Improved strength of Ni and Zn-doped Sn–2.0Ag–0.5Cu lead-free solder alloys under controlled processing parameters

The Sn–Ag–Cu (SAC) solders with low Ag or Cu content have been identified as promising candidates to replace the traditional Sn–Pb solder for flip-chip interconnects. The changes in microstructure, microhardness and mechanical properties associated with the alloying of Ni and Zn to the Sn–2.0Ag–0.5Cu(SAC205) solders were explored in this work. Furthermore, correlations between the mechanical behavior and controlled processing parameters such as temperature, strain and strain rate were analyzed and established. Microstructure analysis showed that additions of Ni and Zn to SAC(205) alloy not only suppressed the formation of brittle Ag3Sn IMC phase but also refined the large β-Sn dendrites, and promoted the formation of new (Cu,Ni)6Sn5 and Cu5Zn8 intermetallic compounds (IMCs) in the SAC(205) solder. Besides, the IMC particles were uniformly distributed in the β-Sn matrix. Accordingly, the mechanical properties, the microhardness and total elongation of Ni and Zn-doped SAC(205) solders were significantly improved. Both the yield strength (YS) and ultimate tensile strength (UTS) were observed to increase with increasing strain rates and decreasing temperature. This behavior was attributed to the competing effects of work hardening and dynamic recovery processes. The Zn-doped solder consistently displayed the highest mechanical properties and ductility due to the formation of hard Cu5Zn8 IMC particles and the refinement of β-Sn grain size, which could provide more obstacles for dislocation pile up in the adjacent grains.