Optimizing the performance of Sn–Cu alloys via microalloying with Ni and Zn: a study on microstructure, thermal, and mechanical properties

Abstract

Microalloying is a critical technique for improving lead-free interconnections in electronic devices, as it selectively incorporates elements and significantly modifies the solidification structure. The current work investigates the effects of microalloying with Ni and Zn on the microstructures, thermal properties, and mechanical properties of Sn–0.7-wt% Cu solder alloy. The following experimental techniques were employed to evaluate the samples of Sn–0.7-wt% Cu alloy: scanning electron microscopy (SEM), optical microscopy (OM), X-ray diffraction (XRD), tensile tests, and differential scanning calorimetry (DSC). The experimental findings indicated that trace addition of Ni (0.05 wt%) could facilitate the formation of (Cu,Ni)₆Sn₅ IMCs in the interdendritic region, consequently refining the coarse β-Sn phase and resulting in a more refined grain structure. The addition of Zn (2.0 wt%) significantly affected the as-solidified microstructure, leading to the dissolution of Zn into Cu₆Sn₅ intermetallic compounds, characterized by both fine and coarse eutectic regions. Moreover, Cu₅Zn₈ phases were generated between the eutectic region and the refined β-Sn phase. The collaborative effect of Ni and Zn on Sn–0.7Cu alloy markedly improves its microstructure, leading to a refined, stable, and fine-grained Cu₆Sn₅ IMC. Additionally, the mechanical properties of the Sn–Cu alloy are enhanced by these structural differences. The results of tensile tests indicate that the Sn–0.7Cu–0.05Ni–2.0Zn solder alloy has superior mechanical properties in comparison to the Sn–Cu alloy. Specifically, the estimated increases in modulus of elasticity (EM), yield strength (YS), and ultimate tensile strength (UTS) are 375.47%, 19%, and 46.67%, respectively. However, this improvement in mechanical properties was accompanied by a decrease in ductility. The increased strength of Ni/Zn alloys was ascribed to the pinning action of (Cu,Ni)₆Sn₅ and Cu₅Zn₈ IMCs, which impede grain growth and the formation of interfacial IMCs. The DSC results showed slightly decrease in melting temperature values, with the additions of Ni and Zn resulting in values that were approximately 2.1 °C lower than those of the binary Sn–Cu alloys. In view of the results, this study offers important perspectives on soldering technology, which will help in the practical aspects of future soldering process strategies.