Investigation of intermetallic compounds in Sn-Cu-Ni lead-free solders

Abstract

Interfacial intermetallic compounds (IMC) play an important role in Sn-Cu lead-free soldering. The size and morphology of the intermetallic compounds formed between the lead-free solder and the Cu substrate have a significant effect on the mechanical strength of the solder joint. In the soldering process of Sn-Cu alloys, Cu 6 Sn 5 intermetallic compounds are formed. The complex structural behaviour of Cu 6 Sn 5 IMC is temperature-and composition-dependent and it is long since subject to scientific research. The Cu 6 Sn 5 phase basically exists in two crystal structures: hexagonal σ-Cu 6 Sn 5 (at temperatures above 186°C) and monoclinic σ'-Cu 6 Sn 5 (at lower temperatures). In the presence of Ni in the solder, the σ-σ' transformation does not occur, therefore, the σ-Cu 6 Sn 5 phase remains stable. In this study the role of Ni in the (Cu,Ni) 6 Sn 5 intermetallic compound in Sn-Cu lead-free solders was examined. Sn-Cu alloys with different Cu content (0.5 to 1 mass%) were modified through Ni addition. The morphology of the intermetallic compounds of the modified Sn-Cu alloys was investigated by optical microscopy (OM) and scanning electron microscopy (SEM), the IMC phases were examined with X-ray diffraction method (XRD).