Evolution of nano-Ag3Sn particle formation on Cu-Sn intermetallic compounds of Sn3.5Ag0.5Cu composite solder/Cu during soldering

A Sn3.5Ag0.5Cu-0.5nano-TiO2 composite lead-free solder was prepared by adding 20 nm TiO2 to Sn3.5Ag0.5Cu (wt.%) solder. This study investigates the morphology of the intermetallic compounds (IMCs) formed during the soldering reactions between Sn3.5Ag0.5Cu-0.5nano-TiO2 solder and Cu substrates at various temperatures ranging from 250 to 325 degrees C. The Cu6Sn5 grains formed in all soldering below 300 degrees C were scallop-type, while those formed at both 300 degrees C and 325 degrees C were prism-type in the early stage of soldering (less than 30 min). Also, Cu6Sn5 grains that formed at both 300 degrees C and 325 degrees C changed from prism-type to scallop-type with increasing soldering time. It is quite interesting that the morphology of Cu6Sn5 grains affects absorption by nano-Ag3Sn particles. Especially, the scallop-type Cu6Sn5 grains formed by the ripening process are likely to be captured by the large amount of nano-Ag3Sn particles. These nanoparticles apparently decrease the surface energy and hinder the growth of the Cu6Sn5 IMC layer. In addition, the grain size of the nano-Ag3Sn compounds increased with increasing soldering temperature and time. All these results indicate that Gibbs absorption theory can be used to explain the formation of these nanoparticles and their effects on the surface energy of the IMCs. (C) 2010 Elsevier B.V. All rights reserved.