An Approach to Board-Level Drop Reliability Evaluation with Improved Correlation with Use Conditions

The work presented in this paper has been carried out in order to find means to improve the existing methods of board-level drop reliability assessment to better represent the use environment loading conditions of modern portable electronic devices. To provide goals and guidance for the development work, eight commercially available smart phones from different manufacturers were evaluated for their free-fall drop response. The results show that the drop response can be divided conceptually into two parts: (i) forced bending/twisting of the component board at the moment of the impact, and (ii) subsequent (resonance) vibration of the component board. The strain magnitudes caused by the forced bending and twisting were much higher than those by the post impact vibrations. Furthermore, sharp strain peaks were often identified within a few milliseconds from the impacts. The results of the Finite Element simulations show that the distribution of strains on the component boards is highly non-uniform; The regions of high strain are localized because they are caused either by the forced bending (by surrounding covers and frames in part i) or by internal collisions between the component board and the surrounding mechanical structures (sharp strain peaks in part ii). The devices were characterized for maximum strain, average rate to maximum strain, vibration frequencies and maximum value of deceleration. The following goals were set for the development of a board-level drop test methodology: a test board that simulates the response of modern portable products should be able to produce board strain well above 3 500 mu ("micro-strain" = [10(-6) m/m]) and mean rate to maximum strain close to 7 s(-1).

Reaching these objectives requires significant changes to the existing JESD22-B111 approach. An investigation on the effects of the shape of deceleration input pulse was carried out first, followed by modifications for the board support to further increase the maximum strain and the average strain-rate of the JESD22-B111 compliant printed wiring board. The support was modified by replacing the four point supports by line supports at both ends of the rectangular shaped board. This approach can achieve the maximum strains of 5 000 mu when coupled with the optimized deceleration input. In addition to the increased strain and strain-rate, dampening of the board vibration became more effective, which is also in a better agreement with the typical drop response of portable electronic products.