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Investigation on Statistical Distribution of Electromigration Lifetime in Flip Chip Solder Bumps
Keywords: Electromigration, solder, flip chip
Electromigration (EM) phenomenon in flip chip solder connection is one of the emerging issues induced by the increasing power demand and the continuing scaling down of IC packaging features. EM induced failures occur mainly by void growth at the interface between the under bump metallurgy and the solder alloy. The corresponding failure times usually follow a lognormal distribution with the median lifetime depending on the current density, the temperature and the diffusion behavior of the interface. Since electromigration experiments are conducted at higher current densities and temperatures compared to operating conditions, extrapolations are needed to assess reliability at operating conditions. The extrapolated lifetimes depend exponentially on the lognormal standard deviation sigma. Existing solder EM studies showed that the standard deviation of ranged from 0.8~1.6, which is significantly higher than the average value for chip-level Cu/Al interconnects (~0.3). The large standard deviation will seriously affect the lifetime expectation when extrapolating the lifetime data to a different level of failure rate. In this study, EM lifetime data from flip chip solder systems with various materials and structure were collected. The relationship between the lifetime standard deviation and the experimental parameters was analyzed. In the mean time, the factors that control the electromigration lifetime statistics were discussed. Their impact was quantified using theoretical models and finite element analysis. The results were compared to the experimental data. Understanding of the sources of the variation will help to set up test guidelines as well as eliminate the extrinsic factors to improve solder EM performance.
Min Ding, Senior Packaging Engineer
Freescale Semiconductor Inc.
Austin, TX

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