Reliability analysis for the design of a multi-layer flexible board

The methodology for reliability analysis on the flexible electronics needs to be studied, both in the aspects of simulation and experimental tests. There appears to be very little work in the open literature on the reliability analysis of flexible electronics. In this article, we discuss FEM simulation in mechanical reliability analysis for the design of polyimide based multi-layer flexible printed wire boards. The reliability of the solder joints used to mount packages onto the flexible board under thermal cycling and drop test has been examined. An approximate simulation based approach has been developed to predict the equivalent material property of the multi-layer flexible board. The histogram feature in Adobe Photoshop has been introduced to evaluate the Cu percentage in a signal layer according to the drawing of the signal layer. Simplified FEA model has been built and a number of basic deformation modes have been simulated to retrieve the equivalent material property of a multi-layer flexible board. This approach has been verified by experimental measurement. In addition, it has been pointed out that for a multi-layer flexible board, it is not appropriate to just use the values of volume percentage of the components in a board as the weighting factors to evaluate the equivalent material property parameters of the board. The thermomechanical analysis has been conducted based on the industrial standard formula for solder fatigue life prediction relevant to the conventional PWB and package assembly. The simulation results are compared with experimental test data. The failure mechanism predicted by simulation is consistent with that observed in experimental tests. However, the simulation results of the solder fatigue life are about 1.8 times the test data. Further study is needed on extending or adjusting the conventional formula for solder fatigue life prediction so that it could be used in the case of multi-layer flexible board. The submodeling technique has been used in LS-DYNA to conduct strain and stress analysis on the critical solder during the board level drop test. Simulation results show that the solder shall fail first at the interface to the PWB side during the drop test. The effect of the change of the board thickness on the reliability of the solder joints has been investigated. It has been shown that the thinner the board, the smaller the peak values of the Von Mise stress and peeling strain at both the package and PWB sides of the solder joint interfaces. In addition, the correlation of the peeling strain on the critical solder with the bending strain on PWB near the critical solder has been examined. It has been shown that for a specific PWB platform, the more the bending strain on the board, the more the peeling strain on the solder during drop test. Based on this study, the solder and/or PWB reliability could be efficiently evaluated by the combination of simulation and experimental measurements. The methods discussed in this article shall be useful in conducting reliability analysis of a general multilayer flexible board.