SMTA International Conference Proceedings


CUMULATIVE AND SYNERGISTIC EFFECTS OF COMBINED MECHANICAL LOADING OF LEAD-FREE BGAS

Authors: Brian Roggeman and Peter Borgesen, Ph.D.
Company: Unovis-Solutions
Date Published: 10/11/2007   Conference: SMTA International


Abstract: With the popularity of portable electronics today the primary reliability concern has shifted from thermal cycling to mechanical loading conditions, such as drop, bend and vibration. Accordingly, there is a need to establish a comprehensive understanding of the mechanical robustness associated with these loading scenarios. This is complicated by various loading conditions as well as several competing failure modes and crack paths.

Current methodologies investigate the mechanical life under a single loading condition. However, actual service may include a combination of loading conditions, with contributions from low and high level drops, vibration and cyclic bend. While current methodologies are intended to model worst-case scenarios, they do not account for cumulative effects of other loading scenarios, or possibly more importantly, non-cumulative effects that may change the behavior of the system and affect reliability result. Solder joint properties may change over the course of cyclic loading, and this adds to the complexity of a general description, especially if it leads to a change in failure mode. Modeling damage from a single loading condition may be misleading. Extension to life in service becomes a large challenge as well, if we do not account for the actual service conditions.

The contribution of mechanical pre-stressing on drop test reliability was investigated by adapting a JEDEC style drop test. The pre-stressing was achieved by a much less severe drop, and was meant to simulate minor handling that an electronic device might see that may otherwise be considered negligible in terms of damage. Test vehicles include ball-grid array devices with SAC-based lead-free solder joints. Interestingly, the pre-stressing was found to have a positive effect on the drop survivability. Detailed failure analysis was conducted and failure modes were determined. The mechanical behavior of the solder joints was investigated, and it was determined through microhardness that the properties of the joints evolved due to the pre-stressing. The results of this study show that extension from board level testing to product lifetime may be more complicated than originally thought. Also, the calibration of Finite Element Modeling may be more challenging, as modeling does not generally consider the effects of solder joint evolution.

Keywords: drop test, pad cratering, failure modes, mechanical reliability



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