Experimental and numerical study of the size effect on microstructure and mechanical behavior of Cu/Sn0.7Cu0.05Ni/Cu joints with very small solder volume

The dimension of solder micro-interconnects has been scaling down to meet the trend of miniaturization of electronic products and devices. In this study, Cu/Sn-0.7Cu-0.05Ni (SCN)/Cu sandwich solder joints with a very small thickness (or height), in the range of 10 to 100 μm, were prepared and the size effect on the microstructure and mechanical behavior of the joints were investigated systematically. The results show that the microstructure of the solder joints got refined with the depressed interfacial intermetallic compound particles in the solder matrix, and the morphology of the interfacial IMC layers changes from rod-like to scallop-like, as the thickness of the joints decreases from 100 to 10 μm. Moreover, the tensile strength of the solder joints increases drastically (by up to 38%) as the ratio of the IMC layer thickness to the joint´s thickness rises. Furthermore, the fracture mode changed from quasi-brittle fracture to brittle fracture while the crack propagated from the interface of solder/IMC to the interior of IMC layers. All these changes can be ascribed by the status of stress distribution and stress triaxiality in the joints, which were simulated by the finite element method (ANSYS 11.0). Finite element analysis results show that there is a high stress concentration at the edge of the interfacial area regardless of the thickness of the joints. Higher stress triaxiality is found in the joint with a smaller thickness, which means the crack may easily initiate and propagate at the interface of solder/IMC rather than in the solder matrix, and the simulation results generally show a good agreement with the experimental results.