Stress/strength and reliability evaluations on UBM in different solder systems

UBM (Under Bump Metallurgy) reliability is one of the critical issues in the total reliability of a flip-chip bumping technology. Since the UBM materials and structures vary for different bumping technologies, the UBM strength and reliability need to be determined for each design and process. In addition, the stress that a UBM experiences during thermal cycles depends on the solder alloy used in the interconnect. Different solder alloys require different UBM structures and strengths to achieve good reliability thermal cycling. In this study, a simplified stress model is developed to determine the UBM stress during thermal cycling. A simplified stress model for the UBM strength is also developed. These models are used to predict the stress and strength of the UBM under the die pull test and the thermal cycle conditions for both eutectic and high lead solder systems. A methodology for using the pull test results to evaluate UBM reliability is also discussed. This methodology can be extended to the studies of UBMs with other solder systems.