A Study of Intermetallics in Cu-Sn system and Development of Sn-Zn Based Lead Free Solders

Abstract

In the electronic industry Pb-Sn solder is a very important material but Pb is toxic and has adverse effects on the environment and human beings. Due to the harmful effects of the Pb the use of the Pb-Sn solder alloys are being avoided and new Pb-free solder alloys are being used for electronic applications. This study is mainly based on the intermetallics that are formed in the Cu-Sn system and development of Sn-Zn based lead free solders. The aim was to understand the solidification of Cu-Sn alloys, the various intermetallics formed and their morphology. Thermal analysis of Sn-Zn based lead free alloys and their wetting characteristics has been analysed. The Pb-Sn solders are being replaced by the Sn-Cu, Sn-Ag, Sn-Ag-Cu, Sn-Zn and Sn-Zn-Bi alloys. Here the intermetallics formed between Cu-Sn during solidification of the molten solder on the Cu electrical contacts are also studied. The intermetallics are brittle in nature and this leads to fracture of the solder. In order to understand the Sn-Cu or Sn-Zn based lead free solder alloys it is essential to is understand the solidification of the molten alloys. This is why a few compositions of Cu-Sn alloy are selected and the various intermetallics formed during the solidification of the molten Cu-Sn alloys are analyzed. At high temperatures diffusion of Cu and Sn increases and as a result intermetallics of Cu-Sn are formed. The common intermetallics formed are Cu3Sn and Cu6Sn5. Sn-8.8Zn and Sn-8Zn-3Bi solder alloys has been developed. DSC analysis of these alloys has carried out to determine their melting points. It has been also observed that the addition of Bi increases the wettability and decreases the melting point of these alloys.