Drop test simulation study of flexible devices

In this paper, a wrist device conception was selected to study the reliability of flexible devices by drop test simulation. In the conception device, all of the components cannot be made flexible immediately. Some of them are still rigid, such as the ball grid array (BGA), the connector, and some other big components like the vibrator. The reliability of the device, where such rigid components are mounted on a flexible plastic substrate, is anything but guaranteed. In this study, a system level simulation model of a drop test was developed. In this simulation model, there are some challenges, such as the uncertain initial shape of the assembled products, the long impact time, and the unknown mechanical behaviours of new materials. To solve these problems, some manual work and some model simplification were performed. For the flexible device, the reliability study focussed on the electrical connection and the assembly processes. Before building the model, structural analysis of this device had been carried out first. Based on this analysis, some points of potential concern were identified. Based on these points, several submodels were refined such as the display, the BGA solder joint, and the flexible printed circuit (FPC) copper trace. Seven drop simulations had been executed with different drop directions. There were six basic orthogonal drop directions and one additional drop direction, that was anticipated to have a serious impact on the reliability of the device. Through the simulations, some weak locations were pointed out by the observation of the stress level of Von Mises in the drop test. The results were delivered to the electronical and mechanical designer for reference in the new design and some actions were taken to optimize the design.