Microstructure Evolution, Thermal and Mechanical Property of Co Alloyed Sn-0.7Cu Lead-Free Solder

The effect of Co addition (0.5-2.0 wt.%) on the microstructure evolution, thermal and mechanical property of Sn-0.7Cu (wt.%) solder was investigated. The microstructure of Sn-Cu alloy was refined with the addition of Co, and CoSn2 phase formed during solidification. The volume fraction of CoSn2 phase increases with the increase of Co content. The melting temperature of Sn-0.7Cu-xCo alloys increases with increasing Co content. The melting range of these alloys is small (< 4°C), and decreased with the increase of Co content. The ultimate tensile strength and microhardness of Sn-Cu-xCo (x = 0.5, 1.0, 1.5, 2.0) alloys increase with the increase of the Co content. The relationships between tensile strength/microhardness and Co content of Sn-Cu-xCo alloy are established. Meanwhile, Sn-0.7Cu-1.5Co (wt.%) and Sn-0.7Cu-2.0Co (wt.%) alloys revealed an indentation size effect. Different ISE behaviors of Sn-0.7Cu-xCo alloys are mainly related to the presence of CoSn2 phase and the volume fraction of it. The indentation creep property of Sn-Cu-xCo alloys is modified with the addition of Co. Enhanced creep property of the alloys is depended on the solid solution of Co and precipitate out of CoSn2 phase.