Development of immersion sn plating process for low-alpha bump formation

As the flip-chip bump process becomes smaller pitched and lead free, several materials have been introduced for UBM preparation on substrate. Au/Ni/Cu and OSP are well known candidates, but it is also known that they have some concerns such as higher cost and temporal stability. Alternatively, immersion tin has recently been focused as an effective UBM preparation process material. During soldering process, this immersion tin melts and mixes with bump materials, but the prevention of its alpha particle emission has been insufficient and misunderstood in the current industry. Even though low-alpha materials are used in bump, conventional tin in UBM can be a major damaging source of alpha ray causing soft errors. In this study, the total low alpha process with our new low alpha immersion tin plating chemical is proposed. Furthermore, some concerns with the immersion tin prepared UBM such as wettability and missing bump are also discussed.