Effect of deep cryogenic treatment on mechanical properties and microstructure of Sn3.0Ag0.5Cu solder

The effect of deep cryogenic treatment on the performance of steels and alloys has attracted wide attention in the past decades. Deep cryogenic treatment can improve the strength and hardness of steel at room temperature, provide microstructure stability and improve wear or fatigue resistance of material. In the current study, the effect of deep cryogenic treatment on the microstructure and mechanical properties of Sn3.0Ag0.5Cu solders are investigated. The influence to microstructure, tensile strength and ductility improvement are discussed. Experimental analysis shows that the tensile strength of Sn3.0Ag0.5Cu solder increases from 36.76 to 46.27 MPa after 600 h of deep cryogenic treatment at 77 K (− 196 °C), the observed strength-time relation is similar to the Taylor theory for the yield strength and dislocation density. Large particles presented in the fracture of Sn3.0Ag0.5Cu samples are caused by the high cooling rate as well as the concentration difference between the β-Sn and the eutectic system. The precipitated Ag₃Sn particles exhibit relatively uniform distribution in deep cryogenic treated Sn-rich matrix, and the size of Ag₃Sn particles becomes smaller with longer deep cryogenic treatment time. It is noted that deep cryogenic treatment can increase the internal stress and the dislocation density, higher dislocation density and good ductility lead to movement of the pre-existing dislocations and specific dislocation configurations. Microscopic experiments on solder joints were performed to investigate the microstructure change. The Intermetallic layers were measured which showed negligible change in thickness. A unified creep and plasticity constitutive model is proposed to simulate the stress–strain relationship under deep cryogenic treatment, the predictions show good agreement compared with experimental results.