Flip-chip reliability: Comparative characterization of lead free (Sn/Ag/Cu) and 63Sn/Pb eutectic solder

There are three main reasons for considering Pb-free solder in flip chip applications: 1) The negative impact of Pb-based solder on the environment is a cornerstone of legislation passed in Europe and Japan, 2) The improvement in soft error rates due to the decrease in solder-bump-based alpha emissions, and 3) The increased reliability that certain of these alloys demonstrate as compared to 63Pb/Sn eutectic solder for flip chip applications. The reliability of a ternary Sn/Ag/Cu alloy for flip chip solder joints will be reported in this paper. Dominant failure mechanisms for given thermal stress regimes are well defined for 63Sn/Pb eutectic solder. Characterizing Sn/Ag/Cu solder reliability in comparison to 63Sn/Pb solder provides a true baseline for these thermal stress regimes and still allows for a broad search of mechanisms due to the change in alloy properties inherent in this new metallurgic system. Reliability characterization must examine both solder bump and under bump metallization (UBM) robustness, because the interaction between the two contributes to the overall efficacy of the structure. Reported in this work are thermal cycle, high temperature storage, and die shear test results demonstrating the solder bump reliability. Electromigration, multiple reflow, and bare die high temperature test results verifying the UBM robustness are also presented. In addition, the assembly-related details are reported in an effort to provide a foundation for improved yield.