Effect of 3.0wt. %Bi and 0.05wt. %Cr additions on the microstructure and tensile strength of the Sn3Ag0.5Cu/Cu solder joint

A new lead-free solder alloy Sn3Ag0.5Cu3Bi0.05Cr (SACBC) was prepared by adding 3.0 wt.% Bi and 0.05 wt.% Cr on the basis of the composition of Sn3Ag0.5Cu(SAC). The tensile strength and the growth behavior of the interfacial intermetallic compound (IMC) of SAC/Cu substrate and the SACBC/Cu solder joints during isothermal aging at 150 °C for 0, 24, 168, 500 and 1000 hours were comparatively investigated, respectively. The results showed that the SACBC alloy has a better melting temperature range of 210°C–217°C, comparing with that of the SAC. Due to the solid solution strengthening of Bi in Sn matrix, the tensile strength of the SACBC/Cu solder joints was obviously superior to that of the Cu/SAC, whether as-soldered or isothermal aged. The tensile strength of the Cu/SAC joints dramatically declined with aging time increasing, even after only aging for 24hr. The strength after 168h aging dropped by 32.1% comparing with that in as-soldered condition. When the aging time extended to 500hr, the strength decline slowed down and it held at 35 MPa, which was only about half of the original strength. On the contrary, the tensile strength decline for the Cu/SACBC joints was not noticeable and the strength almost kept stable from 168h to 1000h. Comparing with the as-soldered, it decreased by less than 5% after aging at 150 °C for 168h and only 5 MPa for 1000h (81.5 MPa). Therefore, 3.0 wt. % Bi and 0.05 wt. % Cr additions could significantly improve the solder joint bonding property and enhance the resistance to high temperature aging. The fracture mechanism for Cu/SACBC joints transformed from a quasi-cleavage to a cleavage-like brittle fracture. The developed new solder can effectively inhibit the overgrowth of IMC at the interface of the solder joints.