High-speed direct writing with metallic microspheres

Abstract

Ultra-small satellite droplets of molten metal, generated from capillary stream break-up, are selectively directed to predetermined locations on a substrate. The satellite droplets are produced at a high rate and are highly uniform because of the nature of capillary stream break-up. Applied harmonic disturbances are used to control and generate the satellite and parent droplets. The satellite droplets are electrostatically charged on a droplet-by-droplet basis and are then deflected by, e.g., an electric field to predetermined locations on a substrate. The substrate is moveable, attached to an x-y table, for facilitating the placement of satellite droplets on the substrate. The satellite droplets can be placed in individual locations on the substrate (e.g., for forming a ball grid array) or can be overlapped to form conductive traces. Because the satellite droplets have small diameters, these traces may have correspondingly small widths or pitches.

What is claimed is:  
     
         1 . A method of depositing metal onto a substrate comprising the steps of: 
 forming a stream of molten metal droplets comprising satellite droplets and parent droplets; and    directing at least a portion of the satellite droplets to predetermined locations on a substrate.    
     
     
         2 . The method of  claim 1  wherein the forming step further includes directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that the parent and satellite droplets form from the stream due to capillary stream break-up.  
     
     
         3 . The method of  claim 1 , wherein the directing step comprises: 
 imparting an electrostatic charge to at least some of the satellite droplets; and    passing the satellite droplets through an electric field,    wherein the electrostatic charge for each satellite droplet and the electric field are determined to deflect each satellite droplet to land at its corresponding predetermined location on the substrate.    
     
     
         4 . The method of  claim 1 , further comprising a step of translating the substrate in at least one direction substantially orthogonal to the capillary stream  
     
     
         5 . The method of  claim 1 , further comprising the step of recycling back into the molten metal the parent droplets and the satellite droplets that are not deposited onto the substrate.  
     
     
         6 . The method of  claim 5 , further comprising the step of filtering the molten metal.  
     
     
         7 . An electrical assembly comprising: 
 a substrate; and    a plurality of metal balls attached to the substrate, wherein each of the plurality of balls has a diameter in a range of about 1.0 to 100 microns.    
     
     
         8 . The electrical assembly of  claim 7 , wherein each of the plurality of balls has a diameter of less than 25 microns.  
     
     
         9 . The electrical assembly of  claim 7 , wherein the electrical assembly is a ball grid array.  
     
     
         10 . An electrical assembly comprising a substrate and a plurality of metallic traces attached thereto, the assembly produced by a process comprising the steps of: 
 directing a capillary stream of molten metal from an orifice by applying an excitation disturbance, wherein the excitation disturbance is determined so that parent and satellite droplets form from the stream due to capillary stream break-up; and    directing at least a portion of the satellite droplets to predetermined locations on the substrate.    
     
     
         11 . A circuit board comprising: 
 a substrate; and    a conductive trace disposed on the substrate, the conductive trace having a pitch in a range of about 5-100 microns, wherein the conductive trace comprises a plurality of solidified metallic droplets fused together to form an electrical connection therebetween.    
     
     
         12 . The circuit board of  claim 11 , wherein the conductive trace has a pitch less than about 25 microns.  
     
     
         13 . The circuit board of  claim 11 , wherein the pitch is within a range of about 3.0 to 5.0 percent of a mean pitch value.