Effect of cryogenic treatment on mechanical properties and microstructure of solder joint

In the current study, the effects of deep cryogenic treatment (DCT) on the microstructure and mechanical properties of Sn-3.0Ag-0.5Cu (SAC30) solder joints were investigated. the mechanism of microstructure, tensile strength and fracture path were discussed. The samples were placed in liquid nitrogen environment at 77 K for 24, 72, 168, and 600 hours. The experimental results show that the strength of SAC305 solder joint increased from 35.54 MPa at 0 h to 41.70 MPa at 72 h, 38.0 MPa at 168h, and 40.51 MPa at 600h. In addition, the ductility of the solder joints after 24 hours of deep cryogenic can be comparable to that of the SAC305 solder joints without cryogenic treatment. However, after a longer period of DCT, the strength and ductility of the solder joint decreased. This is because the cryogenic treatment will lead to an increase of the solder, meanwhile, the mechanical properties of intermetallics (IMC) decreased due to the mismatch of thermal expansion coefficient. The widespread application of DCT suggests that short-term cryogenic treatment may provide a possible processing method for solder joint. The thicknesses of the IMC layer was measured by graphic processing techniques. The experimental data show that the thickness of the IMC layer did not change. The fracture pattern changed from ductile fracture inside the solder to the interlayer fracture near IMC, and finally became brittle fracture of IMC.