Viscoplastic characterization and post-rupture microanalysis of a novel lead-free solder with small additions of Bi, Sb and Ni

Abstract

Due to the RoHS and WEEE legislations for restricting the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment, developing novel Pb-free solders becomes a real challenge for many industrials in recent years. In addition, mechanical properties of the lead-free alloys are very important factors in the design and reliability evaluation of the soldered joints. This paper reports the findings of an investigation into the tensile properties of a new lead-free solder alloy which contains Ni, Bi and Sb additives (SAC387-3Bi-1.5Sb-0.15Ni). The fabrication procedure of the bulk samples is described in this study, as well as the mechanical testing of the obtained specimens. The tensile tests are conducted at temperatures between room temperature and 125 °C and under strain rates between 2.0 × 10−5 and 2.0 × 10−2 s−1. The results show that both temperature and strain rate may have great effects on the mechanical behavior of the solder alloy. The strength was found to decrease with increasing test temperature and decreasing strain rate. It is noted that the mechanical properties of SAC387-3Bi-1.5Sb-0.15Ni solder alloy are strongly dependent on temperature and significantly sensitive to strain rate. Compared with the reference material without additives, additions of Sb, Ni and Bi elements result globally in an increase in strength of the solder alloy. Also, the stress-strain curves from tensile tests are used to identify nine Anand material parameters by using non-linear least square fitting. The identified parameters prove to be in good agreement with those found in literature on other usual solder alloys. Based on the Anand model, a FEM analysis of a multilayered IGBT packaging module under cyclic thermal loading is presented to predict the fatigue life of the solder joints. In addition, SEM and EPMA microanalyses of both as-cast bulk and fracture specimens are carried out to assess effects of testing conditions on microstructure changes in the SAC387-3Bi-1.5Sb-0.15Ni solder material. The improved strength is found to result from the solid solution hardening effect of Sb and Bi in the Sn matrix, together with the formation of (Cu,Ni)6Sn5 and Ag3(Sn,Sb) intermetallic compounds (IMCs).