Abstract
Solidification of 285 μm mono-size copper droplets, produced by controlled laminar-jet breakup, in nitrogen and helium gas and subsequently in oil was investigated. Oxygen content was maintained at 23 or 203 ppm in the nitrogen atmosphere and at 4 ppm in the helium atmosphere. Droplets generated in nitrogen with 203 ppm oxygen solidified into nearly spherical mono-size balls. Droplets generated in nitrogen with 23 ppm oxygen solidified into mono-size balls with poor sphericity and severe surface shrinkage. Droplets generated in helium solidified into mono-size balls with high sphericity and smooth surface. Surface shrinkage was caused by thermal dendritic solidification. A droplet solidification simulation model predicts that spherical balls with good surface are obtained under solidification conditions that permit sufficient liquid redistribution in the solidifying droplets. Controlled laminar jet breakup offers an economical means for the production of mono-size copper balls that are potentially applicable to electronics packaging.
