The microstructure and microhardness of Sn-3.5%Ag solders were explored in the cooling rate ranging from 0.08 to 104 K/s. Under rapid cooling condition, the strong kinetic undercooling effect leads to the actual solidification process starting at the temperature lower than the equilibrium eutectic point, and the actual metastable eutectic point shifts to the higher Ag concentration. Hence, the higher the applied cooling rate is, the more the volume fraction of primary β-Sn crystal forms. At the same time, the separation of primary β-Sn crystal favors restraining the formation of bulk Ag3Sn intermetallic compounds (IMCs) in solder due to the mismatch crystalline orientation relationship, those Ag3Sn phase separating through the eutectic reaction could hardly cling to the primary β-Sn crystal and grow up. Additionally, the Vickers hardness test shows that fine β-Sn and spherical Ag3Sn phase in the rapidly solidified alloy strongly improves the microhardness of the Sn-3.5%Ag solder.
Sn-Ag alloy system has been regarded as one of the most promising lead-free solder to substitute conventional Sn-Pb eutectic solder. But the formation of bulk Ag3Sn intermetallic compounds (IMCs) during reflow and post heattreatment significantly influences the performance of the solder joints. With an effort to clarify its microstructuralevolution as a function of slow cooling rates, the fraction of bulk IMCs within the slowly solidified Sn-4.0 wt pctAg solder was investigated by standard metallographic and compared with that detected by thermal analysis. It wasfound that the bulk IMCs fraction determined by thermal analysis corresponds quite well with the microstructureobservation results. In accordance with the conventional solidification theory, the lower the applied cooling rate, thefewer the amount of bulk Ag3Sn IMCs formed in Sn-4.0 wt pct Ag alloy. In addition, Vickers hardness measurementresults indicated that the relative coarse eutectic Ag3Sn IMCs distributing in the lamellar eutectic structure favoredthe improvement of the mechanical performance.
Jun SHEN+, Yongchang LIU, Yajing HAN, Peizhen ZHANG and Houxiu GAO College of Materials Science & Engineering, Tianjin University, Tianjin 300072, China
The abnormal growth of Ag3Sn inter-metallic compounds in eutectic Sn-3.5% Ag solder was investigated through high-temperature aging treatment. Microstructural evolutions of this solder before and after the aging treatment were observed by optical microscopy and scanning electron mi-croscopy. Precise differential thermal analysis was made to study the changes in enthalpies of the solder under different conditions. The results reveal that the water-cooled solder is in metastable thermodynamic state due to the high free energy of Ag3Sn nanoparti-cles, which sporadically distribute in the matrix as second-phase. The second-phase Ag3Sn nanoparti-cles aggregate rapidly and grow to form bulk inter-metallic compounds due to the migration of grain boundary between primary Sn-rich phase and the Ag3Sn nanoparticles during high temperature aging treatment.
