperature of said second solder.  
     
     
         7 . The reflow method according to  claim 5 , wherein upon the completion said step of reflowing, lowering the temperature of said C4 back to below melting temperature while maintaining the orientation of said first solder layer facing down.