Characterization of solidification process in sn-ag alloys using interrupted tests

Abstract

Recently, lead-free solders have been used in electronic equipments. Sn-Ag-Cu system is an important alloy for lead-free solders. There have been so many reports for mechanical properties of bonding. It is important to understand the evolution of solidified structure of these alloys, though, such reports are few. In order to understand the solidification process of Sn-Ag-Cu alloys, Sn-Ag alloys have been used in this study. Three alloys, hypo-eutectic, eutectic, and hyper-eutectic alloys have been prepared. The specimen was quenched during solidification and the solidified structure was interrupted and compared with thermal history. In the case of hypo-eutectic alloy, the specimen was composed by primary Sn and eutectic. It was found that a small undercooling was necessary for nucleation of eutectic. Such undercooling was not observed in eutectic and hyper-eutectic alloys. This may be interpreted by the difference in the liquid composition when the eutectic solidification starts. In the case of eutectic and hyper-eutectic alloys, the eutectic as well as β may form at almost the same time of recalescence. In the case of hyper-eutectic alloy, it was found that a large undercooling was not necessary for nucleation of Ag 3Sn, even though it was facetted phase. Since the primary Ag 3Sn was surrounded by the halo of β-Sn and eutectic the Ag 3Sn phase may be a site for nucleation of β-Sn and eutectic. © 2009 The Japan Institute of Metals.