Understanding Solidification Kinetics and Microstructural Development of a Lead-Free Composite Solder

This paper presents and discusses the solidification of Sn-Ag eutectic solder and the effect of cooling rate on its microstructure formation with and without addition of nanosized copper powders. The composite solder mixture was mechanically mixed, and subsequently melted in an aluminum crucible placed in a temperature-controlled furnance, prior to promoting solidification. For all cases significant undercooling of the melt was observed prior to solidification. A reduction in the solidification temperature occurred by the addition of copper nanopowders. Optical microscopy observations revealed the microstructure of solders, in the as-solidified condition, to be appreciably altered by the addition of nanosized copper powders. The copper nanopowders were precipitated as an intermetallic compound. The average size of the intermetallic compound decreased as the cooling rate increased. These results suggest that adding nanosize metallic powders may significantly alter and/or improve solder joint properties.