Study on microstructure and properties of Sn-Ag-Cu-Mg lead-free solders

In the present work, Sn-Ag-Cu-Mg (SACM) and Sn-Ag-Cu-Mg-La (SACML) lead-free solders based on near eutectic Sn-Ag-Cu (SAC) alloys were designed and fabricated. Experiments were conducted to investigate the effect of alloying on the microstructure and physical properties in aspects of melting, wetting performance by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and differential thermal analysis (DTA), solderability tester respectively. It was shown that trace addition of Mg presents remarkable effect on modifying the microstructure and lower the melting temperature of the Sn-Ag-Cu solders. With the increase of trace Mg into SAC alloy, the eutectic phases are coarsened and the fraction of eutectic microstructure decrease. With the trace addition of La, the eutectic phases are mainly distributed along the grains boundaries. Though the melting temperature was reduced with the addition of trace Mg, the melting ranges of SACM and SACML are wider than that of SAC solder. The solderability of SACM and SACML solders are inferior to that of SAC solder.