Abstract:Trace amounts of La were utilized to improve the melting behaviors, microstructures, tensile properties and microhardness of Sn-3.0Ag-0.5Cu lead-free solder alloy. La has little effect on the melting behavior of Sn-3.0Ag-0.5Cu alloy according to the differential thermal analysis (DTA) tests. The X-ray diffraction (XRD) patterns show that β-Sn, Ag₃Sn and Cu₆Sn₅ coexist in the as-cast solder alloys and LaSn₃ phases emerge when adding 0.4% La. The microstructures modified by La are more uniform and much finer than that of baseline alloy, and the coarse LaSn₃ particles with complex dendrites are observed by optical microscopy (OM) and scanning electron microscopy (SEM) when the addition of La is up to 0.4%. The composition of the LaSn₃ phases is identified by energy-dispersive spectroscopy (EDS). There are considerable improvements in mechanical properties with 0.05% and 0.1% addition, but degenerations by adding 0.4%La. The Vickers microhardness of β-Sn and eutectic area are both enhanced with the addition of La and the microhardness of LaSn₃ is much larger than those of β-Sn and eutectic area.

Key words: lead-free solder; Sn-Ag-Cu alloy; rare earth element; La; tensile properties