Abstract
Nowadays, due to the trend in having multiple functions and greater performance of consumer electronic products, traditional Al or Cu wire bonding technology is insufficient to serve this high density packaging. Flip chip technology was then developed to meet the requirement to have denser I/O array, smaller feature size of joints, and higher power. At this time, serious reliability problems were discovered as the result of electromigration due to higher current density applied in the solder bumps, and higher operating temperature. For further demands on multifunction, higher performance and rather smaller size, three-dimensional integrated circuit (3D IC) packaging technology is regarded as a promising future to meet those needs. As a result, micro bumps and through silicon via (TSV) are employed; however, with greater increased current density, the performance is highly affected by the reliability problems, and the failure mechanism of electromigration in this newly developed technology requires further studies. In this thesis, electromigration-induced failure on Sn2.5Ag lead-free micro bumps with Cu/Ni under bump metallization (UBM) and chip-on-chip (COC) configuration in three-dimensional integrated circuit samples were studied. Compared to flip chip solder joints, micro bumps in 3D IC samples exhibit better electromigration resistance and are capable to withstand a higher current density, which are attributed to the presence of thicker UBM and short bump height. No exhibited electromigration-induced failure was observed when current density was below 2x104A/cm2. After considering the effect of back stress, threshold current density to trigger electromigration in chip-on-chip samples was estimated to be 7.5x104A/cm2. When current density was higher than 7.5x104A/cm2 at an ambient temperature of 150oC, no void propagation through the whole bump opening was found; instead, electromigration-induced voids were observed at the cathode side of Al trace. The time-to-failure (TTF) of Al trace on three-dimensional integrated circuits was also studied, and the current density exponent (n) and the activation energy (Ea) were calculated as 1.78 and 1.22 eV, respectively.