Microstructure and tensile properties of Sn–Bi–Co solder alloy

Sn–Bi solder is one of the promising lead-free alternatives because it offers advantages such as low melting temperature, low cost, and good tensile strength. Unfortunately, the microstructure coarsening tendency in Sn–Bi solder leads to the embrittlement and reliability. Cobalt (Co) as an alloying element shows potential in improving the properties of Sn–Bi solder. In this study, the effects of Co addition as alloying element using the doped flux technique are investigated. The Sn–58Bi–Co solder melting temperature, microstructure, intermetallic compounds (IMCs) formation, and tensile properties are studied. Differential scanning calorimeter (DSC) analysis shows that, the addition of 1.0–2.0 wt% of Co to Sn–58Bi solder increases the melting temperature marginally in the range of 1.49 to 4.61 °C. The IMC layer thickness between Sn–58Bi–xCo solder and Cu substrate was measured using Olympus SZX10 AnalySIS software. The lowest thickness of Cu6Sn5 and Cu3Sn IMC layer was recorded for Sn–58Bi–1.5Co/Cu solder joint with 3.58 ± 0.33 µm. The presence of Co was also observed the flatten morphology of IMC layers. In terms of mechanical properties, Sn–58Bi–1.5Co solder showed the optimal tensile strength of 60.29 ± 0.18 MPa and modulus of elasticity of 4.14 ± 0.31 GPa, compared to 57.02 ± 0.39 MPa tensile strength and 4.17 ± 0.76 GPa modulus of elasticity of the control pure Sn–Bi solder. By addition of Co alloying element, it is highly anticipated that this new Sn–Bi–Co alloy can be applied in low-temperature soldering and heat-sensitive electronic devices, in an effort to replace lead-containing solder alloy.