| dc.description.abstract | This dissertation investigated the reliability of eutectic PbSn flip chip solder joints under applied current of 0.32 A at 150°C. It includes the discussion of the failure mechanisms and two feasible methods for prolonging the lifetime of solder joints.
We propose a new failure mechanism in this dissertation. Microstructure examinations uncovered that the combined effect of current crowding and the accompanying local Joule heating accelerated the local Ni UBM consumption near the point of electron entrance. Once Ni was depleted at a certain region, this region became non-conductive and the flow of the electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the local Ni UBM consumption was accelerated. This process repeated itself, and the Ni-depleted region extended further on, creating an ever larger non-conductive region. The solder joint eventually failed when the nonconductive region became too large, making the effective current density very high.
To prolong the lifetime of solder joints under current stressing, two kinds of experimental setups were used. One is to experiment the solder joints with different Ni UBM thickness. In this part, three different Ni thicknesses in the Cu/Ni/Al UBM (0.3, 0.5, and 0.8 μm) were used in order to investigate the effect of the Ni thickness on the reliability. The solder joints with 0.8μm Ni UBM have the longest lifetime more than 1000 hours. The failure model is also able to support the observation that the joints with a thicker Ni tended to have a longer average lifetime.
The other is to experiment the solder joint with different UBM and surface finish combinations. In this part, two substrate surface finishes, Au/Ni and organic solderable preservative (OSP), were used to study the effect of the surface finish on the reliability of flip-chip solder joints under electromigration at 150 oC ambient temperature. It was found that the mean-time-to-failure (MTTF) of the OSP joints was six times better than that of the Au/Ni joints (3080 vs. 530 hrs). The key factor determining the MTTF was the Ni consumption rate. The joints with the OSP surface finish had a longer MTTF because Cu released from the substrate was able to reduce the Ni consumption rate.
Besides the study of the reliability of flip chip solder joints under current stressing, we also discuss the existence of melting solder at the last stage of current stressing. By microstructure examination and 3-D coupled thermoelectric simulation, it was found that due to the fact that Ni UBM has been completely consumed and almost replaced by the non-conductive porous structure that the local resistance abruptly raised at the region where there is almost no conduct area and caused excessive Joule heating generation to melt solder joint. | en_US |