Influence Of Strain Rate Effect On Behavior Of Solder Joints Under Drop Impact Loading

Currently, traditional lead-based solders are banned and replaced by lead-free solders due to the harmfulness to the environment. However according to plenty of experiment results, the drop impact reliability of lead-free solder joints, which function as electric signal channels, thermal conductors and mechanical supports in microelectronic packages, exhibit orders of magnitude poorer performance than the lead-based solders. Therefore the mechanics behavior of lead-free solder joint under drop impact has become one crucial problem for the reliability of portable electronic products.In this paper, 4-point dynamic bending tests of board level electronic package were carried out in order to investigate the reliability of solder joints. A high speed camera and the digital image correlation method were used to measure the deflection of the PCB; the strains at PCB center were measured by strain gauges; the failure of solder joints was obtained by experiments. A finite element model of 4-point dynamic bending test was established and analyzed. Two simplified models of solder joints were proposed, and in order to discuss the feasibility of them, the deformation of the PCB and the stresses and strains of solder joints were compared with detailed model. The linear elastic, the rate-independent elastic-plastic material model and the rate-dependent Johnson-Cook model of lead-free solders were used to investigate the strain rate effect on the mechanical behaviors of solder joints under drop impact loadings. The failure of the solder joints was predicted and the results were compared with the experimental observation.The results show that simplified model of solder layer-ball has no influence on the deformation of PCB, but has significant effect on the stress and strain in solder joints. Therefore it is reasonable to apply solder layer-ball model when the failure of solder joints is predicted. The strain rate effect of lead-free solders has little influence on the deflection of the PCB during the drop impact loading, but has significant influence on the stress and strain in the solder joints. The rate-independent elastic-plastic material model always underestimates the stress and overestimates the equivalent plastic strain of the solder joint. The material model that takes the strain rate effect into account can predict more realistic behavior of the solder joints.All the conclusions above could be regarded as the basis for the failure prediction of solder joints in electronic package design. It also provides convenient and feasible method for the study of the reliability of electronic package.