Abstract
Lead-free Sn-3 Ag-x Zn solders and the intermetallic phases in the interface between Sn-3 Ag- 1 Zn solder and Alloy 42 were investigated in this study. The interface between the commercial Sn-3 Ag-0.5 Cu solder and Alloy 42 is also investigated and compared. By a technique of bond opening after taper grinding, the interface between solders and substrate could be observed.With the addition of zinc, the microstructure of Sn-Ag alloy is refined and becomes more uniform. The 1 % Zn-containing alloy has a nearly constant melting point at 217°C and exhibits the best corrosion resistance among the solder alloys studied in this work.On the interface of the Sn-3 Ag-1 Zn solder and Alloy 42, the most noticeable feature is the very long, narrow and thin sweet flag leaf (SFL) particles that densely scattered in a single layer on the interface plane. With a long reflow time of 60 minutes, the SFL particles can grow to a length of about 200μm with a width of several micrometers. There are also some column and cubic shaped phases on the interface, but they are in very small quantity.On the interface between Sn-3 Ag-0.5 Cu solder and Alloy 42, there are SFL phases lying on the interface and a few lump phases among the SFL phases. Similar to the cubic phase in the Sn-3 Ag-1 Zn/ Alloy 42 interface, they are in small quantity compared to SFL particles.The three intermetallic phases on the interface of Sn-3 Ag-1 Zn solder and Alloy 42 were identified as Ni3Sn4 by EPMA analysis and X-ray diffraction. The SFL particles on the interface of Sn-3 Ag-0.5 Cu solder and Alloy 42 were also identified as Ni3Sn4. In comparison, the iron concentration of column phase was a little bit higher than the other two phases. The formation of column phase was probably influenced by the flaws on the continuous FeSn2 layer of the interface.The mechanical properties of soldered specimens were investigated in this study by peel test. The addition of zinc effectively enhances the strength of Sn-Ag alloy. The peak loads of Sn-3 Ag-1 Zn solder joints were 35~50 % higher than that of Sn-3 Ag-0.5 Cu and Sn-3.5 Ag. All the fractures are in the solder region and the strength of the solder joints increases with the decrease in the thickness of solder region.