Effects of intermetallic compounds on properties of Sn–Ag–Cu lead-free soldered joints

The effects of alloy composition on microstructural, especially the formation of large intermetallic compounds, and mechanical properties of various Sn–Ag–Cu solder joints were investigated. The range of Ag–Cu content of Sn–Ag–Cu alloys was from 3.0 wt.%Ag–0.5 wt.%Cu to 3.9Ag–0.7 wt.%Cu. The high Ag content alloys exhibit the formation of large Ag 3Sn platelets especially at the solder–reaction layer interfaces, regardless of the kind of substrates. Long Cu 6Sn 5 whiskers are formed in all the solder–Cu joints and the high Cu content solder joints. Those whiskers have two shapes of needle and of hollow types. For the high Cu content solder joints with a 42 alloy substrate, long Ni–Cu–Sn whiskers are also formed at the interface and in the solder. The presence of large Ag 3Sn platelets does not degrade strength directly, but affects fracture mode. Large Ag 3Sn platelets induce brittle fracture at an interface and provide crack initiation sites. In order to avoid the formation of the large intermetallic compounds, especially Ag 3Sn platelets, the optimum composition of Sn–Ag–Cu alloy lies in the lower Ag–Cu content.