A study of Sn-Bi-Ag-(In) lead-free solders

Sn-Bi-Ag-(In) solder alloys have been extensively studied in the study. The experimental results reveals that the liquidus temperatures of Sn-(1–5) Bi-(2–3.5)Ag-(0–10)In solders are between 201.7 and 225.3°C, which were higher than that of the most popular eutectic Pb-Sn solder (183°C). Additions of (5–10) wt% In into Sn-Bi-Ag solders can effectively decrease the melting point of the solder alloy. However, the gap between Ts and TL temperatures increases with the additions of Bi and In into Sn-Bi-Ag-(In) solders. Although there is no flux applied during soldering, most Sn-Bi-Ag-(In) solder alloys can well bond the Au/Ni metallized copper substrate. 94Sn-3Bi-3Ag solder demonstrates the lowest wetting angle of 45° among all test samples. Thermal expansion coefficients of both 94Sn-3Bi-3Ag and 90Sn-2Bi-3Ag-5In solders are slightly less than that of 63Sn-37Pb. Both 90Sn-2Bi-3Ag-5In/substrate and 94Sn-3Bi-3Ag/substrate interfaces demonstrate similar reaction kinetics in the experiment. The stability of the interface is greatly impaired during 90°C aging. Some locations of the electroless Ni layer break down, and new phases are formed nearby the interface during aging treatment. Initially, the growth of Ni-rich (Ni,Cu)3Sn4 phase dominates the interfacial reaction. However, the growth of Cu-rich (Cu,Ni)6Sn5 phase will dominate the reaction layer for specimens aged at 90°C for long time periods.