Investigation Of Mechanical Behaviors Of Solder Joints Under Drop Impact Loading

Solder joints functioned as mechanical, thermal and electrical interconnections between electronic packages and the printed circuit board (PCB), their failure can lead to critical malfunction of electronic products directly. In recent years, with the increasing demand and popularity of portable telecommunication devices such as mobile phones and PDAs, the mechanical behavior of solder joints under drop impact conditions has drawn more and more attentions of researchers and engineers. As the strain rate have significant effect on the mechanical behavior of lead-containing and lead-free solders, it is important to investigate and understand behaviors of them under high strain rates. On the other hand, there are no applicable material models which can describe the high strain rate behaviors of solders currently. As a result, the linear elastic or rate-independent elastic-plastic material model was applied to solder joints in the drop impact simulations. These models neglect strain rate effects completely and obviously lead to inaccurate predictions of stresses and strains in solder joints. Therefore it is necessary to propose one preferable constitutive model which takes the strain rate effect into account.In this paper, the mechanical behaviors of Sn37Pb, Sn3.5Ag and Sn3.0Ag0.5Cu in quasi-static and at strain rates ranged from 600 s-1 to 2200 s-1 were investigated by a conventional materials tester and by the split Hopkinson pressure and tensile bar technique (SHPB and SHTB). The strain rate sensitivity, microstructures, fractography, tensile strengths and ductility of the three solders were obtained and discussed and compared with results from other researchers. Based on the experimental data we obtained, rate-independent tri-linear elastic-plastic material models and rate-dependent Johnson-Cook models for the three solders were proposed and validated.A 2-D beam model and a 3-D model of board level electronic package were established and the peeling stress in solder joints under static and dynamic loadings was analyzed. The proposed model was used to investigate the effect of PCB deflection induced by drop impact loading on the stress of the soldered joint array. Based on the analysis, an approach was proposed to reduce the computational scale of the problems, and its feasibility was discussed. The Johnson-Cook models were applied to predict behaviors of solder joints in a board level drop impact test specified by the JEDEC. Strain rate, peeling stress and equivalent plastic strain of the solder joints during the drop impact were predicted and compared with that by rate-independent material models. Finally, the cohesive zone model (CZM) was applied to simulate the damage process of solder joint under drop impact loading.The research results provide the mechanical behavior, the failure mechanism of solders and give necessary constitutive model for numerical simulation of solder joints under drop impact loading, which are helpful to reliability design of mobile electronic products.