Mechanical Behavior of Lead-Free Solder Micro-Joints under Electrical Conditions

In this paper, the uniaxial tensile behaviour of micro-scale Sn-3.0Ag-0.5Cu (SAC305) solder joints under electrical-mechanical coupling fields was studied. The diameter is 280 μm and is similar to the dimension of solder joints used in practical electronic devices. Such a microscale for solder joints is rarely investigated due to the preparation challenges especially for the investigations regarding constitutive behaviour. Thanks to the small diameter, the applied electric current density for the solder joints can be up to 8000 A/cm², which is relatively unexplored to date in the reported literature. In the present study, the uniaxial tensile force was measured with the increasing displacement for different electric current densities to address the effect of electric current on the mechanical properties of SAC305 solder joints. A closed relationship between current density and ultimate tensile stress was apparently observed according to the experimental results. In addition, when the fracture occurs in the solder joint, the elongation of the specimen was also collected and analyzed. Compared with previous experimental study, the solder micro-joints own much better tensile properties than the solder joints with larger diameters especially under a high current density. As an interesting phenomenon, an oscillation was observed only for the applied force-displacement responses subjected to the current density of 8000 A/cm². This was explained from the thermo-mechanical point of view but remains to be elucidated in further investigations.