Improvement of strength-ductility trade-off in a Sn-0.7Cu-0.2Ni lead-free solder alloys through Al-microalloying

The enhancement in strength is generally attended by ductility loss in lead-free solder alloys, which is commonly denoted as strength-ductility trade-off (SDT). This work provides new viewpoint on the design of novel Sn-0.7Cu-0.2Ni (SCN) solders with the notably enhanced SDT, possessing yield stress (YS) of 40.8 MPa, tensile strength (UTS) of 42.9 MPa, and large uniform elongation up to 240%. Such breaking between competing properties of SDT is caused by microstructural modification that comprised with the arrangement of fine and coarse-sized phases in complex heterogeneous microstructures. Toward this end, the heterogeneous structures in SCN solder are produced by Al-microalloying, followed by cold-drawn and temperature-annealing. However, microalloying of 0.1% Al decreases the undercooling of SCN solder while 0.2% Al reduces the eutectic temperature. Our results show that the heterogeneous structure design strategy can offer a new generation of enhanced SDT alloys with boosting elastic compliance and plastic energy chock resistance for mobile products industry.