Joint reliability and high temperature stability of Sn–Ag–Bi lead-free solder with Cu and Sn–Pb/Ni/Cu substrates

The mechanical properties and the microstructure were experimentally examined for the Sn–3 mass% Ag solders with 0, 3, and 6 mass% of Bi connecting two Cu substrates or two (Sn–Pb)/Ni/Cu substrates. For the solder between the Cu substrates, the strength increases from 90 to 140 MPa with increasing concentration of Bi from 0 to 6 mass%. On the other hand, the addition of Bi with 6 mass% decreases the strength from 100 to 50 MPa for the solder between the (Sn–Pb)/Ni/Cu substrates. Due to aging at 398 K (125 °C) for 1000 h, the strength remarkably decreases for the 6Bi solder between the Cu substrates but considerably increases for the 3Bi solder between the (Sn–Pb)/Ni/Cu substrates. Such remarkable decrease in the strength may be attributed to the segregation of Bi in the 6Bi solder after aging. At short aging times, the existence of Pb in the substrate may result in the deterioration of the strength of the solder.